Overview

The chromosomes of all life forms naturally undergo recombination, a physical exchange between two DNA molecules that results in new, recombinant DNA molecules. Recombination plays many roles: producing new combinations of gene copies, introducing novel sequences into chromosomes, making novel genes, regulating gene expression, and facilitating some DNA repair. Recombination has a huge impact on the biology of sexually reproducing eukarya.

There are three main kinds of recombination in nature: (1) general recombination, an exchange between homologous DNA molecules, in which the location of the exchange site is not restricted, (2) site-specific recombination, an exchange between nonhomologous DNA molecules and occurring only at specific short sequences, and (3) transposition, the movement of a transposon.

This chapter focuses on the molecular events of recombination, not on its biological consequences, analysis, or practical uses, all very important in genetics.

General Recombination

Homologous DNA molecules (different copies of a double helix, such as the two copies of chromosome 1 that you received from your mother and father), when they are together inside a cell, exchange parts by general recombination (Figure 14.1). In general recombination, portions of double-stranded DNA (dsDNA) chromosomes having extensive homology pair with the help of DNA-binding proteins. At least some homologous sequences in the two DNA molecules are aligned precisely. The two DNA molecules exchange strands in a multistep process catalyzed by many enzymes.

The Aviemore Model of General Recombination

General recombination is not identical in all life forms, and there are many theories of molecular mechanisms.