World of Microbiology and Immunology vol 1 - part 4 pdf

Cech, Thomas R. WORLD OF MICROBIOLOGY AND IMMUNOLOGY 101 • • Both types of conversion take place in the presence and the absence of oxygen. Algal involvement is an aerobic process. The conversion of carbon dioxide to sugar is an energy-requiring process that generates oxygen as a by-product. This evolution of oxygen also occurs in plants and is one of the recognized vital benefits of trees to life on Earth. The carbon available in the carbohydrate sugar molecules is cycled further by microorganisms in a series of reactions that form the so-called tricarboxylic acid (or TCA) cycle. The breakdown of the carbohydrate serves to supply energy to the microorganism. This process is also known as respiration. In anaerobic environments, microorganisms can cycle the carbon compounds to yield energy in a process known as fer- mentation . Carbon dioxide can be converted to another gas called methane (CH 4 ). This occurs in anaerobic environments, such as deep compacted mud, and is accomplished by bacteria known as methanogenic bacteria. The conversion, which requires hydrogen, yields water and energy for the methanogens. To complete the recycling pattern another group of methane bacteria called methane-oxidizing bacteria or methanotrophs (literally “methane eaters”) can convert methane to carbon dioxide. This conversion, which is an aerobic (oxygen-requiring) process, also yields water and energy. Methanotrophs tend to live at the boundary between aerobic and anaerobic zones. There they have access to the methane produced by the anaerobic methanogenic bacteria, but also access to the oxygen needed for their conversion of the methane. Other microorganisms are able to participate in the cycling of carbon. For example the green and purple sulfur bacteria are able to use the energy they gain from the degradation of a compound called hydrogen sulfide to degrade carbon compounds. Other bacteria such as Thiobacillus ferrooxidans uses the energy gained from the removal of an electron from iron-containing compounds to convert carbon. The anerobic degradation of carbon is done only by microorganisms. This degradation is a collaborative effort involving numerous bacteria. Examples of the bacteria include Bacteroides succinogenes, Clostridium butyricum, and Syntrophomonas sp. This bacterial collaboration, which is termed interspecies hydrogen transfer, is responsible for the bulk of the carbon dioxide and methane that is released to the atmosphere. See also Bacterial growth and division; Chemoautotrophic and chemolithotrophic bacteria; Metabolism; Methane oxidizing and producing bacteria; Nitrogen cycle in microorganisms CAULOBACTER Caulobacter Caulobacter crescentus is a Gram-negative rod-like bacterium that inhabits fresh water. It is noteworthy principally because of the unusual nature of its division. Instead of dividing two form two identical daughter cells as other bacteria do (a process termed binary division), Caulobacter crescentus undergoes what is termed symmetric division. The parent bacterium divides to yield two daughter cells that differ from one another structurally and functionally. When a bacterium divides, one cell is motile by virtue of a single flagellum at one end. This daughter cell is called a swarmer cell. The other cell does not have a flagellum. Instead, at one end of the cell there is a stalk that terminates in an attachment structure called a holdfast. This daughter cell is called the stalk cell. The stalk is an outgrowth of the cell wall, and serves to attach the bacterium to plants or to other microbes in its natural environment (lakes, streams, and sea water). Caulobacter crescentus exhibits a distinctive behavior. The swarmer cell remains motile for 30 to 45 minutes. The cell swims around and settles onto a new surface where the food supply is suitable. After settling, the flagellum is shed and the bacterium differentiates into a stalk cell. With each division cycle the stalk becomes longer and can grow to be several times as long as the body of the bacterium. The regulation of gene expression is different in the swarmer and stalk cells. Replication of the genetic material occurs immediately in the stalk cell but for reasons yet to be determined is repressed in the swarmer cell. However, when a swarmer cell differentiates into a stalk cell, replication of the genetic material immediately commences. Thus, the transition to a stalk cell is necessary before division into the daughter swarmer and stalk cells can occur. The genetics of the swarmer to stalk cell cycle are complex, with at least 500 genes known to play a role in the struc- tural transition. The regulation of these activities with respect to time are of great interest to geneticists. Caulobacter crescentus can be grown in the laboratory so that all the bacteria in the population undergoes division at the same time. This type of growth is termed synchronous growth . This has made the bacterium an ideal system to study the various events in gene regulation necessary for growth and division. See also Bacterial appendages; Bacterial surface layers; Cell cycle (prokaryotic), genetic regulation of; Phenotypic variation CDC • see CENTERS FOR DISEASE CONTROL (CDC) CECH , THOMAS R. (1947- ) Cech, Thomas R. American biochemist The work of Thomas R. Cech has revolutionized the way in which scientists look at RNA and at proteins. Up to the time of Cech’s discoveries in 1981 and 1982, it had been thought that genetic coding, stored in the DNA of the nucleus, was imprinted or transcribed onto RNA molecules. These RNA molecules, it was believed, helped transfer the coding onto proteins produced in the ribosomes. The DNA/RNA nexus was thus the information center of the cell, while protein molecules in the form of enzymes were the workhorses, catalyz- ing the thousands of vital chemical reactions that occur in the cell. Conventional wisdom held that the two functions were womi_C 5/6/03 2:04 PM Page 101 Cech, Thomas R. WORLD OF MICROBIOLOGY AND IMMUNOLOGY 102 • • separate—that there was a delicate division of labor. Cech and his colleagues at the University of Colorado established, however, that this picture of how RNA functions was incorrect; they proved that in the absence of other enzymes RNA acts as its own catalyst. It was a discovery that reverberated through- out the scientific community, leading not only to new tech- nologies in RNA engineering but also to a revised view of the evolution of life. Cech shared the 1989 Nobel Prize for Chemistry with Sidney Altman at Yale University for their work regarding the role of RNA in cell reactions. Cech was born in Chicago, Illinois, to Robert Franklin Cech, a physician, and Annette Marie Cerveny Cech. Cech recalled in an autobiographical sketch for Les Prix Nobel, he grew up in “the safe streets and good schools” of Iowa City, Iowa. His father had a deep and abiding interest in physics as well as medicine, and from an early age Cech took an avid interest in science, collecting rocks and minerals and speculating about how they had been formed. In junior high school he was already conferring with geology professors from the nearby university. Cech went to Grinnell College in 1966; at first attracted to physical chemistry, he soon concentrated on biological chemistry, graduating with a chemistry degree in 1970. It was at Grinnell that he met Carol Lynn Martinson, who was a fellow chemistry student. They married in 1970 and went together to the University of California at Berkeley for graduate studies. His thesis advisor there was John Hearst who, Cech recalled in Les Prix Nobel, “had an enthusiasm for chromosome structure and function that proved infectious.” Both Cech and his wife were awarded their Ph.D. degrees in 1975, and they moved to the east coast for postdoctoral positions— Cech at the Massachusetts Institute of Technology (MIT) under Mary Lou Pardue, and his wife at Harvard. At MIT Cech focused on the DNA structures of the mouse genome, strength- ening his knowledge of biology at the same time. In 1978, both Cech and his wife were offered positions at the University of Colorado in Boulder; he was appointed assistant professor in chemistry. By this time, Cech had decided that he would like to investigate more specific genetic material. He was particularly interested in what enables the DNA molecule to instruct the body to produce the various parts of itself—a process known as gene expression. Cech set out to discover the proteins that govern the DNA transcription process onto RNA, and in order to do this he decided to use nucleic acids from a single-cell protozoa, Tetrahymena ther- mophila. Cech chose Tetrahymena because it rapidly repro- duced genetic material and because it had a structure which allowed for the easy extraction of DNA. By the late 1970s, much research had already been done on DNA and its transcription partner, RNA. It had been determined that there were three types of RNA: messenger RNA, which relays the transcription of the DNA structure by attach- ing itself to the ribosome where protein synthesis occurs; ribosomal RNA, which imparts the messenger’s structure within the ribosome; and transfer RNA, which helps to establish amino acids in the proper order in the protein chain as it is being built. Just prior to the time Cech began his work, it was discovered that DNA and final-product RNA (after copying or transcription) actually differed. In 1977, Phillip A. Sharp and others discovered that portions of seemingly noncoded DNA were snipped out of the RNA and the chain was spliced back together where these intervening segments had been removed. These noncoded sections of DNA were called introns. Cech and his coworkers were not initially interested in such introns, but they soon became fascinated with their function and the splicing mechanism itself. In an effort to understand how these so-called nonsense sequences, or introns, were removed from the transcribed RNA, Cech and his colleague Arthur Zaug decided to investigate the pre-ribosomal RNA of the Tetrahymena, just as it underwent transcription. In order to do this, they first isolated unspliced RNA and then added some Tetrahymena nuclei extract. Their assumption was that the catalytic agent or enzyme would be present in such an extract. The two scientists also added small molecules of salts and nucleotides for energy, varying the amounts of each in subsequent experiments, even excluding one or more of the additives. But the experiment took a different turn than was expected. Cech and Zaug discovered instead that RNA splicing occurred even without the nucleic material being present. This was a development they did not understand at first; it was a long-held scientific belief that proteins in the form of enzymes had to be present for catalysis to occur. Presenting itself was a situation in which RNA appeared to be its own catalytic moti- vator. At first they suspected that their experiment had been contaminated. Cech did further experiments involving recom- binant DNA in which there could be no possibility of the presence of splicing enzymes, and these had the same result: the RNA spliced out its own intron. Further discoveries in Cech’s laboratory into the nature of the intron led to his belief that the intron itself was the catalytic agent of RNA splicing, and he decided that this was a sort of RNA enzyme which they called the ribozyme. Cech’s findings of 1982 met with heated debate in the scientific community, for it upset many beliefs about the nature of enzymes. Cech’s ribozyme was in fact not a true enzyme, for thus far he had shown it only to work upon itself and to be changed in the reaction; true enzymes catalyze repeatedly and come out of the reaction unchanged. Other critics argued that this was a freak bit of RNA on a strange microorganism and that it would not be found in other organisms. The critics were soon proved wrong, however, when scientists around the world began discovering other RNA enzymes. In 1984, Sidney Altman proved that RNA carries out enzyme-like activities on substances other than itself. The discovery of catalytic RNA has had profound results. In the medical field alone RNA enzymology may lead to cures of viral infections. By using these rybozymes as gene scissors, the RNA molecule can be cut at certain points, destroying the RNA molecules that cause infections or genetic disorders. In life sciences, the discovery of catalytic RNA has also changed conventional wisdom. The old debate about whether proteins or nucleic acids were the first bit of life form seems to have been solved. If RNA can act as a catalyst and a genetic template to create proteins as well as itself, then it is rather certain that RNA was first in the chain of life. Cech and Altman won the Nobel Prize for chemistry in 1989 for their independent discoveries of catalytic RNA. Cech womi_C 5/6/03 2:04 PM Page 102 Cell cycle and cell division WORLD OF MICROBIOLOGY AND IMMUNOLOGY 103 • • has also been awarded the Passano Foundation Young Scientist Award and the Harrison Howe Award in 1984; the Pfizer Award in Enzyme Chemistry in 1985; the U. S. Steel Award in Molecular Biology; and the V. D. Mattia Award in 1987. In 1988, he won the Newcombe-Cleveland Award, the Heineken Prize, the Gairdner Foundation International Award, the Louisa Gross Horwitz Prize, and the Albert Lasker Basic Medical Research Award; he was presented with the Bonfils- Stanton Award for Science in 1990. Cech was made full professor in the department of chemistry at the University of Colorado in 1983. Cech and his wife have two daughters. In the midst of his busy research career, Cech finds time to enjoy skiing and backpacking. See also Viral genetics CELL-MEDIATED IMMUNE RESPONSE • see IMMUNITY, CELL MEDIATED CELL CYCLE AND CELL DIVISION Cell cycle and cell division The series of stages that a cell undergoes while progressing to division is known as cell cycle. In order for an organism to grow and develop, the organism’s cells must be able to duplicate themselves. Three basic events must take place to achieve this duplication: the deoxyribonucleic acid DNA, which makes up the individual chromosomes within the cell’s nucleus must be duplicated; the two sets of DNA must be packaged up into two separate nuclei; and the cell’s cytoplasm must divide itself to create two separate cells, each complete with its own nucleus. The two new cells, products of the single original cell, are known as daughter cells. Although prokaryotes (e.g. bacteria, non-nucleated unicellular organisms) divide through binary fission, eukaryotes (including, of course, human cells) undergo a more complex process of cell division because DNA is packed in several chromosomes located inside a cell nucleus. In eukaryotes, cell division may take two different paths, in accordance with the cell type involved. Mitosis is a cellular division resulting in two identical nuclei that takes place in somatic cells. Sex cells or gametes (ovum and spermatozoids) divide by meiosis. The process of meiosis results in four nuclei, each containing half of the original number of chromosomes. Both prokaryotes and eukaryotes undergo a final process, known as cytoplasmatic division, which divides the parental cell in new daughter cells. Mitosis is the process during which two complete, identical sets of chromosomes are produced from one original set. This allows a cell to divide during another process called cytokinesis, thus creating two completely identical daughter cells. During much of a cell’s life, the DNA within the nucleus is not actually organized into the discrete units known as chromosomes. Instead, the DNA exists loosely within the nucleus, in a form called chromatin. Prior to the major events of mitosis, the DNA must replicate itself, so that each cell has twice as much DNA as previously. Cells undergoing division are also termed competent cells. When a cell is not progressing to mitosis, it remains in phase G0 (“G” zero). Therefore, the cell cycle is divided into two major phases: interphase and mitosis. Interphase includes the phases (or stages) G1, S and G2 whereas mitosis is subdivided into prophase, metaphase, anaphase and telophase. Interphase is a phase of cell growth and metabolic activ- ity, without cell nuclear division, comprised of several stages or phases. During Gap 1 or G1 the cell resumes protein and RNA synthesis, which was interrupted during previous mitosis, thus allowing the growth and maturation of young cells to accomplish their physiologic function. Immediately following is a variable length pause for DNA checking and repair before cell cycle transition to phase S during which there is synthesis or semi-conservative replication or synthesis of DNA. During Gap 2 or G2, there is increased RNA and protein synthesis, followed by a second pause for proofreading and eventual repairs in the newly synthesized DNA sequences before transition to mitosis. The cell cycle starts in G1, with the active synthesis of RNA and proteins, which are necessary for young cells to grow and mature. The time G1 lasts, varies greatly among eukaryotic cells of different species and from one tissue to another in the same organism. Tissues that require fast cellular renovation, such as mucosa and endometrial epithelia, have shorter G1 periods than those tissues that do not require frequent renovation or repair, such as muscles or connective tissues. The first stage of mitosis is called prophase. During prophase, the DNA organizes or condenses itself into the specific units known as chromosomes. Chromosomes appear as double-stranded structures. Each strand is a replica of the other and is called a chromatid. The two chromatids of a chromosome are joined at a special region, the centromere. Structures called centrioles position themselves across from each other, at either end of the cell. The nuclear membrane then disappears. During the stage of mitosis called metaphase, the chromosomes line themselves up along the midline of the cell. Fibers called spindles attach themselves to the centromere of each chromosome. During the third stage of mitosis, called anaphase, spindle fibers will pull the chromosomes apart at their centromere (chromosomes have two complementary halves, similar to the two nonidentical but complementary halves of a zipper). One arm of each chromosome will migrate toward each centriole, pulled by the spindle fibers. During the final stage of mitosis, telophase, the chromosomes decondense, becoming unorganized chromatin again. A nuclear membrane forms around each daughter set of chromosomes, and the spindle fibers disappear. Sometime during telophase, the cytoplasm and cytoplasmic membrane of the cell split into two (cytokinesis), each containing one set of chromosomes residing within its nucleus. Cells are mainly induced into proliferation by growth factors or hormones that occupy specific receptors on the surface of the cell membrane, being also known as extra-cellular womi_C 5/6/03 2:04 PM Page 103 Cell cycle and cell division WORLD OF MICROBIOLOGY AND IMMUNOLOGY 104 • • ligands. Examples of growth factors are as such: epidermal growth factor (EGF), fibroblastic growth factor (FGF), platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), or by hormones. PDGF and FGF act by regulating the phase G2 of the cell cycle and during mitosis. After mitosis, they act again stimulating the daughter cells to grow, thus leading them from G0 to G1. Therefore, FGF and PDGF are also termed competence factors, whereas EGF and IGF are termed progression factors, because they keep the process of cellular progression to mitosis going on. Growth factors are also classified (along with other molecules that promote the cell cycle) as pro-mitotic signals. Hormones are also pro- mitotic signals. For example, thyrotrophic hormone, one of the hormones produced by the pituitary gland, induces the proliferation of thyroid gland’s cells. Another pituitary hormone, known as growth hormone or somatotrophic hormone (STH), is responsible by body growth during childhood and early adolescence, inducing the lengthening of the long bones and protein synthesis. Estrogens are hormones that do not occupy a membrane receptor, but instead, penetrate the cell and the nucleus, binding directly to specific sites in the DNA, thus inducing the cell cycle. Anti-mitotic signals may have several different origins, such as cell-to-cell adhesion, factors of adhesion to the extra- cellular matrix, or soluble factor such as TGF beta (tumor growth factor beta), which inhibits abnormal cell proliferation, proteins p53, p16, p21, APC, pRb, etc. These molecules are the products of a class of genes called tumor suppressor genes. Oncogenes, until recently also known as proto-oncogenes, synthesize proteins that enhance the stimuli started by growth factors, amplifying the mitotic signal to the nucleus, and/or promoting the accomplishment of a necessary step of the cell cycle. When each phase of the cell cycle is completed, the proteins involved in that phase are degraded, so that once the next phase starts, the cell is unable to go back to the previous one. Next to the end of phase G1, the cycle is paused by tumor suppressor gene products, to allow verification and repair of DNA damage. When DNA damage is not repairable, these genes stimulate other intra-cellular pathways that induce the cell into suicide or apoptosis (also known as programmed cell death). To the end of phase G2, before the transition to mitosis, the cycle is paused again for a new verification and “deci- sion”: either mitosis or apoptosis. Along each pro-mitotic and anti-mitotic intra-cellular sig- naling pathway, as well as along the apoptotic pathways, several gene products ( proteins and enzymes) are involved in an orderly sequence of activation and inactivation, forming complex webs of signal transmission and signal amplification to the nucleus. The general goal of such cascades of signals is to achieve the orderly progression of each phase of the cell cycle. Mitosis always creates two completely identical cells from the original cell. In mitosis, the total amount of DNA doubles briefly, so that the subsequent daughter cells will ultimately have the exact amount of DNA initially present in the original cell. Mitosis is the process by which all of the cells of the body divide and therefore reproduce. The only cells of the body that do not duplicate through mitosis are the sex cells (egg and sperm cells). These cells undergo a slightly different type of cell division called meiosis, which allows each sex cell produced to contain half of its original amount of DNA, in anticipation of doubling it again when an egg and a sperm unite during the course of conception. Meiosis, also known as reduction division, consists of two successive cell divisions in diploid cells. The two cell divisions are similar to mitosis, but differ in that the chromosomes are duplicated only once, not twice. The result of meiosis is four haploid daughter cells. Because meiosis only occurs in the sex organs (gonads), the daughter cells are the gametes (spermatozoa or ova), which contain hereditary material. By halving the number of chromosomes in the sex cells, meiosis assures that the fusion of maternal and paternal gametes at fertilization will result in offspring with the same chromosome number as the parents. In other words, meiosis compensates for chromosomes doubling at fertilization. The two successive nuclear divisions are termed as meiosis I and meiosis II. Each is further divided into four phases (prophase, metaphase, anaphase, and telophase) with an intermediate phase (interphase) preceding each nuclear division. The events that take place during meiosis are similar in many ways to the process of mitosis, in which one cell divides to form two clones (exact copies) of itself. It is important to note that the purpose and final products of mitosis and meiosis are very different. Meiosis I is preceded by an interphase period in which the DNA replicates (makes an exact duplicate of itself), resulting in two exact copies of each chromosome that are firmly attached at one point, the centromere. Each copy is a sister chromatid, and the pair are still considered as only one chromosome. The first phase of meiosis I, prophase I, begins as the chromosomes come together in homologous pairs in a process known as synapsis. Homologous chromosomes, or homo- Segregation of eukaryotic genetic material during mitosis. womi_C 5/6/03 2:04 PM Page 104 Cell cycle and cell division WORLD OF MICROBIOLOGY AND IMMUNOLOGY 105 • • logues, consist of two chromosomes that carry genetic information for the same traits, although that information may hold different messages (e.g., when two chromosomes carry a mes- sage for eye color, but one codes for blue eyes while the other codes for brown). The fertilized eggs (zygotes) of all sexually reproducing organisms receive their chromosomes in pairs, one from the mother and one from the father. During synapsis, adjacent chromatids from homologous chromosomes “cross over” one another at random points and join at spots called chiasmata. These connections hold the pair together as a tetrad (a set of four chromatids, two from each homologue). At the chiasmata, the connected chromatids randomly exchange bits of genetic information so that each contains a mixture of maternal and paternal genes. This “shuffling” of the DNA produces a tetrad, in which each of the chromatids is different from the others, and a gamete that is different from others produced by the same parent. Crossing over does explain why each person is a unique individual, different even from those in the immediate family. Prophase I is also marked by the appearance of spindle fibers (strands of microtubules) extend- ing from the poles or ends of the cell as the nuclear membrane disappears. These spindle fibers attach to the chromosomes during metaphase I as the tetrads line up along the middle or equator of the cell. A spindle fiber from one pole attaches to one chromosome while a fiber from the opposite pole attaches to its homologue. Anaphase I is characterized by the separa- tion of the homologues, as chromosomes are drawn to the opposite poles. The sister chromatids are still intact, but the homologous chromosomes are pulled apart at the chiasmata. Telophase I begins as the chromosomes reach the poles and a nuclear membrane forms around each set. Cytokinesis occurs as the cytoplasm and organelles are divided in half and the one parent cell is split into two new daughter cells. Each daughter cell is now haploid (n), meaning it has half the number of chromosomes of the original parent cell (which is diploid-2n). These chromosomes in the daughter cells still exist as sister chromatids, but there is only one chromosome from each original homologous pair. The phases of meiosis II are similar to those of meiosis I, but there are some important differences. The time between the two nuclear divisions (interphase II) lacks replication of DNA (as in interphase I). As the two daughter cells produced in meiosis I enter meiosis II, their chromosomes are in the form of sister chromatids. No crossing over occurs in prophase II because there are no homologues to synapse. During metaphase II, the spindle fibers from the opposite poles attach to the sister chromatids (instead of the homologues as before). The chromatids are then pulled apart during anaphase II. As the centromeres separate, the two single chromosomes are drawn to the opposite poles. The end result of meiosis II is that by the end of telophase II, there are four haploid daughter cells (in the sperm or ova) with each chromosome now represented by a single copy. The distribution of chromatids during meiosis is a matter of chance, which results in the concept of the law of independent assortment in genetics. The events of meiosis are controlled by a protein enzyme complex known collectively as maturation promoting factor (MPF). These enzymes interact with one another and with cell organelles to cause the breakdown and reconstruction of the nuclear membrane, the formation of the spindle fibers, and the final division of the cell itself. MPF appears to work in a cycle, with the proteins slowly accumulating during interphase, and then rapidly degrading during the later stages of meiosis. In effect, the rate of synthesis of these proteins controls the frequency and rate of meiosis in all sexually reproducing organisms from the simplest to the most complex. Meiosis occurs in humans, giving rise to the haploid gametes, the sperm and egg cells. In males, the process of gamete production is known as spermatogenesis, where each dividing cell in the testes produces four functional sperm cells, all approximately the same size. Each is propelled by a prim- itive but highly efficient flagellum (tail). In contrast, in females, oogenesis produces only one surviving egg cell from each original parent cell. During cytokinesis, the cytoplasm and organelles are concentrated into only one of the four daughter cells—the one that will eventually become the female ovum or egg. The other three smaller cells, called polar bodies, die and are reabsorbed shortly after formation. The concentration of cytoplasm and organelles into the oocyte greatly enhances the ability of the zygote (produced at fertilization from the unification of the mature ovum with a spermatozoa) to undergo rapid cell division. The control of cell division is a complex process and is a topic of much scientific research. Cell division is stimulated by certain kinds of chemical compounds. Molecules called cytokines are secreted by some cells to stimulate others to begin cell division. Contact with adjacent cells can also control cell division. The phenomenon of contact inhibition is a process where the physical contact between neighboring cells prevents cell division from occurring. When contact is interrupted, however, cell division is stimulated to close the gap between cells. Cell division is a major mechanism by which organisms grow, tissues and organs maintain themselves, and wound healing occurs. Cancer is a form of uncontrolled cell division. The cell cycle is highly regulated by several enzymes, proteins, and cytokines in each of its phases, in order to ensure that the resulting daughter cells receive the appropriate amount of genetic information originally present in the parental cell. In the case of somatic cells, each of the two daughter cells must contain an exact copy of the original genome present in the parental cell. Cell cycle controls also regulate when and to what extent the cells of a given tissue must proliferate, in order to avoid abnormal cell proliferation that could lead to dysplasia or tumor development. Therefore, when one or more of such controls are lost or inhibited, abnormal overgrowth will occur and may lead to impairment of function and disease. See also Amino acid chemistry; Bacterial growth and division; Cell cycle (eukaryotic), genetic regulation of; Cell cycle (prokaryotic), genetic regulation of; Chromosomes, eukaryotic; Chromosomes, prokaryotic; DNA (Deoxyribonucleic acid); Enzymes; Genetic regulation of eukaryotic cells; Genetic regulation of prokaryotic cells; Molecular biology and molecular genetics womi_C 5/6/03 2:04 PM Page 105 Cell cycle (eukaryotic), genetic regulation of WORLD OF MICROBIOLOGY AND IMMUNOLOGY 106 • • C ELL CYCLE (EUKARYOTIC), GENETIC REGULATION OF Cell cycle (eukaryotic), genetic regulation of Although prokaryotes (i.e., non-nucleated unicellular organisms) divide through binary fission, eukaryotes undergo a more complex process of cell division because DNA is packed in several chromosomes located inside a cell nucleus. In eukaryotes, cell division may take two different paths, in accordance with the cell type involved. Mitosis is a cellular division resulting in two identical nuclei is performed by somatic cells. The process of meiosis results in four nuclei, each containing half of the original number of chromosomes. Sex cells or gametes (ovum and spermatozoids) divide by meiosis. Both prokaryotes and eukaryotes undergo a final process, known as cytoplasmatic division, which divides the parental cell into new daughter cells. The series of stages that a cell undergoes while progressing to division is known as cell cycle. Cells undergoing division are also termed competent cells. When a cell is not progressing to mitosis, it remains in phase G0 (“G” zero). Therefore, the cell cycle is divided into two major phases: interphase and mitosis. Interphase includes the phases (or stages) G1, S and G2 whereas mitosis is subdivided into prophase, metaphase, anaphase and telophase. The cell cycle starts in G1, with the active synthesis of RNA and proteins, which are necessary for young cells to grow and mature. The time G1 lasts, varies greatly among eukaryotic cells of different species and from one tissue to another in the same organism. Tissues that require fast cellular renovation, such as mucosa and endometrial epithelia, have shorter G1 periods than those tissues that do not require frequent renovation or repair, such as muscles or connective tissues. The cell cycle is highly regulated by several enzymes, proteins, and cytokines in each of its phases, in order to ensure that the resulting daughter cells receive the appropriate amount of genetic information originally present in the parental cell. In the case of somatic cells, each of the two daughter cells must contain an exact copy of the original genome present in the parental cell. Cell cycle controls also regulate when and to what extent the cells of a given tissue must proliferate, in order to avoid abnormal cell proliferation that could lead to dysplasia or tumor development. Therefore, when one or more of such controls are lost or inhibited, abnormal overgrowth will occur and may lead to impairment of function and disease. Cells are mainly induced into proliferation by growth factors or hormones that occupy specific receptors on the surface of the cell membrane, and are also known as extra-cellular ligands. Examples of growth factors are as such: epidermal growth factor (EGF), fibroblastic growth factor (FGF), platelet-derived growth factor (PDGF), insulin-like growth factor (IGF), or by hormones. PDGF and FGF act by regulating the phase G2 of the cell cycle and during mitosis. After mitosis, they act again stimulating the daughter cells to grow, thus leading them from G0 to G1. Therefore, FGF and PDGF are also termed competence factors, whereas EGF and IGF are termed progression factors, because they keep the process of cellular progression to mitosis going on. Growth factors are also classified (along with other molecules that promote the cell cycle) as pro-mitotic signals. Hormones are also pro-mitotic signals. For example, thyrotrophic hormone, one of the hormones produced by the pituitary gland, induces the proliferation of thyroid gland’s cells. Another pituitary hormone, known as growth hormone or somatotrophic hormone (STH), is responsible by body growth during childhood and early adolescence, inducing the lengthening of the long bones and protein synthesis. Estrogens are hormones that do not occupy a membrane receptor, but instead, penetrate the cell and the nucleus, binding directly to specific sites in the DNA, thus inducing the cell cycle. Anti-mitotic signals may have several different origins, such as cell-to-cell adhesion, factors of adhesion to the extra- cellular matrix, or soluble factor such as TGF beta (tumor growth factor beta), which inhibits abnormal cell proliferation, proteins p53, p16, p21, APC, pRb, etc. These molecules are the products of a class of genes called tumor suppressor genes. Oncogenes, until recently also known as proto-oncogenes, synthesize proteins that enhance the stimuli started by growth factors, amplifying the mitotic signal to the nucleus, and/or promoting the accomplishment of a necessary step of the cell cycle. When each phase of the cell cycle is completed, the proteins involved in that phase are degraded, so that once the next phase starts, the cell is unable to go back to the previous one. Next to the end of phase G1, the cycle is paused by tumor suppressor gene products, to allow verification and repair of DNA damage. When DNA damage is not repairable, these genes stimulate other intra-cellular pathways that induce the cell into suicide or apoptosis (also known as programmed cell death). To the end of phase G2, before the transition to mitosis, the cycle is paused again for a new verification and “deci- sion”: either mitosis or apoptosis. Along each pro-mitotic and anti-mitotic intra-cellular sig- naling pathway, as well as along the apoptotic pathways, several gene products ( proteins and enzymes) are involved in an orderly sequence of activation and inactivation, forming complex webs of signal transmission and signal amplification to the nucleus. The general goal of such cascades of signals is to achieve the orderly progression of each phase of the cell cycle. Interphase is a phase of cell growth and metabolic activ- ity, without cell nuclear division, comprised of several stages or phases. During Gap 1 or G1 the cell resumes protein and RNA synthesis, which was interrupted during mitosis, thus allowing the growth and maturation of young cells to accomplish their physiologic function. Immediately following is a variable length pause for DNA checking and repair before cell cycle transition to phase S during which there is synthesis or semi- conservative replication or synthesis of DNA. During Gap 2 or G2, there is increased RNA and protein synthesis, followed by a second pause for proofreading and eventual repairs in the newly synthesized DNA sequences before transition to Mitosis. At the start of mitosis the chromosomes are already duplicated, with the sister-chromatids (identical chromosomes) clearly visible under a light microscope. Mitosis is subdivided into prophase, metaphase, anaphase and telophase. During prophase there is a high condensation of chromatids, with the beginning of nucleolus disorganization and nuclear membrane disintegration, followed by the start of cen- womi_C 5/6/03 2:04 PM Page 106 Cell cycle (eukaryotic), genetic regulation of WORLD OF MICROBIOLOGY AND IMMUNOLOGY 107 • • trioles’ migration to opposite cell poles. During metaphase the chromosomes organize at the equator of a spindle apparatus (microtubules), forming a structure termed metaphase plate. The sister-chromatids are separated and joined to different centromeres, while the microtubules forming the spindle are attached to a region of the centromere termed kinetochore. During anaphase there are spindles, running from each opposite kinetochore, that pull each set of chromosomes to their respective cell poles, thus ensuring that in the following phase each new cell will ultimately receive an equal division of chromosomes. During telophase, kinetochores and spindles disin- tegrate, the reorganization of nucleus begins, chromatin becomes less condensed, and the nucleus membrane start forming again around each set of chromosomes. The cytoskeleton is reorganized and the somatic cell has now dou- bled its volume and presents two organized nucleus. Cytokinesis usually begins during telophase, and is the process of cytoplasmatic division. This process of division Scanning electron micrograph of eukaryotic cell division. womi_C 5/6/03 2:04 PM Page 107 Cell cycle (prokaryotic), genetic regulation of WORLD OF MICROBIOLOGY AND IMMUNOLOGY 108 • • varies among species but in somatic cells, it occurs through the equal division of the cytoplasmatic content, with the plasma membrane forming inwardly a deep cleft that ultimately divides the parental cell in two new daughter cells. The identification and detailed understanding of the many molecules involved in the cell cycle controls and intra- cellular signal transduction is presently under investigation by several research groups around the world. This knowledge is crucial to the development of new anti-cancer drugs as well as to new treatments for other genetic diseases, in which a gene over expression or deregulation may be causing either a chronic or an acute disease, or the impairment of a vital organ function. Scientists predict that the next two decades will be dedicated to the identification of gene products and their respective function in the cellular microenvironment. This new field of research is termed proteomics. See also Cell cycle (Prokaryotic) genetic regulation of; Genetic regulation of eukaryotic cells; Genetic regulation of prokaryotic cells CELL CYCLE (PROKARYOTIC), GENETIC REGULATION OF Cell cycle (prokaryotic), genetic regulation of Although prokaryotes do not have an organized nucleus and other complex organelles found in eukaryotic cells, prokaryotic organisms share some common features with eukaryotes as far as cell division is concerned. For example, they both replicate DNA in a semi conservative manner, and the segregation of the newly formed DNA molecules occurs before the cell division takes place through cytokinesis. Despite such similarities, the prokaryotic genome is stored in a single DNA molecule, whereas eukaryotes may contain a varied number of DNA molecules, specific to each species, seen in the interpha- sic nucleus as chromosomes. Prokaryotic cells also differ in other ways from eukaryotic cells. Prokaryotes do not have cytoskeleton and the DNA is not condensed during mitosis. The prokaryote chromosomes do not present histones, the complexes of histonic proteins that help to pack eukaryotic DNA into a condensate state. Prokaryotic DNA has one single promoter site that initiates replication, whereas eukaryotic DNA has multiple promoter sites. Prokaryotes have a lack of spindle apparatus (or microtubules), which are essential structures for chromosome segregation in eukaryotic cells. In prokaryotes, there are no membranes and organelles dividing the cytosol in different compartments. Although two or more DNA molecules may be present in a given prokaryotic cell, they are genetically identical. They may contain one extra circular strand of genes known as plasmid, much smaller than the genomic DNA, and plasmids may be transferred to another prokaryote through bacterial conjugation, a process known as horizontal gene transfer. The prokaryotic method of reproduction is asexual and is termed binary fission because one cell is divided in two new identical cells. Some prokaryotes also have a plasmid. Genes in plasmids are extra-chromosomal genes and can either be separately duplicated by a class of gene known as transposons Type II, or simply passed on to another individual. Transposons Type I may transfer and insert one or more genes from the plasmid to the cell DNA or vice-versa causing muta- tion through genetic recombination. The chromosome is attached to a region of the internal side of the membrane, forming a nucleoide. Some bacterial cells do present two or more nucleoides, but the genes they contain are identical. The prokaryotic cell cycle is usually a fast process and may occur every 20 minutes in favorable conditions. However, some bacteria, such as Mycobacterium leprae (the cause of leprosy), take 12 days to accomplish replication in the host’s leprous lesion. Replication of prokaryotic DNA, as well as of eukaryotic DNA, is a semi- conservative process, which means that each newly synthesized strand is paired with its complementary parental strand. Each daughter cell, therefore, receives a double-stranded circular DNA molecule that is formed by a new strand is paired with an old strand. The cell cycle is regulated by genes encoding products (i.e., enzymes and proteins) that play crucial roles in the main- tenance of an orderly sequence of events that ensures that each resultant daughter cell will inherit the same amount of genetic information. Cell induction into proliferation and DNA replication are controlled by specific gene products, such as enzyme DNA polymerase III, that binds to a promoter region in the circular DNA, initiating its replication. However, DNA polymerase requires the presence of a pre-existing strand of DNA, which serves as a template, as well as RNA primers, to initiate the polymerization of a new strand. Before replication starts, timidine-H 3 , (a DNA precursor) is added to a Y-shaped site where the double helices were separated, known as the replicating fork. The DNA strands are separated by enzyme helicases and kept apart during replication by single strand proteins (or ss DNA-binding proteins) that binds to DNA, while the enzyme topoisomerase further unwinds and elon- gates the two strands to undo the circular ring. DNA polymerase always makes the new strand by start- ing from the extremity 5’ and terminating at the extremity 3’. Moreover, the two DNA strands have opposite directions (i.e., they keep an anti-parallel arrangement to each other). Therefore, the new strand 5’ to 3’ that is complementary to the old strand 3’ to 5’ is synthesized in a continuous process (leading strand synthesis), whereas the other new strand (3’ to 5’) is synthesized in several isolated fragments (lagging strand synthesis) that will be later bound together to form the whole strand. The new 3’ to 5’ strand is complementary to the old 5’ to 3’. However, the lagging fragments, known as Okazaki’s fragments, are individually synthesized in the direction 5’ to 3’ by DNA polymerase III. RNA polymerases produce the RNA primers that help DNA polymerases to synthesize the leading strand. Nevertheless, the small fragments of the lagging strand have as primers a special RNA that is synthesized by another enzyme, the primase. Enzyme topoisomerase III does the proofreading of the newly transcribed sequences and elimi- nates those wrongly transcribed, before DNA synthesis may continue. RNA primers are removed from the newly synthesized sequences by ribonuclease H. Polymerase I fills the gaps and DNA ligase joins the lagging strands. womi_C 5/6/03 2:04 PM Page 108 Cell membrane transport WORLD OF MICROBIOLOGY AND IMMUNOLOGY 109 • • After DNA replication, each DNA molecule is segre- gated, i.e., separated from the other, and attached to a different region of the internal face of the membrane. The formation of a septum, or dividing internal wall, separates the cell into halves, each containing a nucleotide. The process of splitting the cell in two identical daughter cells is known as cytokinesis. See also Bacterial growth and division; Biochemistry; Cell cycle (eukaryotic), genetic regulation of; Cell cycle and cell division; Chromosomes, eukaryotic; Chromosomes, prokaryotic; DNA (Deoxyribonucleic acid); Enzymes; Genetic regulation of eukaryotic cells; Genetic regulation of prokaryotic cells; Genotype and phenotype; Molecular biology and molecular genetics CELL MEMBRANE TRANSPORT Cell membrane transport The cell is bound by an outer membrane that, in accord with the fluid mosaic model, is comprised of a phospholipid lipid bilayer with proteins—molecules that also act as receptor sites—interspersed within the phospholipid bilayer. Varieties of channels exist within the membrane. There are a number of internal cellular membranes that partially partition the inter- cellular matrix, and that ultimately become continuous with the nuclear membrane. There are three principal mechanisms of outer cellular membrane transport (i.e., means by which molecules can pass through the boundary cellular membrane). The transport mechanisms are passive, or gradient diffusion, facilitated diffusion, and active transport. Diffusion is a process in which the random motions of molecules or other particles result in a net movement from a region of high concentration to a region of lower concentration. A familiar example of diffusion is the dissemination of floral perfumes from a bouquet to all parts of the motionless air of a room. The rate of flow of the diffusing substance is proportional to the concentration gradient for a given direction of diffusion. Thus, if the concentration of the diffusing substance is very high at the source, and is diffusing in a direction where little or none is found, the diffusion rate will be maxi- mized. Several substances may diffuse more or less independ- ently and simultaneously within a space or volume of liquid. Because lightweight molecules have higher average speeds than heavy molecules at the same temperature, they also tend to diffuse more rapidly. Molecules of the same weight move more rapidly at higher temperatures, increasing the rate of diffusion as the temperature rises. Driven by concentration gradients, diffusion in the cell usually takes place through channels or pores lined by proteins. Size and electrical charge may inhibit or prohibit the passage of certain molecules or electrolytes (e.g., sodium, potassium, etc.). Osmosis describes diffusion of water across cell membranes. Although water is a polar molecule (i.e., has overall partially positive and negative charges separated by its molecular structure), transmembrane proteins form hydrophilic (water lov- ing) channels to through which water molecules may move. Facilitated diffusion is the diffusion of a substance not moving against a concentration gradient (i.e., from a region of low concentration to high concentration) but which require the assistance of other molecules. These are not considered to be energetic reactions (i.e., energy in the form of use of adenosine triphosphate molecules (ATP) is not required. The facilitation or assistance—usually in physically turning or orienting a molecule so that it may more easily pass through a membrane—may be by other molecules undergoing their own random motion. Transmembrane proteins establish pores through which ions and some small hydrophilic molecules are able to pass by diffusion. The channels open and close according to the phys- iological needs and state of the cell. Because they open and close transmembrane proteins are termed “gated” proteins. Control of the opening and closing mechanism may be via mechanical, electrical, or other types of membrane changes that may occur as various molecules bind to cell receptor sites. Active transport is movement of molecules across a cell membrane or membrane of a cell organelle, from a region of low concentration to a region of high concentration. Since these molecules are being moved against a concentration gradient, cellular energy is required for active transport. Active transport allows a cell to maintain conditions different from the surrounding environment. There are two main types of active transport; movement directly across the cell membrane with assistance from transport proteins, and endocytosis, the engulfing of materials into a cell using the processes of pinocytosis, phagocytosis, or receptor-mediated endocytosis. Transport proteins found within the phospholipid bilayer of the cell membrane can move substances directly across the cell membrane, molecule by molecule. The sodium- potassium pump, which is found in many cells and helps nerve cells to pass their signals in the form of electrical impulses, is a well-studied example of active transport using transport proteins. The transport proteins that are an essential part of the sodium-potassium pump maintain a higher concentration of potassium ions inside the cells compared to outside, and a higher concentration of sodium ions outside of cells compared to inside. In order to carry the ions across the cell membrane and against the concentration gradient, the transport proteins have very specific shapes that only fit or bond well with sodium and potassium ions. Because the transport of these ions is against the concentration gradient, it requires a signifi- cant amount of energy. Endocytosis is an infolding and then pinching in of the cell membrane so that materials are engulfed into a vacuole or vesicle within the cell. Pinocytosis is the process in which cells engulf liquids. The liquids may or may not contain dis- solved materials. Phagocytosis is the process in which the materials that are taken into the cell are solid particles. With receptor-mediated endocytosis the substances that are to be transported into the cell first bind to specific sites or receptor proteins on the outside of the cell. The substances can then be engulfed into the cell. As the materials are being carried into the cell, the cell membrane pinches in forming a vacuole or other vesicle. The materials can then be used inside the cell. womi_C 5/6/03 2:04 PM Page 109 Centers for Disease Control WORLD OF MICROBIOLOGY AND IMMUNOLOGY 110 • • Because all types of endocytosis use energy, they are considered active transport. See also Bacterial growth and division; Biochemistry; Cell cycle and cell division; Enzymes; Molecular biology and molecular genetics CENTERS FOR DISEASE CONTROL Centers for Disease Control The Centers for Disease Control and Prevention (CDC) is one of the primary public health institutions in the world. CDC is headquartered in Atlanta, Georgia, with facilities at 9 other sites in the United States. The centers are the focus of the United States government efforts to develop and implement prevention and control strategies for diseases, including those of microbiological origin. The CDC is home to 11 national centers that address various aspects of health care and disease prevention. Examples of the centers include the National Center for Chronic Disease Prevention and Health promotion, National Center for Infectious Diseases, National Immunization Program, and the National Center for HIV, STD, and TB Prevention. CDC was originally the acronym for The Communi- cable Disease Center. This center was a redesignation of an existing facility known as the Malaria Control in War Areas. The malaria control effort had been mandated to eradicate View down the channel of the matrix porin of Escherichia coli. womi_C 5/6/03 2:04 PM Page 110 [...]... the Board of Governors of the Weizmann Institute in Israel, and was an outspoken supporter of the importance of providing Jewish education for young Jewish children in England and abroad—all three of his children received part of their education in Israel In addition to the Nobel Prize, Chain received the Berzelius Medal in 19 46 , and was made a commander of the Legion d’Honneur in 19 47 In 19 54, he was... 19 92, and the Helmut Research Award in 19 93 Cohen has held member numerous professional societies, including the N Academy of Sciences (chairing the genetics sectio 19 88 to 19 91) , the Institute of Medicine of the N Academy, and the Genetics Society of America In a he served on the board of the Journal of Bacteriolog 19 70s, and was associate editor of Plasmid from 19 86 Since 19 77, he has been a member of. .. University of Maryland, Colwell was director of the Sea Grant College from 19 77 to 19 83 She served as president of Sigma Xi, the American Society for Microbiology, and the International Congress of Systematic and Evolutionary Biology, and was president-elect of the American Association for the Advancement of Science Colwell has written and edited more than sixteen books and over four hundred papers and articles... mass– the substance Florey and Heatley went to the United 19 41 to enlist the aid of the government and of pharma houses New ways were found to yield more and strong cillin from mold broth, and by 19 43 , the drug went in lar medical use for Allied troops After the war, penic of the heat-shock or cell-stress response, changes in the sion of certain proteins, and the unraveling of the fun proteins that mediate... American Irving S Johnson (19 25– ) They prevent mitosis (division) in cancer cells VP -1 6 and VM -1 6 are derived from the roots and rhizomes of the may apple or mandrake plant, and are used to treat various cancers Taxol, which is derived from the bark of several species of yew trees, was discovered in 19 78, and is used for treatment of ovarian and breast cancer Another class of naturally occurring substances... antineoplastic (anti-cancer) drugs, and, in 1 forerunner of the National Cancer Institute was estab Bethesda, MD Leading the research efforts were the s 4- H Club” of cancer chemotherapy: the Americans Huggins (19 01 19 97), who worked with hormones; Hitchings (19 05 19 98), purines and pyrimidines to with cell metabolism; Charles Heidelberger, fluorinat pounds; and British scientist Alexander Haddow (19 07 antigens,... twenty-five years, serving of the Department of Genetics from 19 78 to 19 86 H author of more than two hundred papers, and has many awards for his scientific contributions, among t Albert Lasker Basic Medical Research Award in 1 Wolf Prize in Medicine in 19 81, both the National M Science and the LVMH Prize of the Institut de la Vie the National Medal of Technology in 19 89, the A Chemical Society Award in 19 92,... into the doctoral program in 18 46 because of his Jewish heritage, Cohn moved to Berlin There he completed his doctoral degree in 18 47 , at the age of 19 , on the structure and germination of seeds After returning to Breslau in 18 49 , Cohn was presented with a top of the line microscope from his father There he studied the cell biology of plants including the growth and division of plant cells, plasma streaming,... Arthritis and Metabolic Diseases in Be Maryland, and Duke University Hospital in Durham Carolina Cohen completed postdoctoral research in the Albert Einstein College of Medicine in the Bron York He joined the faculty at Stanford University i was appointed professor of medicine in 19 75, profe genetics in 19 77, and became Kwoh-Ting Li profe genetics in 19 93 At Stanford Cohen began the study of plasmid of DNA... sulfonamide drugs developed in the 19 30s, penicillin and other antibiotics of the 19 40 s, hormones in the 19 50s, and more recent drugs that interfere with cancer cell metabolism and reproduction have all been part of the chemotherapeutic arsenal The first drug to treat widespread bacteria was developed in the mid -1 9 30s by the German physician-chemist Gerhard Domagk In 19 32, he discovered that a dye named . surface of the cell membrane, being also known as extra-cellular womi_C 5/6/03 2: 04 PM Page 10 3 Cell cycle and cell division WORLD OF MICROBIOLOGY AND IMMUNOLOGY 10 4 • • ligands. Examples of. compounds) are present in both natural and man-made environments and products. womi_C 5/6/03 2: 04 PM Page 11 4 Chemoautotrophic and chemolithotrophic bacteria WORLD OF MICROBIOLOGY AND IMMUNOLOGY 11 5 • • Many. Heidelberger, fluorinated compounds; and British scientist Alexander Haddow (19 07 19 76), womi_C 5/6/03 2: 04 PM Page 11 6 Chitin WORLD OF MICROBIOLOGY AND IMMUNOLOGY 11 7 • • who worked with various

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