14.7 Molecular Mechanisms

199

majuscule and the recessive allele written in miniscule. Ifasterisk∗represents a chromosome

break, we have for the duplex (concerning DNA strands numbered i to iv)

StartLayout 1st Row 1st Column i 2nd Column upper A upper B upper C 2nd Row 1st Column i i 2nd Column upper A upper B upper C 3rd Row 1st Column i i i 2nd Column a b c 4th Row 1st Column i v 2nd Column a b c EndLayout right arrow StartLayout 1st Row 1st Column i 2nd Column upper A upper B asterisk upper C 2nd Row 1st Column i i 2nd Column upper A asterisk upper B upper C 3rd Row 1st Column i i i 2nd Column a asterisk b c 4th Row 1st Column i v 2nd Column a b asterisk c EndLayout right arrow StartLayout 1st Row 1st Column i 2nd Column upper A upper B c 2nd Row 1st Column i i 2nd Column upper A b c 3rd Row 1st Column i i i 2nd Column a upper B upper C 4th Row 1st Column i v 2nd Column a b upper C EndLayout period

i

ABC

ii ABC

iii abc

iv

abc

→

i

AB ∗C

ii A ∗BC

iii a ∗bc

iv

ab ∗c

→

i

ABc

ii

Abc

iii aBC

iv abC

.

(14.10)

There is supposed to be about one crossover per chromosome per meiosis. In more

detail, the stages of recombination are the following:

1. Alignment of two homologous double-stranded molecules;

2. Breakage of the strands to be exchanged;

3. Approach of the broken ends to their new partners and formation of a fork (also

known as a Holliday junction);

4. Joining of broken ends to their new partners;

5. Prolongation of the exchange via displacement of the fork;

6. End of displacement;

7. Breakage of the 3 prime3^' extremities;

8. Separation of the two recombinant double strands;

9. Repair of the breaks via reading from the complementary strand.

The process is drawn in Fig. 14.4.

Unlike replication, in which occasional single-site (“point”) mutations occur due

to isolated errors, recombination results in changes in large blocks of nucleotides.

Correlations between mutations greatly depend on the number of chromosomes. In

species with few chromosomes, reshuffling is combinatorially limited and muta-

tions in different genes are likely to be transmitted together from one generation to

Fig. 14.4 Strand exchange

in homologous

recombination. The numbers

refer to the stages described

in the text