Fertilization and completion of meiosis. The somatic cell cycles discussed so far in this chapter result in diploid daughter cells with identical genetic complements. For this reason, only a single, well-fortified egg is produced by each round of meiosis. These cells develop into sperm or ova. Meiosis and the Formation of Eggs and Sperm March 1, In most cases, the embryo is not viable and dies. Sister chromatids of each chromosome are joined at the centromere.
There are four kinetochores on each tetrad, but the pair of kinetochores on each sister chromatid fuses and functions as a unit during meiosis I. Homologous pairs move together along the metaphase plate: As kinetochore microtubules from both centrosomes attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle, due to continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the two kinetochores of homologous chromosomes. The diploid organism's germ-line cells undergo meiosis to produce spores. The new combinations of DNA created during crossover are a significant source of genetic variation , and result in new combinations of alleles , which may be beneficial.
Meiosis generates gamete genetic diversity in two ways: 1 Law of Independent Assortment. Only cells in the ovaries or testes undergo meiosis. The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Spermatogonia 2n are the cells in the testes that will undergo meiosis. This critical pairing of homologous chromosomes takes place during an extended prophase of meiosis I, which is divided into five stages leptotene , zygotene , pachytene , diplotene , and diakinesis on the basis of chromosome morphology Figure
The synaptonemal complex. The illustration below shows the 8 phases of spermatogenesis. All rights reserved. During meiosis, a diploid parent cell, which has two copies of each chromosome, undergoes one round of DNA replication followed by two separate cycles of nuclear division to produce four haploid cells. This critical pairing of homologous chromosomes takes place during an extended prophase of meiosis I, which is divided into five stages leptotene , zygotene , pachytene , diplotene , and diakinesis on the basis of chromosome morphology Figure Excluding mutation and mistakes, these sperm are identical except for their individual, unique genetic load.