Which statement best describes how restriction enzymes likely evolved?
A They are part of DNA repair mechanisms in eukaryotic cells
B They are a defense mechanism against viruses in bacteria
C They are replication enzymes of yeast
D They are transposable elements of Drosophila
E All of the above
The correct answer and explanation is :
Correct Answer: B – They are a defense mechanism against viruses in bacteria
Explanation:
Restriction enzymes, also known as restriction endonucleases, are proteins that cut DNA at or near specific recognition nucleotide sequences known as restriction sites. These enzymes are found naturally in prokaryotes, particularly bacteria and archaea, and their evolutionary origin is tightly linked to the bacterial immune defense system against viral infections.
When viruses called bacteriophages infect bacteria, they inject their DNA into the bacterial cell with the intent of hijacking the host’s cellular machinery to replicate the viral genome. To combat this, bacteria evolved restriction enzymes as a primitive immune system. These enzymes recognize specific DNA sequences (typically palindromic sequences) and cleave foreign DNA, preventing viral replication.
To protect their own DNA from being degraded by their restriction enzymes, bacteria also produce methyltransferases, which methylate (add methyl groups to) the host DNA at the recognition sites, making it unrecognizable to their own restriction enzymes. This dual system is known as the restriction-modification (R-M) system, an elegant form of self versus non-self recognition.
The incorrect answer choices can be ruled out as follows:
- A (DNA repair in eukaryotes): Restriction enzymes are not primarily involved in DNA repair mechanisms in eukaryotic cells. While eukaryotes do have enzymes that cut DNA (like during recombination or repair), they do not use restriction enzymes like those in bacteria.
- C (Replication enzymes of yeast): Yeast, a eukaryote, does not naturally possess restriction enzymes. Yeast relies on different replication machinery.
- D (Transposable elements of Drosophila): Transposable elements are segments of DNA that can move around within the genome, but they are not the same as restriction enzymes.
- E (All of the above): Since only one option is correct, this choice is invalid.
In summary, restriction enzymes most likely evolved as a defense mechanism in bacteria against viral infection, and this function has been co-opted by molecular biologists for a wide range of genetic engineering techniques.