How does meiosis ensure that chromosome number is halved in gametes?

Meiosis ensures that the chromosome number is halved in gametes through two rounds of cell division, meiosis I and meiosis II.

Meiosis is a type of cell division that produces four daughter cells, each with half the number of chromosomes as the parent cell. This process is crucial for sexual reproduction, as it ensures that offspring inherit the correct number of chromosomes from each parent.

The process begins with a diploid cell, which contains two sets of chromosomes. In humans, for example, diploid cells have 46 chromosomes: 23 from the mother and 23 from the father. The first stage of meiosis, meiosis I, separates these homologous pairs of chromosomes. This is achieved through a process called recombination or crossing over, where sections of DNA are swapped between each pair of chromosomes. This results in two daughter cells, each with a mix of maternal and paternal chromosomes.

The second stage, meiosis II, is similar to mitosis (the process of cell division in body cells). The sister chromatids of each chromosome are separated, resulting in four haploid cells. These cells have only one set of chromosomes, so in humans, they would have 23 chromosomes each.

It's important to note that meiosis introduces genetic variation through the process of crossing over and independent assortment. During crossing over, sections of DNA are swapped between homologous chromosomes, creating new combinations of genes. Independent assortment refers to the random distribution of maternal and paternal chromosomes into the daughter cells. This ensures that each gamete (sperm or egg cell) is genetically unique.

In summary, meiosis ensures that the chromosome number is halved in gametes through two rounds of cell division. The first round separates homologous pairs of chromosomes, and the second round separates sister chromatids. This process results in four haploid cells, each with one set of chromosomes, ready for fertilisation.