Meiosis and Genetic Variation

Overview

Meiosis involves two rounds of nuclear division resulting in four haploid gametes from one diploid cell.
It's a reduction division; chromosome number is halved.
Genetic variation arises through crossing over and independent assortment.

Interphase: Chromosomes and organelles are copied; chromosomes are not visible.

Prophase I:
- Chromosomes condense and become visible.
- Homologous chromosomes pair up forming a bivalent.
- Crossing over of alleles can occur.
- Nuclear membrane breakdown and centrioles move to poles.
- Assembly of spindle apparatus begins.
Metaphase I:
- Homologous chromosome pairs line up on spindle equator.
Anaphase I:
- Spindle fibers shorten.
- Homologous chromosomes move to opposite poles after chiasmata break.
Telophase I:
- Chromosomes reach poles.
- Nuclear membranes reform.
- Chromosomes uncoil into chromatin.
- Cytokinesis splits the cell into two haploid cells.

Prophase II:
- Chromosomes condense and become visible again.
- Nuclear membrane breaks down.
- Development of spindle fibers.
Metaphase II:
- Chromosomes align on spindle equator.
Anaphase II:
- Centromeres divide.
- Chromatids pulled to opposite poles.
Telophase II:
- Chromatids reach poles and become chromosomes.
- Nuclear membranes reform.
- Chromosomes uncoil into chromatin.
- Cytokinesis results in four haploid cells.

Crossing Over:
- Occurs during Prophase I.
- Exchange of genetic material between homologous chromosomes.
Independent Assortment:
- Occurs during Metaphase I.
- Random orientation of homologous chromosomes leads to genetic diversity.
- Formula: 2^n, where n = number of chromosome pairs (e.g., humans: 2^23 > 8 million combinations).
Random Fertilization:
- Fusion of male and female gametes is random.
- Adds another layer of genetic diversity.

Meiosis produces genetically diverse gametes through crossing over, independent assortment, and random fertilization.
Key to understanding genetic variation in sexually reproducing organisms.