Concepts Summary

• 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-stranded DNA. Plasmids are small pieces of bacterial DNA that can replicate independently of the chromosome.
• DNA can be transferred between bacteria by conjugation, transformation, or transduction.
• Conjugation is the union of two bacterial cells and the transfer of genetic material between them. It is controlled by an episome called F. The rate at which individual genes are 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.
• Complete DNA sequences of many bacterial species have been determined. This sequence information indicates that horizontal gene transfer—the movement of DNA between species—is common in bacteria.
• Viruses are replicating structures with DNA or RNA genomes that may be double stranded or single stranded and 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.
• Benzer mapped a large number of mutations that occurred within the rII region of phage T4. The results of his complementation studies demonstrated that the rII region consists of two functional units that he called cistrons. He showed that intragenic recombination takes place.
• A number of viruses have RNA genomes. Retroviruses encode reverse transcriptase, an 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 through small changes taking place by mutation (antigenic drift) and through major changes taking place by the reassortment of the genetic material from different strains.