17.3 Chromosome Transmission During Cell Division & Sexual Reproduction

Chromosome Transmission During Cell Division & Sexual Reproduction

Quick Answer: Mitosis makes two identical diploid cells; meiosis makes four genetically distinct haploid gametes. The reductional division (meiosis I) separates homologs and is where crossing over and independent assortment occur. Fertilization fuses two haploid gametes to restore diploidy. Per the PA-CAT Bulletin of Information, rev. 20240815, chromosome transmission underlies the Patterns of Inheritance grouping.

The Cell Cycle and Its Checkpoints

The cell cycle has four phases: G1 (growth, decision to commit), S (DNA replication), G2 (growth and repair), and M (mitosis and cytokinesis). Quiescent cells exit to G0. Progression is governed by cyclin–CDK complexes: cyclin D–CDK4/6 drives G1 progression, cyclin E–CDK2 fires S phase, cyclin A–CDK1/2 runs S/G2, cyclin B–CDK1 triggers mitosis.

Three main checkpoints enforce order and fidelity:

• G1/S (Restriction point) — is the cell big enough, are growth factors present, is DNA undamaged? p53 can halt progression or trigger apoptosis.
• G2/M — is DNA fully and accurately replicated?
• Spindle assembly checkpoint (M) — are all kinetochores attached and under tension before anaphase?

p53 ("guardian of the genome") arrests the cycle at G1/S in response to DNA damage, inducing p21. Loss of p53 (found in ~50% of cancers) allows damaged cells to divide.

Mitosis Overview

Mitosis conserves chromosome number. One diploid cell (2n = 46) yields two diploid daughter cells, each genetically identical to the parent. Phases:

• Prophase — chromatin condenses into visible chromosomes; the nucleolus fades; the mitotic spindle (made of microtubules polymerized from α/β-tubulin) forms from the centrosomes.
• Prometaphase — nuclear envelope breaks down; spindle microtubules capture chromosomes at kinetochores.
• Metaphase — chromosomes align at the metaphase plate (equator); spindle assembly checkpoint satisfied.
• Anaphase — sister chromatids pulled to opposite poles (kinesin and dynein motor proteins).
• Telophase — nuclear envelopes reform around two sets of chromosomes; chromatin decondenses.
• Cytokinesis — actin–myosin contractile ring pinches the cytoplasm (animals) or cell plate forms (plants).

Meiosis: Two Divisions, One DNA Replication

Meiosis consists of meiosis I (reductional) and meiosis II (equational), with no DNA replication between them. A single pre-meiotic S phase replicates DNA, so each chromosome enters meiosis as two sister chromatids.

Prophase I substages

• Leptotene — chromosomes condense; homologs begin to pair.
• Zygotene — synapsis pairs homologs gene-by-gene; the synaptonemal complex (a protein zipper) forms between them, aligning them in perfect register.
• Pachytene — crossing over occurs at chiasmata; recombination nodules contain Spo11 (which makes double-strand breaks) and recombination machinery (Rad51/Dmc1) exchanges DNA between non-sister chromatids of homologs. Each crossover is reciprocal.
• Diplotene — synaptonemal complex dissolves; homologs partially separate but remain held at chiasmata (visible cytologically).
• Diakinesis — chiasmata terminalize; nucleolus and nuclear envelope break down; spindle forms.

Metaphase I: homologous pairs (bivalents/tetrads) align at the metaphase plate. Anaphase I: homologs segregate to opposite poles; sister chromatids remain together. Telophase I: two haploid cells, each chromosome still two chromatids.

Meiosis II proceeds like mitosis on the haploid cells: sister chromatids separate in anaphase II, yielding four haploid gametes.

Independent Assortment

At metaphase I, each homologous pair orients independently of every other pair — the maternal homolog of pair 1 may face either pole regardless of which pole pair 5's maternal homolog faces. For n pairs, there are 2^n possible orientations. With n = 23, that is 2^23 ≈ 8.4 million gamete combinations from assortment alone; crossing over multiplies this further, making every gamete effectively unique.

Gametogenesis

• Spermatogenesis: continuous from puberty in seminiferous tubules; one primary spermatocyte → four functional, equal-sized sperm. Spermatogonia (stem cells) → primary spermatocyte (2n, replicated) → meiosis I → two secondary spermatocytes (n) → meiosis II → four spermatids → differentiation (spermiogenesis) → mature sperm.
• Oogenesis: begins in fetal life; oogonia → primary oocytes arrest at diplotene of prophase I until puberty. Each cycle, one oocyte resumes; meiosis I completes at ovulation, producing a large secondary oocyte and a small first polar body. Meiosis II arrests at metaphase II and completes only if fertilized. Result: one functional ovum + polar bodies — cytoplasm is conserved to provision the zygote.

Fertilization

Fertilization of two haploid (n = 23) gametes restores the diploid (2n = 46) zygote. Random fertilization multiplies assortment diversity: ~8.4 million × 8.4 million ≈ 7 × 10¹³ zygote combinations, before even counting crossing over.

Nondisjunction and Aneuploidy

Failed chromosome segregation (nondisjunction) produces aneuploidy. Meiosis I nondisjunction (homologs fail to separate) yields gametes with both homologs (n+1) or neither (n−1); meiosis II nondisjunction (sister chromatids fail) yields gametes with both sister chromatids or none. The extra or missing chromosome causes dosage imbalance.

Clinical examples:

Maternal age raises nondisjunction risk (especially meiosis I errors), due to the decades-long arrest of oocytes in prophase I.

PA-CAT Application

The PA-CAT Bulletin of Information, rev. 20240815 groups chromosome transmission under Patterns of Inheritance. Expect items distinguishing reductional vs. equational division, computing gamete diversity from 2^n, identifying which meiotic stage a given event (crossing over = pachytene; sister-chromatid separation = anaphase II; homolog separation = anaphase I) occurs in, and recognizing nondisjunction outcomes from karyotypes.