The molecular mechanism is thought to comprise twokey steps:1.A double-strand break. One key piece of evidencehere was that in yeast transformation theincorporation of a circular plasmid into the yeastgenome is stimulated 1000-fold when the plasmid iscut to become linear. Broken DNA ends seem to berecombinogenic; that is, they promote recombination.2.The formation of heteroduplex DNA.This is ahybrid type of DNA molecule that is composed of asingle DNA strand from a chromatid derived fromone parent, and a single strand from a chromatidderived from the other parent.The first evidence for heteroduplex DNA was alsoprovided by genetics, specifically ascus analysis. Octadsare particularly informative in pointing to the existenceof heteroduplexes in crossing-over. We saw in Chapter2 that in fungi a cross A3awill create a monohybridmeiocyte A±athat is expected to segregate in a 1:1 ra-tio in the meiotic products according to the law ofequal segregation. Indeed the 1:1 ratio is found inmost fungal meiocytes: we see 4 Aand 4 a. However, inrare meiocytes (generally on the order of 0.1 to 1 per-
This is the end of the preview. Sign up
access the rest of the document.
This note was uploaded on 01/10/2011 for the course BIOL BIOL taught by Professor Johnson during the Spring '08 term at Aberystwyth University.