Question

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?

Short answer

Answer: Bacteria undergo genetic recombination, as demonstrated by the Lederberg and Tatum experiment, where mixed bacterial strains formed colonies by exchanging genetic material and creating new recombinant strains. The role of cell contact in conjugation was demonstrated by Bernard Davis's U-tube experiment, which showed that genetic recombination did not occur when bacteria were physically separated in the growth medium. Transduction, where genetic material is transferred by bacteriophages, does not require cell-to-cell contact, as demonstrated in experiments by Joshua Lederberg and Norton Zinder.

Step-by-step solution

(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.

(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.

(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.

Key concepts

Genetic Recombination

Genetic recombination in bacteria is a process where DNA is exchanged between different organisms, resulting in new genetic combinations. This concept is vital for understanding how bacteria evolve and adapt to their environments.

A classic experiment by Lederberg and Tatum illustrated this phenomenon. They took two bacterial strains with different nutritional needs and grew them together in minimal media. Without the exchange of DNA, neither strain could survive on its own. However, the appearance of new colonies indicated that genetic material had been transferred, forming recombinant bacteria capable of independent growth. This demonstrated that bacteria could exchange genes, providing evidence for genetic recombination.

Such recombination enables bacteria to acquire new traits, such as antibiotic resistance, highlighting its importance in microbiology and medicine.

Chromosome Mapping

Chromosome mapping refers to determining the positions of genes on a chromosome. In bacteria, this process is intricately linked with genetic recombination since it involves tracking how genes are transferred during recombination events.

Using methods like conjugation and transduction, scientists can map bacterial chromosomes by observing the order in which genetic markers are exchanged. By analyzing recombinants from controlled experiments, researchers can create detailed maps that show the relative positions of genes.

This information is crucial for understanding bacterial genetics and aiding in gene manipulation, improving our ability to develop treatments for bacterial infections.

Bacteriophages

Bacteriophages, or phages, are viruses that infect bacteria. These unique entities play a significant role in genetic recombination, particularly through the process of transduction.

Phages attach to a bacterial cell and inject their DNA into it. Sometimes, during this process, pieces of bacterial DNA are accidentally packaged into new phage particles. When these phages infect another bacterium, they introduce not only viral DNA but also fragments of bacterial DNA from their previous host.

This transfer of genetic material by bacteriophages bridges bacterial cells, providing a method of genetic exchange that does not require direct cell-to-cell contact. Understanding phages is important for genetic engineering and developing bacteriophage therapy to treat bacterial infections.

Conjugation

Conjugation is a mechanism of genetic recombination in bacteria that involves the direct transfer of DNA from one cell to another through cell-to-cell contact.

The U-tube experiment by Bernard Davis elegantly demonstrated this need for contact. Bacteria were placed in a two-chamber tube with a filter that allowed the movement of fluids but not cells. No genetic recombination occurred across the filter, emphasizing that physical contact is essential for conjugation.

In conjugation, a donor bacterium extends a pilus to connect to a recipient. DNA is then transferred, usually in the form of a plasmid, allowing the recipient to acquire new genetic traits. This ability to share genes, such as those conferring antibiotic resistance, is key to bacterial adaptation.

Transduction

Transduction is a form of genetic recombination where phages facilitate the transfer of genetic material between bacteria, eliminating the need for direct contact.

In the experiments by Lederberg and Zinder, it was shown that even after bacterial cells were eliminated by filtration, genetic exchange occurred, thanks to bacteriophages acting as intermediaries. The phages could transfer bits of DNA from one bacterium to another during infection.

This process allows for horizontal gene transfer, contributing to genetic diversity in bacterial populations. Transduction shows how viruses can affect bacterial evolution and has implications in gene therapy and the development of tools for genetic engineering.