Cell Cycle and Checkpoints - Quick Reference

  • DNA as Genetic Material
    • DNA stores genetic information in nucleotide sequences; most biological traits are coded by DNA.
  • DNA Packaging and Chromatin
    • DNA + histone proteins form chromatin; during interphase, chromatin is dispersed inside the nucleus.
    • Chromatin condenses into chromosomes during cell division; a chromosome has two chromatids (sister chromatids) when replicated.
  • Chromosomes in Humans
    • Humans have 46 chromosomes (23 pairs): autosomes 22 pairs and sex chromosomes (2).
    • Males: XY; Females: XX.
  • The Cell Cycle: Overview
    • Three main stages: Interphase, M phase, Cytokinesis.
    • Interphase includes three subphases: Gap 1 (G1), Synthesis (S), Gap 2 (G2).
    • M phase includes Mitosis or Meiosis; Cytokinesis is cytoplasm division.
  • Interphase Details
    • G1 (Gap 1)
    • Produce proteins/enzymes, nutrients, and energy.
    • Growth of cytoplasm and doubling of organelles; centrioles move to the centrosomes.
    • S (Synthesis)
    • DNA replication occurs; semiconservative replication.
    • Result: two identical DNA molecules, each with one original strand and one new strand.
    • G2 (Gap 2)
    • Continued growth; production of materials for division; protein synthesis continues but not as high as in G1.
  • DNA Replication: Semi-Conservative
    • DNA replication is semi-conservative: each daughter DNA molecule contains one original strand and one newly synthesized strand.
  • M Phase and Cytokinesis
    • M phase: mitosis or meiosis (mitosis for somatic cells; meiosis for gametes).
    • Cytokinesis: division of cytoplasm; cleavage furrow in animal cells; cell plate in plant cells.
  • Checkpoints: Purpose and Location
    • Checkpoints prevent progression if errors or unfavorable conditions exist; repair attempts occur, or apoptosis may ensue.
    • Main checkpoints: G1-to-S (G1 checkpoint), G2-to-M (G2 checkpoint), Metaphase (M) checkpoint.
    • G1-to-S and G2-to-M occur during interphase; Metaphase checkpoint occurs during mitosis/meiosis.
  • G1-to-S Checkpoint (G1 checkpoint)
    • Checks: cell size and energy reserves; availability of doubled organelles; absence of DNA damage.
    • If not ready, cell cannot proceed to S phase and may enter G0
    • G0: resting-like state; some cells (e.g., cardiac and nerve cells) stay permanently in G0.
  • G2-to-M Checkpoint (G2 checkpoint)
    • Checks: DNA damage; whether DNA replication is complete; environmental conditions favorable for division.
    • If conditions are good, cell proceeds to mitosis or meiosis.
  • Metaphase Checkpoint (M checkpoint)
    • Checks: spindle attachment to all chromosomes (kinetochores) before anaphase.
    • If all spindles are properly attached, proceed to anaphase; otherwise, mitosis pauses to fix errors.
  • G0 and Apoptosis
    • G0 represents a long-term resting state for cells not currently dividing.
    • If DNA damage is irreparable, cells may undergo apoptosis to prevent transmission of errors.
  • Quick Takeaways
    • The cell cycle drives growth, development, and reproduction of cells via Interphase, M phase, and Cytokinesis.
    • Interphase prepares the cell for division (G1, S, G2); DNA replication is semi-conservative.
    • Mitosis/meiosis distributes genetic material; Cytokinesis completes cell division.
    • Checkpoints (G1-to-S, G2-to-M, M) ensure proper size, DNA integrity, and spindle attachment; failures can halt the cycle or trigger apoptosis.