genetic recombination in eukaryotes · the mechanism of crossing-over two types of homologous...
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Genetic Recombinationin Eukaryotes
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In meiosis, recombinant products with new combinations of parental alleles aregenerated by:
1. independent assortment (segregation) of alleles on nonhomologouschromosomes.
2. crossing-over in meiotic synaptonemal complexes between nonsisterhomologs.
MessageRecombinants are thoseproducts of meiosis withallelic combinationsdifferent from those ofthe haploid cells thatformed the meiotic diploid.
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Meiosis in a diploid dihybrid cell.
Genotype A/a; B/b
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The detection of recombination in diploid organisms.The advantage of a testcross (homozygote recessive tester)
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Independent assortment
Two unlinked genes producealways a recombinant frequencyof 50%.
(Testcross of a dihybrid)
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Self of a dihybrid
Prunett square showing thegenotypic and phenotypicratios.
2783
...
3n2nn
942
321
GenotypesPhenotypesGenes
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Cross between two A/a ; B/b dihybrids– recombination occurs in both members of cross– recombination frequency is 50%
a/a ; b/ba/a ; B/bA/a ; b/bA/a ; B/ba ; b
a/a ; B/ba/a ; B/BA/a ; B/bA/a ; B/Ba ; B
A/a ; b/bA/a ; B/bA/A ; b/bA/A ; B/bA ; b
A/a ; B/bA/a ; B/BA/A ; B/bA/A ; B/BA ; B
a ; ba ; BA ; bA ; B
1 a/a ; b/b3 a/a ; B/–3 A/– ; b/b9 A/– ; B/–Ratio:
Dihybrid selfing
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9:3:3:1 segregation in maize
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Crossing-overChiasmata at meiosis.
Each line represents a chromatid of a pair of synapsed chromosomes
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In dihybrids for linked genes, recombinants arise from meioses in which nonsister chromatids cross over between the genes under study.
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Recombinants produced by crossing-over
Linkage Symbolism:
A B a bA B A B
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Genetic Maps (linkage maps)
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Message
Recombination between linked genes can be used to map theirrelative distance on the chromosome. The map unit (1m.u. or 1cM)is defined as a recombinant frequency of 1%.
In a dihybrid of linked genes the RF will be between 0% and 50%.
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Recombination frequency (RF)
• Experimentally determined from frequency ofrecombinant phenotypes in testcrosses
• Roughly proportional to physical length of DNAbetween loci
• Greater physical distance between two loci,greater chance of recombination by crossing-over
• 1% recombinants = 1 map unit (m.u.)• 1 m.u. = 1 centiMorgan (cM)
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Linkage maps
• RF is (60+50)/400=27.5%, clearly less than 50%• Map is given by:
# observed
140
50
60
150
A B
27.5 m.u.
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Mapping
• RF analysis determines relative geneorder
• RF between same two loci may bedifferent in different strains or sexes
• RF values are roughly additive up to 50%– multiple crossovers essentially uncouple loci,
mimicking independent assortment• Maps based on RF can be combined with
molecular and cytological analyses toprovide more precise locations of genes
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Genetic maps
• Useful in understanding andexperimenting with the genome oforganisms
• Available for many organisms in theliterature and at Web sites
• Maps based on RF are supplemented withmaps based on molecular markers,segments of chromosomes with differentnucleotide sequences
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Comparison of physical andgenetic maps
The yeast chromosome 1 is shown.
A) indicates a region where thegenetic map is contracted owing todecreased frequency of crossing-over.
B) indicates a region where thegenetic map is expanded owing toincreased frequency of crossing-over.
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The mechanism of crossing-over
Three types of DNA recombination:
1. Homologous recombination2. Site-specific recombination3. Illegitimate recombination
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The mechanism of crossing-over
Two types of homologous recombination.
Crossover between two dsDNA molecules results in the reciprocal exchange ofDNA. Gene conversion involves a nonreciprocal transfer. The donor sequenceremains unchanged, while the recipient sequence is changed.
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The Meselson-Raddingheteroduplex model.
a) single stranded nickb) DNA polymerasec) ssDNA displaces its
counterpart in thehomologue
d) displaced ssDNA isdigested
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The Meselson-Raddingheteroduplex model.
e) ligation completes theformation of a Hollidayjunction
f) resolution according toHolliday model in twoalternative ways creats eithera crossover chromatid (V) or anon-crossover chromatid (H).
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Repair of mismatched nucleotides in heteroduplex DNA
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fungal tetrads for segregation analysis
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An A/a meiocyte undergoesmeiosis, resulting in an equalnumber of A and a products.
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The abberant 5:3 octad is explained by aheteroduplex formed during meiosis. In this case thenucleotide differences in the heteroduplex are notrepaired.
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The role of RecA in strand transfer.
The E. coli RecA protein binds to ssDNA.The resulting nucleoprotein complexaggregates with dsDNA in a triple-stranded DNA complex in which thebases do not pair. This complexfacilitates invasion of the ssDNA.Strands are subsequently exchanged anda heteroduplex can be formed.
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Site-specific recombinationinvolves defined DNA sites, is independent of RecA, and requires specificenzymes. (Examples: bacteriphage λ)
Integration of λ DNA into the E. coli chromosomeinvolves site-specific recombination between the attPsequence of the phage and the bacterial attBsequence. The recombination is catalysed by anintegrase.
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Illegitimate recombinationdoes not require segments of homologous DNA.
(Examples: transposable elements, T-DNA)
T
T
T
1
2
3
T = transposable element
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Mitotic Crossing-over
MessageA mitotic crossover generates homozygosity ofalleles of heterozygous loci distal to the crossover.
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Mitotic Recombination in Drosophila
Cross: y+ sn / y+ sn X y sn+ / y sn+
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y sn+ / y sn y sn+ / y sn+ y+ sn / y+ sn
Mitotic Recombination in Drosophila