Recombination

Recombination

Recombination is a process by which genetic material is shuffled during reproduction. It is important from an evolutionary standpoint because it allows the mixing of different traits. This trait mixing has been crucial for organisms that have had to adapt to a changing environment. Recombination has also allowed for the separation of favorable and unfavorable genetic mutations. In this way, harmful mutations are minimized in the gene pool.

Recombination involves a physical exchange of nucleotides between duplicate strands of deoxyribonucleic acid (DNA). There are three known types of recombination--homologous recombination, specific recombination and transposition. Each type occurs under different circumstances.

In eukaryotes, homologous recombination typically occurs during meiosis, which is a special form of cell division. It takes place during the first phase of meiosis, and involves a physical exchange of parts between chromosomes. In most eukaryotic cells, genetic material is organized as chromosomes in the nucleus. During meiosis, every chromosome is duplicated, or replicated. Each duplicate condenses and forms two identical structures known as chromatids, which are joined at a point called the centromere. Homologous chromosomes pair up with each other and form bivalent structures. It is while the chromosomes are aligned that recombination occurs.

To begin the exchange of genetic pieces between chromosomes, a nick is made on the chromosomal DNA of corresponding strands. This breakage lets each strand move at the free ends. The broken strands then cross over, or exchange, with each other. The recombinant region is extended until a whole gene is transferred. At this point, further recombination can occur or it can be stopped. Both processes require the creation of another break in the DNA strand and subsequent sealing of the nicks.

Site specific recombination is the type of crossing over that usually occurs in prokaryotes. It is the mechanism by which phage, or viral, genomes are incorporated into bacterial chromosomes. When phage DNA infects a bacteria, it exists in two states: lysogenic or lytic. During the lytic stage, the phage DNA exists apart from the bacterial DNA. In the lysogenic stage, the two DNAs are combined. The integration of the bacterial and viral DNAs occurs at a specific location, called an attachment site, on the bacterial DNA. The attachment site has a homologous nucleotide sequence with the phage genome. When conditions are right, the two pieces of genetic material align and enzymes cause them to merge.

Transposition is a third type of recombination. It involves transposable elements called transposons-short segments of mobile DNA. They are found in both prokaryotes and eukaryotes. The process of transposition begins when an enzyme cuts DNA at a target site. This leaves a section thathas unpaired nucleotides. Another enzyme called transposase inserts the transposon at this site and fills the gap. Through the process of transposition certain genes are made to move throughout the cell's genome. Transposition is the mechanism by which immunocytes manufacture the millions of antibodies required to protect vertebrates from antigens.