Unequal crossing over: Difference between revisions
No edit summary |
No edit summary |
||
| Line 2: | Line 2: | ||
[[Image:Holliday Junction.svg|thumb|200px|right|Holliday Junction]] |
[[Image:Holliday Junction.svg|thumb|200px|right|Holliday Junction]] |
||
[[Image:Holliday junction coloured.png|thumb|200px|right|Molecular structure of a Holliday junction. From {{PDB|3CRX}}.]] |
[[Image:Holliday junction coloured.png|thumb|200px|right|Molecular structure of a Holliday junction. From {{PDB|3CRX}}.]] |
||
'''''Unequal Crossing Over''''' is a type of reciprocal [[recombination]] that deletes a sequence in one strand and replaces it with a duplication from its sister [[chromatid]] in [[meiosis]] or from its homologous chromosome during [[mitosis]]. It is a type of [[Chromosomal crossover]] between homologous sequences that are not paired precisely. It exchanges sequences of different links between chromosomes. Along with [[gene conversion]] it is believed to be the main driver for the generation of [[gene duplication]]s and is a source of mutation in the genome.<ref name="Dan">[[Dan Graur]] and [[Wen-Hsiung Li]]. Fundamentals of Molecular Evolution: Second Edition. Sinauer Associates, Inc. 2000</ref>. |
'''''Unequal Crossing Over''''' is a type of reciprocal [[recombination]] that deletes a sequence in one strand and replaces it with a duplication from its sister [[chromatid]] in [[meiosis]] or from its homologous chromosome during [[mitosis]]. It is a type of [[Chromosomal crossover]] between homologous sequences that are not paired precisely. It exchanges sequences of different links between chromosomes. Along with [[gene conversion]] it is believed to be the main driver for the generation of [[gene duplication]]s and is a source of mutation in the genome.<ref name="Dan">{{cite book[[Dan Graur]] and [[Wen-Hsiung Li]]. Fundamentals of Molecular Evolution: Second Edition. Sinauer Associates, Inc. 2000}}</ref>. |
||
==Mechanisms== |
==Mechanisms== |
||
Revision as of 21:12, 26 April 2011
Unequal Crossing Over is a type of reciprocal recombination that deletes a sequence in one strand and replaces it with a duplication from its sister chromatid in meiosis or from its homologous chromosome during mitosis. It is a type of Chromosomal crossover between homologous sequences that are not paired precisely. It exchanges sequences of different links between chromosomes. Along with gene conversion it is believed to be the main driver for the generation of gene duplications and is a source of mutation in the genome.[1].
Mechanisms
During meiosis, the duplicated chromosomes in eukaryotic organism's attach to each other in the centromere region and pair to each other. During this time recombination can take place and leads to reciprocal recombination or nonreciprocal recombination[1]. Unequal crossing over requires a measure of similarity between the sequences for misalignment to occur, the more similar within the sequences the more likely unequal crossing over will occur[1]. One of the sequences is thus lost and replaced with the duplication of another sequence.
When two sequences are misaligned, unequal crossing over may create a tandem repeat on one chromosome and a deletion on the other. The rate of unequal crossing over will increase with the number of repeated sequences around the duplication.
Consequences for the Organism
Unequal crossing over is the process most responsible for creation regional gene duplications in the genome [1]. Repeated rounds of unequal crossing over causes the homogenization of the two sequences. With the increase in the duplicates, unequal crossing over can lead to dosage imbalance in the genome and can be highly deleterious (SOURCE).
Evolutionary implications
Unequal crossing over there can be large sequence exchanges between the chromosomes. Compared with gene conversion which can only transfer can only be a maximum of 1,500 base pairs, unequal crossing over in yeast rDNA genes it has been found that about 20,000 base pairs in a single crossover event [1][2] Can allow for Concerted Evolution.
It has been suggested that longer intron found between two Beta-globin genes are a response to deleterious selection from unequal crossing over in the Beta-globin genes[1][3]. Comparisons between alpha-globin, which does not have long introns, and beta-globin genes show that alpha-globin have 50 times higher concerted evolution.
Genome Size
Gene duplications are the main reason for the increase of genome size, and as unequal crossing over is the main mechanism for gene duplication, unequal crossing over contributes to genome size evolution is the most common regional duplication even that increases the size of the genome.
Junk DNA
When viewing the genome of a eukaryote a striking observation is the large amount of tandem, repetitive DNA sequences that make up a large portion of the genome. For example, over 50% of the Dipodmys ordii genome is made up of three specific repeats, Drosophila virilis has three sequences that make up 40% of the genome, 35% of the Absidia glauca is repetitive DNA sequences [1]. These short sequences have no selection pressure acting on them and the frequency of the repeats can be changed by unequal crossing over.[4].
References
- ^ a b c d e f g {{cite bookDan Graur and Wen-Hsiung Li. Fundamentals of Molecular Evolution: Second Edition. Sinauer Associates, Inc. 2000}}
- ^ Szostak, J. W.; Wu, R. (1980). "Unequal crossing over in the ribosomal DNA of Saccharomyces cerevisiae". Nature. 284: 426–430.
- ^ Zimmer, E. A.; Martin, S. M.; Beverley, S. M.; Kan, Y. W.; Wilson, A. C. (1980). "Rapid duplication and loss of genes coding for the alpha chains of hemoglobin". Proc. Natl. Acad. Sci. USA. 77: 2158–2162.
- ^ Zhang, J. (2003). "Evolution of the human ASPM gene, a major
determinant of brain size". Genetics. 165: 2063–2070.
{{cite journal}}: line feed character in|title=at position 43 (help)
