Cell Cycle and Cell Division Notes

Cell Division and Reproduction

  • In unicellular organisms, cell division reproduces the entire organism.

  • Multicellular organisms rely on cell division for:

    • Development from a fertilized cell

    • Growth

    • Repair

  • Cell division is integral to the cell cycle: the life of a cell from formation to division.

Bacterial Cell Division

  • Bacteria divide via binary fission.

  • A single, circular bacterial chromosome replicates.

  • Replication starts at the origin and proceeds bidirectionally to termination.

  • New chromosomes are partitioned to opposite ends.

  • A septum forms to divide the cell into two.

Eukaryotic Chromosomes

  • The genome comprises all of a cell's DNA.

  • Eukaryotic genomes consist of multiple DNA molecules, packaged into chromosomes.

  • Humans have 46 chromosomes in 23 pairs.

  • Chromosomes consist of DNA wrapped around histone proteins, forming nucleosomes.

  • Histones are positively charged and bind to negatively charged DNA phosphate groups.

  • Nucleosomes coil into solenoids forming chromatin in interphase cells.

  • During mitosis, chromatin condenses via protein scaffolds.

Karyotype

  • A karyotype is an array of an organism's chromosomes, arranged by size and properties.

  • Humans are diploid, possessing two sets of homologous chromosomes (46 total).

Cell Cycle Phases

  • The cell cycle includes:

    • Mitotic (M) phase: mitosis and cytokinesis.

    • Interphase: cell growth and chromosome copying in preparation for division.

Interphase

  • Interphase (90% of cell cycle) includes:

    • G1 phase ("first gap")

    • S phase ("synthesis")

    • G2 phase ("second gap")

  • The cell grows in all phases; chromosomes duplicate only in the S phase.

M Phase: Mitosis

  • Mitosis comprises 5 phases:

    • Prophase

    • Prometaphase

    • Metaphase

    • Anaphase

    • Telophase

Mitosis Stages

  • Prophase: Chromosomes condense, the spindle apparatus assembles from microtubules, and the nuclear envelope breaks down.

  • Prometaphase: Microtubules attach to chromosomes at kinetochores, and chromosomes move toward the cell center.

  • Metaphase: Chromosomes align along the metaphase plate.

  • Anaphase: Centromeres split, cohesin proteins are removed, and sister chromatids are pulled to opposite poles.

  • Telophase: The spindle disassembles, nuclear envelopes form around each set of chromosomes, and chromosomes uncoil.

Cytokinesis

  • Cytokinesis is the division of the cytoplasm.

  • In animal cells, actin filaments constrict to form a cleavage furrow.

  • In plant cells, a cell plate forms between nuclei.

Cell Cycle Control

  • Cell cycle has two irreversible points:

    • Replication of genetic material (DNA)

    • Separation of sister chromatids

  • Checkpoints halt the cell cycle to check for accuracy and respond to signals.

Key Checkpoints

  • G1/S checkpoint: The cell decides to divide; influenced by external signals.

  • G2/M checkpoint: The cell commits to mitosis and assesses DNA replication success.

  • Spindle checkpoint: Occurs in late metaphase, ensures all chromosomes are attached to the spindle.

Cyclin-Dependent Kinases (Cdks)

  • Cdks are enzymes that phosphorylate proteins, controlling the cell cycle.

  • Cdks partner with different cyclins at different stages.

Cancer and Cell Cycle Disruption

  • Cancer involves unrestrained cell growth due to cell cycle control failure.

  • Mutations in tumor-suppressor genes and proto-oncogenes can disrupt the cycle.

Tumor-Suppressor Genes

  • p53 plays a key role in the G1 checkpoint by monitoring DNA integrity.

  • If DNA is damaged, p53 halts division and stimulates repair enzymes; if irreparable, it triggers apoptosis.

  • p53 is often absent or damaged in cancerous cells.

Proto-oncogenes

  • Normal cellular genes that can mutate into oncogenes, causing cancer.

  • Only one copy of a proto-oncogene needs to mutate for uncontrolled division.

  • Some encode receptors for growth factors, overriding cell cycle checkpoints.