Overview

During reproductive cycles genetic information is transferred vertically from parent to offspring, and from progenitor to descendant. Genes can also be transferred horizontally between contemporaneous individuals. In this book, horizontal gene transfer is defined in a broad, inclusive way: the incorporation of DNA from any external source into the genome of the recipient life form – a cell, a virus, a mitochondrion, but excluding genomes of parasites that invade a cell. This chapter focuses on horizontal gene transfer in bacteria.

Types of Horizontal Transfer

In bacteria, there are three kinds of transfer of genes between cells:

• Transformation – the cell takes up exogenous DNA directly from the surrounding medium

• Conjugation – two cells join and a donor cell transfers DNA, usually a plasmid, to a recipient cell

• Transduction – bacteriophages transport DNA from one cell to another

Mitochondria transfer genes horizontally infrequently, when two genetically different mitochondria fuse and their chromosomes undergo recombination. The same is true of chloroplasts. Viruses can exchange genes by recombination, following infection of the host cell by two or more genetically different virions.

Transformation

Transformation is the alteration of a cell's genome by acquiring exogenous DNA. In nature, the acquired DNA is integrated into a main chromosome. The three major events of natural transformation are as follows:

• Binding of double-stranded DNA to receptor proteins

• Uptake of a single strand of the bound DNA

• Integration of that DNA into the genome

Transformation does not include viral infection and does not occur by cell contact.