Students should be able to explain how meiosis halves the numberof chromosomes in gametes and fertilisation restores the fullnumber of chromosomes

Sure, here's a step-by-step explanation:

1. Meiosis begins with a diploid cell, which contains two copies of each chromosome, one from the mother and one from the father. This cell undergoes DNA replication, resulting in two identical copies of each chromosome (sister chromatids).

2. The first division of meiosis (Meiosis I) separates the homologous chromosomes. This is the step that reduces the chromosome number from diploid to haploid. Each resulting cell now has one of each pair of homologous chromosomes, but each chromosome still has its two sister chromatids.

3. The second division of meiosis (Meiosis II) separates the sister chromatids. Each of the four cells resulting from meiosis II has one copy of each chromosome.

4. When gametes (sperm and egg cells) are formed, they are haploid, meaning they have half the number of chromosomes as the original cell. This is important because when fertilization occurs, the sperm and egg combine to form a zygote, which is diploid. This restores the full number of chromosomes.

5. The zygote then undergoes mitosis (cell division) and develops into an organism, with each cell in the organism containing the full number of chromosomes.

This process ensures that the number of chromosomes is maintained from generation to generation.