Question

Assertion -
The sugar-phosphate backbone of two chains in DNA double helix shows antiparallel polarity.
Reason -
The phosphodiester bond in one strand goes from a 3 carbon of one nucleotide to 5 carbon of adjacent nucleotide whereas those in complementary strength go vice-versa.

A. Both assertion and reason are correct, and the reason is the correct explanation of assertion.
B. Both assertion and reason are correct, but the reason is not the correct explanation for the assertion.
C. Assertion is correct, but the reason is incorrect.
D. Both assertion and reason are incorrect.

Answer 1

The DNA is a genetic material found in the nucleus of almost every cell. It is made up of nucleotides that are made up of nitrogenous bases (A, G, C, T), a phosphate group and ribose sugar – a kind of pentose or five-carbon sugar.

Complete answer:
The DNA is made up of two similar strands running in the opposite direction (anti-parallel). The direction of one strand is 5’ to 3’ while the opposite or antiparallel strand is arranged in 3’ to 5’ direction. They are connected with the help of hydrogen bonds.

Each strand of DNA helix is made up of all three basic components:
A nitrogenous base (Adenine, Guanine, Cytosine and Thymine), ribose sugar (a five-carbon sugar) and a phosphate group that joins the two sugar molecules. The nitrogenous base is attached to the first carbon of ribose sugar. The backbone of each strand is made up of ribose sugar. These ribose sugars are connected with the help of a phosphodiester bond.

The phosphodiester bond is formed between 3' carbon atoms of a sugar molecule and 5' carbon atoms of another sugar molecule. The 3rd carbon and 5th carbon of both sugars are connected with the oxygen atom, making an ester bond that is why it is called a phosphodiester bond.

The two strands of DNA made in this way, are held together in opposite or antiparallel directions with the help of hydrogen bonds.

Hence, the correct answer is option (A).

Note: DNA is a double helical molecule which contains different strands running parallel to each other in anti-parallel or opposite direction. These strands are held with each other because of hydrogen bonding between nitrogenous bases.