Question

Strain R of E. coli has an RM system that modifies/cleaves the sequence GAATTC. Sometimes invading phage DNA is methylated by the strain R methylase before the endonuclease can cleave the viral DNA, and then replicates in the methylated form and forms viral particles. If you add some of these viral particles to uninfected bacteria of strain R, what will happen? Select one: a. The invading phage DNA is unmethylated and will be cleaved by the strain Rendonuclease b. Strain R is resistant to all phage, no matter the source C. The invading phage DNA is methylated and protected from the strain Rendonuclease d. The invading phage DNA is methylated and thus cleaved by the strain Rendonuclease e. The invading phage DNA is unmethylated and is protected from the strain Rendonuclease

Answer 1

ANSWER

Option C is correct i.e. invading phage DNA is methylated and is protected from strain R endonuclease.

Explanation- Restriction-modification system (RM system) is type of immune response that bacteria have developed over time to protect their own DNA from foreign or viral DNA. In this system, bacterial DNA sequence is methylated at specific site by the enzyme methyltransferase. Methylation is done to differentiate between ''self'' and phage DNA. Thus, if the bacterium gets infected by viral DNA, it immediately cleaves off the non-methylated phage DNA by the enzyme, R endonuclease and stays protected from phage infection.

However, when phage DNA is already methylated and is grown with the uninfected E.coli strain, then, RM system would not be able to function efficiently. This is because bacterial R endonuclease would recognize methylated phage sequence as ''self DNA''. As a result, phage DNA would remain protected from strain R endonuclease.