12

12. The Cell Cycle

Introduction

1. The continuity of life from one cell to another is based on the reproduction of cells via cell division.

2. This division process occurs as part of the cell cycle, the life of a cell from its origin in the division of a parent cell until its own division into two.

A. The Key Roles of Cell Division

1. Cell division functions in reproduction, growth and repair.

a. Reproduction: The division of a unicellular organism reproduces an entire organism, increasing the population. (Fig. 12.1a)

b. Growth: Cell division is also central to the development of a multicellular organism that begins as a fertilized egg or zygote. (Fig. 12.1b)

c. Repair: Multicellular organisms also use cell division to repair and renew cells that die from normal wear and tear or accidents. (Fig. 12.1)

2. Cell division distributes identical sets of chromosomes to daughter cells.

a. A cell�s genetic information, packaged as DNA, is called its genome.

1. In prokaryotes, the genome is often a single long circular DNA molecule.

2. In eukaryotes, the genome consists of several DNA molecules.

b. DNA molecules are packaged into chromosomes. (Fig. 12.2)

1. Every eukaryotic species has a characteristic number of chromosomes in the nucleus.

2. Human somatic cells (body cells) have 46 chromosomes.

3. Human gametes (sperm or eggs) have 23 chromosomes, half the number in a somatic cell.

c. Chromosome composition

1. Each eukaryotic chromosome consists of a long, linear DNA molecule.

2. Each chromosome has hundreds or thousands of genes, the units that specify an organism�s inherited traits.

3. Associated with DNA are proteins that maintain its structure and help control gene activity.

4. This DNA-protein complex is called chromatin.

d. New cells must contain a copy of the entire genome.

1. Cells must duplicate the DNA and separate the 2 copies so that each daughter cell has with a complete genome (human=about 30m total DNA).

2. Each duplicated chromosome consists of two sister chromatids which contain identical copies of the chromosome�s DNA. (Fig. 12.3)

3. As they condense to prepare for cell division, the region where the strands connect shrinks to a narrow area, the centromere.

4. Later, the sister chromatids are pulled apart and repackaged into two new nuclei at opposite ends of the parent cell.

5. The process of the formation of the two daughter nuclei, mitosis, is usually followed by division of the cytoplasm, cytokinesis.

6. These processes take one cell and produce two cells that are the genetic equivalent of the parent.

e. Each of us inherited 23 chromosomes from each parent: one set in an egg and one set in sperm.

1. The fertilized egg (zygote) underwent trillions of cycles of mitosis and cytokinesis to produce a fully developed multicellular human.

B. The Mitotic Cell Cycle in Eukaryotes

1. The cell cycle

a. Interphase = G1, S and G2

1. Interphase accounts for 90% of the cell cycle in the average cell. (Fig. 12.4)

2. During interphase the cell grows by producing proteins and cytoplasmic organelles, copies its chromosomes and prepares for cell division.

3. Interphase has three subphases

a. The G1 phase (first gap) centered on growth.

b. The S phase (synthesis), when the chromosomes are copied.

c. The G2 phase (second gap), when the cell completes preparations for cell division.

1. By G2 the chromosomes have been duplicated but are loosely packed. (Fig. 12.5)

2. The centrosomes have been duplicated and begin to organize microtubules into an aster (�star�) (Fig. 12.5a-newt lung cell with nucleus (blue) and centrosomes (brown)).

b. Mitotic phase = mitosis + cytokinesis

1. Mitosis = division of the nucleus and is broken into 4 subphases.

a. Prophase

1. The chromosomes are tightly coiled, with sister chromatids joined together.

2. The nucleoli and nuclear envelope disappear.

3. Microtubules from one pole attach to one of two kinetochores, while microtubules from the other pole attach to the other kinetochore and begin to organize the sister chromatids. (Fig. 12.5)

b. Metaphase

1. The spindle fibers push the sister chromatids until they are all arranged at the metaphase plate (equatorial plate), an imaginary plane equidistant between the poles, defining metaphase.

c. Anaphase

1. The centromeres divide, separating the sister chromatids.

2. Each is now pulled toward the pole to which it is attached by spindle fibers.

d. Telophase

1. The 2 sets of chromosomes are grouped at opposite poles.

2. Two nuclei begin to form.

3. Chromatin becomes less tightly coiled.

4. The end of telophase may overlap with the beginning of cytokinesis.

2. Cytokinesis = division of the cytoplasm.

a. In animals, the first sign of cytokinesis is the appearance of a cleavage furrow. (Fig. 12.8)

1. On the cytoplasmic side of the cleavage furrow a contractile ring of actin microfilaments and the motor protein myosin form.

2. Contraction of the ring pinches the cell in two.

b. Cytokinesis in plants, which have cell walls, involves a completely different mechanism.

1. During telophase, vesicles from the Golgi coalesce at the metaphase plate, forming a cell plate.

2. The plate enlarges until its membranes fuse with the plasma membrane at the perimeter, with the contents of the vesicles forming new wall material in between.

C. Bacterial Cell Division

1. Prokaryotes reproduce by binary fission.

a. Prokaryotes reproduce by binary fission, not mitosis.

b. Most bacterial genes are located on a single bacterial chromosome which consists of a circular DNA molecule and associated proteins.

1. While bacteria do not have as many genes or DNA molecules as long as those in eukaryotes, their circular chromosome is still highly folded and coiled in the cell.

2. Chromosome replication in binary fission

a. In binary fission, chromosome replication begins at one point in the circular chromosome, the origin of replication site. (Fig. 12.10)

b. These copied regions begin to move to opposite ends of the cell while the rest of the chromosome is still being copied.

c. As the bacterial chromosome is replicating and the copied regions are moving to opposite ends of the cell, the bacterium continues to grow until it reaches twice its original size.

d. Cell division involves inward growth of the plasma membrane, dividing the parent cell into two daughter cells, each with a complete genome.

3. The mechanism behind the movement of the bacterial chromosome is still an open question. Mitotic spindles and microtubules are unknown in bacteria.