Chapter 12 - The Cell Cycle

Question 1

What is one characteristic that distinguishes living organisms from nonliving matter?

Answer 1

The ability of organisms to produce more of their own kind; a unique capacity to procreate

Question 2

What is the continuity of life based upon?

Answer 2

Cell division; the reproduction of cells

Question 3

In what type of organisms does the division of one cell reproduce the entire organism?

Answer 3

Unicellular organisms

Question 4

What do multicellular eukaryotes depend on cell devision for?

Answer 4

• Development from a fertilized egg
• Growth
• Repair

Question 5

What is the cell cycle?

Answer 5

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;

Cell division is an integral part of the cell cycle. Passing identical genetic material to cellular offspring is a crucial function of cell division.

Question 6

What does most cell division result in?

Answer 6

Genetically identical daughter cells; daughter cells with identical genetic information (DNA)

*Exception - meiosis - a special type of eukaryotic cell division that can produce sperm and eggs

Question 7

What is a genome?

Answer 7

A cells endowment of DNA, its genetic information; all the DNA in a cell

• A genome can consist of a single DNA molecule (common in prokaryotic cells) or a number of DNA molecules (common in eukaryotic cells)

Question 8

What are chromosomes?

Answer 8

DNA molecules in a cell are packaged into structures called chromosomes;

*Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus

Question 9

What is chromatin?

Answer 9

Eukaryotic chromosomes consist of chromatin, a complex of DNA and protein that condenses during cell division

Question 10

What are somatic cells?

Answer 10

Nonreproductive cells; each contain 46 chromosomes, made up of 2 sets of 23, one set inherited from each parent

Question 11

What are gametes?

Answer 11

Reproductive cells; sperm and eggs - have half as many chromosomes as somatic cells;

i.e. human gametes have one set of 23 chromosomes

Question 12

What happens in preparation for cell division?

Answer 12

DNA replicates and the chromosomes condense

Question 13

What are sister chromatids?

Answer 13

Each duplicated chromosome has two sister chromatids (joined copies of the original chromosome), attached along their lengths by protein complexes called cohesins; this attachment is known as sister chromatid cohesion

Question 14

What is the centromere?

Answer 14

Each sister chromatid has a centromere; the region of the chromosomal DNA where the chromatid is attached most closely to its sister chromatid; the “narrow waist” of the duplicated chromosome

Question 15

During cell division, what happens to the two sister chromatids?

Answer 15

The two sister chromatids of each duplicated chromosome separate and move into two nuclei; once separate, the chromatids are called chromosomes

Question 16

Eukaryotic cell division consists of what two phases? (i.e. mitotic phase)

Answer 16

Mitosis and cytokinesis

Question 17

What is mitosis?

Answer 17

The division of the genetic material in the nucleus

Question 18

What is cytokinesis?

Answer 18

The division of the cytoplasm and the organelles within the cytoplasm

Question 19

How are gametes produces?

Answer 19

Gametes are produced by meiosis; meiosis yields nonidentical daughter cells with half as many chromosomes as the parent cell

Question 20

What are the two phases of the cell cycle?

Answer 20

Mitotic (M) phase - mitosis and cytokinesis (the shortest part of the cell cycle)

Interphase - cell growth and copying of chromosomes in preparation of cell division (accounts for 90% of the cell cycle)

Question 21

What are the 3 sub-phases of interphase?

Answer 21

G1 phase (first gap)

S phase (synthesis)

G2 phase (second gap)

*The cell grows through all 3 phases, but chromosomes are duplicated only during the S phase

Question 22

What are the 5 stages of mitosis?

Answer 22

1. Prophase
2. Prometaphase
3. Metaphase
4. Anaphase
5. Telophase

Question 23

What happens during G2 of interphase?

Answer 23

• A nuclear envelope encloses the nucleus; the nucleus contains one or more nucleoli
• 2 centrosomes have formed by duplication of a single centrosome; centrosomes are regions in animals cells that organize the microtubules of the spindle; each centrosome contains two centrioles

Question 24

What happens during prophase?

Answer 24

• The chromatin fibers become more tightly coiled, condensing into discrete chromosomes
• 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 25

What happens during prometaphase?

Answer 25

• 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 oppose pole of the spindle

Question 26

What happens during metaphase?

Answer 26

• The centrosomes are now at opposite poles of the cell
• The chromosomes have all arrived 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 27

What happens during anaphase?

Answer 27

• The shortest stage of mitosis, often lasting only a few minutes
• Begin 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 the opposite ends of the cell as their kinetochore microtubules shorten; because these microtubules are attached at the centromere region, the chromosomes move centromeres first
• 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

Question 28

What happens during telophase?

Answer 28

• 2 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

Question 29

What happens during cytokinesis?

Answer 29

• The division of the cytoplasm is usually under way 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

Question 30

What is the mitotic spindle?

Answer 30

Begins to form in the cytoplasm during prophase; this structure consists of fibers made of microtubules and associated proteins, which control chromosome movement during mitosis.

Question 31

What is the centrosome?

Answer 31

In animal cells, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center;

The centrosome replicates during interphase, forming two centrosomes that migrate to opposite ends of the cell during prophase and prometaphase.

*An aster (a radical array of short microtubules) extends from each centrosome

Question 32

What does the spindle include?

Answer 32

The centrosomes, the spindle microtubules, and the asters

*During prometaphase, some spindle microtubules attach tothe kinetochores of chromosomes and begin to move the chromosomes

Question 33

What are kinetochores?

Answer 33

A structure made up of proteins that have assembled on specific sections of DNA at each centromere; the chromosome’s two kinetochores face in opposite directions

Question 34

What is the metaphase plate?

Answer 34

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

Question 35

Anaphase begins suddenly when…?

Answer 35

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 full fledge chromosomes that move along kinetochore microtubules toward opposite ends of the cell.

*The microtubules shorten by depolymerizing at their kinetochore ends

Question 36

In animal cells, what is responsible for elongating the whole cell during anaphase?

Answer 36

The nonkinetochore microtubules; these overlap and push against each other, elongating the cell.

• At the end of anaphase, duplicate groups of chromosomes have arrived at opposite ends of the elongated parent cell
• In telophase, genetically identical daughter nuclei form at opposite ends of the cell
• Cytokinesis begins during anaphase or telophase and the spindle eventually disassembles

Question 37

What is cleavage?

Answer 37

In animal cells, cytokinesis occurs by a process known as cleavage forming a cleavage furrow;

Characterized by the pinching of the plasma membrane

Question 38

What is the cleavage furrow?

Answer 38

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 myosis; 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 its own share of cytosol, organelles, and other subcellular structures

Question 39

How does cytokinesis occur in plant cells?

Answer 39

In plant cells, a cell plate forms during cytokinesis; during telophase, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, where they coalesce, producing a cell plate