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.
Consider the following statements about the structure of nucleic acids.
(A) A unit formed by the attachment of a base to the 1′ position of the sugar is known as a nucleoside.
(B) A nucleotide is obtained when a nucleoside is linked to phosphoric acid at the 3′ position of the sugar moiety.
(C) In the secondary structure of RNA, a single stranded helix is present which sometimes folds back on itself.
(D) The three types of RNA molecules, m-RNA, r-RNA and t-RNA, perform the same function in the cell.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (C) only
A nucleic acid is built up step by step: a base joins a pentose sugar to give a nucleoside, and the nucleoside joins phosphoric acid to give a nucleotide. The sugar carbons are numbered 1′, 2′, 3′ … to tell them apart from the atoms of the base.
(A) A unit formed by the attachment of a base to the 1′ position of the sugar is known as a nucleoside. — True. The base is joined at C-1′ of the sugar; there is no phosphate in a nucleoside.
(B) A nucleotide is obtained when a nucleoside is linked to phosphoric acid at the 3′ position of the sugar moiety. — False. The phosphoric acid is linked at the 5′ position of the sugar, not at 3′.
(C) In the secondary structure of RNA, a single stranded helix is present which sometimes folds back on itself. — True. DNA is a double strand helix, but RNA has a single strand.
(D) The three types of RNA molecules, m-RNA, r-RNA and t-RNA, perform the same function in the cell. — False. The three types are messenger, ribosomal and transfer RNA, and they perform different functions.
NCERT (2026-27 edition):
10.5.2 Structure of Nucleic Acids – “A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside. … When nucleoside is linked to phosphoric acid at 5′-position of sugar moiety, we get a nucleotide…”
“In secondary structure of RNA single stranded helics is present which sometimes foldsback on itself. RNA molecules are of three types and they perform different functions. They are named as messenger RNA (m-RNA), ribosomal RNA (r-RNA) and transfer RNA (t-RNA).”