Meiosis: Gamete formation

The objective of meiosis is to make four cells from a single somatic cell. The four cells each have half the chromosome number found in the somatic cell. In our human bodies, the four gametes will each have 23 chromosomes which means the 46 chromosomes in the somatic cell must replicate during interphase prior to meiosis just as they would before mitosis. Meiosis occurs in specialized cells of the body called germline cells.

To appreciate meiosis and gamete formation it is important to first understand two ideas, chromosome sets and homologous chromosomes.

Chromosome sets: The 46 chromosomes you have consist of two sets. You are a diploid organism ('di' means two and 'ploid' means sets). One set of chromosomes came from each parent when their gametes fused. Therefore, human gametes are haploid (one set).

Homologous chromosomes: The 46 chromosomes in a somatic cell can be arranged into 23 homologous or similar pairs. One chromosome from each pair came from the male parent, the other from the female. Homologous chromosomes have the same genes although in heterozygous people the genes would be different alleles (A,a). The exception to this would be the sex (X and Y) chromosomes. Passing on a complete set of human genes requires one chromosome from each pair to end up in each gamete.

There are several key differences between meiosis and mitosis that are summarized in the following table:

Mitosis Meiosis
Chromosome number stays the same Chromosome number is halved
One division occurs to make two cells. Four stages of this division. Two divisions occur to make four cells. Eight stages in these divisions.
Similar or homologous chromosomes do not pair. Homologous chromosomes pair during prophase l. Pairing is called synapsis.
Crossover exchanges between homologous chromosomes is rare. Synapsis allows crossing over between homologous chromosomes.
Two cells made are genetically identical. Four cells made are genetically different.

The key events that happen in each of the stages of meiosis are summarized.