Question

Read and select the correct and incorrect statement.
1) The four nucleotide bases are not necessarily present in DNA in exactly equal proportions.
2) The total amount of purines is equal to the total amount of pyrimidines.
3) DNA ligase enzyme act to hydrolyze or breakdown a polynucleotide chain into its component nucleotides.
4) Nuclease enzymes are capable of restoring an intact DNA duplex.

• A. 2 is correct but 1, 3 and 4 are incorrect.
• B. 1 and 2 are incorrect but 3 and 4 are correct.
• C. 1, 2 and 3 are correct.
• D. 2, 3 and 4 are correct but 1 is incorrect.

Answer 1

The correct option is A 2 is correct but 1, 3 and 4 are incorrect.

According to the Chargaff's rule, due to complimentary base pairing, the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine in the DNA from any cell of an organism. Thus, statement 1 is incorrect. Since the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine, the sum of purines is equal to the sum of pyrimidines; statement 2 is correct. DNA ligase enzyme joins two DNA fragments together via phosphodiester bond between 3' hydroxyl at the end of one fragment and 5' phosphate at the end of another fragment; statement 3 is incorrect. Nuclease enzymes cleave the DNA either within the strand or from one end, depending if it is endo- or exonuclease, thus breaking it into small fragments of nucleotides respectively. So, the correct answer is option A.