Question

In this chapter, we have focused on genetic systems present in bacteria and on the viruses that use bacteria as hosts (bacteriophages). In particular, we discussed mechanisms by which bacteria and their phages undergo genetic recombination, which allows geneticists to map bacterial and bacteriophage chromosomes. In the process, we found many opportunities to consider how this information was acquired. From the explanations given in the chapter, what answers would you propose to the following questions? (a) How do we know that genes exist in bacteria and bacteriophages? (b) How do we know that bacteria undergo genetic recombination, allowing the transfer of genes from one organism to another? (c) How do we know whether or not genetic recombination between bacteria involves cell-to-cell contact? (d) How do we know that bacteriophages recombine genetic material through transduction and that cell-to-cell contact is not essential for transduction to occur? (e) How do we know that intergenic exchange occurs in bacteriophages? (f) How do we know that in bacteriophage T4 the \(r I I\) locus is subdivided into two regions, or cistrons?

Short answer

Question: List some experiments that provide evidence for genetic recombination in bacteria and bacteriophages. Answer: The Avery-MacLeod-McCarty experiment provided evidence for DNA being the genetic material in bacteria, while Lederberg and Tatum's experiments demonstrated the occurrence of genetic recombination in bacteria through conjugation. Studies on bacteriophage transduction and intergenic exchange revealed the exchange of genetic material between bacteriophages, and the subdivision of the rII locus in bacteriophage T4 provided further insight into bacteriophage genetic systems.

Step-by-step solution

(a) Evidence for genes in bacteria and bacteriophages

Genes exist in bacteria and bacteriophages because of the presence of nucleic acids, specifically DNA, that have been observed and characterized in them. Many experiments, such as the Avery-MacLeod-McCarty experiment, have demonstrated that DNA is the genetic material that allows for inheritance and the passing of traits in these organisms. Additionally, the study of bacterial and bacteriophage mutations has provided further evidence for the existence of genes and their role in the inheritance of traits.

(b) Evidence for bacterial genetic recombination

Experiments conducted by Joshua Lederberg and Edward Tatum have shown that bacteria can undergo genetic recombination through a process called conjugation. In their classic experiments using Escherichia coli strains, they demonstrated that bacteria could exchange genetic material through physical contact between cells. This transfer of genes from one organism to another results in new combinations of traits, providing evidence for the occurrence of genetic recombination in bacteria.

(c) Cell-to-cell contact in genetic recombination

Research on bacterial conjugation has shown that genetic recombination between bacteria involves cell-to-cell contact. Experiments using a U-tube apparatus, where two different bacterial strains are separated by a filter that prevents physical contact but allows liquid exchange, failed to produce recombinant bacteria. This finding supports the idea that direct contact between bacterial cells is necessary for genetic recombination to occur.

(d) Bacteriophage transduction and cell-to-cell contact

Studies on bacteriophage genetic recombination have revealed that a process called transduction is responsible for the exchange of genetic material between bacteriophages. During transduction, a bacteriophage infects a bacterial cell and inadvertently packages a piece of bacterial DNA into its capsid. When this bacteriophage infects another bacterial cell, it transfers the packaged bacterial DNA, allowing for recombination. Since bacteriophages don't require cell-to-cell contact for infection, this process occurs without any direct interaction between bacterial cells themselves.

(e) Intergenic exchange in bacteriophages

Intergenic exchange in bacteriophages is evidenced by the discovery of hybrid bacteriophages containing genetic material from different parental phages. This recombination process has been demonstrated in experiments where two genetically distinct bacteriophages infect the same bacterial cell. The subsequent offspring phages can display a combination of characteristics from both parental phages, providing evidence for intergenic exchange.

(f) Subdivision of the \(r I I\) locus in bacteriophage T4

The subdivision of the \(r I I\) locus in bacteriophage T4 has been shown through genetic studies and observations of recombination frequencies. Mutations at different sites within the \(r I I\) locus have been identified, and these mutations lead to defects in different aspects of the phage's development. By analyzing the recombination frequencies between these different mutations, researchers have been able to infer the existence of two separate regions, or cistrons, within the \(r I I\) locus.