In this chapter, we have focused on genetic systems present in bacteria and the viruses that use bacteria as hosts (bacteriophages). In particular, we discussed mechanisms by which bacteria and their phages undergo genetic recombination, the basis of chromosome mapping. Based on your knowl- edge of these topics, answer several fundamental questions: (a) How do we know that bacteria undergo genetic recombination, allowing the transfer of genes from one organism to another? (b) How do we know that conjugation leading to genetic recombination between bacteria involves cell contact, which precedes the transfer of genes from one bacterium to another? (c) How do we know that during transduction bacterial cell-tocell contact is not essential?

Step by step solution

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(a) Evidence for genetic recombination in bacteria

To establish that bacteria undergo genetic recombination, one must demonstrate the transfer of genetic material from one organism to another, resulting in the formation of new genotypic combinations. The Lederberg and Tatum experiment provides evidence for this. In their experiment, they mixed two bacterial strains, each with different nutritional requirements, in a minimal medium. The mixture resulted in the formation of colonies, which could only grow if genetic material had been exchanged between the strains, forming a new recombinant bacteria capable of synthesizing all the required nutrients for growth.

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(b) Conjugation and cell contact in genetic recombination

Scientists know that conjugation leading to genetic recombination between bacteria involves cell contact through famous U-tube experiment conducted by Bernard Davis. In this experiment, bacteria were separated by a filter which allowed the passage of medium but not actual cells. It was observed that no genetic recombination took place when the bacteria were physically separated but were allowed to share the medium. This demonstrated that conjugation, leading to genetic recombination, requires cell-to-cell contact, and the genetic material is not transferred through the growth medium.

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(c) Transduction and the non-requirement of cell contact

Transduction is a process where genetic material is transferred from one bacterium to another by a bacteriophage (virus that infects bacteria). In the experiments that discovered transduction (by Joshua Lederberg and Norton Zinder), they observed that genetic material was transferred between bacteria even after filtration, which would have removed any bacterial cells. This indicated the presence of an intermediate agent, like a virus, responsible for transferring genes. The subsequent experiments showed that the intermediate agents were bacteriophages, and cell-to-cell contact was not necessary for transduction to take place. Instead, bacteriophages transmit the genetic material between bacterial cells, entirely bypassing the need for direct contact.

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