Page 50 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

6.6 Site-Specific Recombination 209

Figure 6.29 Detection of gene conversion in yeast • Recombination events result in crossing-over only part of
tetrads. Throughout this figure, the blue and red lines represent the time because helicases can disentangle the
single DNA strands. (a) Recombination during meiosis in an A B C / chromatids before Holliday junctions form.
a b c diploid yeast cell generates heteroduplex regions in which • Gene conversion, a process whereby one allele in a
each DNA strand has different alleles of gene B. Conversion of b to heterozygote is physically changed into the other,
B by mismatch repair (black Bs) results in an unusual tetrad with a provides evidence for heteroduplex formation during
3:1 ratio of B:b alleles instead of 2:2. In this case, the recombination
event resulted in crossing-over and thus recombination of the recombination events.
alleles of the flanking genes A and C. The tetrad is T with respect
to A and C. (b) Here, the recombination event is resolved by the
noncrossover pathway. Because crossing-over does not occur, the
resulting tetrad is PD with respect to genes A and C. However,
mismatch repair of the heteroduplex region converts b into B, so
this tetrad also shows a 3:1 ratio of B:b.
(a) Gene conversion with crossing-over Meiosis II 6.6 Site-Specific Recombination
A B C
A B C
A B C Meiosis I A B C
A B C
A B C
A B c Mismatch repair A B c A B c learning objectives
A b c converts b to B A B c A B c
1. Diagram the possible outcomes of site-specific
a B C
a B C
a b C a B C recombination.
a b c a B C
a B C 2. List the components that would have to be introduced
a b c
a b c a b c to import site-specific recombination into a newly
a b c a b c discovered organism.
Genes A and C: T tetrad
Genes A and B Tetrad neither 3. Contrast the functions of Spo11 and Cas9, two
Genes B and C PD, NPD, nor T enzymes that catalyze the formation of double-
strand breaks.
(b) Gene conversion without crossing-over Meiosis II
A B C
A B C
A B C Meiosis I A B C
A B C
A B C A B C
A B C Mismatch repair A B C Homologous recombination, as discussed in the previous
converts b to B A B C
A B C A B C section, begins with preexisting DNA molecules, breaks
a B c a B c them apart, and then rejoins them to create new se-
a b c a B c quences of DNA. Natural selection then tests these new
a b c a B c
a B c DNA molecules for their ability to help the organisms in
a b c a b c
a b c
a b c which they are found to survive and reproduce in a chang-
a b c
Genes A and C: PD tetrad ing environment. The more types of DNA molecules that
Genes A and B Tetrad neither are created in a population of organisms, the greater is
Genes B and C PD, NPD, nor T
the possibility that the population will continue in future
generations. It is thus not surprising that homologous re-
combination can occur nearly at random at any of a very
large number of sites in a genome, likely between any
two adjacent pairs of nucleotides. In this way, homolo-
essential concepts gous recombination helps to produce an enormous diver-
sity in chromosome base sequences upon which natural
• In tetrad analysis, the existence of Ts and the very low selection can act.
number of NPDs observed establishes that recombination
occurs after chromosome replication, when each pair of
homologs contains four chromatids. T and NPD tetrads
exhibit equal numbers of both classes of recombinants,
indicating reciprocal exchange. Recombinase Enzymes Catalyze
• The exchange of chromosome parts during Recombination Between Specific
recombination involves the breakage and rejoining DNA Sequences
of DNA molecules.
• At the molecular level, crossing-over during meiotic In contrast with this type of nearly random homologous
prophase entails the formation of heteroduplex DNAs recombination, some organisms find it useful to have sys-
between two Holliday junctions and resolution of the tems of site-specific recombination that promote the
junctions by endonucleases and DNA ligase. breakage and rejoining of DNA molecules at particular