Restrictionmodification systems A exist in all organisms as

Restriction-modification systems:

A) exist in all organisms as a protection against foreign DNA invasion.

B) include a site-specific exonuclease to degrade foreign DNA.

C) modify DNA by acetylation.

D) include a DNA methylase and an endonuclease.

E) include three types of endonuclease all of which cleave within a specific DNA target sequence.

Solution

The answer is: D.  include a DNA methylase and an endonuclease

Restriction-modification systems include a DNA methylase and an endonuclease. It prevents invasion of foreign DNA in bacteria. The bacteria recognize the foreign DNA as it is not methylated as the host DNA.