Unequal crossing over is a type of gene duplication or deletion event that deletes a sequence in one strand and replaces it with a duplication from its sister

chromatid A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chro ...

mitosis In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is mainta ...

or from its homologous chromosome during

meiosis Meiosis (; , since it is a reductional division) is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately resu ...

. It is a type of

chromosomal crossover Chromosomal crossover, or crossing over, is the exchange of genetic material during sexual reproduction between two homologous chromosomes' non-sister chromatids that results in recombinant chromosomes. It is one of the final phases of geneti ...

between homologous sequences that are not paired precisely. Normally genes are responsible for occurrence of crossing over. It exchanges sequences of different links between chromosomes. Along with

gene conversion Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces a ...

, it is believed to be the main driver for the generation of

gene duplication Gene duplication (or chromosomal duplication or gene amplification) is a major mechanism through which new genetic material is generated during molecular evolution. It can be defined as any duplication of a region of DNA that contains a gene. ...

s and is a source of mutation in the genome.

Mechanisms

During

, the duplicated chromosomes (

chromatids A chromatid (Greek ''khrōmat-'' 'color' + ''-id'') is one half of a duplicated chromosome. Before replication, one chromosome is composed of one DNA molecule. In replication, the DNA molecule is copied, and the two molecules are known as chro ...

) in eukaryotic organisms are attached to each other in the

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...

region and are thus paired. The maternal and paternal chromosomes then align alongside each other. During this time, recombination can take place via crossing over of sections of the paternal and maternal chromatids and leads to reciprocal recombination or non-reciprocal recombination. Unequal crossing over requires a measure of similarity between the sequences for misalignment to occur. The more similarity within the sequences, the more likely unequal crossing over will occur. 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. This is because these repeated sequences will pair together, allowing for the mismatch in the cross over point to occur.

Consequences for the organism

Unequal crossing over is the process most responsible for creating regional gene duplications in the genome. Repeated rounds of unequal crossing over cause 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.

Evolutionary implications

In unequal crossing over, there can be large sequence exchanges between the chromosomes. Compared with gene conversion, which can only transfer a maximum of 1,500 base pairs, unequal crossing over in yeast rDNA genes has been found to transfer about 20,000 base pairs in a single crossover event Unequal crossover can be followed by the

concerted evolution Concerted Evolution- A definition Concerted evolution is the phenomenon where paralogous genes within one species are more closely related to one another than to members of the same gene family in closely related species. In other terms, when s ...

of duplicated sequences. 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. Comparisons between alpha-globin, which does not have long introns, and beta-globin genes show that alpha-globin have 50 times higher concerted evolution. When unequal crossing over creates a

, the duplicate has 4

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

ary fates. This is due to the fact that

purifying selection In natural selection, negative selection or purifying selection is the selective removal of alleles that are deleterious. This can result in stabilising selection through the purging of deleterious genetic polymorphisms that arise through random ...

acting on a duplicated copy is not very strong. Now that there is a redundant copy,

neutral mutation Neutral mutations are changes in DNA sequence that are neither beneficial nor detrimental to the ability of an organism to survive and reproduce. In population genetics, mutations in which natural selection does not affect the spread of the mutatio ...

s can act on the duplicate. Most commonly the neutral mutations will continue until the duplicate becomes a

pseudogene Pseudogenes are nonfunctional segments of DNA that resemble functional genes. Most arise as superfluous copies of functional genes, either directly by DNA duplication or indirectly by Reverse transcriptase, reverse transcription of an mRNA trans ...

. If the duplicate copy increases the dosage effect of the gene product, then the duplicate may be retained as a redundant copy.

Neofunctionalization Neofunctionalization, one of the possible outcomes of functional divergence, occurs when one gene copy, or paralog, takes on a totally new function after a gene duplication event. Neofunctionalization is an adaptive mutation process; meaning one o ...

is also a possibility: the duplicated copy acquires a mutation that gives it a different function than its ancestor. If both copies acquire mutations, it is possible that a subfunctional event occurs. This happens when both of the duplicated sequences have a more specialized function than the ancestral copy

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 event 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, and 35% of the ''Absidia glauca'' is repetitive DNA sequences. These short sequences have no selection pressure acting on them and the frequency of the repeats can be changed by unequal crossing over.

References

{{DEFAULTSORT:Unequal Crossing Over Evolution Genetics concepts