Biology: Chapter 9: The Cell Cyle

malignant tumor

a cancerous tumor containing cells that have significant genetic and cellular changes and are capable of invading and surviving in new sites. malignant tumors can impair the functions of one or more organs. - based on their ability to divide indefinitely in culture, these cells are also sometimes called transformed cells (although this term is generally restricted to cells in culture). - an individual with a malignant tumor is said to have cancer.

centromere

in a duplicated chromosome, the region on each sister chromatid where it is most closely attached to its sister chromatid by proteins that bind to the centromeric DNA. other proteins condense the chromatin in that region, so it appears as a narrow waist on the duplicated chromosome. (an unduplicated chromosome has a single centromere, identified by the proteins bound there.) - the part of a chromatid to either side of the centromere is referred to as an arm of the chromatid. (an uncondensed, unduplicated chromosome has a single centromere and two arms.) - later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell. Once the sister chromatids separate, they are no longer called sister chromatids but are considered individual chromosomes; this is the step that essentially doubles the number of chromosomes during cell division. Thus, each new nucleus receives a collection of chromosomes identical to that of the parent cell.

phases of the cell cycle

mitosis is just one part of the cell cycle.

origin of replication

site where the replication of a DNA molecule begins, consisting of a specific sequence of nucleotides. - in some bacteria, the process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at a specific place on the chromosome called the origin of replication, producing two origins. - as the chromosome continues to replicate, one origin moves rapidly toward the opposite end of the cell. while the chromosome is replicating, the cell elongates. - when replication is complete and the bacterium has reached about twice its initial size, proteins cause its plasma membrane to pinch inward, dividing the parent bacterial cell into two daughter cells. in this way, each cell inherits a complete genome. - using the techniques of modern DNA technology to tag the origins of replication with molecules that glow green in fluorescence microscopy, researchers hav directly observed the movmetn of bacterial chromosomes. - this movement is reminiscent of teh poleward movements of the centromere regions of eukaryotic chromosomes during anaphase of mitosis, but bacteria don't have visible mitotic spindles or even microtubules. - in most bacterial species studied, the two origins of replication end up at opposite ends of the cell or in some other very specific location, possibly anchored there by one or more proteins.

chromatin

the complex of DNA and proteins that makes up eukaryotic chromosomes. when the cell is not dividing, chromatin exists in its dispersed form, as a mass of very long, thin fibers that are not visible with a light microscope. - chromatin of a chromosome varies in its degree of condensation during the process of cell division.

cytokinesis

the division of the cytoplasm to form two separate daughter cells immediately after mitosis, meiosis I, or meiosis II.

cytokinesis

the division of the cytoplasm to form two separate daughter cells immediately after mitosis, meiosis I, or meiosis II. list of happenings from chart in book: - the division of the cytoplasm is usually well under way by late telophase, so the two daughter cells appear shortly after the end of mitosis. - in animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two.

telophase

the fifth and final stage of mitosis, in which daughter nuclei are forming and cytokinesis has typically begun. list of happenings from chart in book: - two daughter nuclei form in the cell. nuclear envelopes arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system. - nucleoli reappear. - the chromosomes become less condensed. - any remaining spindle microtubules are depolymerized. - mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

G1 phase (first gap)

the first gap, or growth phase, of the cell cycle, consisting of the portion of interphase before DNA synthesis begins.

cleavage furrow

the first sign of cleavage in an animal cell; a shallow groove around the cell in the cell surface near the old metaphase plate. - on the cytoplasmic side of hte furrow is a contractile ring of actin microfilaments associated with molecules of the protein myosin. - the actin microfilaments interact with the myosin molecules, causing the ring to contract. the contraction of the dividing cell's ring of microfilaments is like the pulling of a drawstring. - the cleavage furrow deepens until the parent cell is pinched in two, producing two completely separated cells, each with its own nucleus and its own share of cytosol, organelles, and other subcellular structures.

prophase

the first stage of mitosis, in which the chromatin condenses into discrete chromosomes visible w/ a light microscope, the mitotic spindle begins to form, and the nucleolus disappears but the nucleus remains in tact: list of happenings from chart in book: - the chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable w/ a light microscope. - the nucleoli disappear. - each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins (sister chromatid cohesion). - the mitoticings spindle (named for its shape) begins to form. it is composed of the centrosomes and the microtubules that extend from them. the radial arrays of shorter microtubules that extend from the centrosomes are called asters ("stars"). - the centrosomes move away from each other, propelled partly by the lengthening microtubules between them.

treatment for cancer

- a tumor that appears to be localized may be treated with high-energy radiation, which damages DNA in cancer cells much more than DNA in normal cells, apparently b/c the majority of cancer cells have lost the ability to repair such damage. - to treat known or suspected metastatic tumors, chemotherapy is used, in which drugs that are toxic to actively dividing cells are administered through the circulatory system. - as you might expect, chemotherapeutic drugs interfere w/ specific steps in the cell cycle. - for example: the drug Taxol freezes the mitotic spindle by preventing microtubule depolymerization, which stops actively dividing cells form proceeding past metaphase and leads to their destruction. - the side effects of chemotherapy are due to the drugs' effect on normal cells that divide frequently. - for example: nausea results from chemotherapy's effects on intestinal cells, hair loss from effects on hair follicle cells, and susceptibility to infection from effects on immune system cells. - over the past several decades, researchers have produced a flood of valuable information about cell-signaling pathways and how their malfunction contributes to the development of cancer through effects on the cell cycle. - coupled w/ new molecular techniques, such as the ability to rapidly sequence the DNA of cells in a particular tumor, medical treatments for cancer are beginning to become more "personalized" to a particular patient's tumor. - for example: the cells of roughly 20% of breast cancer tumors show abnormally high amounts of a cell-surface receptor tyrosine kinase called HER2, and many show an increase in the number of estrogen receptor (ER) molecules, intracellular receptors that can trigger cell division. - based on lab findings, a physician can prescribe chemotherapy w/ a molecule that blocks the function of the specific protein (herceptin for HER2 and tamoxifen for ERs). - treatment using these agents, when appropriate, has led to increased survival rates and fewer cancer recurrences.

abnormal cell behavior in the body

- abnormal cell behavior in the body can be catastrophic. - the problem begins when a single cell in a tissue undergoes the first of many steps that converts a normal cell to a cancer cell. - such a cell often has altered proteins on its surface, and the body's immune system normally recognizes the cell as "nonself" -- an insurgent -- and destroys it. - however, if the cell evades destruction, it may proliferate and form a tumor, a mass of abnormal cells within otherwise normal tissue. - the abnormal cells may remain at the original site if their genetic and cellular changes don't allow them to move to or survive at another site. - in that case, the tumor is called a benign tumor. - in contrast, a malignant tumor can be formed, and a person is said to have cancer.

the key roles of cell division

- the ability of organisms to produce more of their own kind is the one characteristic that best distinguishes living things from nonliving matter. - this unique capacity to procreate, like all biological functions, has a cellular basis. - the continuity of life is based on the reproduction of cells, or cell division.

example of an internal signal occurs at the third important checkpoint, the M phase checkpoint

- anaphase, the separation of sister chromatids, does not begin until all the chromosomes are properly attached to teh spindle at the metaphase plate. - researchers have learned that as long as some kinetochores are unattached to spindle microtubules, the sister chromatids remain together, delaying anaphase. - only when the kinetochores of all the chromosomes are properly attached to the spindle does the appropriate regulatory protein complex become activated. - once activated, the complex sets off a chain of molecular events that activates the enzyme separase, which cleaves the cohesins, allowing teh sister chromatids to separate. - the mechanism ensures that daughter cells do not end up with missing or extra chromosomes. - there are checkpoints in addition to those in G1, G2, and M.

how do the kinetochore microtubules function in this poleward movement of chromosomes?

- apparently, two mechanisms are in play that both involve motor proteins. - it is suggested that motor proteins on the kinetochores "walk" the chromosomes along the microtubules, which depolymerize at their kinetochore ends after the motor proteins have passed. (this is referred to as the Pac-man mechanism b/c of its resemblance to the arcade game.) - however, other researchers workign with different cell types or cells from other species, have shown that chromosomes are "reelin in" by motor proteins at teh spindle poles and that the microtubules depolymerize after they pass by these motor proteins at the poles. - the general consensus now is that both mechanisms are used and that their relative contributions vary among cell types.

loss of cell cycle controls in cancer cells

- cancer cells do not need the normal signals that regulate the cell cycle. - in culture, they do not stop dividing when growth factors are depleted. - logical hypothesis: cancer cells do not need growth factors in tehri culture medium to grow and divide. - they may make a required growth factor themselves, or they may have an abnormality in the signaling pathway that conveys the growth factor's signal to the cell cycle control system even in the absence of that factor. - another possibility is an abnormal cell cycle control system. - in these scenarios, the underlying basis of the abnormality is almost always a change in one or more genes (for example, a mutation) that alters the function of their protein products, resulting in faulty cell cycle control.

cytokinesis in plant cells

- cytokinesis in plant cells, which have cell walls, is remarkably different. there is no cleavage furrow. - instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to teh middle of the cell, where they coalesce, producing a cell plate. - cell wall materials carried in the vesicles collect inside the cell plate as it grows. the cell plate enlarges until its surrounding membrane fuses with teh plasma membrane along the permieter of the cell. - two daughter cells result, each with its own plasma membrane. meanwhile, a new cell wall arising from the contents of the cell plate forms between teh daughter cells.

evolution of mitosis

- given that prokaryotes preceded eukaryotes on earth by more than a billion years, we might hypothesize that mitosis evolved from simpler prokaryotic mechanisms of cell reproduction. the fact that some of the proteins involved in bacterial binary fission are related to eukaryotic proteins that function in mitosis supports that hypothesis. - as eukaryotes with nuclear envelopes and larger genomes evolved, the ancestral process of binary fission, seen today in bacteria, somehow gave rise to mitosis. - variations on cell division exist in different groups. these variant processes may be similar to mechanisms used by ancestral species and thus may resemble steps in the evolution of mitosis from a binary fission-like process carried out by very early bacteria. - possible intermediate stages are suggested by two unusual types of nuclear division found today in certain unicellular eukaryotes -- dinoflagellates, diatoms, and some yeasts. - these are thought to be cases where ancestral mechanisms have remained relatively unchanged over evolutionary time. - in both types, the nuclear envelop remains intact, in contrast to what happens in most eukaryotic cells.

differences between normal cells and cancer cells that reflect derangements of the cell cycle

- if and when they stop dividing, cancer cells do so at random points in the cycle, rather than at the normal checkpoints. - moreover, cancer cells can go on dividing indefinitely in culture if they are given continual supply of nutrients; in essence, they are "immortal." - a striking example is a cell line that has been reproducing in culture since 1951. cells of this line are called HeLa cells b/c their original source was a tumor removed from a woman named Henrietta Lacks.

nonkinetochore microtubules elongating cell during anaphase in dividing animal cell

- in a dividing animal cell, the nonkinetochore microtubules are responsible for elongating the whole cell during anaphase. - nonkinetochore microtubules from opposite poles overlap each other extensively during metaphase. - during anaphase, the region of overlap is reduced as motor proteins attached to teh microtubules walk them away from one another, using energy from ATP. - as the microtubules push apart from each other, their spindle poles are pushed apart, elongating the cell. at the same time, the microtubules lengthen somewhat by the addition of tubulin subunits to their overlapping ends. - as a result, the microtubules continue to overlap. - at the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell. nuclei re-form during telophase. - cytokinesis generally begins during anaphase or telophase, and the spindle eventually disassembles by depolymerization of microtubules.

most cell division results in genetically identical daughter cells

- in prokaryotes and eukaryotes, most cell division involves the distribution of identical genetic material -- DNA -- to two daughter cells. (the exception is meiosis, the special type of eukaryotic cell division that can produce sperm and eggs.)

what about stop and go-ahead signals themselves -- what are the signaling molecules?

- studies using animal cells in culture have led to the identification of many external factors, both chemical and physical, thta can influence cell division. - for example: cells fail to divide if an essential nutrient is lackign in the culture medium. (this is analogous to trying to run a washing machine without the water supply hooked up; an internal sensor won't allow the machine to continue past hte point where water is needed.) - and even if all other conditions are favorable, most types of mammalian cells divide in culture only if the growth medium includes specific growth factors.

the eukaryotic cell cycle is regulated by a molecular control system.

- the timing and rate of cell division in different parts of a plant or animal are crucial to normal growth, development, and maintenance. - the frequency of cell division varies with the type of cell. - for example: - human skin cells divide frequently throughout life. - liver cells maintain the ability to divide but keep it in reserve until an appropriate need arises -- such as to repair a wound. - some of the most specialized cells (full formed nerve cells and muscle cells) do not divide at all in a mature human.

checkpoint in S phase + more checkpoints than just those in G1, G2, and M phases.

- there are checkpoints in addition to those in G1, G2, and M. - a checkpoint in S phase stops cells with DNA damage from proceeding in the cell cycle. - and, in 2014, researchers presented evidence for another checkpoint between anaphase and telophase that ensures anaphase is completed and the chromosomes are well separated before cytokinesis can begin, thus avoiding chromosomal damage.

the changes that have occurred in cells of malignant tumors show up in many ways besides excessive proliferation.

- these cells may have unusual numbers of chromosomes, though whether this is a cause or an effect of tumor-related changes is an ongoing debate. - their metabolism may be altered, and they may cease to function in any constructive way. - abnormal changes on the cell surface cause cancer cells to lose attachments to neighboring cells and the extracellular matrix, allowing them to spread into nearby tissues. - cancer cells may also secrete signaling molecules that cause blood vessels to grow toward the tumor. - a few tumor cells may separate from the original tumor, enter blood vessels and lymph vessels, and travel to other parts of the body. there, they may proliferate and form a new tumor. this spread of cancer cells to locations distant from their original site is called metastasis.

distribution of chromosomes during eukaryotic cell division

- when a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosomes is in the form of a long, thin chromatin fiber. - after DNA replication, however, the chromosomes condense as a part of cell division: - each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and so thick that we can see them with a light microscope.

growth factor

1) a protein that must be present in the extracellular environment (culture medium or animal body) for the growth and normal development of certain types of cells. 2) a local regulator that acts on nearby cells to stimulate cell proliferation and differentiation. - a growth factor is a protein released by certain cells that stimulates other cells to divide. different cell types respond specifically to different growth factors or combinations of growth factors. - for example: platelet-derived growth factor (PDGF), which is made by blood cell fragments called platelets. PDGF is required for the division of cultured fibroblasts, a type of connective tissue cell. fibroblasts have PDGF receptors on their plasma membranes. - the binding of PDGF molecules to these receptors triggers a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide. - PDGF stimulates fibroblast division not only in the artificial conditions of cell culture, but also in an animal's body. when an injury occurs, platelets release PDGF in the vicinity. the resulting proliferation of fibroblasts helps heal the wound.

cleavage

1) the process of cytokinesis in animal cells, characterized by pinching of the plasma membrane. - in animal cells, cytokinesis occurs by a process known as cleavage. the first sing of cleavage is the appearance of a cleavage furrow.

chromosomes

a cellular structure consisting of one DNA molecule and associated protein molecules. - eukaryotic cell: typically has multiple, linear chromosomes, which are located in the nucleus. - prokaryotic cell: often has a single, circular chromosome, which is found in the nucleoid, a region that is not enclosed by a membrane. - each eukaryotic chromosome consists of one very long, linear DNA molecule associated with many proteins. DNA molecule carries several hundred to a few thousand genes (the units of information that specify an organism's inherited traits). - the associated proteins maintain the structure of the chromosome and help control the activity of the genes.

checkpoint

a control point in the cell cycle where stop and go-ahead signals can regulate the cycle. - animal cells generally have built-in stop signals that halt the cell cycle at checkpoints until overridden by go-ahead signals. many signals registered at checkpoints come from cellular surveillance mechanisms inside the cell. - these signals report whether crucial cellular processes that should have occurred by that point have in fact been completed correctly and thus whether or not the cell cycle should proceed. - checkpoints also register signals from outside the cell. - for many cells, the G1 checkpoint -- dubbed the "restriction point" in mammalian cells -- seems to be the most important. - if a cell receives a go-ahead signal at G1 checkpoint, it will usually complete the G1, S, G2, and M phases and divide. - if it does not receive a go-ahead signal at that point, it may exit the cycle, switching into a non-dividing state called the G0 phase. - (the cell cycle is regulated at teh molecular level by a set of regulatory proteins and protein complexes, including proteins called cyclins, and other proteins interacting with cyclins that are kinases (enzymes that activate or inactivate other proteins by phosphorylating them).

cell cycle control system

a cyclically operating set of molecules in the eukaryotic cell that both triggers and coordinates key events in the cell cycle. -the sequential events of the cell cycle are directed by a distinct cell cycle control system. - the cell cycle control system has been compared to teh control device of a washing machine. Like the washer's timing device, teh cell cycle control system proceeds on its own, according to a built-in clock. However, just as a washer's cycle is subject to both internal control (such as the sensor that detects when the tub is filled with water) and external adjustment (such as starting the machine), the cell cycle is regulated at certain checkpoints by both internal and external signals.

gamete

a haploid reproductive cell, such as an egg or sperm, that is formed by meiosis or is the descendant of cells formed by meiosis. gametes untie during sexual reproduction to produce a diploid zygote. - gametes have half as many chromosomes as somatic cells, or one set of 23 chromosomes in humans.

benign tumor

a mass of abnormal cells with specific genetic and cellular changes such that the cells are not capable of surviving at a new site and generally remain at the site of the tumor's origin. - most benign tumors do not cause serious problems and can be removed by surger

cell plate

a membrane-bounded, flattened sac located at the midline of a dividing plant cell, inside which the new cell wall forms during cytokinesis.

binary fission

a method of asexual reproduction in single-celled organisms in which the cell grows to roughly double its size and then divides into two cells. in prokaryotes (bacteria and archaea), binary fission does not involve mitosis, but in single-celled eukaryotes (such as an amoeba) that undergo binary fission, mitosis is part of the process. - in bacteria, most genes are carried on a single bacterial chromosome that consist of a circular DNA molecule and associated proteins. - although bacteria are smaller and simpler than eukaryotic cells, the challenge of replicating their genomes in an orderly fashion and distributing the copies equally to two daughter cell sis still formidable. (for example: when it is fully stretched out, the chromosome of the bacterium Escherichia coli is about 500 times as long as the cell. for such a long chromosome to fit within the cell, it must be highly coiled and folded.

G0 phase

a nondividing state occupied by cells that have left the cell cycle, sometimes reversibly. - most cells of the human body are in the G0 phase.

mitosis

a process of nuclear division in eukaryotic cells conventionally divided into five stages: - prophase - prometaphase - metaphase - anaphase - telophase. mitosis conserves chromosome number by allocating replicated chromosomes equally to each of the daughter nuclei. - Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by cytokinesis.

kinetochore

a structure of proteins attached to the centromere that links each sister chromatid to the mitotic spindle. - each of the two sister chromatids of a duplicated chromosome has a kinetochore, a structure made up of proteins that have assembled on specific sections of chromosomal DNA at each centromere. - the chromosome's two kinetochores face in opposite directions. - during prometaphase, some of teh spindle microtubules attach to the kinetochores; these are called kinetochore microtubules. (the number of microtubules attached to a kinetochore varies among species, from one in yeast cells to about 40 in some mammalian cells.) - the kinetochore acts as a coupling device that attaches the motor of the spindle to the cargo that it moves -- the chromosome. - when one of a chromosome's kinetochores is "captured" by microtubules, the chromosome begins to move toward the pole from which those microtubules extend. - however, this movement comes to a halt as soon as microtbuesl from the opposite pole attach to teh kinetochore on the other chromatid. what happens next is like a tug-of-war that ends in a draw. the chromosome moves first in one direction and then in the other, back and forth, finally settling midway between the two ends fo the cell. - at metaphase, the centromeres of all the duplicated chromosomes are on a plane midway between the spindle's two poles. this plane is called the metaphase plate. - meanwhile, microtubules that do not attach to kinetochores have been elongating, and by metaphase, they overlap and interact with other nonkinetochore microtubules from the opposite pole of the spindle. - by metaphase, the microtubules of the asters have also grown and are in contact with the plasma membrane. the spindle is now complete. - the structure of the completed spindle correlates well with its function during anaphase. anaphase begins suddenly when the cohesins holding together the sister chromatids of each chromosome are cleaved by an enzyme called separase. once separated, the chromatids become individual chromosomes that move toward opposite ends of the cell.

centrosome

a structure present in the cytoplasm of animal cells that functions as a microtubule-organizing center and is important during cell division. a centrosome has two centrioles. - in animal cells, the assembly of spindle microtubules starts at the centrosome. - a pair of centrioles is located at the center of the centrosome, but they are not essential for cell division: if the centrioles are destroyed with a laser microbeam, a spindle nevertheless forms during mitosis. in fact, centrioles are not even present in plant cells, which do form mitotic spindles. - during interphase in animal cells, the single centrosome duplicates, forming two centromeres, which remain near the nucleus. - the two centromeres move apart during prophase and prometaphase of mitosis as spindle microtubules grow out of them. by the end of prometaphase, the two centrosomes, one at each pole of the spindle, are at opposite ends of the cell. - an aster, a radial array of short microtubules, extends from each chromosome. the spindle includes centrosomes, the spindle of microtubules, and the asters. (continued below on kinetochore)

mitotic spindle

an assemblage of microtubules and associated proteins that is involved in the movement of chromosomes during mitosis. - many of the events of mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. - while the mitotic spindle assembles, the other microtubules of the cytoskeleton partially disassemble, providing the material used to construct the spindle. - the spindle of microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin and shorten (depolymerize) by losing subunits.

metaphase plate

an imaginary structure located at a plane midway between the two poles of a cell in metaphase on which the centromeres of all the duplicated chromosomes are located. -

cell cyle

an ordered sequence of events in the life of a cell, from its origin in the division of a parent cell until its own division into two. - eukaryotic cell cycle includes: - Interphase (including G1, S, and G2 phases) - M phase (including mitosis and cytokinesis) - passing identical genetic material to cellular offspring is a crucial function of cell division.

somatic cells

any cell in a multicellular organism except a sperm or egg or their precursors. - every eukaryotic species has a characteristic number of chromosomes in each cell's nucleus. - example: the nuclei of human somatic cells (all body cells except the reproductive cells) each contain 46 chromosomes, made up of two sets of 23, one set is inherited from each parent.

mitosis & its stages

broken down into five stages: - Prophase - Prometaphase - Metaphase - Anaphase - Telophase (overlapping with the latter stages of mitosis, cytokinesis completes the mitotic phase.)

anaphase

the fourth stage of mitosis, in which the chromatids of each chromosome have separated and the daughter chromosomes are moving to the poles of the cell. list of happenings from chart in book: - anaphase is the shortest stage of mitosis, often lasting only a few minutes. - anaphase begins when the cohesin proteins are cleaved. this allows the two sister chromatids of each pair to part suddenly. each chromatid thus becomes an independent chromosome. - the two new daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. B/c these microtubules are attached at the centromere region, the centromeres are pulled ahead of the arms, moving at a rate of about 1 um/min. - the cell elongates as the nonkinetochore microtubules lengthen. - by the end of anaphase, the two ends of the cell have equivalent -- and complete -- collections of chromosomes.

genome

the genetic material of an organism or virus; the complete complement of an organism's or virus's genes along with its noncoding nucleic acid sequences. - prokaryotic genome is often a single DNA molecule. - eukaryotic genomes usually consist of a number of DNA molecules. - the overall length of DNA in a eukaryotic cell is enormous: - typical human cell has about 2 m of DNA (a length that is 250,000 times greater than the cell's diameter). - before the cell can divide to form genetically identical daughter cells, all of this DNA must be copied (replicated), and then the two copies must be separated so that each daughter cell ends up with a complete genome.

interphase

the period in the cell cycle when the cell is not dividing. during interphase, cellular metabolic activity is high, chromosomes and organelles are duplicated, and cell size may increase. interphase often accounts for about 90% of the cell cycle. - interphase is divided into three phases: - G1 phase (first gap) - S phase (synthesis) - G2 phase (second gap) - the G phases were originally misnamed as "gaps" b/c the cells appeared inactive, but we no we know that intense metabolic activity and growth occur throughout interphase. - during all three stages of interphase, in fact, a cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum. - duplication of the chromosomes, crucial for eventual division of the cell, occurs entirely during the S phase. - thus, a cell grows (G1), continues to grow as it copies its chromosomes (S), grows more as it completes preparations for cell division (G2), and divides (M). the daughter cells may then repeat the cycle. - a particular human cell might undergo one division in 24 hours. - of this time, the M phase would occupy less than 1 hour. - the S phase might occupy about 10 - 12 hours (about half the cycle). - the rest of the time would be apportioned between the G1 and G2 phases. - the G2 phase usually takes 4 - 6 hours. - in our ex, the G1 would occupy about 5 - 6 hours. (G1 is the most variable in length in different types of cells) - some cells in a multicellular organism divide very in frequently or not at all. these cells spend their time in G1 (or a related phase called G0) doing their job in the organism -- a cell of the pancreas secretes digestive enzymes, for example.

mitotic (M) phase

the phase of the cell cycle that includes mitosis and cytokinesis. - this phase is usually the shortest part of the cell cycle.

density-dependent inhibition

the phenomenon observed in normal animal cells that causes them to stop dividing when they come into contact with one another. = the effect of an external physical factor on cell division is claerly seen in density-dependent inhibition, a phenomenon in whihc crowded cells stop dividing. - cultured cells normally divide until they form a single layer of cells on the inner surface of a culture flask, at which point the cells stop dividign. if some cells are removed, those bordering teh open space begin dividign again and continue until the vacancy is filled. - the bindign of a cell-surface protein to its counterpart on an adjoining cell sends a signal to both cells that inhibits cell division, preventing them from moving forward in the cell cycle, even in the presence of growth factors.

transformation

the process by which a cell in culture acquires the ability to divide indefinitely, similar to the division of cancer cells. - by contrast, nearly all normal, non-transformed mammalian cells growing in culture divide only about 20 to 50 times before they stop dividing, age, and die. - finally, cancer cells evade the normal controls that trigger a cell to undergo a type of programmed cell death called apoptosis when something is wrong -- for example, when an irreparable mistake has occurred during DNA replication preceding mitosis.

cell division

the reproduction of cells. - cell division plays several very important roles in life: - the division of one prokaryotic cell reproduces an entire organism and same is true of unicellular eukaryotes. - cell division enables multicellular eukaryotes to develop from a single cell (example: like the fertilized sand dollar egg that gave rise to two-celled embryo in figure 9.2b on pg. 187). - after such an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from accidents or normal wear and tear (example: dividing cells in your bone marrow continuously make new blood cells). - the cell division process is an integral part of the cell cycle!

anchorage dependence

the requirement that a cell must be attached to a substratum in order to initiate cell division. - most animal cells exhibit anchorage dependence. - to divide, they must be attached to something, such as the inside fo a culture flask or the extracellular matrix of a tissue. - experiments suggest that like cell density, anchorage is signaled to the cell cycle control system via pathways involving plasma membrane proteins and elements of the cytoskeleton linked to them. - cancer cells exhibit neither density-dependent inhibition nor anchorage dependence.

G2 phase (second gap)

the second gap, or growth phase, of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs. - - a nuclear envelope encloses the nucleus. - the nucleus contains one or more nucleoli (singular, nucleolus). - two centrosomes have formed by duplication of a single centrosome. (centrosomes are regions in animal cells that organize the microtubules of the spindle. each centrosome contains two centrioles.) - chromosomes, duplicated during S phase, cannot be seen individually b/c they have not yet condensed.

propmetaphase

the second stage of mitosis, in which the nuclear envelope fragments and the spindle microtubules attach to the kinetochores of the chromosomes. list of happenings from chart in book: - the nuclear envelope fragments. - the microtubules extending from each centrosome can now invade the nuclear area. - the chromosomes have become even more condensed. - a kinetochore (a specialized protein structure) has now formed at the centromere of each chromatid (thus, two per chromosome). - some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules," which jerk the chromosomes back and forth. - nonkinetochore microtubules interact w/ those from the opposite pole of the spindle, lengthening the cell.

metastasis

the spread of cancer cells to locations distant from their original site.

S phase (synthesis)

the synthesis phase of the cell cycle; the portion of interphase during which DNA is replicated.

metaphase

the third stage of mitosis, in which the spindle is complete and the chromosomes, attached to microtubules at their kinetochores, are all aligned at the metaphase plate. list of happenings from chart in book: - the centrosomes are now at opposite poles of the cell. - the chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle's two poles. the chromosomes' centromeres lie at the metaphase plate. - for each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles.

sister chromatids

two copies of a duplicated chromosome attached to each other by proteins at the centromere and, sometimes, along the arms. While joined, two sister chromatids make up one chromosome. Chromatids are eventually separated during mitosis or meiosis II. - the two chromatids, each containing an identical DNA molecule, are typically attached all along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion.