Page 43 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

202 Chapter 6 DNA Structure, Replication, and Recombination

in the remainder of this chapter, we use recombination is lost from one chromatid is gained from the other, and
more narrowly to mean the generation of new allelic com- vice versa. We know that this assumption is legitimate be-
binations through genetic exchange between homologous cause of the results of tetrad analysis.
chromosomes. In this discussion, we refer to the products Suppose you are following linked genes A and B in a
of crossing-over as recombinants: chromosomes that cross between A B and a b strains of yeast. If the recombina-
carry a mix of alleles derived from different homologs. tion that occurs during meiosis is reciprocal, every tetrad with
In eukaryotic organisms, recombination has an addi- recombinant progeny should contain equal numbers of both
tional essential function beyond generating new combina- classes of recombinants. Observations have in general con-
tions of alleles: It helps ensure proper chromosome firmed this prediction: Every T tetrad carries one A b and one
segregation during meiosis. Chapter 4 has already de- a B spore, while every NPD tetrad contains two of each type
scribed how crossovers, in combination with sister chroma- of recombinant (recall Fig. 5.24). We can thus conclude that
tid cohesion, keep homologous chromosomes together as meiotic recombination is almost always reciprocal. We say
bivalents during the period between prophase I and meta- almost always because as you will see later in this chapter, rare
phase I. If homologs fail to recombine, they often cannot exceptions exist in which a tetrad cannot be classified PD,
orient themselves toward opposite poles of the meiosis I NPD, or T. These exceptional tetrads helped scientists to un-
spindle, resulting in nondisjunction. derstand key features of the recombination mechanism.
As we examine recombination at the molecular level,
we look first at experiments establishing the basic parame-
ters of crossing-over. We will then describe the molecular DNA Molecules Break and Rejoin
details of a crossover event. During Recombination

When viewed through the light microscope, recombinant
Tetrad Analysis Illustrates Key chromosomes bearing physical markers appear to result from
Aspects of Recombination two homologous chromosomes breaking and exchanging
parts as they rejoin (see Fig. 5.7). Because the recombined
You saw in Chapter 5 that some fungi like yeast and Neu- chromosomes, like all other chromosomes, are composed of
rospora generate asci that contain in one sac all the prod-
ucts of individual meioses—that is, tetrads. Analysis of
these tetrads allowed geneticists to infer basic information Figure 6.25 Recombination after chromosome replication.
about recombination. (a) Because recombination occurs after chromosomes have replicated,
most tetrads containing recombinant spores are Ts. (b) A disproven
model. If recombination occurred before chromosomes replicated and
if two genes were linked, most tetrads containing recombinant spores
Evidence that recombination takes would be NPDs instead of Ts.
place at the four-strand stage (a)
Recall that in tetrad analysis, the hallmark of linkage be- Duplication Recombination Meiosis I Meiosis II
tween two genes is that very few NPD tetrads are produced: a b
The number of PDs, and also Ts, is always greater than the a b a b a b
number of NPDs. This outcome makes sense because all a b +
SCO and some DCO meioses yield Ts, while only one-quarter a b a b a b +
of the rare DCOs yield NPDs (review Figs. 5.23 and 5.24). a + b + a + b + a + b a + b
The very low number of NPDs actually observed in
crosses establishes that recombination occurs after the a + b + a + b + a + b +
chromosomes have replicated, when four chromatids exist a + b +
for each pair of homologs (Fig. 6.25a). If recombination T
instead took place before chromosome duplication, every
single crossover event (that is, every SCO) would yield four (b)
recombinant chromatids and generate an NPD tetrad; such Recombination Duplication Meiosis I Meiosis II
a model for recombination would be unable to account for a b +
any T tetrads (Fig. 6.25b). a b +
a b +
a b a b + a b +
a b +
Evidence that recombination is usually reciprocal a + b a + b
The discussion in Chapter 5 assumed that recombination is a + b + a + b a + b
reciprocal, with nonsister chromatids from homologous a + b a + b
chromosomes exchanging parts equally. That is, whatever NPD