B

Meiotic tetrad f Tetratype a b

Crossing-over after replication a B

Meiotic tetrad

Figure 5.5 Meiotic recombination between linked loci. Crossing-over (shown as a vertical line between homologous chromosomes) between genes AB and alleles ab occurs at the four-strand stage; that is, after homologous chromosomes had each duplicated into two chromatids. Products of meiotic division will divide by mitosis and the mature ascus will have eight ascospores.

occurred before replication, no tetratypes would have been found. Neurospora began to be used for understanding recombination mechanisms (Figure 5.6). Infrequently, a small proportion of aberrant 6 + : 2 mutant and 2 + : 6 mutant asci instead of normal 4 + : 4 mutant asci were detected (Mitchell, 1955). Such aberrant segregation ratios were explained by gene conversion according to which allele at a locus converts corresponding allele at the same locus into its own type. This observation provided clues on molecular models of crossing over that could account for gene conversion (Holliday, 1964).

Centromere -o

Chromosome

Parents

Chromatids

No crossing over Crossing over

4:4; First division segregation on o <nnp

4:4; First division segregation

Chromatids

No crossing over Crossing over

First Meiotic Division

First meiotic division

Second meiotic division

4 products of meiosis

Mitosis

First meiotic division

Second meiotic division

4 products of meiosis

Mitosis

2:2:2:2; Second division segregation

Figure 5.6 Scheme of first- and second-division segregation in Neurospora. Segregation of two + and - alleles can be deferred to the second division of meiosis, a phenomenon known as "second-division segregation."

0 0

Post a comment