Monday, 30 December 2019

Research paper of asthma and other algeric reaction.

Everyone reacts to medications differently. One person may develop a rash while taking a certain medication, while another person on the same drug may have no adverse reaction. Does that mean the person with the rash has an allergy to that drug? All medications have the potential to cause side effects, but only about 5 to 10% of adverse reactions to drugs are allergic. Whether allergic or not, reactions to medications can range from mild to life-threatening. It is important to take all medications exactly as your physician prescribes. Call your doctor if you have side effects that concern you, or you suspect a drug allergy has occurred. If your symptoms are severe, seek medical help immediately. Allergic Reactions Allergy symptoms are the result of a chain reaction that starts in the immune system. Your immune system controls how your body defends itself. For instance, if you have an allergy to a particular medication, your immune system identifies that drug as an invader or allergen. Your immune system may react to medications in several ways. One type of immune reaction is due to production of antibodies called Immunoglobulin E (IgE) specific to the drug. These antibodies travel to cells that release chemicals, triggering an immediate allergic reaction. This reaction causes symptoms in the nose, lungs, throat, sinuses, ears, lining of the stomach or on the skin and usually occurs within minutes to a few hours of taking the drug. The most common immune response to a drug is due to the expansion of T cells, a type of white blood cell that recognize the drug as foreign. These T cells orchestrate a delayed immune response that most often affects the skin, causing itchy rashes, and occurs days to weeks after exposure to the drug. Most allergic reactions occur within hours to two weeks after taking the medication and most people react to medications to which they have been exposed in the past. This process is called "sensitization." However, rashes may develop up to six weeks after starting certain types of medications. The most severe form of immediate allergic reactions is anaphylaxis (an-a-fi-LAK-sis). Symptoms of anaphylaxis include hives, facial or throat swelling, wheezing, light-headedness, vomiting and shock. Most anaphylactic reactions occur within one hour of taking a medication or receiving an injection of the medication, but sometimes the reaction may start several hours later. Anaphylaxis can result in death, so it is important to seek immediate medical attention if you experience these symptoms. Antibiotics are the most common culprit of anaphylaxis, but more recently, chemotherapy drugs and monoclonal antibodies have also been shown to induce anaphylaxis. The most severe form of delayed drug reactions not only cause rashes but may also involve other organs including the liver, kidneys, lungs, and heart. Blisters may be a sign of serious drug reactions called Stevens-Johnson Syndrome and Toxic epidermal necrolysis (TEN), where the surfaces of your eye, lips, mouth and genital region may be eroded. You should seek medical help immediately if you experience any of these. Many medications can cause these severe delayed reactions including antibiotics, medications for epilepsy (seizures), depression and gout. However, not all drug allergic reactions involve a specific immune reaction. Some people experience flushing, itching or a drop in blood pressure from intravenous dyes used in x-rays or CT scans. If you take angiotensin converting enzyme (ACE) inhibitors for high blood pressure, you may develop a cough or facial and tongue swelling. In addition, some people are sensitive to aspirin, ibuprofen or other non-steroidal anti-inflammatory drugs (NSAIDs). One type of aspirin or NSAID sensitivity may cause a stuffy nose, wheezing and difficulty breathing. This is most common in adults with asthma and in people with nasal polyps (benign growths). Other reactions to NSAIDs can result in hives or in rare instances, severe reactions can result in shock. A number of factors influence your chances of having an adverse reaction to a medication. These include: genetics, body chemistry, frequent drug exposure or the presence of an underlying disease. Also, having an allergy to one drug predisposes an individual to have an allergy to another unrelated drug. Contrary to popular myth, a family history of a reaction to a specific drug typically does not increase your chance of reacting to the same drug. Non-Allergic Reactions Non-allergic reactions are much more common than drug allergic reactions. These reactions are usually predictable based on the properties of the drugs involved. Symptoms of non-allergic drug reactions vary, depending on the type of medication. People being treated with chemotherapy often suffer from vomiting and hair loss. Certain antibiotics irritate the intestines, which can cause stomach cramps and diarrhea. Taking Precautions It is important to tell your physician about any adverse reaction you experience while taking a medication. Be sure to keep a list of any drugs you are currently taking and make special note if you have had past reactions to specific medications. Share this list with your physician and discuss whether you should be avoiding any particular drugs or if you should be wearing a special bracelet that alerts people to your allergy. When to See an Allergist / Immunologist If you have a history of reactions to different medications, or if you have a serious reaction to a drug, an allergist / immunologist, often referred to as an allergist, has specialized training to diagnose the problem and help you develop a plan to protect you in the future. Healthy Tips • Allergic drug reactions account for 5 to 10% of all adverse drug reactions. Any drug has the potential to cause an allergic reaction. • Symptoms of adverse drug reactions include cough, nausea, vomiting, diarrhea, and headaches. • Skin reactions (i.e. rashes, itching) are the most common form of allergic drug reaction. • Non-steroidal anti-inflammatory drugs, antibiotics, chemotherapy drugs, monoclonal antibodies, anti-seizure drugs and ACE inhibitors are frequent causes of allergic drug reactions. • Contrary to popular myth, a family history of a reaction to a specific drug typically does not increase your chance of reacting to the same drug. • If you have a serious adverse reaction, it is important to contact your physician immediately.

Everyone reacts to medications differently. One person may develop a rash while taking a certain medication, while another person on the same drug may have no adverse reaction. Does that mean the person with the rash has an allergy to that drug? All medications have the potential to cause side effects, but only about 5 to 10% of adverse reactions to drugs are allergic. Whether allergic or not, reactions to medications can range from mild to life-threatening. It is important to take all medications exactly as your physician prescribes. Call your doctor if you have side effects that concern you, or you suspect a drug allergy has occurred. If your symptoms are severe, seek medical help immediately. Allergic Reactions Allergy symptoms are the result of a chain reaction that starts in the immune system. Your immune system controls how your body defends itself. For instance, if you have an allergy to a particular medication, your immune system identifies that drug as an invader or allergen. Your immune system may react to medications in several ways. One type of immune reaction is due to production of antibodies called Immunoglobulin E (IgE) specific to the drug. These antibodies travel to cells that release chemicals, triggering an immediate allergic reaction. This reaction causes symptoms in the nose, lungs, throat, sinuses, ears, lining of the stomach or on the skin and usually occurs within minutes to a few hours of taking the drug. The most common immune response to a drug is due to the expansion of T cells, a type of white blood cell that recognize the drug as foreign. These T cells orchestrate a delayed immune response that most often affects the skin, causing itchy rashes, and occurs days to weeks after exposure to the drug. Most allergic reactions occur within hours to two weeks after taking the medication and most people react to medications to which they have been exposed in the past. This process is called "sensitization." However, rashes may develop up to six weeks after starting certain types of medications. The most severe form of immediate allergic reactions is anaphylaxis. Symptoms of anaphylaxis include hives, facial or throat swelling, wheezing, light-headedness, vomiting and shock. Most anaphylactic reactions occur within one hour of taking a medication or receiving an injection of the medication, but sometimes the reaction may start several hours later. Anaphylaxis can result in death, so it is important to seek immediate medical attention if you experience these symptoms. Antibiotics are the most common culprit of anaphylaxis, but more recently, chemotherapy drugs and monoclonal antibodies have also been shown to induce anaphylaxis. The most severe form of delayed drug reactions not only cause rashes but may also involve other organs including the liver, kidneys, lungs, and heart. Blisters may be a sign of serious drug reactions called Stevens-Johnson Syndrome and Toxic epidermal necrolysis (TEN), where the surfaces of your eye, lips, mouth and genital region may be eroded. You should seek medical help immediately if you experience any of these. Many medications can cause these severe delayed reactions including antibiotics, medications for epilepsy (seizures), depression and gout. However, not all drug allergic reactions involve a specific immune reaction. Some people experience flushing, itching or a drop in blood pressure from intravenous dyes used in x-rays or CT scans. If you take angiotensin converting enzyme (ACE) inhibitors for high blood pressure, you may develop a cough or facial and tongue swelling. In addition, some people are sensitive to aspirin, ibuprofen or other non-steroidal anti-inflammatory drugs (NSAIDs). One type of aspirin or NSAID sensitivity may cause a stuffy nose, wheezing and difficulty breathing. This is most common in adults with asthma and in people with nasal polyps (benign growths). Other reactions to NSAIDs can result in hives or in rare instances, severe reactions can result in shock. A number of factors influence your chances of having an adverse reaction to a medication. These include: genetics, body chemistry, frequent drug exposure or the presence of an underlying disease. Also, having an allergy to one drug predisposes an individual to have an allergy to another unrelated drug. Contrary to popular myth, a family history of a reaction to a specific drug typically does not increase your chance of reacting to the same drug. Non-Allergic Reactions Non-allergic reactions are much more common than drug allergic reactions. These reactions are usually predictable based on the properties of the drugs involved. Symptoms of non-allergic drug reactions vary, depending on the type of medication. People being treated with chemotherapy often suffer from vomiting and hair loss. Certain antibiotics irritate the intestines, which can cause stomach cramps and diarrhea. Taking Precautions It is important to tell your physician about any adverse reaction you experience while taking a medication. Be sure to keep a list of any drugs you are currently taking and make special note if you have had past reactions to specific medications. Share this list with your physician and discuss whether you should be avoiding any particular drugs or if you should be wearing a special bracelet that alerts people to your allergy. When to See an Allergist / Immunologist If you have a history of reactions to different medications, or if you have a serious reaction to a drug, an allergist / immunologist, often referred to as an allergist, has specialized training to diagnose the problem and help you develop a plan to protect you in the future. Healthy Tips • Allergic drug reactions account for 5 to 10% of all adverse drug reactions. Any drug has the potential to cause an allergic reaction. • Symptoms of adverse drug reactions include cough, nausea, vomiting, diarrhea, and headaches. • Skin reactions (i.e. rashes, itching) are the most common form of allergic drug reaction. • Non-steroidal anti-inflammatory drugs, antibiotics, chemotherapy drugs, monoclonal antibodies, anti-seizure drugs and ACE inhibitors are frequent causes of allergic drug reactions. • Contrary to popular myth, a family history of a reaction to a specific drug typically does not increase your chance of reacting to the same drug. • If you have a serious adverse reaction, it is important to contact your physician immediately. The AAAAI's Find an Allergist / Immunologist service is a trusted resource to help you find a specialist close to home. This article has been reviewed by Thanai Pongdee, MD, FAAAAI

Thursday, 26 December 2019

History of biology

History of biology

The history of biology traces the study of the living world from ancient to modern times. Although the concept of biology as a single coherent field arose in the 19th century, the biological sciences emerged from traditions of medicine and natural history reaching back to ayurveda, ancient Egyptian medicine and the works of Aristotle and Galen in the ancient Greco-Roman world. This ancient work was further developed in the Middle Ages by Muslim physicians and scholars such as Avicenna. During the European Renaissance and early modern period, biological thought was revolutionized in Europe by a renewed interest in empiricism and the discovery of many novel organisms. Prominent in this movement were Vesalius and Harvey, who used experimentation and careful observation in physiology, and naturalists such as Linnaeus and Buffon who began to classify the diversity of life and the fossil record, as well as the development and behavior of organisms. Antonie van Leeuwenhoek revealed by means of microscopy the previously unknown world of microorganisms, laying the groundwork for cell theory. The growing importance of natural theology, partly a response to the rise of mechanical philosophy, encouraged the growth of natural history (although it entrenched the argument from design).

Before biology, there were several terms used for the study of animals and plants. Natural history referred to the descriptive aspects of biology, though it also included mineralogy and other non-biological fields; from the Middle Ages through the Renaissance, the unifying framework of natural history was the scala naturae or Great Chain of Being. Natural philosophy and natural theology encompassed the conceptual and metaphysical basis of plant and animal life, dealing with problems of why organisms exist and behave the way they do, though these subjects also included what is now geology, physics, chemistry, and astronomy. Physiology and (botanical) pharmacology were the province of medicine. Botany, zoology, and (in the case of fossils) geology replaced natural history and natural philosophy in the 18th and 19th centuries before biology was widely adopted.[4][5] To this day, "botany" and "zoology" are widely used, although they have been joined by other sub-disciplines of biology.

Prior to embarking for Europe for a medical conference in 1999, I was handed a book by one of my surgical residents entitled, “A Traveler’s Guide to the History of Biology and Medicine” by Dr. Eric T. (Ted) Pengelley. As I had already planned on touring some historical sites during my trip, I packed the guide and on the flight read the section on Italy, my intended destination. Intrigued by what I read, I visited three of the featured sites in the guide and found their descriptions to be highly accurate and informative. I felt that I had gained much from incorporating these sites into my trip and that the guide’s information greatly enhanced the experience of the actual visit. Eager to learn more, I finished Pengelley’s guide upon my return to the United States. However, on subsequent trips both here and abroad, I found that some of the information in the guide was dated or changed. I researched to see if another edition of the guide was available, and my search led me directly to the author himself. I contacted Dr. Pengelley directly and was quite pleased when he graciously accepted my call. During our conversation, he informed me he was no longer interested in updating the guide because his wife Daphne, who had co-authored the book, had passed away some years earlier and that he himself was then 82 years old. I asked Dr. Pengelley if he would allow me the opportunity to update and rewrite the book, and he quite willingly granted my request, transferring the rights to the work to me. The information you will be perusing is almost entirely the original work by Ted and Daphne Pengelley. Their obvious passion for the subject resulted in beautifully styled descriptions that I do not believe could be improved or enhanced in any way. Instead, what I have done is update areas where needed. I have verified site information and have obtained additional pictures to supplement the descriptions. Where appropriate, I have noted changes in museum or exhibit locations and included additional points of interest not covered in the first edition. And for medically-minded travelers, I have highlighted and added sites dealing directly with the history of medicine and, in particular, my own field of surgery. I would have preferred, out of my deep respect and admiration of such a learned and charitable man, that Dr. Pengelley be the one to write this introduction. Sadly, though, he passed away before I had the chance to approach him to consider it or for him to see how the culmination of his life’s research and personal discoveries will now guide even more individuals through its publication on the Internet. I hope this guide will enhance your studies and travels and ignite a passion for the rich history of medicine both in the United States and abroad, and may the remarkable discoveries of those who have ventured before propel you in your own quests to advance modern science and medicine. Sir Isaac Newton once said, “If I have seen further, it is by standing upon the shoulders of giants.” Dr. Pengelley, I thank you for just those shoulders. Julius P. Bonello, M.D. Email comments to Dr. Bonello at jbonello48@hotmail.com This site is sponsored by:

Aristotle, and nearly all Western scholars after him until the 18th century, believed that creatures were arranged in a graded scale of perfection rising from plants on up to humans: the scala naturae or Great Chain of Being.[22] Aristotle's successor at the Lyceum, Theophrastus, wrote a series of books on botany—the History of Plants—which survived as the most important contribution of antiquity to botany, even into the Middle Ages. Many of Theophrastus' names survive into modern times, such as carpos for fruit, and pericarpion for seed vessel. Dioscorides wrote a pioneering and encyclopaedic pharmacopoeia, De Materia Medica, incorporating descriptions of some 600 plants and their uses in medicine. Pliny the Elder, in his Natural History, assembled a similarly encyclopaedic account of things in nature, including accounts of many plants and animals.[23]

Systematizing, naming and classifying dominated natural history throughout much of the 17th and 18th centuries. Carl Linnaeus published a basic taxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced scientific names for all his species.[34] While Linnaeus conceived of species as unchanging parts of a designed hierarchy, the other great naturalist of the 18th century, Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility of common descent. Though he was opposed to evolution, Buffon is a key figure in the history of evolutionary thought; his work would influence the evolutionary theories of both Lamarck and Darwin.[35]

Up through the 19th century, the scope of biology was largely divided between medicine, which investigated questions of form and function (i.e., physiology), and natural history, which was concerned with the diversity of life and interactions among different forms of life and between life and non-life. By 1900, much of these domains overlapped, while natural history (and its counterpart natural philosophy) had largely given way to more specialized scientific disciplines—cytology, bacteriology, morphology, embryology, geography, and geology.

Wednesday, 25 December 2019

Vaccination and it's benefits.

Vaccination and it's benefits

There are many reasons to get an influenza (flu) vaccine each year. Flu vaccination can keep you from getting sick with flu. Flu vaccine prevents millions of illnesses and flu-related doctor’s visits each year. For example, during 2017-2018, flu vaccination prevented an estimated 6.2 million influenza illnesses, 3.2 million influenza-associated medical visits, 91,000 influenza-associated hospitalizations, and 5,700 influenza-associated deaths. During seasons when the flu vaccine viruses are similar to circulating flu viruses, flu vaccine has been shown to reduce the risk of having to go to the doctor with flu by 40 percent to 60 percent. Flu vaccination can reduce the risk of flu-associated hospitalization for children, working age adults, and older adults. Flu vaccine prevents tens of thousands of hospitalizations each year. For example, during 2017-2018, flu vaccination prevented an estimated 91,000 flu-related hospitalizations. A 2014 studyexternal icon showed that flu vaccine reduced children’s risk of flu-related pediatric intensive care unit (PICU) admission by 74% during flu seasons from 2010-2012. In recent years, flu vaccines have reduced the risk of flu-associated hospitalizations among older adultsexternal icon on average by about 40%. A 2018 study showed that from 2012 to 2015, flu vaccination among adults reduced the risk of being admitted to an intensive care unit (ICU) with flu by 82 percent. Flu vaccination is an important preventive tool for people with chronic health conditions. Flu vaccination has been associated with lower rates of some cardiac eventsexternal icon among people with heart disease, especially among those who had had a cardiac event in the past year. Flu vaccination can reduce worsening and hospitalization for flu-related chronic lung disease, such as in persons with chronic obstructive pulmonary disease (COPD). Flu vaccination also has been shown in separate studies to be associated with reduced hospitalizations among people with diabetesexternal icon and chronic lung diseaseexternal icon. Flu vaccination helps protect women during and after pregnancy. Vaccination reduces the risk of flu-associated acute respiratory infection in pregnant women by about one-half. A 2018 studyexternal icon that included influenza seasons from 2010-2016 showed that getting a flu shot reduced a pregnant woman’s risk of being hospitalized with flu by an average of 40 percent. A number of studies have shown that in addition to helping to protect pregnant women, a flu vaccine given during pregnancy helps protect the baby from flu for several months after birth, when he or she is not old enough to be vaccinated. Flu vaccine can be life-saving in children. A 2017 study was the first of its kind to show that flu vaccination can significantly reduce a child’s risk of dying from flu. Flu vaccination has been shown in several studies to reduce severity of illness in people who get vaccinated but still get sick. A 2017 study showed that flu vaccination reduced deaths, intensive care unit (ICU) admissions, ICU length of stay, and overall duration of hospitalization among hospitalized flu patients. A 2018 studyexternal icon showed that among adults hospitalized with flu, vaccinated patients were 59 percent less likely to be admitted to the ICU than those who had not been vaccinated. Among adults in the ICU with flu, vaccinated patients on average spent 4 fewer days in the hospital than those who were not vaccinated. Getting vaccinated yourself may also protect people around you, including those who are more vulnerable to serious flu illness, like babies and young children, older people, and people with certain chronic health conditions.
1. IntroductionImmunization methods cover :routine vaccinations in children and adolescents under national immunization programs,vaccinations in adults from risk groups (due to clinical recommendations, e.g. chronic diseases, and epidemiological recommendations, e.g. occupation, scheduled travels),ring vaccination strategy (vaccination of a ring of close contacts of an ill person; it is a strategy used to stop an epidemic, as in the case of smallpox eradication in India) andcocoon vaccination strategy.A cocoon vaccination strategy refers to vaccinations in persons from the immediate environment of those patients who might develop an illness (they are susceptible to illnesses) but cannot be vaccinated due to permanent or temporary medical contraindications to a vaccination (e.g. patients in immunosuppression) or are too young to have a vaccination .Most frequently, a cocoon vaccination strategy is associated with vaccinations in adults aimed at preventing the spread of an illness in children (e.g. pertussis vaccination or influenza vaccination), but it is worth considering whether this strategy should not be understood also as vaccinations in children with the view of protecting adults and the elderly against illnesses (e.g. influenza or pneumococcal diseases) .The aim of the cocoon strategy is to minimize the risk of the transmission of pathogens in the environment of a patient who is susceptible to an infection. A vaccinated patient is not a source of infection any more for a non-vaccinated patient .

A cocoon vaccination strategy refers to vaccinations in persons from the immediate environment of those patients who might develop an illness (they are susceptible to illnesses) but cannot be vaccinated due to permanent or temporary medical contraindications to a vaccination (e.g. immunosuppressed patients) or are too young to have a vaccination. Most frequently, a cocoon vaccination strategy is associated with vaccinations in adults aimed at preventing the spread of an illness in children (e.g. pertussis vaccination or influenza vaccination), but it is worth considering whether this strategy should not be understood also as vaccinations in children with the view of protecting adults and the elderly against illnesses (e.g. influenza or pneumococcal diseases). The aim of the cocoon strategy is to minimize the risk of the transmission of pathogens in the environment of a patient who is susceptible to an infection. A vaccinated patient is not a source of infection any more for a non-vaccinated patient. The chapter presents a history, current implementation of the strategy in different countries, its benefits and limitations.

AbstractA cocoon vaccination strategy refers to vaccinations in persons from the immediate environment of those patients who might develop an illness (they are susceptible to illnesses) but cannot be vaccinated due to permanent or temporary medical contraindications to a vaccination (e.g. immunosuppressed patients) or are too young to have a vaccination. Most frequently, a cocoon vaccination strategy is associated with vaccinations in adults aimed at preventing the spread of an illness in children (e.g. pertussis vaccination or influenza vaccination), but it is worth considering whether this strategy should not be understood also as vaccinations in children with the view of protecting adults and the elderly against illnesses (e.g. influenza or pneumococcal diseases). The aim of the cocoon strategy is to minimize the risk of the transmission of pathogens in the environment of a patient who is susceptible to an infection. A vaccinated patient is not a source of infection any more for a non-vaccinated patient. The chapter presents a history, current implementation of the strategy in different countries, its benefits and limitations.

Various disputes have arisen over the morality, ethics, effectiveness, and safety of vaccination. Some vaccination critics say that vaccines are ineffective against disease  or that vaccine safety studies are inadequate.  Some religious groups do not allow vaccination and some political groups oppose mandatory vaccination on the grounds of individual liberty. In response, concern has been raised that spreading unfounded information about the medical risks of vaccines increases rates of life-threatening infections, not only in the children whose parents refused vaccinations, but also in those who cannot be vaccinated due to age or immunodeficiency, who could contract infections from unvaccinated carriers (see herd immunity).Some parents believe vaccinations cause autism, although there is no scientific evidence to support this idea. In 2011, Andrew Wakefield, a leading proponent of the theory that MMR vaccine causes autism, was found to have been financially motivated to falsify research data and was subsequently stripped of his medical license.[80] In the United States people who refuse vaccines for non-medical reasons have made up a large percentage of the cases of measles, and subsequent cases of permanent hearing loss and death caused by the disease.

Sunday, 22 December 2019

Notes about DNA class 10 and 12 ICSE board

DNA



DNA oligos are short, single- and double-stranded synthetic DNA sequences that can be used in nearly any molecular biology application. Using the Oligo Entry ordering tool, you can design your oligos to contain any DNA sequence you require and select from hundreds of modifications to suit your specific research needs.Every DNA oligo you receive will be deprotected and desalted to remove small molecule impurities. In addition, your oligos will be quality controlled via proprietary ESI-mass spectrometry methods *, and quantified twice by UV spectrophotometry to provide accurate yield measurements.* With the exception of mixed base oligos, which could potentially represent multiple sequences and therefore cannot be accurately evaluated by ESI mass spectrometry.For standard, desalted oligos ≥20 bases, we offer the following yield guarantees:ProductAvailable lengthGuaranteed yield*25 nmol DNA oligo15–60 bases10 nmol100 nmol DNA oligo10–90 bases30 nmol250 nmol DNA oligo5–100 bases50 nmol1 µmol DNA oligo5–100 bases200 nmol5 µmol DNA oligo5–100 bases1000 nmol10 µmol DNA oligo5–100 bases2000 nmol* Guaranteed yield for unmodified oligos 25–50 bases; will vary with oligo composition, length, and purification.

Biologists in the 1940s had difficulty in accepting DNA as the genetic material because of the apparent simplicity of its chemistry. DNA was known to be a long polymer composed of only four types of subunits, which resemble one another chemically. Early in the 1950s, DNA was first examined by x-ray diffraction analysis, a technique for determining the three-dimensional atomic structure of a molecule (discussed in Chapter 8). The early x-ray diffraction results indicated that DNA was composed of two strands of the polymer wound into a helix. The observation that DNA was double-stranded was of crucial significance and provided one of the major clues that led to the Watson-Crick structure of DNA. Only when this model was proposed did DNA's potential for replication and information encoding become apparent. In this section we examine the structure of the DNA molecule and explain in general terms how it is able to store hereditary information.

Scientists have been storing digital data in DNA since 2012. That was when Harvard University geneticists George Church, Sri Kosuri, and colleagues encoded a 52,000-word book in thousands of snippets of DNA, using strands of DNA’s four-letter alphabet of A, G, T, and C to encode the 0s and 1s of the digitized file. Their particular encoding scheme was relatively inefficient, however, and could store only 1.28 petabytes per gram of DNA. Other approaches have done better. But none has been able to store more than half of what researchers think DNA can actually handle, about 1.8 bits of data per nucleotide of DNA. (The number isn’t 2 bits because of rare, but inevitable, DNA writing and reading errors.)

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA). Mitochondria are structures within cells that convert the energy from food into a form that cells can use.

Within a cell, DNA is organized into dense protein-DNA complexes called chromosomes. In eukaryotes, the chromosomes are located in the nucleus, although DNA also is found in mitochondria and chloroplasts. In prokaryotes, which do not have a membrane-bound nucleus, the DNA is found as a single circular chromosome in the cytoplasm. Some prokaryotes, such as bacteria, and a few eukaryotes have extrachromosomal DNA known as plasmids, which are autonomous, self-replicating genetic material. Plasmids have been used extensively in recombinant DNA technology to study gene expression.

A DNA molecule consists of two long polynucleotide chains composed of four types of nucleotide subunits. Each of these chains is known as a DNA chain, or a DNA strand. Hydrogen bonds between the base portions of the nucleotides hold the two chains together (Figure 4-3). As we saw in Chapter 2 (Panel 2-6, pp. 120-121), nucleotides are composed of a five-carbon sugar to which are attached one or more phosphate groups and a nitrogen-containing base. In the case of the nucleotides in DNA, the sugar is deoxyribose attached to a single phosphate group (hence the name deoxyribonucleic acid), and the base may be either adenine (A), cytosine (C), guanine (G), or thymine (T). The nucleotides are covalently linked together in a chain through the sugars and phosphates, which thus form a “backbone” of alternating sugar-phosphate-sugar-phosphate (see Figure 4-3). Because only the base differs in each of the four types of subunits, each polynucleotide chain in DNA is analogous to a necklace (the backbone) strung with four types of beads (the four bases A, C, G, and T). These same symbols (A, C, G, and T) are also commonly used to denote the four different nucleotides—that is, the bases with their attached sugar and phosphate groups.

The chemical DNA was first discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943. In 1953 James Watson and Francis Crick, aided by the work of biophysicists Rosalind Franklin and Maurice Wilkins, determined that the structure of DNA is a double-helix polymer, a spiral consisting of two DNA strands wound around each other. The breakthrough led to significant advances in scientists’ understanding of DNA replication and hereditary control of cellular activities.

The presence or absence of DNA evidence at a crime scene could mean the difference between a guilty verdict and an acquittal. DNA is so important that the United States government has spent enormous amounts of money to unravel the sequence of DNA in the human genome in hopes of understanding and finding cures for many genetic diseases. Finally, from the DNA of one cell, we can clone an animal, a plant or perhaps even a human being.

DNA has many advantages for storing digital data. It’s ultracompact, and it can last hundreds of thousands of years if kept in a cool, dry place. And as long as human societies are reading and writing DNA, they will be able to decode it. “DNA won’t degrade over time like cassette tapes and CDs, and it won’t become obsolete,” says Yaniv Erlich, a computer scientist at Columbia University. And unlike other high-density approaches, such as manipulating individual atoms on a surface, new technologies can write and read large amounts of DNA at a time, allowing it to be scaled up.

Notes about Eukaryotes. Class 10 and 12 notes

Eukaryotes

The earliest widely accepted fossil evidence of photosynthetic eukaryotes is Bangiomorpha, a red alga deposited ∼1.1 billion y ago (Bya) . However, recent reports of multicellular photosynthetic eukaryotes at ∼1.6 Bya provide evidence for an earlier establishment of photosynthesis within the eukaryotes . Currently, the oldest reliable evidence for eukaryotes as a whole is found in ∼1.7 billion-y-old rocks . These cyst-like microfossils occur in low-diversity assemblages that potentially include stem group eukaryotes or stem representatives of extant major taxa (14⇓⇓–17). Sterane biomarkers originally viewed as evidence for 2.7 Ga eukaryotes have now been reinterpreted as younger contaminants (15, 16). Only around 750–800 Mya do fossils show a major increase in eukaryotic diversity that includes recognizable green algae (e.g., Cladophorales) (14, 17, 18), radiations possibly related to the evolution of eukaryovores—eukaryotes that eat other eukaryotes .

Early Evolution of the Eukaryote Lineage.Recent years have seen significant improvements in our understanding of eukaryotic origins. It is now evident that eukaryotes do not constitute a third primary lineage of life, at least not as originally proposed by Woese and Fox . Instead, phylogenomic analyses minimally agree that the eukaryotic lineage evolved from within the Archaea (71⇓⇓–74). Furthermore, it is uncontroversial that the establishment of the crown eukaryotes involved a stable endosymbiosis between an archaeon and an alpha-proteobacterium . A topic of current debate in eukaryotic evolution is whether an amitochondriate protoeukaryotic lineage ever existed. Bioenergetic arguments have been marshalled to disfavor this view , suggesting that only a mitochondriate cell can acquire the complexity observed in living eukaryotes, but these arguments are not universally accepted (78, 79). A recent genomic study suggested that the mitochondrion might have entered the protoeukaryotic lineage late, when the stem eukaryotic cell was already cytologically rather complex . However, these results have been shown to represent a methodological artifact . Irrespective of how eukaryote-like the stem eukaryotes were, all crown eukaryotes are mitochondriate, and therefore, their last common ancestor must also have contained mitochondria . It is not yet clear when the protomitochondrial endosymbiosis was established . However, because the free-living ancestor of the mitochondrion was an alpha-proteobacterium, at the very least, this event must have postdated the separation of the alpha-Proteobacteria from their sister lineage (the group including the beta- and gamma-Proteobacteria and possibly other less well-known lineages, like the Acidithiobacillia and Zetaproteobacteria) . Given the capacity of mitochondria for aerobic respiration, this might well have happened after the Great Oxygenation Event.Our analyses help to constrain the origin of eukaryotes in so far as they suggest that the endosymbiotic event between a crown eukaryote and a cyanobacterium resulted in the origin of the Archaeplastida by ∼1.9 Bya. This is consistent with currently available fossil evidence and the view that eukaryotes, minimally, must postdate the diversification of Archaea and the node separating the alpha-Proteobacteria from their sister lineage. How much older the base of the eukaryotes might be from the base of the Archaeplastida is unknown and will, in part, depend on the topology of the eukaryotic tree, which is still not fully resolved .

Elongation Factors; TranslationJ. Parker, in Encyclopedia of Genetics, 2001Elongation Factors in Archaea and EukaryotesThe eukaryotes have elongation factors that perform the same functions as EF-Tu, EF-Ts, and EF-G. The eukaryotic equivalent of EF-Tu is EF-1α, and there is high sequence conservation between EF-Tu and EF-1α. EF-1α is also one of the most abundant cytoplasmic proteins in eukaryotes. Genes for this protein are often present in more than one copy and may have cell-type or stage-specific regulation.The eukaryotes have a complex of proteins, EF-1β, EF-1γ, and EF-1δ, which function in a nucleotide exchange reaction like that involving EF-Ts. The factors EF-1β and EF-1δ are closely related to each other, but none of these proteins is closely related to EF-Ts.The eukaryotic equivalent of EF-G is called EF-2. Like EF-G, it is responsible for the GTP-dependent translocation step of the ribosome. It also contains a diphthamide residue, a unique posttranslational modification of a histidine residue, which is the cellular target for ADP ribosylation by diphtheria toxin.Interestingly, the elongation factors of the archaea are more closely related to those of the eukaryotes than they are to those of bacteria, and, therefore, factors from the archaea are given the same nomenclature as those from eukaryotes. The only elongation factor in the archaea that is more closely related to a bacterial factor than to the one from the eukaryotes is the elongation factor that brings selenocysteinyl-tRNA to the ribosome.As for the prokaryotes, there are almost certainly other eukaryotic protein factors involved in elongation. For instance, the fungi have a factor called EF-3 which has both ATPase and GTPase activities.

J. Parker, in Encyclopedia of Genetics, 2001Elongation Factors in Archaea and Eukaryotes says the eukaryotes have elongation factors that perform the same functions as EF-Tu, EF-Ts, and EF-G. The eukaryotic equivalent of EF-Tu is EF-1α, and there is high sequence conservation between EF-Tu and EF-1α. EF-1α is also one of the most abundant cytoplasmic proteins in eukaryotes. Genes for this protein are often present in more than one copy and may have cell-type or stage-specific regulation.The eukaryotes have a complex of proteins, EF-1β, EF-1γ, and EF-1δ, which function in a nucleotide exchange reaction like that involving EF-Ts. The factors EF-1β and EF-1δ are closely related to each other, but none of these proteins is closely related to EF-Ts.The eukaryotic equivalent of EF-G is called EF-2. Like EF-G, it is responsible for the GTP-dependent translocation step of the ribosome. It also contains a diphthamide residue, a unique posttranslational modification of a histidine residue, which is the cellular target for ADP ribosylation by diphtheria toxin.Interestingly, the elongation factors of the archaea are more closely related to those of the eukaryotes than they are to those of bacteria, and, therefore, factors from the archaea are given the same nomenclature as those from eukaryotes. The only elongation factor in the archaea that is more closely related to a bacterial factor than to the one from the eukaryotes is the elongation factor that brings selenocysteinyl-tRNA to the ribosome.As for the prokaryotes, there are almost certainly other eukaryotic protein factors involved in elongation. For instance, the fungi have a factor called EF-3 which has both ATPase and GTPase activities.

You can’t survive without mitochondria, the organelles that power most human cells. Nor, researchers thought, can any other eukaryotes—the group of organisms we belong to along with other animals, plants, fungi, and various microscopic creatures. But a new study has identified the first eukaryote that has ditched its mitochondria, suggesting that our branch on the tree of life may be more versatile than researchers thought. “This is a discovery of fundamental importance,” says evolutionary biologist Eugene Koonin of the National Center for Biotechnology Information in Bethesda, Maryland, who wasn’t connected to the study. “We now know that eukaryotes can live happily without any remnant of the mitochondria.” Mitochondria are the descendants of bacteria that settled down inside primordial eukaryotic cells, eventually becoming the power plants for their new hosts. Although mitochondria are a signature feature of eukaryotes, scientists have long wondered whether some of them might have gotten rid of the organelles. The diarrhea-causing microbe Giardia intestinalis for a time seemed mitochondria-free, but on closer investigation, it and other suspects proved to be false alarms, containing shrunken versions of the organelles. For the new study, a team led by evolutionary biologist Anna Karnkowska, a postdoc, and her adviser, Vladimir Hampl, of Charles University in Prague, checked another candidate, a species in the genus Monocercomonoides. The single-celled organism came from the guts of a chinchilla that belonged to one of the lab members. The team decided to test it because it belonged to a group of microbes that scientists posited had lost their mitochondria. The definition of eukaryotic cells is that they have mitochondria,” says Karnkowska, who is now at the University of British Columbia, Vancouver, in Canada. “We overturn this definition.” Monocercomonoides may not need mitochondria because of where it lives—in the intestines of chinchilla hosts, which it doesn’t appear to harm. Nutrients are abundant there, but oxygen, which mitochondria require to produce energy, is scarce. Instead of relying on mitochondria, the organism likely uses enzymes in its cytoplasm to break down food and furnish energy, the authors suggest. But energy production is not the only problem that Monocercomonoides solved. Mitochondria provide another cellular service: synthesizing clusters of iron and sulfur that are essential helpers for a variety of proteins. It turns out that Monocercomonoides has come up with a workaround by borrowing some bacterial genes that perform the same function, the scientists reveal online today in Current Biology. “It’s a very solid paper experimentally,” says evolutionary genomicist B. Franz Lang of the University of Montreal in Canada. “If you’d like me to bet, I’d give them 90% probability that they are correct.” To strengthen the case, he says, researchers need to perform a detailed microscopic analysis to confirm the absence of the organelles. notes

Monday, 16 December 2019

All about lysosomes ICSE CLASS 10 AND 12 NOES.

Lysosomes
Lysosomes are specialized vesicles within cells that digest large molecules using hydrolytic enzymes. Vesicles are small spheres of fluid surrounded by a lipid bi-layer membrane, and they have roles in transporting molecules within the cell. Lysosomes are only found in animal cells; a human cell contains around 300 of them. Not only do they digest large molecules, they are also responsible for breaking down and getting rid of waste products of the cell. Lysosomes contain over 60 different enzymes that allow them to carry out these processes. Functions of the Lysosomes digest many complex molecules such as carbohydrates, lipids, proteins, and nucleic acids, which the cell then recycles for other uses. The pH of lysosomes is acidic (around pH 5) because their hydrolytic enzymes function best at this pH instead of at the neutral pH of the rest of the cell. Hydrolytic enzymes specifically break down large molecules through hydrolysis. During the process of hydrolysis, a molecule of water is added to a substance, causing it to cleave. Like the digestive system of the human body, which breaks down food using enzymes, the lysosomes can be thought of as the “digestive system” of the cell because it breaks down molecules using enzymes. Lysosomes digest several different kinds of molecules. They can digest food molecules that enter the cell into smaller pieces if an endocrine vesicle (a vesicle that brings particles into the cell) fuses with them. They can also perform autophagy, which is the destruction of improperly functioning organelles. In addition, lysosomes have a role in phagocytosis, which is when a cell engulfs a molecule in order to break it down; it is also known as “cell eating”. For example, the white blood cells called phagocytes ingest invading bacteria in order to break it down and destroy it, and the bacteria is enclosed by a vesicle that lysosomes fuse with. These lysosomes then break down the bacteria. Structure of Lysosomes are generally very small, ranging in size from 0.1-0.5 µm, though they can reach up to 1.2 µm. They have a simple structure; they are spheres made up of a lipid bilayer that encloses fluid that contains a variety of hydrolytic enzymes. The lipids that make up the bilayer are phospholipids, which are molecules that have hydrophilic phosphate group heads, a glycerol molecule, and hydrophobic fatty acid tails. Due to these differences in properties, phospholipids naturally form double-layered membranes when placed in a solution containing water. The phosphate group heads move to the outside of the layer, while the fatty acid tails move to the inside of the layer to be away from water. Phospholipids make up many other membranes in the cell, such as the cell membrane which surrounds the entire cell, the nuclear membrane (or nuclear envelope) that surrounds the nucleus, the Golgi apparatus, and the endoplasmic reticulum. Lysosomes are formed by budding off the Golgi apparatus, and the hydrolytic enzymes within them are formed in the endoplasmic reticulum. The enzymes are tagged with the molecule mannose-6-phosphate, transported to the Golgi apparatus in vesicles, and then packaged into the lysosomes. There are many different types of enzymes in lysosomes including proteases, amylases, nucleases, lipases, and acid phosphatases, among many others. Enzymes are usually named for the molecules that they break down; for example, break down proteins, and nucleases break down nucleic acids. Amylases break down starches into sugars. They are sometimes used to deliver nutrients and pharmaceutical drugs. Lysosomal Storage Diseases Some inherited metabolic disorders can cause defects in the proper functioning of lysosomes. These disorders are called lysosomal storage diseases, or LSDs. There are around 50 different LSDs. Each type of LSD is rare, occurring in less than 1 in 100,000 births; however, as a group, LSDs occur in 1 in 5,000-10,000. LSDs usually occur when a person is deficient in one enzyme that breaks down large molecules like proteins or lipids. Because the enzyme is lacking, the large molecules cannot be broken down, and they eventually build up within the cell and kill it. Most LSDs are inherited in an autosomal recessive pattern. This means that it can be masked by a copy of an allele without the mutation (a dominant allele) and is caused by a mutation on one of the autosomal chromosomes, which are all chromosomes except the sex chromosomes X and Y. Tay-Sachs disease is an example of a well-known LSD that is recessively inherited. Due to insufficient function of the enzyme hexosaminidase A, glycolipids build up in the brain and interfere with normal functioning. This causes nerve cells to break down, and physical and mental functioning to decline. There is no cure, and death usually occurs by age four. A few LSDs are X-linked; they occur because of a mutation on the X chromosome. One such LSD is Fabry disease. Fabry disease is rare, occurring in 1 in 40,000-120,000 live births Symptoms include fatigue, burning pain in the extremities or full body pain, tinnitus, nausea, cardiac and kidney complications, and papules on the skin called angiokeratomas. The mutation that causes Fabry disease is located on the X-chromosome, but females with only one copy of the mutated gene also show symptoms. Since men only have one X chromosome, their symptoms tend to be more severe. Life expectancy for those with this disease in the United States is 58.2 for males and 75.4 for females. Related Biology Terms Vesicle – A small sphere of lipid bilayer in the cell that can transport molecules. Lysosomal storage diseases (LSDs) – A group of about 50 genetic disorders involving abnormal lysosomal function. Autophagy – The degradation of unnecessary or improperly functioning components within a cell. Hydrolytic enzyme – A molecule that speeds up a chemical reaction involving hydrolysis. Most of the lysosomal storage disorders are inherited in an autosomal recessive pattern. However, a few such as Fabry’s disease are X-linked recessive. Lysosomes contain over 60 different hydrolytic enzymes, such as proteases, nucleases, and lipases. These enzymes break down large molecules through hydrolysis. Deficiency in one hydrolytic enzyme in lysosomes leads to a build-up of large molecules, eventually killing the cell.  The phospholipid bilayer of lysosomes does not form correctly, so the lysosomes cannot contain the necessary enzymes.   Most of lysosomal storage disorders occur when a person is deficient in a single hydrolytic enzyme. With low/no enzyme activity, the molecule that the enzyme is supposed to act on does not get broken down, and large molecules accumulate in the cell. Eventually, too many large molecules build up in the cell and it dies.

Sunday, 15 December 2019

All about Ribosomes. (ICSE board ).

Bacterial ribosome, with the small subunit in green and the large subunit in blue. Ribosomes are one of the wonders of the cellular world, and one of the many wonders you can explore yourself at the RCSB PDB. In 2000, structural biologists Venkatraman Ramakrishnan, Thomas A. Steitz and Ada E. Yonath made the first structures of ribosomal subunits available in the PDB, and in 2009, they each received a Nobel Prize for this work. Structures are also available for many of the other players in protein synthesis, including transfer RNA and elongation factors. Building on these structures, there are now hundreds of structures of entire ribosomes in the PDB, revealing the atomic details of many important steps in protein synthesis.Ribosomes in ActionAfter solving the structures of the individual small and large subunits, the next step in ribosome structure research was to determine the structure of the whole ribosome. This work is the culmination of decades of research, which started with blurry pictures of the ribosome from electron microscopy, continued with more detailed cryoelectron micrographic reconstructions, and now includes many atomic structures. By using small pieces of mRNA, various forms of shortened or chemically-modified tRNA, purified protein factors, and modified ribosomes, researchers have solved the structures of ribosomes in the act of building proteins (PDB entry 4v5d).70S RibosomesLooking at all the different forms of life on the Earth.  All living organisms have ribosomes and that they come in two basic sizes. Bacteria and archaebacteria have smaller ribosomes, termed 70S ribosomes, which are composed of a small 30S subunit and large 50S subunit. The "S" stands for svedbergs, a unit used to measure how fast molecules move in a centrifuge. Note that the values for the individual subunits don't add up to the value for the whole ribosome, since the rate of sedimentation is related in a complex way to the mass and shape of the molecule. The ribosomes in our cells, and in other animals, plants and fungi, are larger, termed 80S ribosomes, composed of a 40S small subunit and a 60S large subunit. Strangely, our mitochondria have small 70S ribosomes that are made separately from the larger ones in the cytoplasm. This observation has lead to the hypothesis that mitochondria (and chloroplasts in plant cells) are actually bacteria that were caught inside cells early in the evolution of eukaryotic cells. Now, they live and reproduce happily inside cells, focusing on energy production and relying on the surrounding cell for most of their other needs.The BasicsThe early structures revealed many of the basics of ribosome action. They showed that ribosomes are ribozymes, using RNA and not protein for their reaction, and thus supporting the idea that RNA was central to the early evolution of life. They revealed the importance of the ribosomal proteins for stabilizing and locking the structure of RNA in the ribosome. With the new structures, however, we can start delving into the atomic details of genetic information readout and peptide synthesis. Protein synthesis occurs in three major steps: initiation, elongation, and termination, and structures are available that show aspects of each one.

Yu. T Dyakov, V G Dzhavakhiya, in Comprehensive and Molecular Phytopathology, 2007Ribosome inactivation.Ribosome inhibitory proteins (RIP) are antiviral substances as they inhibit protein synthesis by blocking the final phases of the virus intracellular development. The RIPs specifically inactivate ribosomes, which results in blocking the protein synthesis at the phase of elongation. The primary structure of the RIP isolated from different sources is highly homologous, and this enables in drawing a conclusion that the inhibitory activity of the given peptides correlates to a certain degree with the primary structure of their active sites responsible for ribosome linking.It was initially assumed that the RIPs are inactive in homologous ribosomes. However, the low ribosome activity of the pokeweed (Phytolacca americana) in the extracellular translation experiments led to the discovery that the pokeweed ribosomes are actually inactivated by the pokeweed RIPs (PAP-1) during their release. Other plant ribosomes are also blocked by the effect of their own RIP in similar conditions. However, it should be noted, that the pokeweed ribosomes are still resistant enough against the addition of their own RIPs (PAP-1) and are much more susceptible to the effect of the RIPs isolated from other plants. Yet tritin, a wheat RIP, does not inactivate the wheat ribosomes, and so the isolated wheat ribosomes retain high activity.The experimental data gained to date allows a conclusion that RIPs have a specific activity towards the ribosomes isolated from different plant species. Apparently, ribosomes have certain structural features which can be recognized or not recognized by different RIPs. The mechanisms of functioning of these proteins, however, are not so simple: data are available that PAP-S (RIP from pokeweed seeds) inhibits carrot cell growth in the liquid culture, yet the same concentration of PAP-1 stimulates rice cell growth.The time interval of antiviral activity is rather narrow, therefore, antiviral activity of most RIPs is tested by plant inoculation with a mix of these proteins and the viral preparation or a viral RNA preparation. Plant treatment with RIPs some time after the inoculation will not prevent development of the viral infection. For instance, the protective effect cannot be detected, if pokeweed RIP (PAP-1) is applied 30–50 minutes after the inoculation of tobacco protoplasts with tobacco mosaic virus (TMV). Hence, RIPs are active only at very early stages of the virus life cycle. It is well known, that the host ribosomes can bind a viral RNA almost immediately after the RNA virus has lost its envelope. Possibly, the translation complex (viral RNA–ribosome) is already unsusceptible to the RIP action, and while this complex exists, RIP are inactive.

Ribosomes that stall before completing peptide synthesis must be recycled and returned to the cytoplasmic pool. The protein Dom34 and cofactors Hbs1 and Rli1 can dissociate stalled ribosomes in vitro, but the identity of targets in the cell is unknown. Here, we extend ribosome profiling methodology to reveal a high-resolution molecular characterization of Dom34 function in vivo. Dom34 removes stalled ribosomes from truncated mRNAs, but, in contrast, does not generally dissociate ribosomes on coding sequences known to trigger stalling, such as polyproline. We also show that Dom34 targets arrested ribosomes near the ends of 3' UTRs. These ribosomes appear to gain access to the 3' UTR via a mechanism that does not require decoding of the mRNA. These results suggest that ribosomes frequently enter downstream noncoding regions and that Dom34 carries out the important task of rescuing them.

One notable difference between prokaryotic and eukaryotic ribosomes is size. Ribosomes are measured in Svedberg units, which are a measure of how long it takes a molecule to sediment out of solution in a centrifuge. The larger the number, the larger the molecule. Prokaryotic ribosomes are typically 70S, or Svedberg units. A eukaryotic ribosome is usually 80S. Eukaryotic ribosomes are larger because they contain more proteins and more RNA. Prokaryotic ribosomes contain 3 RNA molecules, while eukaryotic ribosomes contain 4 RNA molecules. The differences are subtle, as the ribosomes of each operate in much the same way.

Looking at all the different forms of life on the Earth, we find that all living organisms have ribosomes and that they come in two basic sizes. Bacteria and archaebacteria have smaller ribosomes, termed 70S ribosomes, which are composed of a small 30S subunit and large 50S subunit. The "S" stands for svedbergs, a unit used to measure how fast molecules move in a centrifuge. Note that the values for the individual subunits don't add up to the value for the whole ribosome, since the rate of sedimentation is related in a complex way to the mass and shape of the molecule. The ribosomes in our cells, and in other animals, plants and fungi, are larger, termed 80S ribosomes, composed of a 40S small subunit and a 60S large subunit. Strangely, our mitochondria have small 70S ribosomes that are made separately from the larger ones in the cytoplasm. This observation has lead to the hypothesis that mitochondria (and chloroplasts in plant cells) are actually bacteria that were caught inside cells early in the evolution of eukaryotic cells. Now, they live and reproduce happily inside cells, focusing on energy production and relying on the surrounding cell for most of their other needs.