Bacteria and viruses are well suited to genetic studies: they are small, have a small haploid genome, undergo rapid reproduction, and produce large numbers of progeny through asexual reproduction.
The bacterial genome normally consists of a single, circular molecule of double-
DNA may be transferred between bacteria by means of conjugation, transformation, and transduction.
Conjugation is the transfer of genetic material from one bacterial cell to another. It is controlled by an episome called the F factor. The time it takes for individual genes to be transferred during conjugation provides information about the order of the genes, and the distances between them, on the bacterial chromosome.
Bacteria take up DNA from the environment through the process of transformation. Frequencies of the cotransformation of genes provide information about the physical distances between chromosomal genes.
The bacterium E. coli is an important model genetic organism that has the advantages of small size, rapid reproduction, and a small genome.
Viruses are replicating structures with DNA or RNA genomes that may be double stranded or single stranded, linear or circular.
Bacterial genes become incorporated into phage coats and are transferred to other bacteria by phages through the process of transduction. Rates of cotransduction can be used to map bacterial genes.
Phage genes can be mapped by infecting bacterial cells with two different phage strains and counting the number of recombinant plaques produced by the progeny phages.
A number of viruses have RNA genomes. Retroviruses encode a reverse transcriptase enzyme used to make a DNA copy of the viral genome, which then integrates into the host genome as a provirus.
HIV is a retrovirus that is the causative agent for AIDS.
Influenza is caused by RNA influenza viruses that evolve both by small changes taking place through mutation (antigenic drift) and by major changes taking place through the reassortment of genetic material from different strains.