Meiosis and Sexual Life Cycles

Core Concept

Meiosis is a special type of cell division that halves the chromosome number, producing four haploid gametes (or spores) from one diploid cell. It achieves this with two rounds of division after a single round of DNA replication: meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids. Because it shuffles and recombines chromosomes, meiosis is the chief source of genetic variation in sexually reproducing organisms.

Key Concepts

Haploid vs. diploid; homologous chromosomes

A diploid (2n) cell has two complete sets of chromosomes — one inherited from each parent. A haploid (n) cell has one set. In a diploid cell, the two chromosomes of a pair are homologous chromosomes (homologs): same length, same centromere position, same genes in the same order, but possibly different alleles — one from each parent. Humans are 2n = 46 (23 pairs); gametes are n = 23.

The sexual life cycle

In a sexual life cycle, meiosis reduces 2n → n (making gametes), and fertilization restores n + n → 2n (making a zygote). The alternation of meiosis and fertilization keeps the chromosome number constant across generations. (Some organisms, like fungi, have other variations, but the core logic is the same.)

Meiosis I (reductional division)

After S phase duplicates the DNA, meiosis I separates homologous chromosomes. Prophase I is long and includes synapsis — homologous chromosomes pair tightly into a tetrad (four chromatids) — and crossing over, in which non-sister chromatids exchange corresponding DNA segments, producing recombinant chromosomes. In metaphase I, tetrads line up at the plate with homologs oriented randomly (the physical basis of independent assortment). In anaphase I, homologs are pulled to opposite poles (sister chromatids remain attached). Telophase I and cytokinesis yield two haploid cells, each with duplicated chromosomes (two chromatids each).

Meiosis II (equational division)

Meiosis II resembles mitosis but without a preceding S phase. In anaphase II, the sister chromatids separate and move to opposite poles. The result is four haploid cells, each with one copy of each chromosome.

Sources of variation

Three mechanisms make offspring genetically unique: (1) crossing over in prophase I creates new combinations of alleles on a chromosome; (2) independent assortment of homologous pairs in metaphase I produces 2ⁿ possible chromosome combinations (for humans, 2²³ ≈ 8.4 million from assortment alone); and (3) random fertilization — any sperm can fuse with any egg. No two gametes (and almost no two siblings) are alike.

How It Works

DNA replicates once (S phase), so each chromosome is a pair of sister chromatids. In meiosis I, homologs find each other and swap segments (crossing over), then the spindle pulls whole homologs apart, halving the chromosome number. In meiosis II, the sister chromatids separate, so each of the four products gets exactly one chromatid-derived chromosome per homolog. The key contrast with mitosis: mitosis does one division and yields two identical diploid cells; meiosis does two divisions and yields four genetically distinct haploid cells.

How it works

DNA replicates once (S phase), so each chromosome is a pair of sister chromatids. In meiosis I, homologs find each other and swap segments (crossing over), then the spindle pulls whole homologs apart, halving the chromosome number. In meiosis II, the sister chromatids separate, so each of the four products gets exactly one chromatid-derived chromosome per homolog. The key contrast with mitosis: mitosis does one division and yields two identical diploid cells; meiosis does two divisions and yields four genetically distinct haploid cells.

Common confusions

• "Meiosis II separates homologous chromosomes." Wrong — meiosis II separates sister chromatids; it is meiosis I that separates homologs.
• "Meiosis produces two diploid cells." Wrong — it produces four haploid cells.
• "DNA replicates between meiosis I and meiosis II." Wrong — replication happens only once, before meiosis I; there is no S phase before meiosis II.
• "Crossing over happens in mitosis too." Wrong — crossing over between homologs is a meiosis I (prophase I) event; sister chromatid exchange in mitosis is a rare, different phenomenon.
• "Homologous chromosomes are identical copies." Wrong — homologs carry the same genes but may carry different alleles; sister chromatids are the identical copies.

Quick review

• Meiosis halves chromosome number: 2n → n.
• Two divisions: meiosis I (homologs apart) and meiosis II (sister chromatids apart).
• Prophase I: synapsis (tetrads) and crossing over (recombination).
• Metaphase I: independent assortment of homolog pairs.
• Result: four genetically unique haploid gametes.
• Variation sources: crossing over, independent assortment, random fertilization.
• Nondisjunction → aneuploidy (trisomy 21 = Down syndrome).