3.4. Meiosis

Meiosis is Sex Cell (Gamete) Formation

In sexually reproducing organisms, some cells are able to divide by another method called meiosis. This type of cell division results in the production of gametes (eggs or sperm).

Meiosis is much more complex than mitosis. Whereas mitosis involves the duplication and subsequent division of chromosomes, meiosis involves two divisions of genetic material. Gametes are haploid (1n) with half the number of chromosomes than the progenitor cell that they arose from. These haploid sex cells arise in specialized reproductive tissue called the gonads. Ovaries (female gonads) and testes (male gonads) are the sites of meiosis.

Ploidy – Diploid & Haploid

Most of the cells in our bodies are somatic, or non sex cells and have a diplod (2n) chromosome number, meaning that chromosomes come in pairs called homologues. Every somatic cell in your body has 46 chromosomes. You received a set of 23 from your mother’s egg and a matching set of 23 via your father’s sperm, and now these chromosomes are the genetic material inside nearly every cell of your body.

Sex cells (sperm or eggs) have half the number of chromosomes as do body (somatic) cells. The merging of sperm and egg at fertilization brings the chromosome count back to 2n diploid number necessary for a zygote to have complete genetic information; 2 sets of genetic instructions in 23 pairs of chromosomes.

STAGES OF THE MEIOSIS

MEIOSIS I

Meiosis I, the first meiotic division, leads thanks to crossing-over to the formation of daughter cells with a distinct genetical compositions.

• Pre-meiosis: DNA replication is accomplished.

• Prophase I. (Gr. pro = before) is divided into: 

Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis 

Meiosis I begins with the condensation of the chromosomes in the leptotene during prophase I. During the pachytene (another stage of prophase I) the pairs of homolog chromosomes align to form tetrads in a process called synapsis. Corresponding segments of DNA of sister chromatids of two homolog chromosomes twist and cross, forming so-called chiasmata. In these regions exchange of DNA between homologe chromosomes  can occur: crossing-over.

Crossing-over (or crossover) requires that the chromosomes break and reconnect to the other chromosome.

• Metaphase I. In metaphase I (Gr meta= middle) the homolog chromosomes with the already exchanged DNA align as bivalents in the equatorial plane. The chromatides are strongly condensed. Bivalents are positioned in such a way that homolog centromeres lay at either side of the equatorial plane.

• Anaphase I. Anaphase I (Gr ana = apart) begins when the chromosomes, including piecesf exchanged DNA, are pulled to the opposite poles in the cell. The units of a homolog pair move apart (separation of bivalents) in opposite direction; however, the chromatides of each chromosomes stay joined.

• Telophase I. In telophase I (Gr telos = end) the bivalents are separated in two opposite domains in the cell(two poles) and the chromosomes decondense. In some species a new nuclear envelop is formed.

The second round of division, also called equatoriale division, resembles a normal division. The division plane is perpendicular to that of meiosis I. Meiosis II begins with two haploid cells (or two domains) that contain sisterchromatids with an own composition, and ends with four haploid cells each with an own genetical composition which is carried by single chromatids.

• Prophase II: typical for this stage is the presence of a haploid number of chromosomes that condense again. The two sister chromatides of a chromosome are still coupled together at the centromere.

• Metaphase II: The chromosomes move again to the equatorial plane between the poles. However, this plane is oriented perpendicularly with respect to the previous one of meiosis I.

• Anaphase II: The centromeres separate and the sister chromatides are pulled apart to opposite poles by the spindle.

• Telophase II: The former sister chromatides have reached the poles. A nuclear envelop is formed around each nucleus, while chromosome despiralize again.

Meiosis animation (Mc Graw-Hill)