17-Meiosis: Mechanisms, Chromosomal Basis, and Genetic Variation

Meiosis: Introduction and Overview

Definition and Purpose

Meiosis is a specialized form of cell division that produces gametes (sperm or egg cells) with half the number of chromosomes as the parent cell. This reduction is essential for sexual reproduction, ensuring that offspring inherit the correct chromosome number.

• Diploid (2n) cells undergo meiosis to produce haploid (n) gametes.

• Meiosis consists of two consecutive divisions: Meiosis I and Meiosis II.

• Occurs only in gonads (testes in males, ovaries in females).

• Male gamete production: spermatogenesis; Female gamete production: oogenesis.

• Meiosis shares similarities with mitosis but includes unique chromosomal events.

Example: Human Spermatogenesis

In humans, a diploid cell (2n = 46) undergoes meiosis to produce four haploid sperm cells (n = 23).

Chromosomal Basis of Meiosis

Interphase I

Before meiosis begins, the cell undergoes interphase, similar to mitosis.

• Chromosomes replicate during the S phase.

• Each duplicated chromosome consists of two identical sister chromatids joined at the centromere.

• Centriole pairs also replicate.

• The nucleus and nucleolus are visible; chromatin is dispersed.

Meiosis I: Reduction Division

Meiosis I reduces the chromosome number by half and consists of four phases:

1. Prophase I

2. Metaphase I

3. Anaphase I

4. Telophase I

Prophase I

• Longest and most complex phase (about 90% of meiosis I).

• Chromosomes condense and become visible.

• Synapsis occurs: homologous chromosomes pair up to form a tetrad (two chromosomes, four chromatids).

• Crossing over may occur between nonsister chromatids at the chiasmata, increasing genetic variation.

Homologous Chromosomes

• Pairs of chromosomes (one maternal, one paternal) with similar shape and size.

• Homologous pairs (tetrads) carry genes for the same inherited traits at the same locus (gene position).

• Humans have 23 pairs of homologous chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes.

Karyotype

A karyotype is a method of organizing the chromosomes of a cell by number, size, and type. It is used to identify chromosomal abnormalities.

Example: Down Syndrome (Trisomy 21)

Individuals with trisomy 21 have an extra copy of chromosome 21, resulting in Down syndrome.

Genetic Variation in Meiosis

Crossing Over

• Occurs during prophase I at the chiasmata.

• Segments of nonsister chromatids exchange genetic material, producing new allele combinations.

• Increases genetic diversity among gametes.

Independent Assortment

• Occurs during metaphase I as tetrads align randomly on the metaphase plate.

• Orientation of homologous pairs to poles is random, leading to variation in gametes.

• Number of possible combinations: $2^n$ (where n = haploid number).

• Example: For humans, $n = 23$, so $2^{23} ext{ (about 8 million combinations)}$.

Random Fertilization

• Any sperm can fertilize any egg, further increasing genetic variation.

Phases of Meiosis I

Metaphase I

• Shortest phase.

• Tetrads align on the metaphase plate.

• Independent assortment occurs.

Anaphase I

• Homologous chromosomes separate and move toward opposite poles.

• Sister chromatids remain attached at their centromeres.

Telophase I

• Each pole has a haploid set of chromosomes.

• Cytokinesis occurs, forming two haploid daughter cells.

Meiosis II: Equational Division

Overview

Meiosis II is similar to mitosis but occurs in haploid cells. There is no interphase II or only a very short one (no DNA replication).

Phases of Meiosis II

• Prophase II: Chromosomes condense; spindle forms.

• Metaphase II: Chromosomes align on the metaphase plate.

• Anaphase II: Sister chromatids separate and move to opposite poles.

• Telophase II: Nuclei reform; cytokinesis produces four haploid daughter cells.

Result of Meiosis

• Four genetically unique haploid gametes are produced from one diploid parent cell.

• Each gamete contains half the chromosome number of the original cell.

Summary Table: Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetic variation	None (identical)	High (crossing over, independent assortment)
Role	Growth, repair	Sexual reproduction

Genetic Variation: Importance and Sources

• Variation is essential for natural selection and evolution.

• Three sexual sources of genetic variation:

  1. Crossing over (prophase I)

  2. Independent assortment (metaphase I)

  3. Random fertilization

Example Calculation

How many different combinations of sperm could a human male produce?

• Formula: $2^n$

• Human chromosomes: $2n = 46$, $n = 23$

• $2^{23} ext{ = ~8 million combinations}$

Example Question

If a cell contains 20 chromosomes (diploid) at the beginning of meiosis, how many chromosomes will each resulting cell have?

• Answer: 10 chromosomes (haploid)

Key Terms

• Meiosis: Cell division producing gametes with half the chromosome number.

• Gamete: Haploid sex cell (sperm or egg).

• Diploid (2n): Cell with two sets of chromosomes.

• Haploid (n): Cell with one set of chromosomes.

• Homologous chromosomes: Chromosome pairs with genes for the same traits.

• Tetrad: Structure of four chromatids during synapsis.

• Crossing over: Exchange of genetic material between nonsister chromatids.

• Independent assortment: Random orientation of homologous pairs during metaphase I.

• Cytokinesis: Division of the cytoplasm.

• Karyotype: Chromosome arrangement by number, size, and type.

• Autosome: Non-sex chromosome.

• Sex chromosome: Chromosome determining sex (X or Y).

• Chiasma: Site of crossing over.

Additional info:

• Meiosis is fundamental for maintaining chromosome number across generations and for generating genetic diversity in sexually reproducing populations.

• Errors in meiosis can lead to chromosomal disorders, such as Down syndrome (trisomy 21).