Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Mitosis and the Cell Cycle - 1 Growth and reproduction are two of the characteristics of life. The cell theory states "All cells come from preexisting cells by a process of cell reproduction, or cell division". Cell division is the process by which all the cells of a multicellular organism are formed during growth and development. Cell division is used for repair and replacement of cells and tissues during one's lifetime. Asexual reproduction, a means of making more individuals for many groups of organisms, is also accomplished by cell division. In our discussion of cell reproduction, we shall focus on the processes of cell reproduction in eukaryotic organisms. We know that all cells of an individual have exactly the same DNA, and their DNA is found in structures called chromosomes. Each species has a fixed chromosome number, a number that does not change from generation to generation. The total of a cell's DNA is called its genome. The DNA must also stay the same from cell to cell within an organism, so that when cells divide, new cells formed will have exactly the same DNA as the original cell. To ensure that chromosomes and DNA remain the same in new cells (the genome remains constant) when cells divide, it is crucial to have a mechanism that exactly replicates (duplicates) the DNA of the original cell and distributes the copied DNA equally to the new cells. To form new cells, we must also separate the cytoplasm and critical organelles, such as mitochondria and chloroplasts, of the original cell into the new cells formed. We will look at the mechanism by which cells duplicate DNA in a later unit. At this time we shall focus on mitosis, the process by which the duplicated chromosomes are equally distributed to new nuclei in eukaryotic organisms. The distribution of the cytoplasm of the original cell into new cells is called cytokinesis. Before discussing how cells divide, it's probably useful to discuss the structure of chromosomes and chromosome terms (of which there are a sufficiency). Structure of Chromosomes Chromosomes are composed of DNA and protein, a mixture called chromatin. DNA, as we know, is a double chain of nucleotides. It's estimated that our human DNA consists of 140 million nucleotides per cell. Each of our chromosomes is about 5cm in length. Chromosomes fit into the nucleus because they are coiled. Every 200 nucleotides coil around a complex of 8 histone protein molecules forming a nucleosome. Histones are positively charged so that they are attracted to the phosphate groups of the nucleotides. Nucleosomes further coil into supercoils. Mitosis and the Cell Cycle - 2 Some DNA gets highly condensed and is known as heterochromatin. Heterochromatin may stay so tightly condensed that its DNA cannot be read (or expressed). The DNA that can be expressed is called euchromatin. How DNA gets expressed is for a later chapter. Chromosomes vary in size and shape. The display of chromosomes of any given individual is known as the karyotype. Karyotypes are made of chromosomes during the metaphase state of mitosis, when they are particularly distinctive. Karyotype analysis can be useful for finding chromosome abnormalities, and will be discussed later. Human Male karyotype Mitosis and the Cell Cycle - 3 Chromosome Terms before and after Duplication • An unduplicated chromosome is one chromosome. A chromosome more or less consists of two arms that extend from a narrow "centralized" region called the centromere. The centromere region is often visible when chromosomes duplicate. • When a chromosome duplicates, it becomes one duplicated chromosome, and the two copies remain attached to each other. Note it is still o n e chromosome. • The two exact copies of the duplicated chromosome, which remain attached at the centromere region, are called "sister" chromatids. They are identical to each other.(Remember this; it is essential!) • At the centromere region of the duplicated chromosome, there are structures (made of protein and DNA) called kinetochores. The kinetochores attach to microtubules of the spindle during mitosis. The kinetochores of the two sister chromatids face in opposite directions. • After the identical sister chromatids are separated during mitosis, each (called a "daughter" chromosome now) becomes a single unduplicated chromosome again. Rule to remember: A chromatid must be attached to its identical chromatid and the two sister chromatids comprise one duplicated chromosome. "Sister" chromatids are not two chromosomes. They are one duplicated chromosome that consists of two identical chromatids. The only time you can use the word chromatid is when you have the two identical chromatids attached to each other. Mitosis is one part of the cell cycle. The cell cycle includes all activities of a cell from the time it is formed until (and if) it divides or dies. Mitosis and the Cell Cycle - 4 To summarize - When cells divide: • We must form two new cells from the original cell. • The new cells formed must have all of the genetic material for the organism, so we need a mechanism that exactly replicates (duplicates) the DNA of the original cell and distributes the copied DNA equally to the new cells. Mitosis is the process by which the duplicated chromosomes are equally distributed to new nuclei. We will see how DNA duplication is accomplished later. • We must also separate the cytoplasm, and critical organelles, such as mitochondria and chloroplasts, of the original cell into the new cells formed so that the new cells can survive, grow and function. The distribution of the cytoplasm of the original cell into new cells is called cytokinesis. Mitosis and the Eukaryotic Cell Cycle. As mentioned, the processes, or events, of cell division can be related to the normal lifetime of a cell. For our convenience, these events are divided into "stages" of a cell's cell cycle. The cell cycle starts when a cell is formed and continues until it divides. Some cells never divide, others are specialized to divide (especially in plants, where virtually all cell division occurs in specialized tissue called meristem. Cell division is a brief part of the life cycle; most of the life of a cell is spent in normal activities of growth and maintenance, a stage called interphase. The events of interphase and cell division are: Interphase Growth (called G1 or First Gap in the cell cycle) DNA Duplication (called S for synthesis in the cell cycle) Preparation for Division (called G 2 ) Note: If a cell never divides, following G1 it will be in a “permanent” state of G0 (or non-dividing state). Cell Division (or cell reproduction) Mitosis, with Prophase Prometaphase Metaphase Anaphase Telophase Cytokinesis Mitosis and the Cell Cycle - 5 Interphase is divided into the respective activities: Growth (called G1 or Gap in the cell cycle) • Normal growth and cell activities • The chromosomes are stretched out and grainy in appearance. They stain well, and early on this material was called chromatin. DNA Duplication (called S for synthesis in the cell cycle) • DNA duplication forming the duplicated chromosomes • This event is triggered in the G1 phase. Once started this process can not be reversed; the cell is committed to divide. Preparation for Division (called G 2 ) • Condensation of chromosomes is initiated during the G2 phase of the cell cycle, although condensed chromosomes are not visible until well into prophase of mitosis. • Synthesis of materials, such as tubulin, needed for mitosis and cytokinesis also takes place during G2, as does the • Duplication of centrioles in those organisms that produce centrioles occurs, and the microtubule organizing center is initiated. Interphase The Stages of Mitosis: Mitosis is a continuum. Humans have decided to separate the process into stages for the convenience of our discussions. Some humans even separate the stages into sub-stages and intermediate stages. For our purposes, cell division in eukaryotes involves three events: DNA Duplication • Process of duplicating the genetic material of the nucleus • This occurs during Interphase, when growth and/or normal metabolic activities take place. Mitosis • Process of distributing the duplicated DNA equally to the two new nuclei. Cytokinesis • Process of separating the cytoplasm contents Mitosis and the Cell Cycle - 6 Prophase Chromosome Condensation The duplicated chromosomes continue condensation and become visible as threadlike structures. Chromosomes continue condensing and thickening as prophase progresses. Special motor proteins are involved in the condensation of chromosomes. The duplicated chromosomes are firmly attached at their centromeres throughout this condensation and coiling. Microtubule Organization and the Mitotic Spindle • Microtubules and associated proteins form the spindle apparatus during prophase and determine the poles of the cell. Some of the cell's cytoskeleton will disassemble to provide spindle microtubules. The spindle also synthesizes additional microtubules from the protein, tubulin, synthesized during interphase. • The spindle apparatus originates from a microtubule organizing center, also called the centrosome. The centrosome is self-duplicating and duplicates during interphase. In animal cells, centrioles are found within the centrosome. Microtubules radiating from the centrosomes are called asters. Centrioles are not essential to this process. Cells which lack centrioles still form the spindle apparatus. Prophase Prometaphase (Late Prophase) Nuclear membrane • The nuclear membrane degrades in prometaphase into small vesicles, used to synthesize new nuclear membrane material in the new cells. Spindle and Duplicated Chromosomes • The spindle apparatus will extend from the poles of the cell through the center of the cell to the opposite pole of the cell. In animals, as the spindle formation is completed, the centrosomes will move to the respective poles of the cell and are called the spindle poles. Mitosis and the Cell Cycle - 7 • Some microtubules from each pole of the cell attach to the kinetochores of each of the sister chromatids. The organization of the spindle microtubules is such that each duplicated chromosome has microtubules from one pole attaching to one of the sister chromatids, and microtubules from the opposite pole attaching to the other sister chromatid. If there are mistakes in attachment, chromatids are not separated correctly during mitosis. • The spindle microtubules pull on the kinetochore regions of the sister chromatids in a "tug-of-war" which eventually leads to a migration of the duplicated chromosomes to the equatorial plane of the cell. Microtubules not attached to kinetochores pass through the equatorial region as they extend from pole to pole of the cell. Prometaphase Metaphase • The spindle apparatus has moved the chromosomes to the equator of the cell, aligning the centromeres of each duplicated chromosome along the equator of the cell. • Centromeres of each sister chromatid are aligned with each other and each sister chromatid is connected at its kinetochore to a microtubule. • This alignment of chromosomes along the equatorial plane of the cell is often called the metaphase plate. Metaphase Mitosis and the Cell Cycle - 8 Anaphase • Centromeres of each duplicated chromosome separate to start anaphase. You can't see this; the separating chromosomes are the first visual sign of anaphase. By definition, each sister chromatid is now a single unduplicated chromosome. • Kinetochore microtubules from each pole pull the chromosomes away from each other and toward the respective poles of the cell. They do so by decomposing at the kinetochore end, hence shortening themselves. The microtubules attach at the centromere region of the chromosomes so that chromosomes are pulled centromere first towards the poles of the cell. • Motor proteins move the kinetochores of the chromosomes along the shortening kinetochore microtubules. • Polar microtubules lengthen, moving the poles of the cell further apart, and, in animal cells elongating the cell. Overlapping Polar Microtubules in Metaphase and Anaphase • Since sister chromatids are identical, each of the two clusters of chromosomes being pulled to the two poles of the cell has one copy of each original chromosome. As the chromosomes are pulled to the poles, they begin to lengthen out. Anaphase Mitosis and the Cell Cycle - 9 Telophase • The cell is elongated more by continued lengthening of the non-kinetochore microtubules (decreasing the overlap). • Membrane vesicles and membrane fragments form new nuclear membranes around each group of chromosomes (at the two poles) • Chromosomes uncoil and become indistinct as chromatin • The spindle microtubules disperse and the spindle apparatus disappears • New nucleoli form Telophase Mitosis in Blood Lily Mitosis and the Cell Cycle - 10 Mitosis in Newt Lung Mitosis in Whitefish Blastula Mitosis and the Cell Cycle - 11 Cytokinesis: Separation of the Cytoplasmic Contents Speaking precisely, mitosis describes events of chromosomes and nuclei. Most cells accompany mitosis with cytokinesis, the separation of the cytoplasm of the original cell into two new cells. This is not always the case. Some organisms (including many fungi and algae) are "multinucleate"; they just have one cell body with many nuclei. Some animal tissues are also multinucleate. Cytokinesis coincides with the events of telophase or occurs immediately after, so that at the completion of mitosis, the original cell is separated into two cells, each with a nucleus and DNA identical to that of the original cell. Although the end result of cytokinesis is always two new cells, the mechanism of separation is different in plants and animals, so we shall discuss them separately. Cytokinesis in Animal Cells The cells of animals lack cell walls. Cytokinesis in animal cells is started with the formation of a cleavage furrow, a depression or pinching in of the plasma membrane. This is caused by a ring of microfilaments, the contractile ring, composed of the protein, actin, associated with myosin, which forms across the middle of the cell after the chromatids are separated in anaphase. This ring contracts, pinching the membrane toward the center of the cell, which eventually pinches the cell in two. The additional membrane surface needed is supplied by membrane material synthesized during interphase. Cytokinesis in Animal Cells Cytokinesis in Plant Cells Cytokinesis in Plant Cells Each cell of a plant is surrounded by a cell wall, which is quite rigid. Plant cells can not form a cleavage furrow. Instead, plant cells are separated by the cell plate formation. Cell plate formation involves making a cross wall at the equator of the original cell. Golgi vesicles containing wall material fuse along microtubules forming a disk-like structure that is called the phragmoplast or cell plate. As cellulose and other fibers are deposited, the cell plate is formed creating a boundary and new cell wall between the two new cells. Membrane material from the original cell fuses to each side of the cell plate forming new cell membranes on the dividing sides of the original cell. Mitosis and the Cell Cycle - 12 Variations in Mitosis in Eukaryotes The process of mitosis in most eukaryotes is remarkably the same. However, some protists, such as the dinoflagellates and some diatoms, do not degrade the nuclear membrane during mitosis. Spindles may not (the dinoflagellates) or may (the diatoms) form within the nucleus. Binary Fission in Prokaryotes The process of cell division in prokaryotes, called binary fission, parallels that of eukaryotes. Bacteria have just one circular molecule of DNA and the absence of a nucleus in the prokaryotic cell account for a number of differences in the "mechanics" of the process. The single DNA molecule is attached to the plasma membrane prior to duplication and cell division. After duplication (which is very similar to that of eukaryotes), the duplicated molecule is also attached to the plasma membrane. The cell elongates separating the two DNA molecules. After a period of elongation, in which the original cell about doubles its length, plasma membrane grows inward, pinching off the two halves of the original cell. New cell wall material is synthesized. Mitosis and the Cell Cycle - 13 When and Where does Mitosis Occur? Growth All growth (increase in numbers of cells) in individual organisms takes place by mitosis, from the fertilized egg (zygote) to death. Replacement Many cells are routinely replaced in organisms. This replacement of cells is done by mitosis. For examples, we replace the cells which line our digestive tract every 1 - 3 days. Repair and Maintenance Mitosis is used for repair and replacement of damaged cells or tissues, whenever possible. This includes regeneration of lost parts for some organisms. Non-Sexual (Asexual) Reproduction Mitosis is used for all asexual reproduction or propagation. This is especially common in plants, fungi and protists. Asexual reproduction produces offspring genetically identical to the original parent. Animals rarely reproduce non-sexually. When this does occur, the offspring are genetically identical to the parent, as would be expected of any mitosis. Cloning, a method of producing genetically identical offspring, uses mitosis, precisely because mitosis duplicates the DNA exactly. Cloning is quite easy to do with many plants, since they are easily propagated non-sexually anyway. We find huge groves of Aspen trees, all of which are root clones. Many agricultural products, such as navel oranges, originate from cloned individuals. Tissue culture is also a popular way of cloning plants. Most cells of plants retain the ability to "dedifferentiate" and become embryonic-like. Most animal cells, once specialized (or differentiated), can not do so. What passes for cloning in animal cells is a different process (to be discussed some other time). We can make genetically identical organisms by removing nuclei from non-fertilized egg cells and replacing the egg cell nuclei with nuclei from diploid somatic cells. This has been done with many organisms, including sheep and cattle. The resulting organisms will have the DNA of the diploid donor and will be genetically identical. This successful "cloning" of mammals has resulted in a flurry of research, and speculation about the ability and desire to clone humans. This is one of the biological issues which has serious ethical consequences. One of the reasons each of us should learn as much as possible about biology is to make informed decisions about the ethical applications of research. Mitosis and the Cell Cycle - 14 Regulating the Cell Cycle The control of cell division is one of the most active areas of biological research if only because cancers are diseases of cell division out of control. In humans, some cells routinely divide; taste bud cells and cells lining the digestive tract divide about every three days, skin cells monthly; red blood cells by the millions are produced each day. Other cells in mature humans never divide. Nerve cells and muscle cells are two examples. Virtually all brain development occurs by the time we are two. Liver cells divide only if damage occurs. In plants, only cells in specialized areas of the plant body, called meristems, normally divide. In fact meristems are specialized for cell division. However, there are numerous exceptions in plants, including some that are a part of normal development. And, as mentioned previously, many plant cells have a remarkable ability to dedifferentiate and become "embryonic", something that does not happen in animals. But what tells a cell to divide - or not to divide? Such activities are part of the normal cell regulation and checks and balances. In cell fusion studies during the 1970's, researchers learned that there are definite cell-cycle controls which direct and coordinate the events of the cell cycle. If one looks at the animal cell cycle, there are a number of "points" where controls keep the cell cycle in the stage it is at until they are over-ridden by chemical signals. One of these is in G1 , another in G2 just prior to mitosis and a third is midpoint in mitosis, (where proteolytic enzymes control the separation of centromeres at the start of anaphase). Cells will stay in G1 until they receive a signal to proceed with DNA duplication. Cells that never leave G1 are said to be in a non-dividing cycle called G0. It should not surprise us that the signals are protein kinase molecules, in this case a class of protein kinases called cyclin-dependent kinases (Cdk), because they are activated by protein called cyclin, whose concentration is cyclical (hence cyclin). In cell division in animals, cyclin concentration rises during interphase and complexes with the cyclin-dependent kinases forming a complex (cyclin-CdK). The first such complex discovered was called the "maturation (or mitosis) promoting factor" (or MPF). This complex triggers mitosis at the end of G2 . Cyclin - Cyclin-dependent Kinase Complex (MPF) Mitosis and the Cell Cycle - 15 The cyclin-dependent kinases phosphorylate a number of proteins needed to do mitosis. They also phosphorylate, and through serine and threonine kinase relay pathways, promote other proteins to phosphorylate the nuclear membrane lamina layer so that it degrades. As mitosis is nearing completion, the CdK complex promotes its own degradation by activating proteolytic enzymes that destroy cyclin. Control of Mitosis by the Cyclin-CdK complex Although the general role of cyclin-dependent kinases is known, specific ways in which they work and their substrates are still mostly mystery. Signals that activate the cyclin-dependent kinases can be internal or external. One signal pathway that is known is the anaphase checkpoint. Mitosis Kinetochore Checkpoint – Anaphase Control The mitosis anaphase checkpoint is regulated by the kinetochores of the sister chromatids. They delay anaphase until all kinetochores are attached to spindle fibers (microtubules). Proteins associated with the kinetochores have a signal pathway that blocks the cyclin breakdown and the associated breakdown of the proteins that hold the sister chromatids together. When all kinetochores are attached, the anaphase promoting complex stops its block, the proteins holding the sister chromatids together are degraded and cyclin associated with the mitosis checkpoint is broken down, permitting mitosis to continue. This control ensures that each cell formed will have the correct complement of chromosomes; no stray chromosome can avoid the metaphase plate. Mitosis and the Cell Cycle - 16 External Signals and Cell Division Control It has long been known that cell division, like many other cell activities, cannot occur if essential nutrients are not available. Growth factors, a class of proteins that are involved with signal transduction pathways, are important in promoting and in arresting cell division, so that cells divide when needed and do not divide inappropriately (except when cancers happen). It takes a certain concentration of growth factor to continue to trigger cell division. It also takes a certain amount of nutrients so that growth factors and nutrient supply are both important in regulating the rate of cell division. Absence of appropriate growth factor is one means of keeping a cell in G0 . In plant tissue culture, a cell will remain undivided until and unless an appropriate mixture of nutrients and growth regulators (plant hormones) is provided. Early plant tissue culture succeeded when researchers added coconut endosperm (coconut milk) to the culture medium. The endosperm is rich in plant hormones because its normal job is providing nutrients to the developing embryo. In animal cell and exceeds divide. As in as a density inhibition. cultures, cell division is halted when the cell population gets too dense the nutrient and growth factor concentration needed to continue to population biology and ecology, such regulation of cell division is known dependent activity or in this case a density dependent Animal cell division in cultures is also arrested when cells do not have an appropriate substrate or the extracellular matrix of cells to attach to during division. Normally, membrane proteins and components of the cytoskeleton trigger anchoring signal pathways that regulate cell division. Such cell division control is known as anchorage dependence. Mitosis and the Cell Cycle - 17 Cancer and the Cell Cycle Cancer is a disease of uncontrolled cell reproduction. The current statistics for cancer say that one in four people in the United States today will get some form of cancer. One in eight will get breast cancer. Almost any male who lives long enough will have prostate cancer. No one knows why any one person gets cancer. Some cancers are probably genetic. Some are related to the environment, especially the smoker's environment. For others, substances in foods may be cancer causing, cancer promoting or, conversely cancer preventing. We will discuss cancer later in the context of mutation and gene alterations, although it is useful to mention some of the features of cancer now, as related to cell division controls. Cancer cells are not inhibited by density factors or the need to be anchored. They freely divide and apparently "enjoy" being crowded. Cancer cells can migrate, particularly through the lymph system from the tissues in which they arise to other tissues and organs of the body. Unless controlled, cancers can and do cause death to the organism. Given adequate nutrients in culture media, cancer cells can divide indefinitely The HeLa cell line has been cultured for about a half century, indicating that telomerases (to be discussed with DNA duplication) are active in cancer cell lines. Cancer cells do not just divide rapidly. They have chromosome abnormalities and abnormal metabolism. Their cell surfaces are altered which promotes the metastasis and migration to other areas of the body, where they can attach to tissues and form more tumors (a mass of abnormal cells). Research indicates that a number of things are at work in cancer cells. Some may make their own growth factors (that promote cell division) and do not need external signals. Some have abnormal cell signal pathways for the cell division checkpoints, so checkpoints do not function properly. Two examples that affect checkpoints are the p53 and Rb protein transcription factors that have mutations in their genes in about 40 – 50% of all cancers. Other proteins involved in the kinase relay pathways also show mutations in many cancers. p53, for example, normally checks the integrity of DNA prior to duplication. If there are errors in DNA, p53 will trigger the enzymes that correct errors. If corrections cannot be made, p53 triggers the destruction of damaged cells. Mutated p53 genes result in failure of p53 to function. Damaged DNA duplicates, and can ultimately lead to cancer. The Rb gene is a tumor suppressing gene that codes for proteins that block cyclin from bonding to the Cdk complex in G1, blocking cell division and keeping cells in G0. The Rb protein interferes with a number of regulatory proteins that would normally promote the cyclin-Cdk complex. Mitosis and the Cell Cycle - 18 Function of p53