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 pair of bases in DNA is held together by
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → Hydrogen bonds
The passage states that in the secondary structure of DNA the two nucleotide chains form a right-handed helix in which the two strands are held together by hydrogen bonds. These hydrogen bonds form between specific pairs of bases: adenine with thymine and cytosine with guanine, which is why the two strands are complementary.