Chapter 12: The Cell Cycle

What occurs during the "preparation for cell division" part of the G2 phase?

*A nuclear envelope encloses the nucleus. *The nucleus contains one or more nucleoli. *Two centrosomes have formed by duplication of a single centrosome. *Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet been condensed. (see Figure 12.7, p. 236)

*QUIZ QUESTION* 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) 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

*ANSWER* b) a plant cell in the process of cytokinesis (view #52-53)

What occurs 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 region, the chromosomes move centromere-first. *The cell elongates as the nonkinetochore microtubles lengthen. *By the end of anaphase, the two ends of the cell have equivalent--and complete--collections of chromosomes. (see Figure 12.7, p. 237, and Figure 12.9, p. 239 to see how kinetochore microtubules shorten during this stage)

What occurs during metaphase?

*The centrosomes are now at the opposite poles of the cell. *The chromosomes have all arrived at the metaphase plate. 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. (see Figure 12.7, p. 237)

What occurs 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 cohesins). *The mitotic spindle (named for its shape) begins to form. *The centrosomes move away from each other, propelled partly by the lengthening microtubles between them (see Figure 12.7, p. 236)

What occurs 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. (see Figure 12.7, p. 236 and Figure 12.10, p. 240)

What occurs during prometaphase?

*The nuclear envelope fragments. *The microtubules extending from each centrosome can now invade the nuclear area. *The chromosomes have become even and more condensed. *Each of the two chromatids of each chromosome now has a kinetochore. *Some of the microtubules attach to the kinetochores, become "kinetochore microtubules," which jerk the chromosomes back and forth. *Nonkinetochore microtubules interact with those from the opposite pole of the spindle. (see Figure 12.7, p. 236)

What occurs during telophase?

*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. (see Figure 12.7, p. 237)

What are the three subphases of interphase?

1) G1 phase "(first gap") 2) S phase ("synthesis") 3) G2 phase ("second gap")

How many chromosomes are in the nuclei of human somatic cells?

46 chromosomes; two sets of 23, one inherited from each parent

What form is a chromosome in when its cell is not dividing?

A long, thin chromatin fiber.

When do sister chromatids form?

After DNA is replicated.

What characteristics does an unduplicated chromosome possess?

An unduplicated chromosome has a single centromere, distinguished by the proteins that bind there, and two arms.

Where is the pair of centrioles located in the centrosome?

At the center.

What is the difference between binary fission in bacteria and binary fission in single-celled eukaryotes?

Binary fission in eukaryotes involves mitosis, while binary fission in prokaryotes does not.

What occurs during the G1 phase of interphase?

Cell growth

What occurs in the G2 phase of interphase?

Cell growth and preparation for cell division

What occurs during the S phase of interphase?

Cell growth and the duplication of a cell's chromosomes (see Figure 12.5, p. 234 for a basic outline)

How does cytokinesis happen in plant cells, since they have cell walls and can't form cleavage furrows?

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 inthe 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 has formed between the daughter cells. (see Figure 12.10b, p. 240)

How is sister chromatid centromeric attachment mediated?

It is mediated by proteins bound to the centromeric DNA.

Does the number of chromosomes in somatic cells vary among species, or is it constant?

It varies.

Are centrioles essential for cell division?

No; plants do not have centrioles and cell division can still be performed when the centrioles in animal cells are removed.

How many DNA molecules are in a typical prokaryote?

One

How is the narrow "waist" of a duplicated chromosome formed?

Other bound proteins "condense" the DNA and give it the "waist."

What are the 5 stages of mitosis, in order of occurrence?

Prophase, prometaphase, metaphase, anaphase, and telophase.

How long is the mitotic (M) phase in comparison with the rest of the phases of the cell cycle?

Quite short.

Which of the checkpoints is the most important for many cells?

The G1 checkpoint; if a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the G1, S, G2, and M phases and divide

Which phase of the cell cycle is most variable in length in different types of cells?

The G1 phase.

What occurs during all three subphases of interphase?

The cell grows by producing proteins and cytoplasmic organelles such as mitochondria and endoplasmic reticulum.

How is the mitotic spindle formed?

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.

What does the term "arm of the chromatid" refer to?

The portion of a chromatid to either side of the centromere.

What do chromosomes do immediately after DNA replication?

They condense as part of cell division: Each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and thick enough to see with a light microscope.

How do prokaryotes and unicellular eukaryotes reproduce?

They divide, since the process gives rise to another organism (a new cell).

What do the proteins in a chromosome do?

They maintain the structure of the chromosome and help control the activity of the genes.

When does cytokinesis occur with respect to mitosis?

Usually immediately follows mitosis.

chromosome

a cellular structure consisting of one DNA molecule and associated protein molecules

anchorage dependence

a characteristic of most animal cells in which they must be attached to substratum (such as the inside of a culture flask or the extracellular matrix of a tissue) to divide

checkpoint

a control point in the cell cycle where stop and go-ahead signals can regulate the cycle (see Figure 12.15, p. 243)

cell cycle control system

a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle; regulated at certain checkpoints by both internal and external signals that stop or restart the cycle (see Figure 12.15, p. 243)

cell plate

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

G0 phase

a phase that most cells of the human body reside in (mature nerve cells, muscle cells); usually a cell goes into this phase if it does not receive a go-ahead signal at the G1 checkpoint

density-dependent inhibition

a phenomenon in which crowded cells stop dividing once they form a single layer of cells on a surface; the binding of a cell-surface protein to its counterpart on an adjoining cell sends a cell division-inhibiting signal forward in the cell cycle, even in the presence of growth factors (see Figure 12.19, p. 246)

metaphase plate

a plane that is equidistant between the spindle's two poles

cyclin

a protein that must be attached to a kinase in order for the kinase to be active

aster

a radial array of short microtubules that extends form each centrosome during the cell cycle

cleavage furrow

a shallow groove in the cell surface near the old metaphase plate

kinetochore

a specialized protein structure at the centromere made up of proteins

centrosome

a subcellular region containing material that organizes the cell's microtubules during cell division; the assembly of spindle microtubules begins here

malignant tumor

a tumor that has genetic and cellular changes that enable them to spread to new tissues and impair the function of one or more cells; an individual with a * is said to have cancer

benign tumor

a tumor that remains at the original cell if they have too few genetic and cellular changes to survive at another site; most do not cause serious problems and can be removed by surgery

binary fission

a type of reproduction in which a bacterium/single-cell eukaryote grows to roughly double its size and then divides to form two cells (view Figure 12.12, p. 241 to see a diagram of bacterial cell division by binary fission; study each step)

*QUIZ QUESTION* If there are 20 centromeres in a cell, how many chromosomes are there? a) 20 b) 40 c) 5 d) 10

a) 20 (Each chromosome has one centromere.)

*QUIZ QUESTION* 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) G1 (After the G1 stage is over, cells go into the S stage, wherein DNA is replicated. The resulting cell contains twice as much DNA as it did in the G1 stage. The other three answer choices all occur after DNA is replicated, meaning that the G1 phase would have half as much DNA as all those other stages. Therefore, the answer A is the only viable option.)

*QUIZ QUESTION* In the cells of some organisms, mitosis occurs without cytokinesis. This will result in a) cells with more than one nucleus b) cells that are unusually small c) cells lacking nuclei d) cells cycles lacking an S phase

a) cells with more than one nucleus (review the processes of mitosis minus cytokinesis)

*QUIZ QUESTION* 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) 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) disruption of mitotic spindle formation (view #42-43)

interphase

accounts for about 90% of the cell cycle; time of intense metabolic activity and growth

somatic cells

all body cells except reproductive cells

*QUIZ QUESTION* All of the following statements are true except: a) All cells come from preexisting cells. b) The mitotic spindles in prokaryotic cells are composed of carbohydrates. c) Mitosis and cytokinesis are required for asexual reproduction. d) Mitosis may occur without cytokinesis. e) Mitosis produces new nuclei with exactly the same chromosomal endowment as the parent nucleus.

b) The mitotic spindles in prokaryotic cells are composed of carbohydrates. (All somatic cells reproduce by asexual reproduction, meaning mitosis and cytokinesis are required. View #77, #50, & #54.

*QUIZ QUESTION* 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) spindle formation

b) replication of the DNA (DNA replication occurs in the S phase of interphase)

mitotic spindle

begins to form in the cytoplasm during prophase; composed of the centrosomes, the spindle microtubules that extend from them, and the asters

*QUIZ QUESTION* 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) continue to divide even when they are tightly packed together (view #69)

*QUIZ QUESTION* 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) the degradation of cyclin (view #64, read p. 243-244, & view Figure 12.16a on p. 244)

*QUIZ QUESTION* As a cell grows, it a) places more demands on its DNA b) uses up food and oxygen more quickly c) has more trouble moving enough materials across its cell membrane d) all of the above

d) all of the above

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

d) cleavage furrow formation and cytokinesis ( )

protein kinases

enzymes that activate or inactivate other proteins by phosphorylating them (see Chapter 11)

mitosis

the division of genetic material in the nucleus

mitotic (M) phase

includes both mitosis and cytokinesis; usually the shortest part of the cell cycle

sister chromatids

joined copies of the original chromosome in a cell, each containing an identical DNA molecule; initially attached all along their lengths

cyclin-dependent kinases (Cdks)

kinases that require a cyclin in order to be active; their activity rises and falls with changes in the concentration of its cyclin partner

How many chromosomes are in the nuclei of human gametes?

one set of 23 chromosomes

cohesins

protein complexes that attach sister chromatids all along their lengths

centromere

region of the chromosomal DNA on sister chromatids where a chromatid is attached most closely to its sister

gametes

reproductive cells; sperm and eggs; contain half as many chromosomes as somatic cells

What enzyme cleaves the cohesins holding together the sister chromatids of each chromosome during anaphase?

separase

origin of replication

site where the replication of a DNA molecule begins, consisting of a specific sequence of nucleotides

genome

the complete set of genes or genetic material present in a cell or organism

transformation

the conversion of a normal cell into a cell that is able to divide indefinitely in culture, thus behaving like a cancer cell

MPF

the cyclin-Cdk complex that was discovered first; "M-phase-promoting-factor"; triggers the cell's passage into the M phase, past the G2 checkpoint; acts bot directly as a kinase and indirectly by activating other kinases (view Figure 12.16, p. 244)

chromatin

the entire complex of DNA and proteins that is the building material of chromosomes

cell cycle

the life of a cell from the time it is first formed during division of a parent cell until its own division into two daughter cells

sister chromatid cohesion

the process by which sister chromatids are attached all along their lengths by cohesins

cell division

the reproduction of cells

metastasis

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