Question

1. Scientists recently identified an extrachromosomal element in a Salmonella bacterium. They suspected that this element is linear. As this goes against the common line of thinking in bacterial genetics, they will have to accumulate a significant body of experimental evidence to support this hypothesis, before it is accepted in the field.

1. Explain why the existence of a linear bacterial extrachromosomal element goes against common knowledge in bacterial genetics.

2. Propose two experiments that the scientists might use to obtain data to support or reject this hypothesis.

3. If the element is indeed linear, what are the features that you would consider important to study in the long term?

Answer 1

a. Viral infection to the bacterial is one of the causes of the presence of extrachromosomal signal stranded DNA. Some viruses integrate their genome into the bacterial genetic material after infection to the bacteria while another class of viruses replicates in the cytoplasm without integrating int he genome of the bacteria. Therefore the existence of hypothesized linear extrachromosomal DNA in the Salmonella sp. could be due to the infection of the virus with linear DNA.

b. Transmission Electron Microscopy (TEM) is one of the best technique to know test genetic material is linear or circular. The DNA will be extracted from the cell and prepare the samples for TEM analysis on the carbon-coated grid and examine under the microscope. Another technique to distinguish between linear and circular DNA is agarose gel electrophoresis. Circular DNA has a tendency to form coiled structures. Depending upon their degree of coiling, circular DNA will give multiple bands on the agarose gel electrophoresis. While linear DNA will only give one band. Therefore by comparing the single band or multiple bands on the agarose gel one can distinguish between single and circular DNA.

c. First I would like to study the stability of the DNA. Then, I would be interested to know what is the expression product of the DNA and its effect on the bacteria. It is also equally important to know whether linear DNA will integrate into the bacterial genome and the impact of integration onto bacterial.