Thomas Hunt Morgan's illustration of crossing over (1916)

Chromosomal crossover (or crossing over) is the process by which two chromosomes pair up and exchange sections of their DNA. A chromosome is an organized structure of DNA and Protein that is found in cells. Deoxyribonucleic acid ( DNA) is a Nucleic acid that contains the genetic instructions used in the development and functioning of all known This often occurs during prophase 1 of meiosis in a process called synapsis. Prophase is a stage of Mitosis in which the Chromatin condenses into a highly ordered structure called a Chromosome In Biology or life science meiosis (pronounced my-oh-sis is a process of reductional division in which the number of chromosomes per cell is cut in half Synapsis (also called syndesis) is the pairing of two Homologous chromosomes that occurs during Meiosis. Synapsis begins before the synaptonemal complex develops, and is not completed until near the end of prophase 1. The synaptonemal complex is a Protein structure that forms between two homologous chromosomes during Meiosis and that is thought to mediate chromosome pairing Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome. A chromosome is an organized structure of DNA and Protein that is found in cells. The result of this process is an exchange of genes, called genetic recombination. History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance Genetic recombination is the process by which a strand of genetic material (usually DNA; but can also be RNA) is broken and then joined to a different DNA molecule Chromosomal crossovers also occur in asexual organisms and in somatic cells, since they are important in some forms of DNA repair. Somatic cells are any cells forming the body of an organism as opposed to Germline cells DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its Genome. ^[1]

A double crossing over

Recombination involves the breakage and rejoining of parental chromosomes

Crossing over was first described, in theory, by Thomas Hunt Morgan. Thomas Hunt Morgan ( September 25, 1866 &ndash December 4, 1945) was an American geneticist and embryologist. The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931. Harriet Baldwin Creighton ( 27 June 1909 &ndash January 9 2004) was an American Botanist, Geneticist and educator Barbara McClintock (June 16 1902 – September 2 1992 the 1983 Nobel Laureate in Physiology or Medicine, was an American scientist and one of the world's most distinguished Year 1931 ( MCMXXXI) was a Common year starting on Thursday (link will display full 1931 calendar of the Gregorian calendar. ^[2]

Chemistry

Holliday Junction

Molecular structure of a Holliday junction.

Meiotic recombination initiates with double-stranded breaks that are introduced into the DNA by the Spo11 protein. ^[3] One or more exonucleases then digest the 5’ ends generated by the double-stranded breaks to produce 3’ single-stranded DNA tails. The meiosis-specific recombinase Dmc1 and the general recombinase Rad51 coat the single-stranded DNA to form nucleoprotein filaments. ^[4] The recombinases catalyze invasion of the opposite chromatid by the single-stranded DNA from one end of the break. A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division Next, the 3’ end of the invading DNA primes DNA synthesis, causing displacement of the complementary strand, which subsequently anneals to the single-stranded DNA generated from the other end of the initial double-stranded break. The structure that results is a cross-strand exchange, also known as a Holliday junction. A Holliday junction is a mobile junction between four strands of DNA. The contact between two chromatids that will soon undergo crossing-over is known as a chiasma. A chiasma (plural chiasmata) in Genetics, is thought to be the point where two homologous chromatids exchange genetic material during Chromosomal crossover The Holliday junction is a tetrahedral structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure. A tetrahedron (plural tetrahedra) is a Polyhedron composed of four triangular faces three of which meet at each vertex.

Consequences

In most eukaryotes, a cell carries two copies of each gene, each referred to as an allele. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called History See also History of genetics The existence of genes was first suggested by Gregor Mendel (1822-1884 who in the 1860s studied inheritance An allele (ˈæliːl (UK /əˈliːl/ (US (from the Greek αλληλος allelos, meaning each other) is one member of a pair or series of different forms Each parent passes on one allele to each offspring. An individual gamete inherits a complete haploid complement of alleles on chromosomes that are independently selected from each pair of chromatids lined up on the metaphase plate. A gamete (from Ancient Greek γαμέτης; translated gamete = wife gametes = husband is a cell that fuses with another gamete A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division Without recombination, all alleles for those genes linked together on the same chromosome would be inherited together. Meiotic recombination allows a more independent selection between the two alleles that occupy the positions of single genes, as recombination shuffles the allele content between sister chromatids. A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division

Recombination does not have any influence on the statistical probability that another offspring will have the same combination. This theory of "independent assortment" of alleles is fundamental to genetic inheritance. Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets relating to the transmission of hereditary characteristics from parent However, there is an exception that requires further discussion.

The difference between gene conversion and chromosomal crossover. Gene conversion is an event in DNA Genetic recombination, which occurs at high frequencies during meiotic division but which also occurs in somatic cells Blue is the two chromatids of one chromosome and red is the two chromatids of another one. A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division

The frequency of recombination is actually not the same for all gene combinations. This leads to the notion of "genetic distance", which is a measure of recombination frequency averaged over a (suitably large) sample of pedigrees. In Genetics, a centimorgan (abbreviated cM) or map unit (mu is a unit of Recombinant frequency for measuring Genetic linkage. Loosely speaking, one may say that this is because recombination is greatly influenced by the proximity of one gene to another. If two genes are located close together on a chromosome, the likelihood that a recombination event will separate these two genes is less than if they were farther apart. Genetic linkage describes the tendency of genes to be inherited together as a result of their location on the same chromosome. Genetic linkage occurs when particular genetic loci or Alleles for genes are inherited jointly Linkage disequilibrium describes a situation in which some combinations of genes or genetic markers occur more or less frequently in a population than would be expected from their distances apart. In Population genetics, linkage disequilibrium is the non-random association of Alleles at two or more loci, not necessarily on the same Chromosome This concept is applied when searching for a gene that may cause a particular disease. A disease is an abnormal condition of an organism that impairs bodily functions and can be deadly This is done by comparing the occurrence of a specific DNA sequence with the appearance of a disease. A DNA sequence or genetic sequence is a succession of letters representing the Primary structure of a real or hypothetical DNA Molecule When a high correlation between the two is found, it is likely that the appropriate gene sequence is really closer.

Problems

Although crossovers typically occur between homologous regions of matching chromosomes, similarities in sequence can result in mismatched alignments. These processes are called unbalanced recombination. Unbalanced recombination is fairly rare compared to normal recombination, but severe problems can arise if a gamete containing unbalanced recombinants becomes part of a zygote. For other meanings see Zygote (disambiguation. A zygote (from Greek ζυγωτός zugōtos "joined" or "yoked" The result can be a local duplication of genes on one chromosome and a deletion of these on the other, a translocation of part of one chromosome onto a different one, or an inversion. Gene duplication (or chromosomal duplication) is any duplication of a region of DNA that contains a Gene; it may occur as an error in Homologous In Genetics, a deletion (also called gene deletion, deficiency, or deletion mutation) is a Mutation (a genetic aberration In Genetics, a chromosome translocation is a Chromosome abnormality caused by rearrangement of parts between nonhomologous Chromosomes. An inversion is a Chromosome rearrangement in which a segment of a chromosome is reversed end to end

References

1. ^ Li X, Heyer WD (2008). "Homologous recombination in DNA repair and DNA damage tolerance". Cell Res. 18 (1): 99-113. doi:10.1038/cr.2008.1. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. PMID 18166982.  
2. ^ Creighton H, McClintock B (1931). "A Correlation of Cytological and Genetical Crossing-Over in Zea Mays". Proc Natl Acad Sci U S A 17 (8): 492-7. PMID 16587654.   (Original paper)
3. ^ Keeney S, Giroux CN, and Kleckner N (1997). "Meiosis-specific DNA double-stranded breaks are catalyzed by Spo11, a member of a widely conserved protein family. " Cell 88(3):375-384. PMID 9039264 doi:10.1016/S0092-8674(00)81876-0
4. ^ Sauvageau S, Stasiak AZ, Banville I, Ploquin M, Stasiak A, and Masson JY (2005). A digital object identifier ( DOI) is a permanent identifier given to an Electronic document. "Fission yeast rad51 and dmc1, two efficient DNA recombinases forming helical nucleoprotein filaments. " Mol Cell Biol 25(11):4377-4387. PMID 15899844 doi:10.1128/MCB.25.11.4377-4387.2005

See also

• Mitotic crossover
• Recombinant frequency
• Independent assortment
• Genetic distance