The Cell Cycle

BIOLOGY

Key Terms• Genome—the total complement

of an organism’s genes• Chromatin—network of DNA and

fibrils in a nucleus that is not dividing.

• Chromosome—structure of DNA that transmits genetic information to the next generation.

• Chromatids—two copies of the same chromosome attached together

• Centromere—constriction where sister chromatids of a chromosome are held together.

Chromatin in a developing salamander ovum

Chromosome Numbers• Somatic cells (body cells)— diploid (2N) 2

of each chromosome

• Gametes (sex cells)— haploid (N)

1 of each chromosome

• Examples: Body cells Sex Cells– Humans 46 23– Fern 1,262 631– Drosophila 8 4– Dog 78 39

Mitosis• A diploid cell divides to produce

diploid daughter cells.

• Ensures that each daughter cell contains the exact same chromosomes as the original mother cell.

• Mitosis can occur in either diploid or haploid cells; maintaining either a 2N or N chromosome number.

Interphase• Most of the cell cycle

is spent in interphase.– G1 - organelles double;

accumulates materials for DNA synthesis.

– S - DNA replication.

– G2 - Cell synthesizes proteins necessary for cell division.

Prophase & Prometaphase• Prophase

– Chromatin has condensed.– Nucleolus disappears.– Nuclear envelope

disintegrates.– Spindle begins to assemble.

• Prometaphase– Kinetochores develop on

centromere.• Attach sister chromatids to

spindle fibers.

Figure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase; prometaphase

Metaphase and Anaphase• Metaphase

– Chromosomes, attached to kinetochore fibers, are in alignment at center of cell.

• Anaphase– Sister chromatids split,

producing daughter chromosomes.

• Daughter chromosomes are pulled to opposite poles.

Telophase• Telophase– Spindle disappears as new nuclear envelopes form

around the daughter chromosomes.• Chromosomes become diffuse chromatin again.• Nucleolus appears in each daughter nucleus.

CytokinesisAnimal Cells:Cleavage furrow begins as

anaphase nears completion.– Deepens when a contractile

ring forms a constriction between the two daughter cells.

– Mitosis without cytokinesis results in a multinucleated cell.

CytokinesisPlant Cells:– Rigid cell wall surrounding

plant cells does not permit cytokinesis by furrowing.

• Begins with formation of a cell plate which eventually becomes new plasma membrane between the daughter cells.

Figure 12.9 Mitosis in a plant cell

Figure 12-09x Mitosis in an onion root

Cell Cycle Control• Protein (cyclin)

increases & decreases as cell cycle continues.– Must be present for the

cell to proceed from the G2 stage to the M stage and from G1 stage to S stage.

• Allows time for any damage to be repaired.

The Cell Cycle and Cancer• Cancer is a growth

disorder that results from the mutation of genes regulating the cell cycle.– Lack differentiation.– Have abnormal nuclei.– Form tumors.– Loss of contact inhibition.– Undergo metastasis

(spread).

Apoptosis• Often defined as programmed

cell death.– Cells routinely harbor enzymes

(caspases) necessary for apoptosis to occur.

• Ordinarily held in check by inhibitors, but can be unleashed by internal or external signals.

• Mitosis increases and apoptosis decreases the number of somatic cells.