Meiosis is a two part nuclear division in which the number of chromosomes is halved during gamete formation. Meiosis I reduces the number of chromosomes and Meiosis II divides double stranded chromosomes to single stranded. Meiosis creates daughter cells which receives half the number of chromosomes of the parent cell.

A human cell contains 46 chromosomes, therefore after the process of meiosis the four daughter cells have 23 chromosomes each. Sex cells or gametes are produced in animals during meiosis. When sexual reproduction occurs, the male and female gametes unite and create a new being called a zygote. The zygote receives two sets of 23 chromosomes from the gametes. These add to become the necessary 46 chromosomes and a diploid or 2n cell.

Meiosis I

Prophase I

Prophase I

• The chromosomes become visible due to the DNA coiling more tightly, called condensation, and the chromonemata, matrix of thin threads, also becomes visible.
• DNA has already completely replicated itself before meiosis began. Each chromosome is made up of two genetically identical chromatid called sister chromosomes, joined at their centromeres.
• Homologous chromosomes become physically associated and line up parallel to each other, which forms bivalents.
• The ends of the chromatids are attached to specific places on the nuclear envelope. Synapsis occurs when homologous chromosomes form tetrads and crossing-over or recombination takes place. A combination of RNA and protein forms between homologous chromosomes and places the two chromonemata directly across. Therefore each gene is exactly across from its sister on the homologous chromosome. The result is a complex called a synaptonemal complex. DNA strands of one homologue can pair with the corresponding DNA strands of the other.

Crossing-Over

Example Chromosomes Before Crossing-Over ABCDEF ABCDEF a b c d e f a b c d e f After the Process of Crossing-Over ABCd e f ABCDe f a b c DEF a b c d EF

Crossing-over is another name for recombination or physical exchange of equal pieces of adjacent non-sister chromatids. When crossing-over occurs chromatids break and may be reattached to a different homologous chromosome.

During the process of crossing-over one of the paired chromosome arms may exchanged physically at one or more locations. If the two chromosomes contain different mutations on each side of the cross-over the exchange of chromosome arms will produce chromosomes that contain different combinations of mutations. When these chromosomes segregate in meiosis, they form gametes that have completely new combinations of alleles.

The Double-Stranded-Break Repair Model was conceived in 1983 at the University of Oregon by Frank Stahl and his fellow coworkers to explain the process of crossing-over. First a break within one of the two homologous chromosomes caused by the synaptonemal complex which the homologous chromosomes are lined up next to each other. A gap is created by enzymes that chew at the break and smooth is down. One of the newly loose ends uncoils and attaches itself with the other undamaged strand that has a similar nucleotide combination to its own. A single strand loop is created by the undamaged strand. The broken strand begins to grow in the undamaged one, adding new nucleotides on the ends based on the undamaged strand's nucleotides. The loop continues to get larger due to this growth. The growth fills up the gap made by the enzymes and at the same time, the used broken strand also fills in the gap. The damaged ends are sealed by other enzymes, making the two pairs that are continuos. However one strand from each pair has exchanged segments with the other in two places. Finally the crossed-over strands break apart, removing the bridges between the two homologous chromosomes.

• When crossing-over has completed the synaptonemal complex falls apart and chromatids start to move apart from each other. At this time there are four chromatids for each chromosome, yet the chromatids do not completely separate. The chromatids are held together in two different ways. The two sister chromatids of each homologue make by DNA replication are joined by their centromere and at the sites where crossing-over occurred in the synaptonemal complex.
• The points where segments of chromosomes have been exchanged can be observed by microscope as an x-shaped structure called a chiasma (plural chiasmata). A chiasma shows that two of the four chromatids have crossed each other. The chiasma structure slowly moves out to the end of the chromosomes as they separate.

In Prophase I, DNA threads of the two homologous chromosomes pair up with each other. Crossing-over may occur between the pair DNA, which creates structures called chiasmata. Their purpose is to secure the two homologues together so they do not separate immediately.

Metaphase I

Metaphase I

• The nuclear envelope disappears and microtubules form a spindle.
• At the same time, the chiasmata continues to move down the paired chromosomes. The chiasmata eventually reaches the end, where it is then called a terminal chiasmata.
• Because of the terminal chiasmata, the two homologous chromosomes are still held together. The microtubules can only attach to the sides of the homologous chromosomes that are facing outward, also called outside centromere faces, due to the presence of the chiasmata.
• The centromere of one homologue attaches to microtubules from one pole, and the other homologue attaches to microtubules from the opposite pole.
• Each joined pair of homologues, called a bivalent, lines up on the metaphase plate.

Anaphase I

Anaphase I

• The homologous chromosome pairs are randomly lined on the metaphase plate.
• After spindles completely attach, the microtubules contract, breaking the chiasmata apart and pulling the centromeres toward the two poles. The chromosomes also are pulled to their respective poles.
• The entire centromere continues to move to one pole, taking both sister chromatids along.
• After the spindle fibers have completely finished contracting, both poles have a complete set of chromosomes.
• Because placement on the metaphase plate is entirely random, the chromosomes that each pole receives is also random.
• The stage is responsible for independent assortment of traits located on chromosomes.

Telophase I

Telophase I

• After Anaphase I, each pole has a complete set of chromosomes.
• Each of the chromosomes has already replicated itself before meiosis even started and therefore has two copies of itself. Every copy is joined by a shared centromere.
• An important fact is that the copies are not identical due to crossing-over in Prophase I.
• Telophase is the stage when the two complements of bivalents congregate at their own pole to make chromosome groups.

Meiosis II

• Meiosis II is a mitotic division.
• At both poles the two bivalent complements divide themselves.
• Spindle fibers bind each side of the centromeres, where they separate and move to opposite poles.
• The ending result is four haploid cells often called daughter cells.