Bio 122 Chapter 12 Study Guide: The Cell Cycle

Together, the entire complex of DNA and proteins that is the building material of chromosomes is referred to as

chromatin.

The replication and distribution of so much DNA are manageable because the DNA molecules are packaged into structures called

chromosomes. Each eukaryotic chromosome consists 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.

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

elongating the whole cell during anaphase.

If a human somatic cell is just about to divide, it has __________ chromatids.

92. Human somatic cells have 92 chromatids just prior to cell division due to the replication of the 46 chromosomes that occurred during the S phase.

Cell A has half as much DNA as cells B, C, and D in a mitotically active tissue. Cell A is most likely in A) G1 B) G2 C) prophase D) metaphase.

A

During interphase, most of the nucleus is filled with a complex of DNA and protein in a dispersed form called

chromatin.

The complex of DNA and protein that makes up a eukaryotic chromosome is properly called __________.

chromatin.

A cell's DNA, its genetic information, is called its

genome.

MPF stand for "maturation-promoting factor," but we can think of MPF as

"M-phase-promoting factor" because it triggers the cell's passage into the M phase, past the G2 checkpoint.

Growth factor

1) A protein that must be present in the extracellular environment 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.

How many maternal chromosomes are present in a somatic human cell when it is not engaged in cell division?

23.

M checkpoint

A cell in mitosis receives a stop signal when any of its chromosomes are not attached to spindle fibers. When all chromosomes are attached to spindle fibers from both poles, a go-ahead signal allows the cell to proceed into anaphase.

A checkpoint in S phase stops cells with

DNA damage from proceeding in the cell cycle.

For many cells, the G1 checkpoint seems to be the most important. If a cell receives a go-ahead signal at the G1G1 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 nondividing state called the

G0 phase.

Which of the following phases of mitosis is essentially the opposite of prometaphase in terms of the nuclear envelope?

Telophase. In prometaphase the nuclear envelope fragments and the kinetochores attach to microtubules. During telophase, fragments of the nuclear envelope begin to reassemble along with portions of the endomembrane system.

Density dependent inhibition

The phenomenon observed in normal animal cells that causes them to stop dividing when they come into contact with one another.

Anchorage dependence

The requirement that a cell must be attached to a substratum in order to initiate cell division. To divide, they must be attached to something, such as the inside of 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.

The function of the mitotic cell cycle is to produce daughter cells that __________.

are genetically identical to the parent cell (assuming no mutation has occurred).

During interphase, the genetic material of a typical eukaryotic cell is __________.

dispersed in the nucleus as long strands of chromatin.

In dividing cells, most of the cell's growth occurs during

interphase.

When cyclins that accumulate during G2G2 associate with Cdk molecules, the resulting MPF complex is active—it phosphorylates a variety of proteins, initiating

mitosis. MPF acts both directly as a kinase and indirectly by activating other kinases.

In telophase of mitosis, the mitotic spindle breaks down and the chromatin uncoils. This is essentially the opposite of what happens in __________.

prophase. During prophase, we observe the formation of the spindle, the condensation of chromatin, and the disappearance of the nucleolus.

Cell division

The reproduction of cells.

Anaphase has two parts: anaphase A and anaphase B.

- In anaphase A, sister chromatids get pulled by the microtubules to each centrosome. - This happens due to the shortening of the microtubules. - At the end of anaphase A, the chromosomes are clustered around each centrosome. - In anaphase B, the cell elongates and the two centrosomes move farther apart (taking the chromosomes). - In anaphase B, the cell elongates and the two centrosomes move farther apart (taking the chromosomes).

The mitotic apparatus consists of:

- microtubules, aka spindle fibers - centrosomes, aka spindle poles - kinetochores (structures attached to the centromere)

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.

Chromosomes become visible during _____. A) prophase B) interphase C) anaphase D) prometaphase E) metaphase

A

Movement of the chromosomes during anaphase would be most directly affected by a drug that prevents which of the following events? A) shortening of microtubules B) nuclear envelope breakdown C) formation of a cleavage furrow D) elongation of microtubules

A

What is the best description of the structure of a chromosome in the nucleus of a non-dividing cell? A) One long DNA molecule, associated with many proteins. B) Two long DNA molecules wound around each other, with no proteins. C) One long DNA molecule, with no proteins. D) Two long DNA molecules wound around each other, associated with many proteins.

A

Which of the following characteristics would be most important for a chemotherapeutic drug designed to treat cancer cells? A) It specifically inhibits the cell division cycle in rapidly dividing cells. B) It does not interfere with metabolically active cells. C) It specifically inhibits cells entering G0. D) It is safe enough to prevent all apoptosis.

A

Which of the following is true of benign tumors, but not malignant tumors? A) they remain confined to their original site B) they are the result of the transformation of normal cells C) they have an unusual number of chromosomes D) they can divide indefinitely if an adequate supply of nutrients is available E) they migrate from the initial site of transformation to other organs or tissues

A

Which of the following occurs during S phase? A) replication of the DNA B) separation of sister chromatids C) spindle formation D) condensation of the chromosomes

A

Which of the following statements best describes cells in culture that do not exhibit either density-dependent inhibition or anchorage dependence? A) The cells show characteristics of tumors. B) The cells follow an altered series of cell cycle phases. C) The cells are unable to form spindle microtubules. D) The cells have nonfunctional MPF.

A

Which of the following statements best describes microtubules? A) Microtubules are made up of subunits of tubulin, and are structures along which substances are transported in the cell. B) Microtubules are made up of subunits of actin, and function in muscle contraction and changes in cell shape. C) Microtubules are fibrous proteins coiled into cables that maintain cell shape and anchor the nucleus. D) Microtubules are permanent cellular structures that help the cell to move from place to place.

A

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, binary fission does not involve mitosis, but in single-celled eukaryotes that undergo binary fission, mitosis is part of the process.

How can a protein outside the cell cause events to happen inside the cell? A) A protein outside the cell can bind to a receptor protein on the cell surface, causing it to change shape and sending a signal inside the cell. B) A protein outside the cell can cause the H+ ion concentration outside the cell to be higher, which allows substances to pass into the cell and transduce a signal. C) A protein can pass through the membrane and into the cell, causing signaling inside the cell. D) A protein outside the cell can phosphorylate a protein inside the cell, which transduces a signal and causes a response.

A. The hydrophobic interior of the membrane impedes direct passage of proteins into the cell. Reception, the target cell's detection of a signaling molecule coming from outside the cell, occurs when a protein binds to a receptor protein located at the cell's surface, changing the shape of the receptor protein and initiating transduction. Transduction occurs within the cell and typically involves several steps, converting the external signal into a form that brings about a specific cellular response inside the cell.

Cell cycle

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. The eukaryotic cell cycle is composed of interphase (including G1, S, and G2 phases) and the M phase (including mitosis and cytokinesis).

Cells will usually divide if they receive the proper signal at a checkpoint in which phase of the cell cycle? A) cytokinesis B) G1 C) S D) M E) G2

B

Centrosomes duplicate during _____. A) prophase B) interphase C) anaphase D) prometaphase E) metaphase

B

One difference between cancer cells and normal cells is that cancer cells A) are unable to synthesize DNA. B) continue to divide even when they are tightly packed together. C) are arrested at the SS phase of the cell cycle. D) cannot function properly because they are affected by density-dependent inhibition.

B

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) spindle formation B) cleavage furrow formation and cytokinesis C) cell elongation during anaphase D) spindle attachment to kinetochores

B

Vinblastine is a standard chemotherapeutic drug used to treat cancer. Because it interferes with the assembly of microtubules, its effectiveness must be related to A) suppression of cyclin production B) disruption of mitotic spindle formation C) myosin denaturation and inhibition of cleavage furrow formation D) inhibition of DNA synthesis

B

What is the name of the region on duplicated chromosomes where the sister chromatids are most closely attached to each other? A) the centrosome B) the centromere C) the chromatin D) the cohesin

B

Which of the following correctly matches a phase of the cell cycle with its description? A) S, immediately precedes cell division B) G1, follows cell division C) G2, cell division D) M, duplication of DNA E) All of the listed choices are correctly matched

B

Which of the following statements best describes density-dependent inhibition? A) As cells become more numerous, the protein kinases they produce begin to compete with each other, such that the proteins produced by one cell essentially cancel those produced by its neighbor. B) As cells become more numerous, the cell surface proteins of one cell contact the adjoining cells, and they signal each other to stop dividing. C) As cells become more numerous, they begin to squeeze against each other, restricting their size. D) As cells become more numerous, the level of waste products increases, which slows metabolism and inhibits growth.

B

How do cancer cells differ from normal cells? A) Cultured cancer cells exhibit anchorage dependence. B) Cancer cells may be immortal. C) Cancer cells trigger chromosomal changes in surrounding cells.

B. Cancer cells can go on dividing indefinitely in culture if they are given a continual supply of nutrients; in essence, they are "immortal."

Cytokinesis accompanies _____. A) metaphase B) prometaphase C) telophase D) anaphase E) interphase

C

Taxol is an anticancer drug extracted from the Pacific yew tree. In animal cells, Taxol prevents microtubule depolymerization, which interferes with which of the following processes? A) cytokinesis B) centriole duplication C) chromosome separation D) chromosome condensation

C

The continuity of life is based on heritable information in the form of DNADNA. How does the process of mitosis faithfully parcel out exact copies of this heritable information in the production of genetically identical daughter cells? A) During mitosis, each chromosome becomes attached to the centromere. At anaphase, centromeres drag random chromosomes toward the poles of the cell, where they become duplicated. As a result, two identical chromosomal sets form at each pole. B) During mitosis, both chromatids becomes attached to the microtubules. At anaphase, microtubules pull both chromatids toward one pole of the cell. C) During mitosis, each chromatid becomes attached to microtubules. At anaphase, microtubules pull the chromatids toward the opposite poles of the cell, therefore forming two identical chromosomal sets at each pole. D) During mitosis, each chromosome becomes attached to the centromere. At anaphase, centromeres drag random chromatids toward the poles of the cell, where they become duplicated. As a result, two identical chromosomal sets form at each pole.

C

The decline of MPF activity at the end of mitosis is due to A) decreased synthesis of Cdk. B) the destruction of the protein kinase Cdk. C) the degradation of cyclin. D) the accumulation of cyclin.

C

Through a microscope, you can see a cell plate beginning to develop across the middle of a cell and nuclei forming on either side of the cell plate. This cell is most likely A) a bacterial cell dividing. B) an animal cell in the process of cytokinesis. C) a plant cell in the process of cytokinesis. D) a plant cell in metaphase.

C

Which of the following molecules is synthesized at specific times during the cell cycle and forms an active complex with a kinase? A) Cdk B) PDGF C) cyclin D) MPF

C

Which of the following properties is associated with a cyclin-dependent kinase (Cdk)? A) The number of Cdk molecules increases during the S and G2 phases and decrease during M. B) A Cdk is inactive, or "turned off," in the presence of a cyclin. C) A Cdk is an enzyme that attaches phosphate groups to other proteins. D) A Cdk is an enzyme that catalyzes the attachment of kinetochores to microtubules.

C

Which of the following statements best explains how neurons and some other specialized cells divide infrequently? A) They can no longer degrade cyclins. B) They no longer have active nuclei. C) They have entered into G0. D) They no longer produce MPF.

C

Which of the following statements describes a characteristic feature of metaphase? A) cytokinesis B) separation of the centromeres C) alignment of chromosomes on the equator of the cell D) separation of sister chromatids

C

Why do some species employ both mitosis and meiosis, whereas other species use only mitosis? A) if they produce egg cells they do not require mitosis B) a single-celled organism only needs mitosis C) they need both if they are reproducing sexually D) if they produce large numbers of sperm cells they do not require meiosis E) they need meiosis if the cells are producing organs such as ovaries

C

If all chromosomes are not attached to the mitotic spindle at the M phase checkpoint, the cells would most likely be arrested at which of the following points in the cell division cycle or mitosis? A) prophase B) telophase C) G2 D) metaphase

D

Microtubules attach to kinetochores during _____. A) telophase B) anaphase C) interphase D) metaphase

D

The mitotic spindle plays a critical role in which of the following processes? A) dissolving the nuclear membrane B) splitting of the cell (cytokinesis) following mitosis C) triggering condensation of chromosomes D) separation of sister chromatids

D

Which of the following cellular events occur in the G1 phase of the cell division cycle? A) DNA replication B) break down of the nuclear membrane C) the beginning of mitosis D) normal growth and cell function

D

Which of the following molecules is maintained at a relatively constant level throughout the cell cycle but requires a cyclin to become catalytically active? A) PDGF B) MPF C) cyclin D) Cdk

D

Which of the following statements describing kinetochores is correct? A) They attach to the ring of actin along the cytoplasmic surface of the plasma membrane, causing the actin to contract to form the cleavage furrow. B) They are the primary centromere structures that maintain the attachment of the sister chromatids prior to mitosis. C) They are located at the center of the centrosome; their function is to organize tubulin into elongated bundles called spindle fibers. D) They are sites at which microtubules attach to chromosomes. E) They interdigitate at the cell's equator and then move apart, causing the cell to elongate.

D

What effect does phosphorylating a protein have on that protein? A) Phosphorylation adds one or more phosphorus atoms to the protein, allowing it to perform a particular function. B) Phosphorylation adds energy to the protein, allowing it to form ATP from ADP and Pi. C) Phosphorylation may have no effect at all on the protein. D) Phosphorylation changes the shape of the protein, most often activating it.

D. Protein phosphorylation causes a shape change in the phosphorylated protein. The shape change results from the interaction of the newly added phosphate groups with charged or polar amino acids on the protein being phosphorylated. The shape change alters the function of the protein, most often activating it, but in some cases phosphorylation instead decreases the activity of the protein.

Eukaryotic chromatin is composed of which of the following macromolecules? A) DNA and phospholipids B) DNA and RNA C) DNA only E) DNA and proteins

E

Prophase

First stage of mitosis, in which the chromatin condenses into discrete chromosomes visible worth a light microscope, the mitotic spindle begins to form, and the nucleolus disappears but the nucleus remains intact.

Sister chromatids are joined along their entire length during

G2 and the first part of mitosis.

Telophase

The fifth and final stage of mitosis, in which daughter nuclei are forming and cytokinesis has typically begun.

G1 phase

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

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.

Cytokinetic ring

The ring of filaments that pinches the cell into two cells, a structure on the inside of the cell. Acts like a drawstring, pinching the cell in the middle until it separates into two cells.

G2 phase

The second gap, or growth phase, of the cell cycle, consisting of the portion of interphase after DNA synthesis occurs.

Prometaphase

The second stage of mitosis, in which the nuclear envelope fragments and the spindle microtubules attach to the kinetochores of the chromosomes.

S phase

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.

What are the genetic or other changes that might cause cells to escape normal cell cycle regulation?

The underlying basis for this altered behavior is a series of genetic and cellular changes, including mutations in genes whose protein products normally regulate the cell cycle. These gene products are often proteins that function in cell signaling pathways.

Each sister chromatid is a

a complete chromosome, with its own centromere

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 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.

Before the cell can divide to form genetically identical daughter 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.

Unlike normal animal cells, cultured cancer cells fail to exhibit

anchorage dependence and can continue to divide despite lack of attachment to a substratum.

Microtubules—hollow rods constructed from globular proteins called tubulins—shape and support the cell and serve

as tracks along which organelles equipped with motor proteins can move. Microfilaments are made up of subunits of actin, and function in muscle contraction and changes in cell shape. Intermediate filaments are fibrous proteins coiled into cables that maintain cell shape and anchor the nucleus.

Prokaryotes (bacteria and archaea) reproduce by a type of cell division called

binary fission. It has only three steps. 1) There is only one chromosome. It replicates. Each copy of the chromosome is attached to the plasma membrane. 2) The two copies of the chromosome move apart. 3) The cell divides into two cells. One copy of the chromosome is in each cell.

Sexually reproducing organisms employ

both mitosis and meiosis. Organisms that reproduce asexually generally use only mitosis.

Each sister chromatid has a

centromere, a region made up of repetitive sequences in the chromosomal DNA where the chromatid is attached most closely to its sister chromatid. This attachment is mediated by proteins that recognize and bind to the centromeric DNA; other bound proteins condense the DNA, giving the duplicated chromosome a narrow "waist."

After chromosomes condense, the

centromeres is the region where the identical DNA molecules are most tightly attached to each other.

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 a type of 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.

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.

A checkpoint in the cell cycle is a

control point where stop and go-ahead signals can regulate the cycle. Three important checkpoints are found in the G1G1, G2G2, and M phases

Many of the kinases that drive the cell cycle are actually present at a constant concentration in the growing cell, but much of the time they are in an inactive form. To be active, such a kinase must be attached to a

cyclin, a protein that gets its name from its cyclically fluctuating concentration in the cell. Because of this requirement, these kinases are called cyclin-dependent kinases, or Cdks.

Mitosis, the division of the genetic material in the nucleus, is usually followed immediately by

cytokinesis, the division of the cytoplasm. One cell has become two, each the genetic equivalent of the parent cell.

In most eukaryotes, division of the nucleus is followed by

cytokinesis, when the rest of the cell divides.

Cytokinesis in plant cells, which have cell walls, is markedly 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 inside 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 forms between the daughter cells.​

For the metaphase or M checkpoint:

if all of the kinetochores attached to microtubules, there is a GO-AHEAD signal. If even one kinetochore is not attached, no signal and mitosis will be stopped.

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.

Many organisms contain cells that do not normally divide. These cells exit the cell cycle before the G1 checkpoint. Once a cell passes the G1 checkpoint,

it usually completes the cell cycle--that is, it divides. The first step in preparing for division is to replicate the cell's DNA in the S phase. In the G2 phase, the centrosome replicates. In early M phase, the centrosomes move away from each other toward the poles of the cell, in the process organizing the formation of the mitotic spindle. At the end of the M phase when mitosis is complete, the cell divides (cytokinesis), forming two genetically identical daughter cells.

The activity of a Cdk rises and falls with changes in the concentration of

its cyclin partner. The cyclin level rises during the S and G2 phases and then falls abruptly during M phase.

During prometaphase, some of the spindle microtubules attach to the kinetochores; these are called

kinetochore microtubules.

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 DNA at each centromere. Sister chromatids have the two kinetochores facing in opposite directions. Microtubules attach to the kinetochore.

During mitosis, microtubules attach to chromosomes at the

kinetochores.

You produce gametes—eggs or sperm—by a variation of cell division called

meiosis, which yields daughter cells with only one set of chromosomes, half as many chromosomes as the parent cell. Meiosis in humans occurs only in special cells in the ovaries or testes (the gonads). Generating gametes, meiosis reduces the chromosome number from 46 (two sets) to 23 (one set). Fertilization fuses two gametes together and returns the chromosome number to 46 (two sets).

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 plate rather than an actual cellular structure.

The centrosomes are the organizing centers for

microtubules involved in separating chromosomes during mitosis.

The mitotic spindles is a cell structure consisting of

microtubules, which forms during early mitosis and plays a role in cell division.

The machinery that separates the chromosomes is called the

mitotic apparatus.

Many of the 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. 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.

During all three phases of interphase, actually, 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.

Mitosis is conventionally broken down into five stages:

prophase, prometaphase, metaphase, anaphase, and telophase. Overlapping with the latter stages of mitosis, cytokinesis completes the mitotic phase.

Rhythmic fluctuations in the abundance and activity of cell cycle control molecules pace the sequential events of the cell cycle. These regulatory molecules are mainly proteins of two types:

protein kinases and cyclins.

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.

The mitotic (M) phase, which includes both mitosis and cytokinesis, is usually the

shortest part of the cell cycle. The mitotic phase alternates with a much longer stage called interphase, which often accounts for about 90% of the cycle.

Each duplicated chromosome consists of two

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

DNA replication produces two identical DNA molecules, called

sister chromatids, which separate during mitosis.

Reproductive cells, or gametes—such as sperm and eggs—have half as many chromosomes as

somatic cells; in our example, human gametes have one set of 23 chromosomes.

During interphase in animal cells, the single centrosome duplicates, forming two centrosomes, which remain near the nucleus. The two centrosomes move apart during prophase and prometaphase of mitosis as spindle microtubules grow out from 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 centrosome. The spindle includes the centrosomes, the spindle microtubules, and the asters.

Sister chromatids form when DNA replicates in the S phase. The sister chromatids become individual chromosomes once

they separate in early anaphase. Similarly, the cellular DNA content doubles in the S phase when the DNA replicates. However, the cell's DNA content does not return to its normal (undoubled) levels until after cytokinesis is complete and two daughter cells have formed. The condensation state of the DNA is not related to the presence or absence of sister chromatids. The DNA condenses in prophase and remains condensed until after the sister chromatids separate and the new daughter cells begin to form. In late telophase/cytokinesis, the emphasis shifts to cell growth and DNA replication for the next cell cycle. For these processes to occur, the DNA needs to be de-condensed so it is accessible to the cellular machinery involved in transcription.

Although a prokaryotic genome is often a single DNA molecule, eukaryotic genomes

usually consist of a number of DNA molecules.