CH 2: CELL DIVISION & PUNNET

What is the purpose of mitosis?

Growth, repair, and maintenance of diploid cells.

What is the purpose of meiosis?

To produce haploid gametes from diploid precursors.

Do bacteria undergo mitosis? Why or why not?

No. They undergo binary fission because they lack spindle fibers.

What do spindle fibers do in eukaryotic cell division?

Attach to centromeres via kinetochore proteins and move chromosomes into position.

What is bi-orientation in mitosis?

When spindle fibers pull chromosomes toward the center, creating tensional equilibrium (bipolarity).

What is disjunction?

Proper separation of sister chromatids during metaphase

What is nondisjunction?

Failure of spindle fibers to attach properly, leading to unequal chromosome distribution.

What are the two main phases of the cell cycle?

Interphase and M Phase (mitosis).

What happens in G1 phase?

Cell grows, organelles produced, and division enzymes made

What happens in S phase?

Centromere duplicates first, then p & q arms, then centrosomes

What happens in G2 phase?

More growth, energy production, and enzyme synthesis.

What is the G1 checkpoint, when does it occur, and what regulates it

Occurs after G1; ensures cell is healthy and ready for division. Passage mediated by cyclins and kinases.

What is the S checkpoint?

Ensures DNA replication forks are stable.

What is the G2 checkpoint?

Ensures DNA was replicated properly during S phase.

What is the M checkpoint? What happens if checkpoints are ignored?

Ensures spindle fibers are properly attached for disjunction.

If check points are ignored: Unregulated cell growth, which can lead to cancer.

What happens in each stage of mitosis?

• Prophase: Chromosomes condense, nuclear envelope breaks down, spindle forms.

• Prometaphase: Spindles attach to kinetochores.

• Metaphase: Chromosomes align at cell center (M checkpoint).

• Anaphase: Sister chromatids separate to opposite poles.

• Telophase: Nuclear membranes reform around chromosomes.

• Cytokinesis: Cytoplasm divides → two identical daughter cells (cleavage furrow in animals, cell plate in plants).

How does cell division differ between mitosis and meiosis in humans?

• Mitosis:

  • 2n = 46 chromosomes (somatic cells).

  • After S phase: 46 chromosomes → 46 pairs of sister chromatids (92 chromatids).

  • Sister chromatids separate → 2 identical diploid daughter cells (46 each).

• Meiosis:

  • 2n = 46 chromosomes.

  • Chromosomes pair into 23 tetrads.

  • Meiosis I: Homologous pairs separate → 2 haploid cells (23 each).

  • Meiosis II: Sister chromatids separate → 4 non-identical haploid gametes (23 each).

What is the purpose of meiosis?

To produce haploid gametes (n = 23) from diploid precursor cells (2n = 46).

What happens in Meiosis I? What are the phases and what do they do?

A: Homologous chromosomes form tetrads, synapse, cross over, and separate → diploid reduced to 2 haploid cells.

1. Prophase I: Homologs condense, synapse, cross over, spindle attaches.

2. Metaphase I: Tetrads align at cell center.

3. Anaphase I: Homologs (tetrads) separate into recombined sister chromatids.

4. Telophase I: Two haploid nuclei form; cytokinesis → 2 haploid cells.

5. Interkinesis: Resting stage (no S phase).

What happens in Meiosis II?

A: Sister chromatids separate → 4 haploid cells formed.

• Prophase II: Chromatids condense, spindle attaches.

• Metaphase II: Sister chromatids align at center.

• Anaphase II: Sister chromatids separate into daughter chromosomes.

• Telophase II: Nuclei reform; cytokinesis produces 4 haploid gametes.

: What’s the difference in meiosis between males and females?

• Males: All 4 haploid cells become gametes (sperm).

• Females: 3 cells degenerate, leaving 1 large oocyte.

When and how does crossing-over occur in meiosis?

Crossing-over occurs in Prophase I after the enzyme SPO11 creates double-stranded DNA breaks, leading to allelic changes between homologous chromosomes.