Cell Cycle Overview

Cell Cycle

• Cell cycle is a series of events that lead to cell division, or a temporary resting stage (G0 phase).
• Two major periods comprise the cell cycle:
  • Interphase (interval between cell division)
  • M phase (mitosis, the period of cell division)

Interphase

• Interphase is a series of changes in a newly formed cell and its nucleus before it can divide again.
• It's also known as the preparatory phase or inter-mitosis.
• Interphase is divided into three phases:
  • G1 Phase:
    • The gap of time between mitosis (M phase) and DNA synthesis (S-phase).
    • RNA, protein, and organelle synthesis occurs in this phase.
    • In a typical mammalian cell with a 16-hour cell cycle, the G1 phase lasts about 5 hours.
    • A G1 checkpoint (Cdk2-cyclin D and Cdk2-cyclin E) regulates the G1 to S phase transition.
  • S Phase (synthesis):
    • DNA synthesis occurs.
    • In a typical mammalian cell with a 16-hour cell cycle, the S phase lasts about 7 hours.
  • G2 Phase:
    • The gap of time between DNA synthesis (S phase) and mitosis (M phase).
    • High levels of ATP synthesis occur in this phase.
    • In a typical mammalian cell with a 16-hour cell cycle, the G2 phase lasts about 3 hours.
    • A G2 checkpoint (Cdk1-cyclin A and Cdk1-cyclin B) regulates the G2 to M phase transition.

Checkpoints

• Checkpoints are specialized signaling mechanisms that control cell response to DNA damage and replication blockages.
• If DNA damage or replication blockage exceeds the threshold, a checkpoint signal is produced, triggering a checkpoint activation.
• Checkpoint activation slows down the cell cycle to allow DNA repair or recovery of blocked replication forks.
• Main protein families controlling the cell cycle: cyclins and cyclin-dependent protein kinases (Cdks).
• Cyclins regulate Cdk activity and undergo synthesis/degradation cycles.
• Cdk-cyclin complexes phosphorylate target proteins, controlled by the specific cyclin.

Mitosis

• Mitosis (M phase) is the process where a cell divides into two identical daughter cells with an identical number of chromosomes within them.
• The human diploid number of chromosomes is 46.
• Six stages:
  • Prophase: Chromatin condenses to form well-defined chromosomes. Each chromosome has been duplicated and has a centromere for proper segregation. The centrosome complex splits. The mitotic spindle forms.
  • Prometaphase: The nuclear envelope is disrupted, microtubules enter the area, and kinetochores (protein complexes) assemble on each chromosome.
  • Metaphase: Chromosomes align at the metaphase plate. Cells can be arrested for karyotype analysis.
  • Anaphase: Centromeres split, kinetochores separate, chromosomes move to opposite poles. Kinetochore microtubules shorten, polar microtubules lengthen.
  • Telophase: Chromosomes decondense to form chromatin. Nuclear envelope reforms. Nucleolus reappears. Kinetochore microtubules disappear. Polar microtubules lengthen.
  • Cytokinesis: The cytoplasm divides into two separate cells by a cleavage furrow, formed by a contractile ring of actin and myosin filaments.

Meiosis

• Meiosis is a specialized cell division that produces gametes (sex cells).

• Meiosis consists of two divisions: meiosis I and meiosis II.

• Only one round of DNA replication.

• Produces four unique haploid gametes each with 23 single chromosomes (half the number of normal chromosomes.

• Meiosis I:

  • Synapsis: Homologous chromosomes pair with their homologue, occurring only in meiosis, not mitosis or meiosis II.
  • Crossover/Recombination: Exchange of large segments of DNA between maternal and paternal chromatids at the chiasma during prophase I (pachytene stage). This introduces genetic variability.
  • Alignment: Homologous duplicated chromosomes align at the metaphase plate.
  • Disjunction: Homologous pairs separate from each other creating secondary gametocytes. The centromeres do not split
  • Cell division: Reduction division; the haploid number of chromosomes is formed

• Meiosis II:

  • Synapsis: Absent
  • Crossover: Absent
  • Alignment: 23 duplicated chromosomes align at the metaphase plate.
  • Disjunction: Sister chromatids separate from each other creating gametes. Centromeres split
  • Cell division: Gametes (23 single chromosomes) are formed

• Key Differences between mitosis and meiosis:

  • Mitosis produces two identical diploid daughter cells; meiosis produces four genetically diverse haploid gametes.
  • Mitosis has one cell division; meiosis has two cell divisions.
  • Mitosis has no crossing over or homologous chromosome pairing; meiosis involves both.

• Chromosome numbers & DNA amounts:

  • Gametes have 23 chromosomes and 1N DNA.
  • Somatic cells have 46 chromosomes and 2N DNA.