Genetic Transfer in Bacteria and Viruses
Humans are not the only organisms with hereditary processes. In fact, bacterias have the ability to pass on their genome from cell to cell. Most bacterial genomes contain a double stranded circular molecule. In addition, some bacterias carry plasmids, which are smaller DNA circles that contains extra DNA that can replicate asexually. In another case, bacterias can be parasitized by certain viruses known as bacteriophages or phages which can have either DNA or RNA as their genetic material.
There are many ways in which bacteria undergo genetic recombination. The first method of gene exchange that will be discussed is conjugation. Conjugation was first experimented using a U-shaped tube where the two ends of the tube were separated by a fine filter. In addition, there were small pores on the filter which small enough to not let bacteria pass. With strain A located in one arm and strain B in the other, Bernard Davis observed that there were no prototrophic cells resent. As a result, after incubation and plating, there were no recombinant colonies present without cell-to-cell contact.During conjugation, the F plasmids are transferred when the pilus of a bacteria pulls the other bacteria from a different cell in which a single stranded copy of plasmid DNA is produced from donor to recipient cell.
The next type of bacterial genetic exchange is transformation where bacteria exchange DNA from the external environment from a different genotypic recipient. This uptake of free DNA is usually from a dead bacterial cell. This process is important because it can be used to measure how closely related two genes are within one bacterial chromosome. The experiment that lead to the first insight on transformation was performed by Frederick Griffith where he demonstrated that DNA was the “transforming principle.” Most importantly, he discovered that the strain of Streptococcus pneumoniae became virulent after interacting with heat-killed virulent strains. As a result, he deduced that the heat-killed strain caused the harmless strain virulent. Through research with transformation in genetic engineering, there was evidence that eukaryotic cells can be transformed as well.
Last, but not least is transduction. This process allows phages to obtain foreign DNA and transfer them from one bacterial cell to another. However, transduction does not require a cell-to-cell contact like conjugation does. It was Joshua Lederberg and Norton Zinder using two different strains led to the discovery in a U-tube experiment the agent responsible for gene transfer. This agent was the same size as the phage Salmonella typhimurium mediated by a virus. There are two types of phage cycles during transduction. First are virulent phages that immediately lyse and kill the host. Second are temperate phages that stays in the host cell for a certain amount of time without killing it. The two kinds of transduction are generalized and specialized. In generalized transduction, a new phage picks up DNA from the host cell and then injecting it into a new cell at random. If recombination occurs, the bacterial DNA is incorporated into the recipient cell’s chromosome. As a result, generalized transduction is important in obtaining bacterial linkage information only when genes are genes are in close contact. However, In specialized transduction, a specific phage will insert at one position of the bacterial chromosome. Research done by Joshua and Esther Lederberg on a type of E.coli phage called lambda inserts by a single crossover at the lambda attachment site which is located between the gal and bio genes. In other words, transduction occurs when faulty outlooping which produces lambda phage. Thus the new phage contains both phage and bacterial genes.