Unequal crossing over
Encyclopedia
Chromatid
A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division . They are called sister chromatids so long as they are joined by the centromeres...
in mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
or from its homologous chromosome during meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. It is a type of chromosomal crossover
Chromosomal crossover
Chromosomal crossover is an exchange of genetic material between homologous chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed...
between homologous sequences that are not paired precisely. It exchanges sequences of different links between chromosomes. Along with gene conversion
Gene conversion
Gene conversion is an event in DNA genetic recombination, which occurs at high frequencies during meiotic division but which also occurs in somatic cells. It is a process by which DNA sequence information is transferred from one DNA helix to another DNA helix, whose sequence is altered.It is one...
, it is believed to be the main driver for the generation of gene duplication
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
s and is a source of mutation in the genome.
Mechanisms
During meiosisMeiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
, the duplicated chromosomes in eukaryotic organisms attach to each other in the centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
region and pair to each other. During this time, recombination can take place 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 evolutionConcerted evolution
Concerted evolution is a process that may explain the observation that paralogous genes within one species are more closely related to each other than to members of the same gene family in another species, even though the gene duplication event preceded the speciation event...
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 gene duplication
Gene duplication
Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...
, the duplicate has 4 evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary fates. This is due the fact that purifying selection acting on a duplicated copy is not very strong. Now that there is a redundant copy, neutral mutation
Neutral mutation
In genetics, a neutral mutation is a mutation that has no effect on fitness. In other words, it is neutral with respect to natural selection.For example, some mutations in a DNA triplet or codon do not change which amino acid is introduced: this is known as a synonymous substitution. Unless the...
s can act on the duplicate. Most commonly the neutral mutations will continue until the duplicate becomes a pseudogene
Pseudogene
Pseudogenes are dysfunctional relatives of known genes that have lost their protein-coding ability or are otherwise no longer expressed in the cell...
. If the duplicate copy increases the dosage effect of the gene product, then the duplicate may be retained as a redundant copy. Neofunctionalization 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
Subfunctionalization
Subfunctionalization is a model that explains the process by which duplicated genes remain functional in a genome. Duplicated genes are frequently formed in eukaryotic genomes and are thought to be initially redundant in function. One of the extra copies is usually under relaxed selection and...
occurs. This happens when both of the duplicated sequences have a more specialized function than the ancestral copy