Download Gene rearrangements occur via various mechanisms

Gene rearrangements occur via various mechanisms and produce
genes causing mutation or favorable traits that can contribute to
evolution.
LEARNING OBJECTIVE [ edit ]
Discuss the ways that gene rearrangement can occur
KEY POINTS [ edit ]
A major mechanism by which gene rearrangements can occur is during prophase I of meiosis I
when pairing of homologouschromosomes can cross-over and exchange genetic materials to form
new genetic combinations.
Gene recombination can occur via various mechanisms including crossing over, gene conversion
and nonhomologous recombination.
It is possible for gene rearrangements to occur and cause a favorable trait to be produced which
can contribute to divergent evolution and formation of new species.
Gene conversion occurs when a portion of the genetic material is copied from one chromosome to
another without the copied chromosome undergoing a change.
Nonhomlogous recombination occurs when DNA sequences are exchanged between
chromosomes without sequence homology.
TERMS [ edit ]
recombination
the formation of genetic combinations in offspring that are not present in the parents
translocation
a transfer of a chromosomal segment to a new position, especially on a nonhomologous
chromosome
synapsis
the association of homologous maternal and paternal chromosomes during the initial part of
meiosis
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GeneRearrangement
The DNA sequences from two individuals
of the same speciesare highly similar,
differing by only about 1 in
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1,000nucleotides. Genetic recombination is the process by which two
DNA molecules exchange genetic information, resulting in the production of a new
combination of alleles. In eukaryotes, genetic recombination during meiosis can lead to a
novel set of genetic information that can be passed on to progeny. Most recombination is
naturally occurring and the rearrangement of DNA can occur by several different
mechanisms and result in structural variation.
Crossing Over
During meiosis in eukaryotes, genetic recombination involves the pairing
of homologous chromosomes. This may be followed by information exchange between the
chromosomes. The information exchange may occur without physical exchange (a section of
genetic material is copied from one chromosome to another, without the donating
chromosome being changed); or by the breaking and rejoining of DNA strands, which forms
new molecules of DNA. Crossing-over (homologous recombination) is one such mechanism
by which DNA variations can occur, and genes can be rearranged. Crossing-over is an event
that occurs during meiosis when chromatids are in contact with each other and may exchange
segments. During meiosis, as chromosomes condense and pair with their homologs
(prophase I), they interact at distinct points. It is known that this pairing and interaction
between homologous chromosomes, known assynapsis, does more than simply organize the
homologs for migration to separate daughter cells. When synapsed, homologous
chromosomes undergo reciprocal physical exchanges at their arms . This event results in
variations ofgametes that can produce variation in species. However, there are circumstances
when the chromosomes do not align properly, and their exchange of genes is not equal. This
can result in one chromosome ending up with more DNA and one ending up with less. These
types of errors can result inmutations.
Crossing Over
Crossing over occurs during meiosis I, and is the process where homologous chromosomes pair up with
each other and exchange different segments of their genetic material to form recombinant chromosomes.
Crossing over is essential for the normal segregation of chromosomes during meiosis. Crossing over also
accounts for genetic variation, because, due to the swapping of genetic material during crossing over,
the chromatids held together by the centromere are no longer identical.
Recombination may also occur during mitosis in eukaryotes where it ordinarily involves the
two sister chromosomes formed after chromosomal replication. In this case, new
combinations of alleles are not produced since the sister chromosomes are usually identical.
In meiosis and mitosis, recombination occurs between similar molecules (homologs) of DNA.
In both meiotic and mitotic cells, recombination between homologous chromosomes is a
common mechanism used in DNA repair. Genetic recombination and recombinational DNA
repair also occurs in bacteria and archaea. Recombination can be artificially induced in
laboratory (in vitro) settings, producingrecombinant DNA for purposes
including vaccinedevelopment. V(D)J recombination in organisms with an adaptive immune
system is a type of site-specific genetic recombination that helps immune cells rapidly
diversify to recognize and adapt to new pathogen.
Nonhomologous Recombination
Recombination can occur between DNA sequences that contain no sequence homology. This
can cause chromosomaltranslocations, sometimes leading to cancer.
Gene Conversion
In gene conversion, a section of genetic material is copied from one chromosome to another,
without the donating chromosome being changed. Gene conversion occurs at
high frequency at the actual site of the recombination event during meiosis. It is a process by
which a DNA sequence is copied from one DNA helix (which remains unchanged) to another
DNA helix, whose sequence is altered. Gene conversion has often been studied in fungal
crosses where the 4 products of individual meioses can be conveniently observed. Gene
conversion events can be distinguished as deviations in an individual meiosis from the
normal 2:2 segregation pattern.