Genetic Recombinationin Eukaryotes

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.

Meiosis in a diploid dihybrid cell.

Genotype A/a; B/b

The detection of recombination in diploid organisms.The advantage of a testcross (homozygote recessive tester)

Independent assortment

Two unlinked genes producealways a recombinant frequencyof 50%.

(Testcross of a dihybrid)

Self of a dihybrid

Prunett square showing thegenotypic and phenotypicratios.

2783

...

3n2nn

942

321

GenotypesPhenotypesGenes

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

9:3:3:1 segregation in maize

Crossing-overChiasmata at meiosis.

Each line represents a chromatid of a pair of synapsed chromosomes

In dihybrids for linked genes, recombinants arise from meioses in which nonsister chromatids cross over between the genes under study.

Recombinants produced by crossing-over

Linkage Symbolism:

A B a bA B A B

Genetic Maps (linkage maps)

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%.

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)

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.

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

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

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.

The mechanism of crossing-over

Three types of DNA recombination:

1. Homologous recombination2. Site-specific recombination3. Illegitimate recombination

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.

The Meselson-Raddingheteroduplex model.

a) single stranded nickb) DNA polymerasec) ssDNA displaces its

counterpart in thehomologue

d) displaced ssDNA isdigested

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).

Repair of mismatched nucleotides in heteroduplex DNA

fungal tetrads for segregation analysis

An A/a meiocyte undergoesmeiosis, resulting in an equalnumber of A and a products.

The abberant 5:3 octad is explained by aheteroduplex formed during meiosis. In this case thenucleotide differences in the heteroduplex are notrepaired.

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.

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.

Illegitimate recombinationdoes not require segments of homologous DNA.

(Examples: transposable elements, T-DNA)

T

T

T

1

2

3

T = transposable element

Mitotic Crossing-over

MessageA mitotic crossover generates homozygosity ofalleles of heterozygous loci distal to the crossover.

Mitotic Recombination in Drosophila

Cross: y+ sn / y+ sn X y sn+ / y sn+

y sn+ / y sn y sn+ / y sn+ y+ sn / y+ sn

Mitotic Recombination in Drosophila