Meiosis - SS1 Biology Lesson Note

Meiosis is a specialized type of cell division that occurs in reproductive cells (germ cells) to produce gametes (sex cells), such as sperm and eggs, in sexually reproducing organisms. Unlike mitosis, which produces genetically identical daughter cells, meiosis results in the formation of cells with half the number of chromosomes as the parent cell. This reduction in chromosome number is essential for sexual reproduction and ensures that the offspring receive a combination of genetic material from both parents.

Meiosis consists of two successive divisions, called meiosis I and meiosis II, each composed of several stages:

1.     Prophase I: Prophase I is the longest and most complex phase of meiosis. It can be further divided into several sub-stages:

§  Leptotene: Chromosomes start to condense and become visible under a microscope.

§  Zygotene: Homologous chromosomes pair up and undergo a process called synapsis. Each pair of homologous chromosomes forms a structure called a bivalent or tetrad.

§  Pachytene: Crossing over occurs, where non-sister chromatids of homologous chromosomes exchange genetic material, leading to genetic recombination.

§  Diplotene: Homologous chromosomes begin to separate but remain attached at points called chiasmata. The chromosomes start to decondense.

§  Diakinesis: Chromosomes further condense, and the nuclear envelope disassembles. The spindle apparatus begins to form.

2.     Metaphase I: Homologous chromosome pairs align along the equator of the cell, forming a metaphase plate. Spindle fibers from opposite poles attach to the homologous chromosomes.

3.     Anaphase I: Homologous chromosomes separate and are pulled toward opposite poles of the cell. Sister chromatids of each chromosome remain attached.

4.     Telophase I: Chromosomes reach the opposite poles of the cell. The nuclear envelopes start to reform, and the cell undergoes cytokinesis, resulting in two haploid cells.

5.     Interkinesis: In some organisms, there is a brief period of rest between meiosis I and meiosis II. During interkinesis, the nuclear envelopes may reform, and the chromosomes may briefly decondense.

6.     Prophase II: Chromosomes recondense, and the nuclear envelopes disassemble again. The spindle apparatus forms.

7.     Metaphase II: Chromosomes align along the equator of each cell.

8.     Anaphase II: Sister chromatids separate and move towards opposite poles of each cell.

9.     Telophase II: Chromosomes reach the opposite poles, and the nuclear envelopes start to reform. Cytokinesis follows, resulting in the formation of four haploid daughter cells.

The resulting four daughter cells are genetically distinct from each other and the parent cell. They are haploid, meaning they contain half the number of chromosomes as the parent cell. In animals, these haploid cells develop into mature gametes (sperm or eggs), which, upon fertilization, combine with a gamete from the other parent to restore the diploid chromosome number in the offspring.

Meiosis ensures genetic diversity by introducing genetic variation through crossing over and independent assortment of chromosomes. These mechanisms contribute to the production of genetically unique offspring and play a crucial role in evolution and adaptation.

Overall, meiosis is a vital process for sexual reproduction, generating genetically diverse gametes, and maintaining the stability of chromosome numbers across generations.