Passage of Information from Parents to Offspring

This section focuses on the crucial process of meiosis and the need for reduction division during gamete formation. Understanding meiosis is fundamental to comprehending how genetic information is passed from one generation to the next, ensuring genetic diversity and maintaining a stable chromosome number.

The Need for Reduction Division in Meiosis

Sexual reproduction involves the fusion of two gametes (sperm and egg), each containing half the number of chromosomes as a somatic (body) cell. If gametes had the same number of chromosomes as somatic cells, the resulting offspring would have double the normal chromosome number, which is not viable.

Meiosis is a specialized type of cell division that occurs in the germline (cells that give rise to gametes). It involves two rounds of division (Meiosis I and Meiosis II) and results in the production of four genetically distinct haploid daughter cells (gametes) from a single diploid parent cell.

Why is Reduction Division Necessary?

• Maintaining Chromosome Number: Diploid organisms (like humans) have two sets of chromosomes – one inherited from each parent. Gametes must be haploid (containing only one set of chromosomes) to prevent the chromosome number from doubling during fertilization.
• Genetic Diversity: Meiosis introduces genetic variation through processes like crossing over and independent assortment, which are essential for evolution and adaptation.

Meiosis I: Separating Homologous Chromosomes

Meiosis I is the reductional division. It's characterized by the separation of homologous chromosomes. Homologous chromosomes are pairs of chromosomes, one inherited from the mother and one from the father, that carry genes for the same traits.

Prophase I: Chromosomes condense, and homologous chromosomes pair up in a process called synapsis, forming a tetrad (a structure with four chromatids). Crossing over occurs during this phase, where homologous chromosomes exchange genetic material.

Metaphase I: Tetrads line up at the metaphase plate.

Anaphase I: Homologous chromosomes are separated and pulled towards opposite poles of the cell. This is where the chromosome number is reduced from diploid to haploid.

Telophase I and Cytokinesis: The cell divides into two haploid daughter cells. Each daughter cell contains one chromosome from each homologous pair.

Meiosis II: Separating Sister Chromatids

Meiosis II is very similar to mitosis. Each daughter cell from Meiosis I undergoes mitosis, where sister chromatids (identical copies of a chromosome) are separated.

Prophase II: Chromosomes condense again.

Metaphase II: Chromosomes line up at the metaphase plate.

Anaphase II: Sister chromatids are separated and pulled towards opposite poles.

Telophase II and Cytokinesis: The cells divide, resulting in four haploid daughter cells (gametes).

Phase	Event
Prophase I	Chromosomes condense, synapsis, crossing over
Metaphase I	Tetrads align at the metaphase plate
Anaphase I	Homologous chromosomes separate
Telophase I & Cytokinesis	Two haploid daughter cells
Prophase II	Chromosomes condense
Metaphase II	Chromosomes align at the metaphase plate
Anaphase II	Sister chromatids separate
Telophase II & Cytokinesis	Four haploid daughter cells (gametes)

The result of meiosis is four genetically unique gametes, each with a haploid number of chromosomes. These gametes are ready for fertilization, where they fuse with another gamete to restore the diploid chromosome number in the offspring.