Cell Cycle

1. The Cell Cycle (Big Picture)

The cell cycle has 2 main parts:

  1. Interphase (G1, S, G2) → cell lives & prepares

  2. Mitosis → cell divides into 2 identical cells

Purpose of mitosis:
➡ growth, maintenance, and repair

2. Interphase (Longest Stage)

Most cells are in interphase because it takes the longest.

G1 Phase

  • Cell grows

  • Does normal functions

  • Checkpoint: decides whether to divide

S Phase

  • DNA is copied

  • Each chromosome becomes two sister chromatids

  • Sister chromatids are identical and joined at the centromere

  • Human cell:

    • Before S: 46 chromosomes

    • After S: 92 chromatids

G2 Phase

  • Cell checks DNA for mistakes

  • Repairs errors before division

3. Mitosis (Division of the Nucleus)

Mitosis has 4 stages: PMAT

Prophase

  • DNA condenses into chromosomes

  • Sister chromatids still attached

  • Nuclear membrane dissolves

Metaphase

  • Chromosomes line up at the equator

  • Spindle fibers attach to centromeres

Anaphase

  • Sister chromatids separate

  • Pulled to opposite ends of the cell

Telophase

  • Nuclear membranes reform

  • Chromosomes uncoil into chromatin

4. Cytokinesis (Division of Cytoplasm)

  • Final step of cell division

Animal cells:
➡ membrane pinches inward

Plant cells:
➡ cell plate forms → becomes a new cell wall

Result:
➡ 2 identical diploid daughter cells

5. When Things Go Wrong (Mutations)

  • DNA mistakes = mutations

  • Usually fixed in G2

  • If centromeres don’t separate properly in anaphase:

    • One cell gets extra chromosome

    • Other is missing one

  • Severe mutations can lead to cancer

6. Diploid vs HaploidDiploid (2n)

  • Full set of chromosomes

  • Humans: 46 chromosomes

  • All somatic (body) cells

Haploid (n)

  • Half the chromosomes

  • Humans: 23 chromosomes

  • Gametes (sperm & egg)

7. Meiosis (Making Gametes)

Purpose:
➡ produce haploid, genetically unique cells

Key differences from mitosis:

  • Two divisions

  • Cells are not identical

  • Results in 4 haploid cells

8. Meiosis I (Reduces Chromosome Number)Prophase I

  • Chromosomes pair with their homologous partner

  • Crossing over occurs

    • DNA is exchanged

    • Increases genetic variation

Metaphase I

  • Homologous pairs line up at equator

  • Independent assortment occurs

    • Pairs line up randomly

Anaphase I

  • Homologous chromosomes separate

  • Centromeres do NOT split

Telophase I

  • 2 haploid cells

  • Chromosomes still have two chromatids

9. Meiosis II (Like Mitosis)Prophase II

  • Nuclear membrane dissolves

Metaphase II

  • Chromosomes line up individually

Anaphase II

  • Sister chromatids separate

Telophase II + Cytokinesis

  • 4 haploid cells

  • Each has 23 chromosomes

  • All are genetically different

10. Genetic Variation (Why We’re All Different)

Variation comes from:

  1. Crossing over (Prophase I)

  2. Independent assortment (Metaphase I)

Possible chromosome combinations:
➡ 8,324,608 (not even counting crossing over)

11. Zygote

  • Formed when sperm + egg combine

  • First diploid cell

  • Grows into a baby