Process of Meiosis: Genetic Recombination

Meiosis Terminology; Chromatid

A chromatid is formed during meiosis and is one of two identical DNA strands formed after DNA synthesis. Sister chromatids are from the same chromosome, and they are connected together at the centromere. The classic drawing of a chromosome shaped like the letter X actually depicts sister chromatids.

Interphase I

A cell that is not dividing is said to be in interphase. It contains 23 pairs of homologous or matching chromosomes - one that was inherited from the person's mother and one that was inherited from the person's father. When referring to one of a pair of homologous chromosomes it is common to refer to this as a homolog, for short. During Interphase I, the cell duplicates its nuclear material. Each homolog replicates, forming two identical sister chromatids. At this point in time, there is the equivalent of 4 copies of each chromosome in the cell. In reality, chromosomes are not yet condensed enough to be visible until the end of prophase 1.

Meiosis Defined

Creates gametes (egg and sperm cells). During meiosis, genetic information is exchanged between the maternally and paternally inherited copies of a pair of chromosomes in order to create new combinations of genes. This process of genetic recombination helps to increase genetic variability within a species. It allows for the transmission of virtually limitless combinations of genes from parent to offspring. The resulting gametes have 23 new chromosomes, one member of each of the 23 pairs, representing unique combinations of the original maternal and paternal copies.

Recombination

Homologous chromosomes switch materials in a process known as recombination, or 'crossing over', which is mediated by the formation of chiasmata. The two chromatids attached to the chiasmata exchange parts. In this way, some of the genes originally on the father's chromosomes are now on the mother's chromosomes, and vice versa. This shuffling process helps to create and maintain variation in a species.

Meiosis Terminology; Homo-log

Homologous chromosomes, or homologs, consist of the members of any given chromosome pair. One member of the homolog is the chromosome from the mother and the other member is from the father. During meiosis, homologs pair together.

Early Prophase I

In prophase I, the chromosomes become shorter and thicker and more easily stained. They can now be viewed underneath a light microscope. The sister chromatids are joined at the centromere(see diagram). The nuclear envelope begins to disappear.

Late Prophase I

Later in prophase I, homologous chromosomes - for example, the maternal chromosome #1 and the paternal chromosome #1 - line up next to one another in a process called synapsis. An aggregate of RNA and protein holds the paired chromosomes together. Homologous chromosomes switch material in a process known as recombination or crossing over, which is mediated by the formation of chiasmata. The two chromatids attached at the chiasma exchange parts. In this way some of the genes originally on the father's chromosome are now on the mother's chromosome and vice versa. This shuffling process helps to create and maintain variation in a species. The nuclear envelope begins to disappear and spindle fibers begin to form at the poles of the cell.

Two Parts of Meiosis; Meiosis I

Meiosis is divided into two parts: meiosis I and meiosis II. In meiosis I, each chromosome in a cell is replicated to yield duplicated sister chromatides for each member of the homologous chromosomes. Physical contact between chromatids may occur, resulting in the formation of chiasmata (from the Greek khiasma: to cross). Chiasmata are thought to represent the process of crossing over, or recombination, in which an exchange of DNA between two of the four chromatids occurs.