Nucleic acids are mainly of two types: deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). A nucleotide is composed of a base, a C5-carbohydrate and phosphate group, which are bonded in the sequence base–carbohydrate–phosphate. The carbohydrate molecule in DNA is \(\beta\)-D-2-deoxyribose, whereas in RNA it is \(\beta\)-D-ribose. DNA contains four bases, viz. two purine bases adenine and guanine and two pyrimidine bases cytosine and thymine. RNA also contains four bases: the same two purine bases adenine and guanine and two pyrimidine bases cytosine and uracil (in the place of thymine). In nucleic acids the nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar. The secondary structure of DNA shows that two nucleotide chains run in opposite directions and form a right-handed helix in which the two strands are held together by hydrogen bonds at definite distances. The double helix of DNA is the storehouse of the hereditary information of the organism. The process in which a DNA molecule is copied to give an identical DNA molecule is known as replication.
The main point of difference between DNA and RNA is the
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Presence of deoxyribose and thymine in DNA, ribose and uracil in RNA
The passage states that the sugar in DNA is β-D-2-deoxyribose and in RNA it is β-D-ribose. It also states that both contain adenine, guanine and cytosine, but the fourth base is thymine in DNA and uracil in RNA (in the place of thymine).
Why the other options are incorrect
Option (1) Presence of thymine in both DNA and RNA — Incorrect. RNA has uracil, not thymine; and a feature common to both cannot be a difference.
Option (3) Presence of ribose and thymine in DNA, deoxyribose and uracil in RNA — Incorrect. The sugars are swapped: DNA has deoxyribose and RNA has ribose.
Option (4) Presence of deoxyribose and uracil in DNA, ribose and thymine in RNA — Incorrect. The sugars are right but the bases are swapped: thymine is in DNA and uracil in RNA.