12.5 Eukaryotic Cell Reproduction & Stages of Meiosis

12.5 Eukaryotic Cell Reproduction & Stages of Meiosis

Quick Answer: The cell cycle alternates interphase (G1 → S → G2) with the M phase (mitosis + cytokinesis). Mitosis produces two genetically identical diploid somatic cells. Meiosis is two divisions—meiosis I (separates homologous chromosomes) and meiosis II (separates sister chromatids)—and produces four genetically distinct haploid gametes. Variation comes from crossing over in prophase I, independent assortment in metaphase I, and random fertilization.

The Cell Cycle

A dividing eukaryotic cell cycles through:

1. G1 (Gap 1) — cell grows; synthesizes proteins and organelles.
2. S (Synthesis) — DNA replicated; centrosome duplicated.
3. G2 (Gap 2) — more growth; checks for DNA damage before mitosis.
4. M phase — mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Non-dividing cells exit to G0, where they perform their function without proliferating (e.g., neurons). The cycle is governed by cyclin-CDK complexes: cyclin levels oscillate while CDKs are constant; binding triggers phosphorylation that drives transitions (G1/S, G2/M). Checkpoints at G1, G2, and the spindle assembly checkpoint (M) halt the cycle if DNA is damaged or chromosomes are misattached. The tumor suppressor p53 mediates G1 arrest in response to DNA damage, triggering either repair or apoptosis; loss of checkpoint control is a hallmark of cancer, which is why cell-cycle regulation appears in medical-relevant PA-CAT items.

Mitosis

Cytokinesis overlaps anaphase/telophase: an actin-myosin contractile ring pinches the membrane (animals) or a cell plate forms (plants). The result is two identical diploid cells.

Meiosis I: Reductional Division

Meiosis I halves the chromosome number by separating homologous chromosomes.

• Prophase I (sub-stages leptotene, zygotene, pachytene, diplotene, diakinesis): homologs pair (synapsis) forming a tetrad (bivalent) of four chromatids; crossing over exchanges segments between non-sister chromatids at chiasmata, generating recombinant chromosomes.
• Metaphase I: homologous pairs align at the metaphase plate; independent assortment means either maternal or paternal homolog can face either pole—with 23 pairs in humans this yields 2²³ (~8.4 million) combinations.
• Anaphase I: homologs separate; sister chromatids remain attached.
• Telophase I / Cytokinesis: two haploid cells, each with one set of duplicated chromosomes (still as sister chromatids).

Meiosis II: Equational Division

Meiosis II resembles mitosis but starts in haploid cells:

• Prophase II: chromosomes recondense in each haploid cell.
• Metaphase II: individual chromosomes align at the metaphase plate.
• Anaphase II: sister chromatids separate.
• Telophase II / Cytokinesis: four haploid gametes, each with one chromatid per chromosome.

Sources of Genetic Variation

1. Crossing over (prophase I) — recombination creates chromosomes with new allele combinations not present in either parent.
2. Independent assortment (metaphase I) — random orientation of homologous pairs multiplies possible gamete genotypes.
3. Random fertilization — any of ~8.4 million sperm can fertilize any of ~8.4 million eggs, for ~70 trillion diploid combinations before even counting recombination.

Mitosis vs Meiosis

Why This Matters for the PA-CAT

The PA-CAT Bulletin of Information (rev. 20240815) lists Cellular Reproduction within General Biology. Expect questions that (a) order the stages of mitosis and meiosis and identify the defining event of each, (b) distinguish reductional (meiosis I) from equational (meiosis II) divisions, (c) compute possible chromosome combinations from independent assortment (2ⁿ), and (d) identify the stage at which crossing over occurs (prophase I, specifically pachytene). A common distractor is attributing crossing over to metaphase I; it occurs in prophase I, while metaphase I is where independent assortment happens.

Nondisjunction: A Worked Chromosome-Count Example

Nondisjunction is the failure of homologs (meiosis I) or sister chromatids (meiosis II) to separate, producing gametes with abnormal chromosome counts. The PA-CAT may give you a diploid number and ask for the chromosome count in each resulting gamete. Work through it for 2n = 6.

Normal meiosis I separates the three homologous pairs, and meiosis II separates sister chromatids, yielding four gametes with n = 3. If nondisjunction occurs in meiosis I (one homologous pair fails to separate), two gametes receive both homologs (n+1 = 4) and two receive neither (n-1 = 2); all four gametes are affected. If nondisjunction occurs in meiosis II (one sister chromatid pair fails to separate in one of the two cells), two gametes are normal (n = 3), one has n+1 = 4, and one has n-1 = 2; only two of four gametes are abnormal, a key distinguishing feature.

Fertilization by a normal gamete (n = 3) then produces a zygote. An n+1 gamete yields 2n+1 = 7 (trisomy); an n-1 gamete yields 2n-1 = 5 (monosomy). In humans, the classic example is trisomy 21 (Down syndrome): nondisjunction produces a gamete with two copies of chromosome 21, and fertilization gives three copies in the zygote.