The Cell Cycle, DNA

Replication and

Mitosis

Chapter 19

Becker’s The World of Cell

Overview of the Cell Cycle

The eukaryotic cell cycle is divided into G1, S,

G2, and M phases. Chromosomal DNA replication takes place during S phase, whereas cell division (mitosis and cytokinesis) occurs in M phase. Interphase (G1, S, and G2) is a time of cell growth and metabolism that typically occupies about 95% of the cycle.

The length of the cell cycle varies greatly, ranging from cells that divide rapidly and continuously to cells that do not divide at all.

DNA Replication

DNA is replicated by a semiconservative mechanism in which the two strands of the double helix unwind and each strand serves as a template for the synthesis of a complementary strand.

Bacterial chromosome replication is typically initiated at a single point and moves in both directions around a circular DNA molecule. In contrast, eukaryotes initiate DNA replication at multiple replicons, and replication proceeds bidirectionally at each replicon. A licensing mechanism involving the binding of MCM proteins to replication origins allows eukaryotes to ensure that DNA is replicated only once prior to each mitosis.

During replication, DNA polymerases synthesize DNA in the direction. Synthesis is continuous along the leading strand but discontinuous along the lagging strand, generating small Okazaki fragments that are subsequently joined by DNA ligase.

DNA Replication

DNA replication is initiated by primase, which synthesizes short RNA primers that are later removed and replaced with DNA. During replication, the double helix is unwound through the combined action of DNA helicases, topoisomerases, and single-stranded DNA binding proteins. As replication proceeds, a proofreading mechanism based on the exonuclease activity of DNA polymerase allows incorrectly base-paired nucleotides to be removed and replaced.

The ends of linear chromosomal DNA molecules are synthesized by telomerase, an enzyme that uses an RNA template for creating short, repeated DNA sequences at the ends of each chromosome.

DNA Damage and Repair

DNA damage arises both spontaneously and

through the action of mutagenic chemicals and radiation.

Some forms of DNA polymerase carry out translesion synthesis of new DNA across regions where the template DNA is damaged.

Excision repair is used to correct DNA damage involving abnormal bases, whereas mismatch repair fixes improperly base-paired nucleotides. Nonhomologous end-joining and homologous recombination are used to repair double-strand DNA breaks.

Nuclear and Cell Division

Mitosis is subdivided into prophase, prometaphase,

metaphase, anaphase, and telophase. During prophase, replicated chromosomes condense into paired sister chromatids while centrosomes initiate assembly of the mitotic spindle. In prometaphase, the nuclear envelope breaks down and chromosomes then attach to spindle microtubules and move to the spindle equator, where they line up at metaphase. At anaphase, sister chromatids separate and the resulting daughter chromosomes move toward opposite spindle poles. During telophase the chromosomes decondense, and a nuclear envelope reassembles around each daughter nucleus.

Chromosome movements are driven by three groups of motor proteins. Motor proteins located at the kinetochores and spindle poles move chromosomes toward the spindle poles, accompanied by disassembly of the microtubules at their plus and minus ends. Motor proteins that crosslink the polar microtubules move overlapping microtubules in opposite directions, thereby pushing the spindle poles apart. The final group of motor proteins move astral microtubules toward the plasma membrane, thereby pulling the spindle poles apart.

Cytokinesis usually begins before mitosis is

complete. In animal cells, an actomyosin

filament network forms a cleavage furrow

that constricts the cell at the midline and

separates the cytoplasm into two

daughter cells. In plant cells, a cell wall

forms through the middle of the dividing

cell.

Regulation of the Cell Cycle

Progression through the eukaryotic cell cycle is regulated by Cdk-cyclin complexes.

At the restriction point (Start in yeast), a Cdk-cyclin complex catalyzes the phosphorylation of the Rb protein to trigger passage into S phase.

At the G2-M boundary, another Cdk-cyclin complex triggers entry into mitosis by catalyzing the phosphorylation of proteins that promote nuclear envelope breakdown, chromosome condensation, and spindle formation.

At the metaphase-anaphase boundary, activation of the anaphase-promoting complex triggers a protein degradation pathway that initiates chromatid separation and targets mitotic cyclin for breakdown. The resulting loss of mitotic Cdk activity leads to events associated with the exit from mitosis, including cytokinesis, chromatin decondensation, and reassembly of the nuclear envelope.

Checkpoint pathways monitor intracellular conditions and temporarily halt the cell cycle if conditions are not suitable for proceeding. The DNA replication checkpoint verifies that DNA synthesis has been completed before allowing the cell to exit from G2 and begin mitosis. DNA damage checkpoints involving the p53 protein halt the cell cycle at various points if DNA damage is detected. Finally, the mitotic spindle checkpoint prevents chromosomes from moving to the spindle poles before all chromosomes are attached to the spindle.

Growth Factors and Cell

Proliferation

The cells of multicellular organisms do not normally proliferate unless they are stimulated by an appropriate growth factor.

Many growth factors bind to receptors that activate the Ras pathway, which culminates in passage through the restriction point and into S phase. Growth factor receptors also activate the P1 3-kinase–Akt pathway, which promotes cell survival and proliferation by phosphorylating and thereby activating target proteins that suppress apoptosis and inhibit cell cycle arrest.

Some growth factors inhibit (rather than stimulate) cell proliferation by triggering the production of Cdk inhibitors.

Apoptosis

Apoptosis is a form of cellular death triggered by

activation of death receptors, withdrawal of survival factors, or as a result of DNA damage.

Apoptosis involves the orderly dismantling of a dying cell’s contents.

Proteases called caspases are key mediators of apoptosis. Initiator caspases activate executioner caspases, which in turn activate other apoptosis proteins.

Initiator caspases can be activated in several ways, including through the release of cytochrome c from mitochondria. Pro- and anti-apoptotic proteins at the mitochondrion regulate release of cytochrome c.