Chapter 12 - The Cell Cycle

Question 1

a. How many chromosomes do you have in your somatic cells?
b. How many chromosomes do you have in your gametes?
c. How many chromatids are in one of your body cells that has duplicated its chromosomes prior to mitosis?

Answer 1

a. 46
b. 23
c. 92

Question 2

a. How are the three subphases of interphase alike?
b. What key event happens during the S phase?

Answer 2

a. Growth — most organelles and cell components are produced continuously throughout these subphases.
b. DNA synthesis

Question 3

These diagrams will depict interphase and the five subphases of mitosis in an animal cell, after you draw in the missing chromosomes.

For simplicity, assume that this cell has four chromosomes. Sketch the chromosomes as they would appear in each subphase.

Identify the stages and label the indicated structures.

(2nd half of picture is in answer portion)

Answer 3

a. G₂ of interphase
b. prophase
c. prometaphase
d. metaphase
e. anaphase
f. telophase and cytokinesis
g. centrosomes (with centrioles)
h. chromatin (duplicated)
i. nuclear envelope
j. nucleolus
k. early mitotic spindle
l. aster
m. nonkinetochore microtubulesn
n. kinetochore microtubules
o. metaphase plate
p. spindle
q. cleavage furrow
r. nuclear envelope forming

Question 4

a. What is MPF?
b. Describe the relative concentrations of MPF and its constituent molecules throughout the cell cycle:

MPF

Cdk

cyclin

Answer 4

a. MPF is a complex of cyclin and Cdk that initiates mitosis by phosphorylating proteins and other kinases.
b. MPF concentration is high as it triggers the onset of mitosis but is reduced at the end of the mitosis because it depends on the concentration of cyclin in the cell.

The Cdk level is constant through the cell cycle, but the level of cyclin varies because active MPF starts a process that degrades cyclin.

Thus, MPF regulates its own level and can only become active when sufficient cyclin accumulates after being synthesized in the S and G₂ phases of the next interphase.

Question 5

Describe the life of one chromosome as it proceeds through an entire cell cycle, starting with G₁ of interphase and ending with telophase of mitosis.

Answer 5

Interphase: 90% of cell cycle; growth and DNA replication.

• G₁ phase: the chromosome consists of a long, thin chromatin fiber made of DNA and associated proteins. RNA molecules are being transcribed from genes that are switched on.
• S phase — synthesis of DNA: The chromosome is duplicated; two exact copies, called sister chromatids, are produced and held together by cohesins along their length. Growth continues.
• G₂ phase: Growth continues.

Mitosis phase: cell division

• Prophase: The sister chromatids, held together by sister chromatid cohesion and at the centromere, become tightly coiled and folded.
• Prometaphase: Kinetochore fibers from opposite ends of the mitotic spindle attach to the kinetochores of the sister chromatids; the chrmosome moves toward midline.
• Metaphase: The centromere of the chromosome is aligned at the metaphase plate along with the centromeres of the other chromosomes.
• Anaphase: Cohesions are cleaved and the sister chromatids separate (now considered to be individual chromosomes) and move to the opposite poles.
• Telophase: Chromatin fiber of chromosome uncoils and is surrounded by reforming nuclear membrane.

Question 6

Draw a sketch of one half a mitotic spindle. Identify and list the functions of the components.

Answer 6

a. nonkinetochore microtubule: push poles apart by elongating and “walking” past microtubules from the opposite pole
b. kinetochore: protein and DNA structure in region of centromere where microtubules attach
c. kinetochore microtubule: move chromosomes to metaphase plate and separate chromosomes as motor proteins of kinetochores “walk” toward the pole and the microtubules disassemble
d. centrosome and centrioles: region of mitotic spindle formation; organized microtubules
e. aster: radiating spindle fibers (in animal cells)

Question 7

In this photomicrograph of cells in an onion root tip, identify the cell cycle phases for the indicated cells.

Answer 7

a. anaphase
b. interphase
c. late telophase
d. metaphase

Question 8

1. most cells that will no longer divide are in this phase
2. sister chromatids separate and chromosomes move apart
3. mitotic spindle begins to form
4. cell plate forms or cleavage furrow pinches cells apart
5. chromosomes duplicate
6. chromosomes line up at equatorial plane
7. nuclear membranes form around separated chromosomes
8. chromosomes become visible
9. kinetochore-microtubule interactions move chromosomes to midline
10. restriction point occurs in this phase

Answer 8

1. G₀
2. anaphase
3. prophase
4. cytokinesis
5. S phase
6. metaphase
7. telophase
8. prophase
9. prometaphase
10. G₁ phase

Question 9

One of the major differences in the binary fission of prokaryotic cells compared to mitosis of eukaryotic cells is that

a. cytokinesis does not occur in prokaryotic cells.
b. eukaryotic cell division involves microtubules composed of tubulin and actin microfilaments, whereas no such molecules are involved in prokaryotic binary fission.
c. the duplicated chromosomes are attached to the plasma membrane in prokaryotic cells and are separated from each other as the membrane splits apart.
d. the duplicated single chromosome does not separate along a mitotic spindle in prokaryotic cells.
e. the chromosome number is reduced by half in eukaryotic cells but not in prokaryotic cells.

Answer 9

d. the duplicated single chromosome does not separate along a mitotic spindle in prokaryotic cells.

Question 10

A plant cell has 12 chromosomes at the end of mitosis. How many chromosomes would it have in the G₂ phase of its next cell cycle?

a. 6
b. 9
c. 12
d. 24
e. It depends on whether it is undergoing mitosis or meiosis.

Answer 10

c. 12

Question 11

How many chromatids would this plant cell have in the G₂ phase of its cell cycle?

a. 6
b. 9
c. 12
d. 24
e. 48

Answer 11

d. 24

Question 12

The longest part of the cell cycle is

a. prophase.
b. G₁ phase.
c. G₂ phase.
d. mitosis.
e. interphase.

Answer 12

e. interphase.

Question 13

In animal cells, cytokinesis involves

a. the separation of sister chromatids.
b. the contraction of a ring of actin microfilaments.
c. depolymerization of kinetochore microtubules.
d. a protein kinase that phosphorylates other enzymes.
e. sliding of nonkinetochore microtubules past each other.

Answer 13

b. the contraction of a ring of actin microfilaments.

Question 14

Humans have 46 chromosomes. That number of chromosomes will be found in

a. cells in anaphase.
b. the egg and sperm cells.
c. the somatic cells.
d. all the cells of the body.
e. only cells in G₁ of interphase.

Answer 14

c. the somatic cells.

Question 15

Sister chromatids

a. have one-half the amount of genetic material as does the original chromosome.
b. start to move along kinetochore microtubules toward opposite poles during telophase.
c. each have their own kinetochore.
d. are formed during S phase but do not join by sister chromatid cohesion until prophase.
e. slide past each other as nonkinetochore microtubules elongate.

Answer 15

c. each have their own kinetochore.

Question 16

Which of the following would not be exhibited by cancer cells?

a. changing levels of MPF concentration
b. passage through the restriction point
c. density-dependent inhibition
d. metastasis
e. G₁ phase of the cell cycle

Answer 16

c. density-dependent inhibition

Question 17

Which of the following is not true of a cell plate?

a. It forms at the site of the metaphase plate.
b. It results from the fusion of microtubules.
c. It fuses with the plasma membrane.
d. A cell wall is laid down between its membranes.
e. It forms during telophase in plant cells.

Answer 17

b. It results from the fusion of microtubules.

Question 18

A cell that passes the restriction point in G₁ will most likely

a. undergo chromosome duplication.
b. have just completed cytokinesis.
c. continue to divide only if it is a cancer cell.
d. show a drop in MPF concentration.
e. move into the G₀ phase.

Answer 18

a. undergo chromosome duplication.

Question 19

The rhythmic changes in cyclin concentration in a cell cycle are due to

a. its increased production once the restriction point is passed.
b. the cascade of increased production once its enzyme is phosphorylated by MPF.
c. its degradation, which is initiated by active MPF.
d. the correlation of its production with the production of Cdk.
e. the binding of the growth factor PDGF.

Answer 19

c. its degradation, which is initiated by active MPF.

Question 20

A cell in which of the following phases would have the least amount of DNA?

a. G₀
b. G₂
c. prophase
d. metaphase
e. anaphase

Answer 20

a. G₀

Question 21

What initiates the separation of sister chromatids in anaphase?

a. the drop in MPF concentration
b. a rapid rise in Cdk concentration
c. movement past the G₂ checkpoint
d. a signal pathway initiated by the binding of a growth factor
e. activation of a regulatory protein following the attachment of all kinetochores to microtubules

Answer 21

e. activation of a regulatory protein following the attachment of all kinetochores to microtubules

Question 22

Knowledge of the cell cycle control system will be most beneficial to the area of

a. human reproduction.
b. plant genetics.
c. prokaryotic growth and development.
d. cancer prevention and treatment.
e. prevention and treatment of cardiovascular disease.

Answer 22

d. cancer prevention and treatment.

Question 23

Cell division creates duplicate offspring in unicellular organisms and provides for growth, development, and repair in multicellular organisms. The cell cycle extends from the creation of a new cell by the division of its parent cell to its own division into two cells.

Cell division results in genetically identical daughter cells.

The process of recreating a structure as intricate as a cell necessitates the exact duplication and equal division of the DNA containing the cell’s genetic program.

A cell’s complete complement of DNA is called its genome. Each diploid eukaryotic species has a characteristic number of chromosomes in each somatic cell; reproductive cells, or gametes (egg and sperm), have half that number of chromosomes.

Each chromosome is a very long DNA molecule with associated proteins that help to structure the chomosome and control the activity of the genes. This DNA-protein complex is called chromatin.

Question 24

Prior to cell division, a cell copies its DNA and each chromosome densely coils and shortens. Duplicated chromosomes consist of two identical sister chromatids, initially attached by proteins called cohesins, which hold them together in sister chromatid cohesion. A duplicated chromosome has a centromere where the chromatids separate during mitosis (the division of the nucleus), and then the cytoplasm divides during cytokinesis, producing two separate, genetically equivalent daughter cells.

A type of cell division called meiosis produces daughter cells that have half the number of chromosomes (one set) of the parent cell. With the fertilization of egg and sperm, which were formed (in animals) by meiosis, the chromosome number is restored as the somatic cells of the new offspring again have two sets of chromosomes.

Question 25

The mitotic phase alternates with interphase in the cell cycle.

The cell cycle consists of the mitotic (M) phase, which includes mitosis and cytokinesis, and interphase, during which the cell grows and duplicates its chromosomes. Interphase, usually lasting 90% of the cell cycle, includes the G₁ phase, the S phase, and the G₂ phase. Mitosis is conventionally described in five subphases: prophase, prometaphase, metaphase, anaphase, and telophase.

Question 26

The mitotic spindle consists of fivers made of microtubules and associated proteins. The assembly of the spindle begins in the centrosome, or mictrotubule-organizing center. A pair of centrioles is centered in each centrosome of an animal cell, but centrioles are not required for normal spindle operation. The single centrosome replicateds during interphase. As spindle microtubules grow out from them, the two centrosomes move to opposite poles of the cell. Radial arrays of shorter microtubules, called asters, extend from the centrosomes.

During prophase, the nucleoli disappear and the chromatin fibers coil and fold into visible chromosomes, consisting of sister chromatids joined by sister chromatid cohesion. During prometaphase, some of the spindle microtubules attach to each chromatid’s kinetochore, a structure of protein associated with DNA located at the centromere region. Alternate tugging on the chromosome by opposite kinetochore microtubules moves the chromosome to the midline of the cell. At metaphase, the chromosomes are aligned at the metaphase plate, across the midline of the spindle. Nonkinetochore microtubules (or “polar” microtubules) extend out from each centrosome and overlap at the midline. Aster microtubules contact the plasma membrane.

The cohesins joining sister chromatids are cleaved in anaphase, and the now separate chromosomes move toward the poles. Motor proteins appear to “walk” a chromosome along the kinetochore microtubules as these shorten by depolymerizing at the kinetochore end. Motor proteins at the spindle poles also appear to “reel in” the chromosomes, with the microtubules depolymerizing at the poles. The extension of the spindle poles away from each other as an animal cell elongates is probably due to the overlapping nonkinetochore microtubules walking past each other, also using motor proteins.

During telophase, equivalent sets of chromosomes gather at the two poles of the cell. Nuclear envelopes form, nucleoli reappear, cytokinesis begins, and the spindle disassembles.

Question 27

Cleavage is the process that separates the two daughter cells in animals. A cleavage furrow forms, as a ring of actin microfilaments interacting with myosin proteins begins to contract on the cytoplasmic side of the membrane. The cleavage furrow deepens until the dividing cell is pinched in two.

In plant cells, a cell plate forms from the fusion of membrane vesicles derived from the Golgi apparatus. The membrane of the enlarging cell plate joins with the plasma membrane, separating the two daughter cells. A new cell wall develops between the cells from the contents of the cell plate.

Question 28

Single-celled eukaryotes reproduce asexually by a process known as binary fission, which includes mitosis. The binary fission of prokaryotes does not include mitosis. The bacterial chromosome, a single circular DNA molecule, begins to replicate at the origin of replication and one of the duplicated origins moves to the opposite pole of the cell. Replication is completed as the cell doubles in size, and the plasma membrane grows inward to divide the two identical daughter cells. The mechanism of chromosome movement is not fully understood.

Question 29

Evidence for the evolution of mitosis from prokaryotic cell division includes the relatedness of several proteins involved in both types of division and the possible intermediate stages seen in some unicellular algae, in which chromosomal division takes place within an intact nuclear envelope.

Question 30

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

Normal growth, development, and maintenance depend on proper control of the timing and rate of cell division.

Evidence for Cytoplasmic Signals.

Experiments that fuse two cells at different phases of the cell cycle indicate that cytoplasmic chemical signals drive the cell cycle.

Question 31

A cell cycle control system, consisting of a set of molecules that function cyclically, coordinates the events of the cell cycle.

Important internal and external signals are monitored to determine whether the cell cycle will proceed past the three main checkpoints in the G₁, G_2, and M phases. If a mammalian cell does not receive a go-ahead signal at the G₁ checkpoint, called the “restriction point”, the cell will usually exit the cell cycle to a nondividing state called the G₀ phase.

Protein kinases are enzymes that activate or inactivate other proteins by phosphorylating them. The changing concentrations of cyclins, regulatory proteins that attach to these kinases, affect the activity of cyclin-dependent kinases, or Cdks.

A cyclin-Cdk complex called MPF, for maturation or M-phase-promoting factor, triggers passage past the G₂ checkpoint into M phase. In addition to phosphorylating proteins and other kinases that initiate mitotic events, MPF activates a protein breakdown process that detroys its cyclin and this MPF activity during anaphase. The Cdk portion of the complex remains to associate with new cyclin synthesized during S and G₂ phases of the next cycle.

Other Cdk proteins and cyclins appear to control the movement of a cell past the G₁ checkpoint.

An internal signal is required to move past the M phase checkpoint into anaphase. The cohesins holding sister chromatids together are not cleaved until all chromosomes are attached at their kinetochores to spindle microtubules.

Growing cells in cell culture has allowed researchers to identify chemical and physical factors that affect cell division. Certain nutrients and regulatory proteins called growth factors have been found to be essential for cells to divide in culture. Mammalian fibroblast cells have receptors on their plasma membranes for platelet-derived growth factor (PDGF), which is released from blood platelets at the site of an injury. Binding of PDGF initiates a signal-transduction pathway that enables cells to pass the G₁ checkpoint.

Density-dependent inhibition of cell division appears to involve the binding of cell-surface proteins of adjacent cells, which sends a growth-inhibiting signal to both cells. Most animal cells also show anchorage dependence and must be attached to a substratum in order to survive.

Question 32

When grown in cell culture, cancer cells do not exhibit density-dependent inhibition, do not depend on growth factors to divide, and may continue to divide indefinitely instead of stopping after the typical 20 to 50 divisions of normal mammalian cells.

When a normal cell is transformed or converted to a cancer cell, the body’s immune system may destroy it. If it proliferates, a mass of abnormal cells develops within a tissue. Benign tumors remain at their original site and can be removed by surgery. Malignant tumors cause cancer as they invade and disrupt the functions of one or more organs. Malignant tumor cells may have abnormal metabolism and unusual numbers of chromosomes. They lose their attachments to other cells and may metastasize, entering the blood and lymph systems and spreading to other sites. Radiation and chemicals are used to treat tumors that metastasize. Much remains to be learned about control of the cell division processes of both normal and cancerous cells.