Recombination

This wiki will be discussing recombination of chromosome. It is the exchange of genetic material between two homologous chromosomes. One example of the recombination of chromosomes is crossing over. As we learned in class, crossing over happens during meiosis and can be equal or unequal. The diagram below shows equal or normal crossing over. With normal crossing over, the chromosomes are rearranged without deleting or duplicating material.



Unequal crossing over usually results in a deletion or duplication and is a cause of some genetic disorders. One example of a genetic disorder which is due to unequal crossing over is color blindness.

Genetic recombination can also refer to geneticist deliberately combining DNA sequences that are not found in nature or occur in the wild-type. DNA sequences can be taken from genomes of the same species or genomes from different species as well. To alter genome sequences and bring forth a new production of different proteins that would not otherwise be formed.

There are three recombinational processes: non-homologous, site-specific, and homologous. Below is the description of the non-homologous recombination. 1. Non-homologous recombination is the breaking and joining of deoxyribonucleic acid (DNA) at non site-specific locations. There are two ways for non-homologous recombination to occur in bacteria: transposons and integrons
 * Transposons- which can also referred to as "jumping genes", due to the fact that they have to ability to transpose from one region of DNA to another region, encode their own transposase and regulatory genes, are flanked by inverted repeats, and enter cells via plasmids or transducing phages.
 * Discovered by Barbara McClintock, won a Nobel Prize in Physiology or Medicine 1983
 * Barbara McClintock discovered transposons in corn
 * Types of Transposons: IS elements, composite, and non-composite
 * IS elements (insertional elements)- have the ability to generate mutations by insertional inactivation (literally jumps into a gene), has the ability to encode a transposase, do not carry selectable gene (aka drug resistance), and also acts as a marker for RFLP (restriction fragment length polymorphism)[[image:webkit-fake-url://306FD056-B1A3-4FFA-9BC3-CCADAD26A46F/Fig_32_3.jpg caption="Fig_32_3.jpg"]]Example of a IS element
 * Composite Transposons- is a gene which is flanked with numerous IS elements in an direct or indirect orientation. Transposition can occur outside-end transposition or inside-end transposition
 * [[image:webkit-fake-url://262AC61E-277B-44B9-A13F-5093C624908E/Tn10.gif caption="Tn10.gif"]]Example of Composite Transposon
 * Non-composite Transposons- has a single IR on each end, is site specific in structural aspects of target DNA, and has the ability to carry resistance genes
 * [[image:webkit-fake-url://9D1CCE9B-DB6E-47A4-85C0-9CB6FC19A96E/Tn3.gif caption="Tn3.gif"]]Example of Non-composite Transposon

2. Homologous Recombination is another type of genetic recombination when two identical or similar DNA strands are broken and then reattached so that their is an exchange of material or to repair during meiosis or mitosis.
 * Integrons- are genetic units that are capable of capturing and mobilizing gene cassettes, mobile DNA elements which are found in gram negative elements, and provide promoters for the expression of the inserted cassettes
 * If an integron contains a large number of the gene cassettes it can be called a super-integron.



Recombination can also occur in bacterial cells. Since bacteria don't have two homologous chromosomes (they are haploid) they have different mechanisms to obtain extra DNA. They can obtain this by using transformation, conjugation, and transduction. In transformation, the DNA is aquired from the environment. Conjugation involves physical cell contact and genetic exchange through the tube-like pilus. Transduction occurs from genetic exchange between two cells by using a bacteriophage (bacterial virus).

Homologous chromosomes separating in the production of sex cells || Genetic linkage continues as homologous chromosomes separate in the formation of sex cells || Crossing-over unlinks alleles of genes as homologous chromosomes separate in the formation of sex cells. ||
 * [[image:http://anthro.palomar.edu/biobasis/images/paired_chromosomes_separate.gif width="237" height="235" caption="drawing of homologous chromosomes separating in the production of sex cells"]] ||
 * [[image:http://anthro.palomar.edu/biobasis/images/linked_genes.gif width="324" height="165" caption="drawing of genetic linkage continuing as homologous chromosomes separate in the formation of sex cells"]] ||
 * [[image:http://anthro.palomar.edu/biobasis/images/linked_genes.gif width="324" height="165" caption="drawing of genetic linkage continuing as homologous chromosomes separate in the formation of sex cells"]] ||
 * || [[image:http://anthro.palomar.edu/biobasis/images/recombination.gif width="328" height="164" caption="drawing of crossing-over unlinking genes as homologous chromosomes separate in the formation of sex cells"]] ||

NOTE:  At both ends of chromosomes there are telomere caps, which are sections of several thousand repeat DNA base unit sequences that are not part of genes (TTAGGG, TTAGGG, TTAGGG, etc.). Telomeres help protect critical DNA sequences from being changed by recombination. However, the telomeres become slightly shorter every time cell division occurs. When they have only around 77 base units remaining, they become unstable, leading to the fusion of chromosomes and cell death. This is a normal consequence of aging. Very short telomeres are also associated with early stages of cancer. (Base units and DNA are described and explained in the last section of this tutorial.)  3. Site-specific recombination is a mobile DNA element inserted into a strand of DNA by means similar to that seen in crossover. A segment of DNA on the mobile element matches exactly with a segment of DNA on the target, allowing enzymes called integrases to insert the rest of the mobile element into the target.

Another form of site-specific recombination, transpositional recombination does not require an identical strand of DNA in the mobile element to match with the target DNA. Instead, the integrases involved introduce nicks in both the mobile element and the target DNA, allowing the mobile DNA to enter the sequence. The nicks are then removed by ligases.

The link below goes to an animation that will clearly show how recombination occurs in a cell. []

Genetic recombination happens during meiosis, a special type of cell division that occurs during formation of sperm and egg cells and gives them the correct number of chromosomes. Since a sperm and egg unite during fertilization, each must have only half the number of chromosomes other body cells have. Otherwise, the fertilized cell would have too many. Genes that tend to stay together during recombination are said to be linked. Sometimes, one gene in a linked pair serves as a "marker" that can be used by geneticists to infer the presence of the other (often, a disease-causing gene). After the chromosomes separate, they are parceled out into individual sex cells. Each chromosome moves independently of all the others - a phenomenon called independent assortment. So, for example, the copy of chromosome 1 that an egg cell receives in no way influences which of the two possible copies of chromosome 5 it gets.