Chap 12 For Bio FInal

What is an aster? -

An aster, a radial array of short microtubles, extends from each centrosome. The spindle includes the centrosomes, the spindle microtubules, and the asters.

What are the characteristics of the cell during anaphase? -

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 a full-fledged chromosome. The two liberated daughter chromosomes begin moving toward opposite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere first (at about 1 micrometer/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.

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

Apparently two mechanisms are at play, both involving motor proteins. A clever experiment carried out in 1987 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 "Pacman" mechanism because of its resemblence to the arcade game character that moves by eating all the dots in its path.) However, other researchers, working with different cell types or cells from other species, have shown that chromosomes are "reeled in" by motor proteins at the spindle poles and the microtubules depolymerize after they pass by these motor proteins. The general consensus now is that both mechanisms are used and that their relative contributions vary among cell types.

How does cell division occur in bacteria? -

Bacteria - During binary fission in bacteria, the origins of the daughter chromosomes move to opposite ends of the cell. The mechanism is not fully understood, but proteins may anchor the daughter chromosomes to specific sites on the plasma membrane.

One difference between cancer cells and normal cells is that cancer cells a. are unable to synthesize DNA. b. are arrested at the S phase of the cell cycle. c. continue to divide even when they are tightly packed together. d. cannot function properly because they are affected by density-dependent inhibition. -

C

The decline of MPF activity at the end of mitosis is due to a. the destruction of the protein kinase Cdk. b. decreased synthesis of Cdk. c. the degradation of cyclin. d. the accumulation of cyclin. -

C

How are the bacterial chromosomes being moved to their specific locations? -

In most bacterial species studied, the two origins of replication end up at opposite ends of the cell or in some other specific location, possible anchored there by one or more proteins. How bacterial chromosomes move and how their specific location is established and maintained are still not fully understood. However, several proteins have been identified that play important roles: One resembling eukaryotic actin apparently functions in bacterial chromosome movement during cell division, and another that is related to tubulin seems to help pinch the plasma membrane inward, separating the two bacterial daughter cells.

How does cell division occur in most eukaryotes? -

In most eukaryotes, including plants and animals, the spindle forms outside the nucleus, and the nuclear envelope breaks down mitosis. Microtubules separate the chromosomes, and two nuclear envelopes then form.

How is interphase divided? -

Interphase can be divided into subphases: the G1 phase ("first gap"), the S phase (synthesis", and the G2 phase (second gap). During all three subphases, a cell that will eventually divide grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

What is mitosis?

Later in the cell division process, the two sister chromatids of each duplicated chromosome seperate 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. Thus each new nucleus receives a collection of chromosomes identical to that of the parent cell. Mitosis, is the division of the genetic material in the nucleus, which is usually followed immediately by cytokinesis, the division of the cytoplasm. Once cell has become two, each the genetic equivalent of the parent cell.

What is the mitotic spindle? -

Many events of mitosis depend on the mitotic spindle, which begins to form in the cytoplasm during prophase. This structure consists of fibers made of microtubules and associated proteins.

Where does meiosis occur in humans?

Meiosis in humans occurs only in the gonads (ovaries or testes). In each generation, meiosis reduces the chromosome number from 46 (two sets of chromosomes) to 23 (one set). Fertilization fuses two gametes together and returns the chromosome number to 46, and mitosis conserves that number in every somatic cell nucleus of the new individual.

What other functions do actin and tubulin carry out? Name the proteins they interact with to do so. -

Microtubules made up of tubulin in the cell provide "rails" along which vesicles and other organelles can travel, based on interactions of motor proteins with tubulin in the microtubules. In muscle cells, actin in micro- filaments interacts with myosin filaments to cause muscle contraction.

What other functions do actin and tubulin carry out? Name the proteins they interact with to do so. -

Microtubules made up of tubulin in the cell provide "rails" along which vesicles and other organelles can travel, based on interactions of motor proteins with tubulin in the microtubules. In muscle cells, actin in microfilaments interacts with myosin filaments to cause muscle contraction.

What happens to the microtubules that do not attach to kinetochores? -

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. (These are sometimes called "polar" microtubules.) By metaphase, the microtubules of the asters have also grown and are in contact with the plasma membrane. The spindle is now complete.

How many somatic cells does mitosis and cytokinesis produce by the time you are an adult?

Mitosis and cytokinesis produced the 200 trillion somatic cells that now make up your body and the same processes continue to generate new cells to replace dead and damaged ones.

What happens during the mitotic (M) phase? -

Mitosis is just one part of the cell cycle. In fact, the mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the shortest path of the cell cycle.

What happens during interphase? -

Mitotic cell division alternates with a much longer stage called interphase, which accounts for about 90% of the cycle. During Interphase, a cell that is about to divide grows and copies its chromosomes in preparation for cell division.

What is the purpose of the cleavage furrow? -

On the cytoplasmic side of the 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 share of cytosol, organelles, and other subcellular structures.

PART B - Mechanisms underlying the events of mitosis Mitosis unfolds through a sequence of stages marked by specific events in the cell. The structural changes in the cell are brought about by a series of tightly coordinated underlying mechanisms. Sort each process into the appropriate bin to indicate the stage of mitosis in which it occurs. If a process occurs in more than one stage, sort it to the stage when it first occurs. -

PROPHASE: -Tubulins assemble into spindle microtubules -Cohesins join sister chromatids of duplicated chromosomes PROMETAPHASE: -Microtubules attach to kinetochores METAPHASE: -Kinetochores are motionless in relation to cell poles ANAPHASE: -Kinetochores move toward poles of the cells -Cohesins break down TELOAPHASE: -Spindle microtubules disassemble [The micrographs in Part A show some of the cellular processes that occur during the stages of mitosis. -In prophase, the microtubules of the spindle apparatus begin to assemble from individual tubulin subunits. As the identical chromatids of each pair of sister chromatids condense during this stage, they are held together by cohesin proteins. -Prometaphase is marked by fragmentation of the nuclear envelope, expansion of the spindle into the nuclear region, and attachment of some spindle fibers to the chromosomes via the kinetochores. -Metaphase, marked by the alignment of chromsomes along the metaphase plate, is brought about by kinetochores aligning and then remaining motionless relative to the poles of the cell. -In anaphase, the cohesin proteins are cleaved, and the kinetochores move toward the poles of the cell, separating the sister chromatids. -As telophase proceeds, the kinetochore microtubules of the spindle disassemble. As the chromosomes reach the poles of the cell, the nuclear envelopes of the two new daughter nuclei form.]

What is binary fission? -

Prokaryotes (bacteria and archaea) can undergo a type of reproduction in which the cell grows to roughly double its size and then divides to form two cells. This term binary fission, meaning "division in half," refers to this process and to the asexual reproduction of single-celled eukaryotes, such as amoeba. However, the process in eukaryotes involves mitosis, while that in prokaryotes does not.

What are gametes?

Reproductive cells, or gametes - sperm and eggs - have half as many chromosomes as somatic cells, or one set of 23 chromosomes in humans. The number of chromosomes in somatic cells varies widely among species: 18 in cabbage plants, 48 in chimpanzees, 56 in elephants, 90 in hedgehogs, and 148 in one species of alga.

What cells divide very infrequently/not at all? -

Some cells in a multicellular organism divide very infrequently or not at all. These cells spend their time in G1 (or a related phase called G0) doing their job in the organism. An example of this type of cell is a nerve cell which carries out impulses.

How does cell division occur in dinoflagellates? -

Dinoflagellates - In unicellular protists called dinoflagellates, the chromosomes attach to the nuclear envelope, which remains intact during cell division. Microtubules pass through the nucleus inside cytoplasmic tunnels, reinforcing the spatial orientation of the nucleus, which then divides in a process reminiscent of bacterial binary fission.

What were the conclusions drawn from Gary Borisy's experiment? -

During anaphase in this cell type, chromosome movement is correlated with kinetochore shortening at their kinetochore ends and not at their spindle pole ends. This experiment supports the hypothesis that during anaphase, a chromosome is walked along a microtubule as the microtubule depolymerizes at its kinetochore end, releasing tubulin subunits.

Compare the roles of tubulin and actin during eukaryotic cell division with the roles of tubulin-like and actin-like proteins during bacterial binary fission. -

During eukaryotic cell division, tubulin is involved in spindle forma- tion and chromosome movement, while actin functions during cytokinesis. In bacterial binary fission, it's the opposite: Tubulin-like molecules are thought to act in daughter cell separation, and actin-like molecules are thought to move the daughter bacterial chromosomes to opposite ends of the cell.

Compare the roles of tubulin and actin during eukaryotic cell division with the roles of tubulin-like and actin-like proteins during bacterial binary fission. -

During eukaryotic cell division, tubulin is involved in spindle formation and chromosome movement, while actin functions during cytokinesis. In bacterial binary fission, it's the opposite: Tubulin-like molecules are thought to act in daughter cell separation, and actin-like molecules are thought to move the daughter bacterial chromosomes to opposite ends of the cell.

What are kinetochore microtubules? -

During prometaphase, some of the spindle microtubules attach to the kinetochores; these are called kinetochore microtubules. The number of microtubules attached to a kinetochore varies among species, from one microtubule in yeast cells to 40 or so in some mammalian cells.

What happens to the single centrosome during interphase in animals? -

During the interphase in animals, the single centrosome duplicates, forming two centrosomes, which remain together near the nucleus. The two centrosomes 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.

What are sister chromatids?

Each duplicated chromosome has two sister chromatids, which are joined copies of the original chromosome. The two chromatids, each containing an identical DNA molecule, are initially attached along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion.

What is a kinetochore? -

Each of the two sister chromatids of a duplicated chromosome has a kinetochore, a structure of proteins associated with specific sections of chromosomal DNA at each centromere. The chromosome's two kinetochores face in opposite directions.

What is a centromere?

Each sister chromatid has a centromere, a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid. This attachment is mediated by proteins bound to the centromeric DNA sequences and gives the condensed, duplicated chromosome a narrow "waist". The part of a chromatid on 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.)

What are somatic cells?

Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. For 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 inherited from each parent.

How did Gary Borisy go about carrying out his experiment? -

First they labeled the microtubules of a pig kidney cell in early anaphase with a yellow fluorescent dye. Then they marked a region of the kinetochore microtubules between one spindle pole and the chromosome by using a laser to eliminate the fluorescence from that region, while leaving the microtubules intact. As anaphase proceeded, they monitored the changes in microtubule length on either side of the mark.

Compare cytokinesis in animal cells and plant cells. -

Following mitosis, cytokinesis results in two genetically identical daughter cells in both plant cells and animal cells. However, the mechanism of dividing the cytoplasm is different in animals and plants. In an animal cell, cytokinesis occurs by cleavage, which divides the parent cell in two with a contractile ring of actin filaments. In a plant cell, a cell plate forms in the middle of the cell and grows until its membrane fuses with the plasma membrane of the parent cell. A new cell wall grows inside the cell plate.

Compare cytokinesis in animal cells and plant cells. -

Following mitosis, cytokinesis results in two genetically identical daughter cells in both plant cells and animal cells. However, the mechanism of dividing the cytoplasm is different in animals and plants. In animal cell, cytokinesis occurs by cleavage, which divides the parent cell in two with a contractile ring of actin filaments. In a plant cell, a cell plate forms in the middle of the cell and grows until its membrane fuses with the plasma membrane of the parent cell. A new cell wall grows inside the cell plate.

During which stages of the cell cycle does a chromo- some consist of two identical chromatids? -

From the end of S phase in interphase through the end of metaphase in mitosis.

During which stages of the cell cycle does a chromosome consist of two identical chromatids? -

From the end of S phase in interphase through the end of metaphase in mitosis. In anaphase the sister chromatids are pulled apart.

What did Gary Borisy want to find out about kinetochore microtubules? -

Gary Borisy and colleagues at the University of Wisconsin wanted to determine whether kinetochore microtubules depolymerize at the kinetochore end or the pole end as a chromosome move toward the poles during mitosis.

By how many years did prokaryotes precede eukaryotes on Earth? -

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.

Why did the German anatomist named Walther Flemming develop dyes in 1882? -

He developed dyes that allowed him to observe, for the first time, the behavior of chromosomes during mitosis and cytokinesis. (In fact, Flemming coined the terms mitosis and chromatin.) During the period between one cell division and the next, it appeared to Flemming that the cell was simply growing larger.

How many chromatids are in a duplicated chromosome?

I believe two.

What is the origin of replication? -

In E.coli, the process of cell division is initiated when the DNA of the bacterial chromosome begins to replicate at the specific place on the chromosome called the origin of replication, producing two origins.

What is the first sign of cleavage? -

In animal cells, cytokinesis occurs by a process known as cleavage. The first sign of cleavage is the appearance of a cleavage furrow, a shallow groove in the cell surface near the old metaphase plate.

Where is the centrosome located? -

In animal cells, the assembly of spindle microtubules starts at the centrosome, a subcellular region containing material that functions throughout the cell cycle to organize the cell's microtubules. (It is also called the microtubule organizing center). 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.

How long is the E.coli chromosome when it is stretched out? -

In bacteria, most genes are carried on a single bacterial chromosome that consists 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 cells is still formidable. The chromosome of the bacterium E.coli, for example when it is stretched out is about 500 times as long as the cell. For such a long chromosome to fit within the cell requires that it be highly coiled and folded.

How many chromosomes do the gametes in humans carry?

In contrast, you produce gametes-eggs or sperm- by a variation of cell division called meiosis, which yields nonidentical daughter cells that have only one set of chromosomes, half as many chromosomes as the parent cell.

Differentiate between these terms: chromosome, chromatin, and chromatid. -

The DNA of a eukaryotic cell is packaged into structures called chromosomes. Each chromosome is a long molecule of DNA, which carries hundreds to thou- sands of genes, with associated proteins that maintain chromosome structure and help control gene activity. This DNA-protein complex is called chromatin. The chromatin of each chromosome is long and thin when the cell is not dividing. Prior to cell division, each chromosome is duplicated, and the resulting sister chromatids are attached to each other by proteins at the centromeres and, for many species, all along their lengths (a phenomenon called sister chromatid cohesion).

What is one of the characteristics that best distinguishes living things from nonliving matter?

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. Rudolf Virchow, a German physician, put it this way in 1855: "Where a cell exists, there must have been a preexisting cell, just as the animal arises only from an animal and the plant from a plant". He summarized this concept with the Latin axiom "Omnis cellula e cellula," meaning "Every cell from a cell." The continuity of life is based on the reproduction of cells, or cell division.

What happens to DNA when a cell experiences cell division?

The reproduction of an assembly as complex as a cell cannot occur by a mere pinching in half; a cell is not like a soap bubble that simply enlarges and splits in two. In both 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 is most remarkable about cell division is the fidelity (faithfulness) with which the DNA is passed along from one generation of cells to the next. A dividing cell duplicates its DNA, allocates the two copies to opposite ends of the cell, and only then splits into daughter cells.

What is the function of the enzyme separase during anaphase? -

The structure of the completed spindle correlates well with its function during anaphase. Anaphase commences suddenly when the cohesins holding together the sister chromatids of each chromosome are cleaved by an enzyme called separase. Once the chromatids become separate, full-fledged chromosomes, they move toward opposite ends of the cell.

What are the characteristics of the cell during telophase? -

The 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 reamining spindle microtubules are depolymerized. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

What are the five stages that make up mitosis? -

They are prophase, prometaphase, metaphase, anaphase, and telophase.

In a dividing animal cell, what are the nonkinetochore microtubules responsible for? -

They 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 the 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.

What is the function of nonkinetochore microtubules? -

They elongate the cell during anaphase.

What is chromatin?

Together, the entire complex of DNA and proteins that is the building material of chromosomes is referred to as chromatin. As you will see soon, the chromatin of a chromosome varies in its degree of condensation during the process of cell division.

Describe the process of E.coli replication -

1. Chromosome replication begins. Soon after, one copy of the origin moves rapidly toward the other end of the cell by a mechanism not yet fully understood (wow - if we could understand we could stop bacteria from replicating - that would be an amazing antibiotic) 2. Replication continues. One copy of the origin is now at each end of the cell. Meanwhile, the cell elongates. 3. Replication finishes. The plasma membrane grows inward, and the new cell wall is deposited. 4. This results in two daughter cells being formed.

A particular cell has half as much DNA as some other cells in a mitotically active tissue. The cell in question is most likely in a. G1. b. G2. c. prophase d. metaphase. -

A

How does a chromosome get duplicated into two daughter cells?

1. One of the multiple chromosomes in a eukaryotic cell is represented here, not yet duplicated. Normally it would be a long, thin chromatin fiber containing one DNA molecule and associated proteins. The chromosome duplicates (which includes DNA replication) and condenses. 2. Once duplicated, a chromosome consists of two sister chromatids connected along their entire lengths by sister chromatid cohesion. Each chromatid contains a copy of the DNA molecule. The sister chromatids will then separate into two chromosomes. 3. Molecular and mechanical processes separate the sister chromatids into two chromosomes and distribute them to to daughter cells.

What are the 5 steps of mitosis that occur in plant cells? -

1. Prophase - The chromatin is condensing and the nucleolus is beginning to disappear. Although not yet visible in the migrograph, the mitotic spindle is starting to form. 2. Prometaphase - Discrete chromosomes are now visible; each consists of two aligned, identical sister chromatids; Later in prometaphase, the nuclear envelop will fragment. 3. Metaphase - The spindle is complete and the chromosomes, attached to microtubules at their kinetochores, are all at the metaphase plate. 4. Anaphase - The chromatids of each chromosome have separated, and the daughter chromosomes are moving to the ends of the cell as their kinetochore microtubules shorten. 5. Telophase - Daughter nuclei are forming. Meanwhile, cytokinesis has started: The cell plate which will divide the cytoplasm in two, is growing toward the perimeter of the parent cell.

In the cells of some organisms, mitosis occurs without cytoki- nesis. This will result in a. cells with more than one nucleus. b. cells that are unusually small. c. cells lacking nuclei. d. cell cycles lacking an S phase. -

A

Vinblastine is a standard chemotherapeutic drug used to treat cancer. Because it interferes with the assembly of microtu- bules, its effectiveness must be related to a. disruption of mitotic spindle formation. b. suppression of cyclin production. c. myosin denaturation and inhibition of cleavage furrow formation. d. inhibition of DNA synthesis. -

A

How long is the DNA in a typical human cell?

A cell's endowment of DNA, its genetic information, is called its genome. Although a 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. A typical human cell, for example, has about 2 meters of DNA - a length about 250,000 times greater than the cell's diameter. Yet before the cell can divide to form genetically identical cells, all of this DNA must be copied or replicated, and then the two copies must be separated so that each daughter cell ends up with a complete genome.

A kinetochore has been compared to a coupling device that connects a motor to the cargo that it moves. Explain. -

A kinetochore connects the spindle (a motor; note that it has motor proteins) to a chromosome (the cargo it will move).

What are the characteristics of G2 of interphase in the cell? -

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 because they have not yet condensed.

What phase has the most variability in terms of time to take place? -

A particular human cell might undergo one division in 24 hours. Of this time, the M phase would occupy less than 1 hour, while the S phase might occupy about 10-12 hours, or 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 example, G1 would occupy about 5-6 hours. G1 is the most variable in length in different types of cells.

How does MPF allow a cell to pass the G2 phase check- point and enter mitosis? -

A sufficient amount of MPF has to exist for a cell to pass the G2 checkpoint; this occurs through the accumulation of cyclin proteins, which combine with Cdk to form (active) MPF.

How many chromosomes are drawn in each part of Figure 12.5? -

1; 1; 2

A chicken has 78 chromosomes in its so- matic cells. How many chromosomes did the chicken inherit from each parent? How many chromosomes are in each of the chicken's gametes? How many chro- mosomes will be in each somatic cell of the chicken's offspring? -

39; 39; 78

How many chromosomes are drawn in Figure 12.8? Are they duplicated? How many chromatids are shown? -

6 chromosomes, duplicated; 12 chromatids

What can diatoms and dinoflagellates tell us about the evolution of cellular division in prokaryotes? -

As eukaryotes evolved along with their larger genomes and nuclear envelopes, the ancestral process of binary fission, seen today in bacteria, somehow gave rise to mitosis. These 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 presumably 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 two modes of nuclear division are thought to be cases where ancestral mechanisms have remained relatively unchanged over evolutionary time. In both types, the nuclear envelope remains intact, in contrast to what happens in most eukaryotic cells.

What happens to the chromosome in E.coli as it continues to replicate? -

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 the replication is complete and the bacterium has reached about twice its initial size, its plasma membrane pinches inward, dividing the parent E.coli cell into two daughter cells. In this way, each cell inherits a complete genome.

What were the results of Gary Borisy's experiment? -

As the chromosomes moved poleward, the microtubule segments on the kinetochore side of the mark shortened, while those on the spindle pole side stayed the same length.

What is the metaphase plane? -

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, which is an imaginary rather than an actual cellular structure.

What happens at the end of anaphase? -

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.

Through a microscope, you can see a cell plate beginning to develop across the middle of a cell and nuclei forming on ei- ther side of the cell plate. This cell is most likely a. an animal cell in the process of cytokinesis. b. a plant cell in the process of cytokinesis. c. a bacterial cell dividing. d. a plant cell in metaphase. -

B

Which of the following does not occur during mitosis? a. condensation of the chromosomes b. replication of the DNA c. separation of sister chromatids d. spindleformation -

B

What roles does cell division play in life?

Cell division plays several important roles in life. The division of one prokaryotic cell reproduces an entire organism. The same is true of a unicellular eukaryote. Cell division also enables multicellular eukaryotes to develop from a single cell, like the fertilized egg that gives rise to a two-celled embryo. And after such an organism is fully grown, cell division continues to function in renewal and repair, replacing cells that die from normal wear and tear or accidents. For example, dividing cells in your bone marrow continuously make new blood cells.

Explain the significance of the G1, G2, and M checkpoints and the go-ahead signals involved in the cell cycle control system. -

Checkpoints allow cellular surveillance mechanisms to determine whether the cell is prepared to go to the next stage. Internal and external signals move a cell past these checkpoints. The G1 checkpoint, called the "restriction point" in mammalian cells, determines whether a cell will com- plete the cell cycle and divide or switch into the G0 phase. The signals to pass this checkpoint often are external—such as growth factors. Passing the G2 checkpoint requires sufficient numbers of active MPF complexes, which in turn orchestrate several mitotic events. MPF also initiates degradation of its cyclin component, ter- minating the M phase. The M phase will not begin again until sufficient cyclin is produced during the next S and G2 phases. The signal to pass the M phase check- point is not activated until all chromosomes are attached to kinetochore fibers and are aligned at the metaphase plate. Only then will sister chromatid separation occur.

In which of the three subphases of interphase and the stages of mitosis do chromosomes exist as single DNA molecules? -

Chro- mosomes exist as single DNA molecules in G1 of interphase and in anaphase and telophase of mitosis. During S phase, DNA replication produces sister chromatids, which persist during G2 of interphase and through prophase, prometaphase, and metaphase of mitosis.

How does cytokinesis work in plant cells? -

Cytokinesis in plant cells, which have cell walls, is remarkedly different. There is no cleavage furrow. Instead, during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate. Cell wall materials carried in the vesicles collect the cell plate as it grows. The cell plate enlarges until its surrounding membrane fuses with the plasma membrane along the perimeter 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 has formed between

The drug cytochalasin B blocks the function of actin. Which of the following aspects of the animal cell cycle would be most disrupted by cytochalasin B? a. spindleformation b. spindle attachment to kinetochores c. cell elongation during anaphase d. cleavage furrow formation and cytokinesis -

D

What will you learn in chapter 12?

The cell division process is an integral part of the cell cycle, the life of a cell from the time it is first formed from a dividing parent cell into its own division into two daughter cells. (Our use of the words daughter or sister in relation to cells is not meant to imply gender.) Passing identical genetic material to cellular offspring is a crucial function of cell division. In this chapter, you will learn how this process occurs. After studying the cellular mechanics of cell division in eukaryotes and bacteria, you will learn about the molecular control system that regulates progress through the eukaryotic cell cycle and what happens when the control system malfunctions. Because a breakdown in cell cycle control plays a major role in cancer development, this aspect of cell biology is an active area of research.

What are the characteristics of the cell during metaphase? -

The centrosomes are now at opposite poles of the cell. The chromosomes convene 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.

A chicken has 78 chromosomes in its somatic cells. How many chromosomes did the chicken inherit from each parent? How many chromosomes are in each of the chicken's gametes? How many chromosomes will be in each somatic cell of the chicken's offspring?

The chicken inherited 39 chromosomes from each parent. There will be 39 chromosomes in each of the chicken's gametes. There will be 78 chromosomes in each somatic cell of the chicken's offspring. This will all happen if the chicken does contract any disease or have any mutations, that would take away or add chromosomes.

What are the characteristics of the cell during prophase? -

The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with 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 mitotic 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 centrosome move away from each other, propelled partly by the lengthening microtubules between them.

When are the chromosomes duplicated? -

The chromosomes are duplicated only 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.

What are the characteristics of the cell during cytokinesis? -

The division of the cytoplasm is usually well underway 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.

Explain in general how recep- tor tyrosine kinases and intracellular receptors might function in triggering cell division. -

The intracellular estrogen receptor, once activated, would be able to act as a transcription factor in the nucleus, turning on genes that may cause the cell to pass a checkpoint and divide. The HER2 receptor, when activated by a ligand, would form a dimer, and each subunit of the dimer would phosphorylate the other. This would lead to a series of signal transduction steps, ultimately turning on genes in the nucleus. As in the case of the estrogen receptor, the genes would code for proteins necessary to commit the cell to divide.

What if this experiment had been done on a cell type in which "reeling in" at the poles was the main cause of the chromosome movement, how would the mark have moved relative to the poles? How would the microtubule lengths have changed? -

The mark would have moved toward the nearer pole. The lengths of fluorescent microtubules between that pole and the mark would have decreased, while the lengths between the chromosomes and the mark would have remained the same.

How does cell division occur in diatoms and some yeasts? -

The nuclear envelope also remains intact during cell division in diatoms and some yeasts. In these organisms the microtubules form a spindle within the nucleus. Microtubules separate the chromosomes, and the nucleus splits into two daughter nuclei.

What are the characteristics of the cell during prometaphase? -

The nuclear envelope fragments. The microtubules extending from each centrosome can now invade the nuclear area. The chromosomes have become even more condensed. Each of the two chromatids of each chromosome now has a kinetochore, a specialized protein structure at the centromere. Some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules," which jerk the chromosomes back and forth. Nonkinetochore microtubules interact with those from the opposite pole of the spindle.

Why do the nuclei resulting from experi- ment 2 contain different amounts of DNA? -

The nucleus on the right was originally in the G1 phase; therefore, it had not yet duplicated its chromosomes. The nucleus on the left was in the M phase, so it had already duplicated its chromosomes.

What are chromosomes?

The replication and distribution of so much DNA is manageable because the DNA molecules are packaged into structures called chromosomes, so named because they take up certain dyes used in microscopy (from the Greek "chroma", color, and "soma", body. Each eukaryotic chromosome consist of one very long, linear DNA molecule associated with many proteins. The 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.

How are researchers able to observe the movement of bacterial chromosomes? -

Using the technique of modern DNA technology to tag the origins of replication with molecules that glow green in fluorescence microscopy, researchers have directly observed the movement of bacterial chromosomes. This movement is reminiscent of the poleward movements of the centromere region of eukaryotic chromosomes during the anaphase of mitosis, but bacteria don't have visible mitotic spindles or even microtubules.

Are chromosomes long and thin or densely coiled after DNA replication?

When a cell is not dividing, and even as it replicates its DNA in preparation for cell division, each chromosome is in the form of a long, thin chromatin fiber. After DNA replication, however, the chromosome condense as 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.

What happens when a chromosome's kinetochores become "captured" by microtubules? -

When the kinetochore becomes captured by microtubules, the chromosome begins to move toward the pole from which those microtubules extend. However, this movement is checked as soon as microtubules from the opposite pole attach to the other kinetochore. What happens next is a like a tug-of-war that ends in a draw. The chromosome moves first in one direction, then the other, back and forth, finally settling midway between the two ends of the cell.

How does the mitotic spindle get formed? -

While the mitotic spindle assembles, the other microtubules of the cytoskeleton partially disassemble, providing the material used to construct the spindle. The spindle microtubules elongate (polymerize) by incorporating more subunits of the protein tubulin and shorten (depolymerize) by losing subunits.

How does a zygote get formed?

You inherited 46 chromosomes, one set of 23 from each parent. They were combined in the nucleus of a single cell when a sperm from your father united with an egg from your mother, forming a fertilized egg, or zygote.

PART A - Identifying the stages of mitosis The stages of mitosis were originally defined by cellular features observable through a light microscope. The six micrographs below show animal cells (lung cells from a newt) during the five stages of mitosis, plus cytokinesis. (Note that interphase is not represented in these micrographs.) In these images, the chromosomes have been stained blue, microtubules green, and microfilaments red. Drag each micrograph to the target that indicates the stage of mitosis or cytokinesis it shows -

[As these six micrographs demonstrate, cellular events observable by light microscopy can be used to define the six stages of mitosis and cytokinesis. However, deciphering which stage is which in real cells can be much more challenging than in the drawings of idealized cells you see in your textbook. Thus, it is important to carefully observe the completeness of the mitotic spindle and the location of the chromosomes, as well as how condensed the chromosomes are.]