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. Keeping the number of chromosomes in somatic cells from generation-to-generation constant happens because gametes contain half the chromosomes found in somatic cells. It is also key to genetic stability that meiosis cell division works to deliver one complete set of human genes from each human parent to create an offspring that has two complete sets of genes. Because genes are a part of chromosomes, this genetic stability is connected to the reliability of meiosis.
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 the pattern that homologous chromosomes have the same genes is the sex (X and Y) chromosomes. Female humans (and other mammals) have two X chromosomes while the males have one X and one Y. 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:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Chromosome number | Chromosome number stays the same. | Chromosome number is halved. |
| Number of divisions and stages | One division occurs to make two cells. Four stages of this division. | Two divisions occur to make four cells. Eight stages in these divisions. |
| Chromosome pairing | Similar or homologous chromosomes do not pair. | Homologous chromosomes pair during prophase l. Pairing is called synapsis. |
| Crossover | Crossover exchanges between homologous chromosomes is rare. | Synapsis allows crossing over between homologous chromosomes. |
| Final cells | Two cells made are genetically identical. | Four cells made are genetically different. |