Wikia

Psychology Wiki

Changes: Chromosomal crossover

Edit

Back to page

 
 
Line 1: Line 1:
 
{{BioPsy}}
 
{{BioPsy}}
[[Image:Morgan crossover 1.jpg|thumb|Thomas Hunt Morgan's illustration of crossing over (1916)]]
+
[[File:Chromosomal Crossover.svg|thumbnail|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. So, when the chromosomes go on to meiosis II and separate, some of the daughter cells receive daughter chromosomes with recombined alleles. Due to this genetic recombination, the offspring have a different set of alleles and genes than their parents do. In the diagram, genes B and are a crossed over with each other, making the resulting recombinants after meiosis Ab, AB, ab, and aB.]]
'''Homologous Recombination''' is the process by which two [[chromosomes]], paired up during [[prophase]] 1 of [[meiosis]], exchange some distal portion of their [[DNA]]. Crossover occurs when two [[chromosome]]s, normally two [[homology (biology)|homologous]] instances of the same chromosome, break and then reconnect but to the different end piece. If they break at the same place or [[locus (genetics)|locus]] in the sequence of base pairs, the result is an exchange of [[gene]]s, called [[genetic recombination]]. This outcome is the normal way for crossover to occur. If they break at slightly different loci, the result can be a duplication of genes on one chromosome and a deletion of these on the other. This is known as an '''unequal crossover'''. If chromosomes break and rejoin on opposite sides of the [[centromere]], the result can be one chromosome being lost during [[cell division]].
+
[[Image:Morgan crossover 1.jpg|thumb|200px|Thomas Hunt Morgan's illustration of crossing over (1916)]]
[[Image:Morgan crossover 2.jpg|thumb|A double crossing over]]
+
[[Image:Morgan crossover 2.jpg||thumb|200px|A double crossing over]]
Any pair of homologous chromosomes may be expected to cross over multiple times during meiosis, depending on the species and length of the chromosome. The recombination is activily assisted in the cell by machinery that has been well conserved through evolution. This reduces the [[genetic linkage]] between genes on the same chromosome. The genetic variation of a population is thereby increased through chromosomal crossover. [[Independent assortment]] is a somewhat related process operating on the complete set of chromosomes.
 
 
[[Image:Chromosomal Recombination.svg|thumb|Recombination involves the breakage and rejoining of parental chromosomes]]
 
[[Image:Chromosomal Recombination.svg|thumb|Recombination involves the breakage and rejoining of parental chromosomes]]
Crossing over was first described by [[Thomas Hunt Morgan]], and the physical basis of crossing over was first demonstrated by [[Harriet Creighton]] and [[Barbara McClintock]] in [[1931]].
 
   
The production of [[knockout mice]], for example, uses this machinery to incorporate altered DNA sequences into the genome.
+
'''Chromosomal crossover''' (or '''crossing over''') is the exchange of genetic material between [[homologous chromosome]]s that results in recombinant chromosomes. It is one of the final phases of [[genetic recombination]], which occurs during [[prophase I]] of [[meiosis]] (pachytene) in a process called [[synapsis]]. Synapsis begins before the [[synaptonemal complex]] develops, and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching [[chromosome]]s break and then reconnect to the other chromosome.
   
  +
Crossing over was described, in theory, by [[Thomas Hunt Morgan]]. He relied on the discovery of the Belgian Professor [[Frans Alfons Janssens]] of the [[Catholic University of Leuven (1834–1968)|University of Leuven]] who described the phenomenon in 1909 and had called it 'chiasmatypie'. The term ''[[chiasma (genetics)|chiasma]]'' is linked if not identical to chromosomal crossover. Morgan immediately saw the great importance of Janssens' cytological interpretation of chiasmata to the experimental results of his research on the heredity of ''[[Drosophila]]''. The physical basis of crossing over was first demonstrated by [[Harriet Creighton]] and [[Barbara McClintock]] in 1931.<ref>{{cite journal |author=Creighton H, McClintock B |title=A Correlation of Cytological and Genetical Crossing-Over in Zea Mays |journal=Proc Natl Acad Sci USA |volume=17 |issue=8 |pages=492–7 |year=1931 |pmid=16587654 |doi=10.1073/pnas.17.8.492 |pmc=1076098}} (Original paper)</ref>
   
The basis for this is double strand break (DSB) repair. DSB repair encompasses the related pathways of:
+
==Chemistry==
* Non-Homologous End Joining (NHEJ)
 
* Synthesis-Dependent Strand Annealing (SDSA)
 
* Break Induced Replication (BIR)
 
* Singel Strand Annealing (SSA)
 
   
Depending on topology requirements, these mechanisms may or may not involve a [[Holliday junction]] (HJ).
+
Meiotic recombination initiates with double-stranded breaks that are introduced into the DNA by the [[Spo11]] protein.<ref>{{cite journal |doi=10.1016/S0092-8674(00)81876-0 |title=Meiosis-Specific DNA Double-Strand Breaks Are Catalyzed by Spo11, a Member of a Widely Conserved Protein Family |year=1997 |author=Keeney, S |journal=Cell |volume=88 |pages=375–84 |pmid=9039264 |last2=Giroux |first2=CN |last3=Kleckner |first3=N |issue=3}}</ref> One or more [[exonuclease]]s 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.<ref>{{cite journal |doi=10.1128/MCB.25.11.4377-4387.2005 |year=2005 |month=Jun |author=Sauvageau, S; Stasiak, Az; Banville, I; Ploquin, M; Stasiak, A; Masson, Jy |title=Fission Yeast Rad51 and Dmc1, Two Efficient DNA Recombinases Forming Helical Nucleoprotein Filaments |volume=25 |issue=11 |pages=4377–87 |issn=0270-7306 |pmid=15899844 |pmc=1140613 |journal=Molecular and Cellular Biology |url=http://mcb.asm.org/cgi/pmidlookup?view=long&pmid=15899844 |format=Free full text}}</ref> The recombinases catalyze invasion of the opposite [[chromatid]] by the single-stranded DNA from one end of the break. 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]]. The contact between two chromatids that will soon undergo crossing-over is known as a ''[[chiasma (genetics)|chiasma]]''. The Holliday junction is a [[Tetrahedron|tetrahedral]] structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure.
  +
{{gallery
  +
|width=200
  +
|Image:Holliday Junction.svg|Holliday Junction
  +
|Image:Holliday junction.jpg|Molecular structure of a Holliday junction.
  +
}}
   
  +
==Consequences==
  +
[[Image:Conversion and crossover.jpg|thumb|right|250px|The difference between [[gene conversion]] and '''chromosomal crossover'''. Blue is the two [[chromatid]]s of one chromosome and red is the two chromatids of another one.]]
  +
In most [[eukaryote]]s, a [[cell (biology)|cell]] carries two versions of each [[gene]], each referred to as an [[allele]]. 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 [[chromatid]]s lined up on the metaphase plate. 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 homologous chromosomes.
   
  +
Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome. Although the same genes appear in the same order, the alleles are different. In this way, it is theoretically possible to have any combination of parental alleles in an offspring, and the fact that two alleles appear together in one offspring does not have any influence on the statistical probability that another offspring will have the same combination. This theory of "[[Mendelian inheritance|independent assortment]]" of alleles is fundamental to genetic inheritance.<ref>{{cite web|url= http://www.daviddarling.info/encyclopedia/G/genetic_recombination.html|title= genetic recombination}}</ref>
  +
However, the frequency of recombination is actually not the same for all gene combinations. This leads to the notion of "[[centiMorgan|genetic distance]]", which is a measure of recombination frequency averaged over a (suitably large) sample of pedigrees. 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. [[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. This concept is applied when searching for a gene that may cause a particular [[disease]]. This is done by comparing the occurrence of a specific [[DNA sequence]] with the appearance of a disease. When a high correlation between the two is found, it is likely that the appropriate gene sequence is really closer.<ref name=" Genetic Recombination "> Genetic Recombination </ref>
  +
  +
==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]]. The result can be a local [[Gene duplication|duplication]] of genes on one chromosome and a [[Gene deletion|deletion]] of these on the other, a [[Chromosomal translocation|translocation]] of part of one chromosome onto a different one, or an [[Chromosomal inversion|inversion]].
  +
  +
==References==
  +
{{reflist}}
   
 
==See also==
 
==See also==
  +
*[[Unequal crossing over]]
  +
*[[Coefficient of coincidence]]
  +
*[[Genetic distance]]
  +
*[[Independent assortment]]
 
*[[Mitotic crossover]]
 
*[[Mitotic crossover]]
 
*[[Recombinant frequency]]
 
*[[Recombinant frequency]]
*[[Independent assortment]]
 
   
[[Category:Cell biology]][[Category:Molecular genetics]]
+
{{Genetic recombination}}
  +
  +
[[Category:Cellular processes]]
  +
[[Category:Molecular genetics]]
   
  +
<!--
 
[[cs:Crossing-over]]
 
[[cs:Crossing-over]]
 
[[de:Crossing over]]
 
[[de:Crossing over]]
Line 35: Line 50:
 
[[sr:Кросинг-овер]]
 
[[sr:Кросинг-овер]]
 
[[sv:Överkorsning]]
 
[[sv:Överkorsning]]
  +
-->
 
{{enWP|Chromosomal crossover}}
 
{{enWP|Chromosomal crossover}}

Latest revision as of 22:25, September 27, 2013

Assessment | Biopsychology | Comparative | Cognitive | Developmental | Language | Individual differences | Personality | Philosophy | Social |
Methods | Statistics | Clinical | Educational | Industrial | Professional items | World psychology |

Biological: Behavioural genetics · Evolutionary psychology · Neuroanatomy · Neurochemistry · Neuroendocrinology · Neuroscience · Psychoneuroimmunology · Physiological Psychology · Psychopharmacology (Index, Outline)


File:Chromosomal Crossover.svg
Morgan crossover 1

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

Morgan crossover 2

A double crossing over

Chromosomal Recombination

Recombination involves the breakage and rejoining of parental chromosomes

Chromosomal crossover (or crossing over) is the exchange of genetic material between homologous chromosomes that results in recombinant chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase I of meiosis (pachytene) in a process called synapsis. Synapsis begins before the synaptonemal complex develops, and is not completed until near the end of prophase I. Crossover usually occurs when matching regions on matching chromosomes break and then reconnect to the other chromosome.

Crossing over was described, in theory, by Thomas Hunt Morgan. He relied on the discovery of the Belgian Professor Frans Alfons Janssens of the University of Leuven who described the phenomenon in 1909 and had called it 'chiasmatypie'. The term chiasma is linked if not identical to chromosomal crossover. Morgan immediately saw the great importance of Janssens' cytological interpretation of chiasmata to the experimental results of his research on the heredity of Drosophila. The physical basis of crossing over was first demonstrated by Harriet Creighton and Barbara McClintock in 1931.[1]

ChemistryEdit

Meiotic recombination initiates with double-stranded breaks that are introduced into the DNA by the Spo11 protein.[2] 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.[3] The recombinases catalyze invasion of the opposite chromatid by the single-stranded DNA from one end of the break. 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. The contact between two chromatids that will soon undergo crossing-over is known as a chiasma. The Holliday junction is a tetrahedral structure which can be 'pulled' by other recombinases, moving it along the four-stranded structure.

Holliday Junction  
Molecular structure of a Holliday junction.  

ConsequencesEdit

File:Conversion and crossover.jpg

In most eukaryotes, a cell carries two versions of each gene, each referred to as an allele. 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. 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 homologous chromosomes.

Recombination results in a new arrangement of maternal and paternal alleles on the same chromosome. Although the same genes appear in the same order, the alleles are different. In this way, it is theoretically possible to have any combination of parental alleles in an offspring, and the fact that two alleles appear together in one offspring 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.[4] However, 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. 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. 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. This concept is applied when searching for a gene that may cause a particular disease. This is done by comparing the occurrence of a specific DNA sequence with the appearance of a disease. When a high correlation between the two is found, it is likely that the appropriate gene sequence is really closer.[5]

ProblemsEdit

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. 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.

ReferencesEdit

  1. Creighton H, McClintock B (1931). A Correlation of Cytological and Genetical Crossing-Over in Zea Mays. Proc Natl Acad Sci USA 17 (8): 492–7. (Original paper)
  2. Keeney, S (1997). Meiosis-Specific DNA Double-Strand Breaks Are Catalyzed by Spo11, a Member of a Widely Conserved Protein Family. Cell 88 (3): 375–84.
  3. Sauvageau, S; Stasiak, Az; Banville, I; Ploquin, M; Stasiak, A; Masson, Jy (Jun 2005). Fission Yeast Rad51 and Dmc1, Two Efficient DNA Recombinases Forming Helical Nucleoprotein Filaments. Molecular and Cellular Biology 25 (11): 4377–87.
  4. genetic recombination.
  5. Genetic Recombination

See alsoEdit

Template:Genetic recombination

This page uses Creative Commons Licensed content from Wikipedia (view authors).

Around Wikia's network

Random Wiki