Chapter 12 The Cell Cycle

Question 1

What are the three key functions of cell division?

Answer 1

reproduction
growth and development
tissue renewal

Question 2

What is meant by “cell cycle”?

Answer 2

The cell cycle is the life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells. This process is cyclical because every cell forms from a preexisting cell.

Question 3

What is the meaning of “genome”? Compare your genome to that of a prokaryotic cell.

Answer 3

A cell’s endowment of DNA, its genetic information, is called its genome. While a prokaryotic genome is often a single DNA molecule, a typical human cell has 46 molecules of DNA.

Question 4

How many chromosomes are in a human somatic cell?

Answer 4

The nuclei of human somatic cells each contain 46 chromosomes, made up of two sets of 23, one set inherited from each parent.

Question 5

Name two types of somatic cells in your body.

Answer 5

Somatic cells include bone cells and liver cells.

Question 6

What is a gamete?

Answer 6

Gametes are reproductive cells.

Question 7

Name the two types of gametes.

Answer 7

Sperm are the gametes in males and eggs are the gametes in females.

Question 8

How many chromosomes are in a human gamete?

Answer 8

A human gamete contains one set of 23 chromosomes.

Question 9

Define chromatin.

Answer 9

Chromatin is the entire complex of DNA and proteins that makes up chromosomes.

Question 10

How many DNA molecules are in each of your somatic cells?

Answer 10

Since each chromosome consists of one DNA molecule, each human somatic cell contains 46 chromosomes.

Question 11

What is a chromosome?

Answer 11

A chromosome is a packaged gene-carrying structure consisting of chromatin.

Question 12

What is a chromatid?

Answer 12

A chromatid is one copy of a duplicated chromosome.

Question 13

What is a centromere?

Answer 13

A centromere is a region containing specific DNA sequences where the chromatid is attached most closely to its sister chromatid (represented by the narrow “waist”).

Question 14

Describe DNA level in each stage of chromosome duplication

Answer 14

[1] One of the multiple chromosomes in a eukaryotic cell, a long, thin chromatin fiber containing one DNA molecule and associated proteins, is duplicated, involving the replication of DNA, and condensed. [2] Once duplicated, the chromosome consists of two sister chromatids connected along their entire lengths by sister chromatid cohesion. Each chromatid contains a copy of the DNA molecule. [3] Molecular and mechanical processes separate the sister chromatids into two distinct chromosomes and distribute them to two daughter cells.

Question 15

What is mitosis? How is it different from cytokinesis?

Answer 15

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

Question 16

What is the attachment of two sister chromatids and what is the attaching protein?

Answer 16

sister chromatid cohesion, cohesins

Question 17

What is the arm of a chromatid?

Answer 17

the part of a chromatid on either side of the centromere

Question 18

What occurs in meiosis? How is the chromosome number of daughter cells different?

Answer 18

Meiosis is the variation of cell division that produces gametes, which yields nonidentical daughter cells that have only one set of chromosomes, half as many chromosomes as the parent cell.

Question 19

Compare mitotic and meiotic processes.

Answer 19

Mitosis is the process by which identical daughter cells are produced, the damaged cells are replaced in a wound, and a zygote develops into a multicellular organism. Meiosis is the process by which eggs are formed and the chromosome number of daughter cells is reduced.

Question 20

Give a brief explanation of what happens in each subphase of the interphase.

Answer 20

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).

Question 21

What are the two parts of interphase?

Answer 21

G1 and G2

Question 22

What are the components of the mitotic spindle? What is the source of these components?

Answer 22

In animal cells, the mitotic spindle, a structure consisting of fibers made of microtubules and associated proteins, emerges from the centrosome, a subcellular region containing material that functions throughout the cell to organize the cell’s microtubules.

Question 23

Where does the assembly of spindle microtubules start?

Answer 23

In animal cells, the assembly of spindle microtubules starts at the centrosome.
The microtubule-organizing center is another name for the centrosome.

Question 24

Describe what happens to the centrosome during interphase and then prophase.

Answer 24

During interphase in animal cells, 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 from them. By the end of prometaphase, the two centrosomes, one at each pole of the spindle, are at opposite ends of the cell.

Question 25

What is a kinetochore?

Answer 25

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.

Question 26

Describe the difference between kinetochore and nonkinetochore microtubules and the function of each.

Answer 26

Unlike nonkinetochore microtubules, kinetochore microtubules attach to the kinetochores and jerk the chromosomes back and forth, eventually aligning them along the metaphase plate. In a dividing animal cell, the nonkinetochore microtubules are responsible for elongating the whole cell during anaphase.

Question 27

At which end do kinetochore microtubules shorten during anaphase?

Answer 27

A 1987 experiment to determine whether kinetochore microtubules depolymerize at the kinetochore end or the pole end as chromosomes move toward the poles during mitosis concluded that the microtubule segments on the kinetochore side of the mark shorten, while those on the spindle pole side stay the same length. The experimenters marked a region of the kinetochore microtubules between one spindle pole and the chromosomes, then monitored the changes in microtubule length on either side of the mark. 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.

Question 28

Describe cytokinesis in an animal cell.

Answer 28

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. 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.

Question 29

Describe cytokinesis in a plant cell.

Answer 29

Cytokinesis in plant cells, which have cell walls, does not involve a 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 in 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.

Question 30

How is the cell plate formed? What is the source of material for the cell plate?

Answer 30

Vesicles from the Golgi apparatus containing cellulose move along microtubules to the center of the cell, where they coalesce, forming the cell plate, as new cell wall materials fuse with the plasma membrane and the old cell wall.

Question 31

Prokaryote reproduction does not involve mitosis, but instead occurs by binary fission. Describe binary fission.

Answer 31

In binary fission, a prokaryotic cell grows to roughly double its size, then divides to form two cells.

Question 32

Besides the fact that prokaryotes lack a membrane-bounded nucleus, contrast prokaryotic and eukaryotic cells.

Answer 32

Prokaryotes reproduce through binary fission, while eukaryotes reproduce through mitosis or meiosis. Prokaryotes typically contain a single bacterial chromosome, while eukaryotes usually contain several chromosomes. In prokaryotes, chromosomes are not condensed; in eukaryotes, chromosomes are condensed into discrete chromosomes with certain genes.

Question 33

What controls the cell cycle?

Answer 33

The sequential events of the cell cycle are directed by a distinct cell cycle control system, a cyclically operating set of molecules in the cell that both triggers and coordinates key events in the cell cycle.

Question 34

What is a cell cycle checkpoint?

Answer 34

A checkpoint in the cell cycle is a control point where stop and go signals can regulate the cycle.

Question 35

What happens at each checkpoint?

Answer 35

G1: “restriction point” in animal cells; continues on to G2 if go, will usually complete cycle; exits cell cycle and enters G0, a nondividing state, if no go; regulated by the activity of cyclin-Cdk protein complexes
G2: MPF triggers cell’s passage past G2 checkpoint into M phase if all chromosomes have been replicated
M: irreversible anaphase stage entered only if all sister chromatids correctly attached to spindle microtubules

Question 36

Describe the G0 phase.

Answer 36

a non dividing state

Question 37

What is a protein kinase?

Answer 37

Protein kinases are enzymes that activate or inactivate other proteins by phosphorylating them. Particular protein kinases give the go-ahead signals at the G1 and G2 checkpoints.

Question 38

Kinases drive the cell cycle, but they must be activated by the attachment of…

Answer 38

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

Question 39

Why does the activity of cyclin-dependent kinases (Cdks) rise and fall?

Answer 39

The activity of a Cdk rises and falls with changes in the concentration of its cyclin partner.

Question 40

What does MPF trigger? What are some specific activities that it triggers?

Answer 40

MPF (maturation-promoting factor) triggers the cell’s passage past the G2 checkpoint into M phase. For example, MPF causes phosphorylation of various proteins of the nuclear lamina, which promotes fragmentation of the nuclear envelope during prometaphase of mitosis.

Question 41

What happens if all the chromosome kinetochores are not attached to spindle fibers?

Answer 41

This condition keeps the cell from passing the M checkpoint. Anaphase, the separation of sister chromatids, does not begin until all the chromosomes are properly attached to the spindle at the metaphase plate.

Question 42

What are growth factors? How does PDGF stimulate fibroblast division?

Answer 42

Growth factors are proteins released by certain cells that stimulate other cells to divide. Fibroblasts have PDGF (platelet- derived growth factor) receptors on their plasma membranes. The binding of PDGF molecules to these receptor tyrosine kinases triggers a signal transduction pathway that allows the cells to pass the G1 checkpoint and divide.

Question 43

Cancer cells exhibit different behaviors than normal cells. Explain two normal behaviors they no longer show.

Answer 43

Density-dependent inhibition is a phenomenon in which crowded cells stop dividing. The bindings of a cell-surface protein to its counterpart on an adjoining cell sends a growth-inhibiting signal to both cells, preventing them from moving forward in the cell cycle, even in the presence of growth factors. Moreover, in a phenomenon known as anchorage dependence, most cells must be attached to a substratum, such as the extracellular matrix of a tissue, in order to divide. Anchorage is probably signaled to the cell cycle control system via pathways involving plasma membrane proteins and elements of the cytoskeleton linked to them.

Question 44

What is transformation? What is metastasis?

Answer 44

Transformation is the process that converts a normal cell to a cancer cell. Cancer cells may secrete signaling molecules that cause blood vessels to grow toward the tumor. A few tumor cells may separate from the original tumor, enter blood vessels and lymph vessels, and travel to other parts of the body. There, they may proliferate and form a new tumor. This spread of cancer cells to locations distant from their original site is called metastasis.

Question 45

Distinguish between a benign tumor and a malignant tumor.

Answer 45

Most benign tumors do not cause serious problems and can be completely removed by surgery. In contrast to benign tumors, whose abnormal cells remain at the original site if they have too few genetic and cellular changes to survive at another site, malignant tumors include cells whose genetic and cellular changes enable them to spread to new tissues and impair the functions of one or more organs. An individual with a malignant tumor is said to have cancer.

Question 46

List two specific cancer treatments and explain how each treatment works.

Answer 46

The chemotherapeutic drug Taxol freezes the mitotic spindle by preventing microtubule depolymerization, which stops actively dividing cells from proceeding past metaphase. Also, the cells of about 20–25% of breast cancer tumors show abnormally high amounts of a cell-surface receptor tyrosine kinase called HER2, and many show an increase in the number of estrogen receptor (ER) molecules, intracellular receptors that can trigger cell division. In this case, chemotherapy can be prescribed with a molecule that blocks the function of the specific protein (herceptin for HER2 and tamoxifen for ERs).

Question 47

What happens in G2 of interphase?

Answer 47

In G2 of interphase, a nuclear envelope encloses the nucleus, which contains one or more nucleoli. Two centrosomes, each containing two centrioles, are formed by the duplication of a single centrosome. Chromosomes, duplicated during S phase, cannot be individually distinguished because they have not yet condensed.

Question 48

What happens in prophase?

Answer 48

In prophase, the chromatin fibers condense into discrete chromosomes and 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. The mitotic spindle 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. The centrosomes move away from each other, propelled partly by the lengthening microtubules between them.

Question 49

What happens in prometaphase?

Answer 49

In prometaphase, the nuclear envelope fragments, and 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.

Question 50

What happens in metaphase?

Answer 50

In metaphase, the centrosomes reach the opposite poles of the cell while 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.

Question 51

What happens in anaphase?

Answer 51

Anaphase, the shortest stage of mitosis, begins when the cohesin proteins are cleaved, allowing 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 microtubules lengthen. By the end of anaphase, the two ends of the ell have equivalent and complete collections of chromosomes.

Question 52

What happens in telophase?

Answer 52

In 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 and the chromosomes become less condensed. Any remaining spindle microtubules are depolymerized. Mitosis, the division of one nucleus into two genetically identical nuclei, is complete. In animal cells, cytokinesis involves the formation of a cleavage furrow, which pinches the cell in two.

Question 53

Which 4 organisms support the evolution of mitosis?

Answer 53

bacteria, dinoflagellates, diatoms and yeast, eukaryotes

Question 54

What site is important for binary fission

Answer 54

origin of replication