The nucleus of all living cells contain macromolecular species called chromosomes made up of proteins and another type of biomolecules called nucleic acids. The chromosomes are responsible for transmission of inherent characters called heredity. Nucleic acids are mainly two types: deoxyribonucleic acids (DNA) and ribonucleic acids (RNA). Nucleic acids are long chain polymers of nucleotides, so they are also known as polynucleotides. A nucleotide is composed of a base, a C₅-carbohydrate and phosphate group (orthophosphate, pyrophosphate or triphosphate), 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. The nucleotide act as a energy carrier and a cofactor or coenzyme for a redox enzymes. The removal of phosphate moiety from a nucleotide produces nucleoside. The N-glycoside of purine or pyrimidine bases with pentose sugars are known as nucleosides. DNA contains four bases, viz. two purine bases adenine and guanine and two pyrimidine bases cystosine and thyamine. RNA also contains four bases: the same two purine bases adenine and guanine and two pyrimidine bases cystosine and uracil (in the place of thyamine). In nucleic acids the nucleotides are joined together by phosphodiester linkage between 5′ and 3′ carbon atoms of the pentose sugar. The primary structure of nucleic acid gives the information regarding the sequence of nucleotides in the chain. The secondary structure of DNA shows that two nucleotide chains running in opposite directions and giving a right handed helix in which the two strands are held together by hydrogen bonds at definite distances.

In the secondary structure of RNA, helices are present which are only single-stranded. RNA molecules are three types: (i) messenger RNA (m–RNA) (ii) ribosomal RNA (r–RNA) and (iii) transfer RNA (t–RNA). These perform different functions. The tertiary structure gives the information of the orientation of the planar aromatic rings of the bases causing the DNA double helix to twist about the same axis. Due to twisting, a large number of hydrogen bonds are formed. The double helix of DNA is the store house of the hereditary information of the organism. The process of synthesis of RNA involving direct copying of one DNA molecule from another DNA molecule is known as replication. The process of synthesis of RNA in which a single strand DNA can act as a template is called transcription. The proteins are synthesized by various RNA molecules in the cell but the message for the synthesis of particular protein is present in DNA. This process is known as translation. In molecular biology, transcription is used as a synonym for RNA synthesis and translation is a synonym for protein synthesis. Translation is unidirectional but transcription can sometimes be reversed, i.e., RNA is copied into DNA and is known as reverse transcription.

The pair of bases in DNA is held together by

hydrogen bonds

The correct answer is option 4. hydrogen bonds.

In DNA, the bases adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). These base pairs are held together by hydrogen bonds. Specifically, adenine forms two hydrogen bonds with thymine, while cytosine forms three hydrogen bonds with guanine.

Hydrogen bonds are relatively weak compared to covalent bonds but are crucial for the stability and structure of DNA. The hydrogen bonds between the base pairs provide the complementary pairing that allows the DNA strands to form a double helix structure. The hydrogen bonds can be easily broken and reformed, allowing DNA replication and transcription processes to occur.

Ionic compounds (1), phosphate groups (2), and oxygen bonds (3) are not primarily responsible for holding the base pairs together in DNA. While phosphate groups form the backbone of the DNA molecule, hydrogen bonds specifically mediate the pairing of the bases.

Therefore, the correct answer is (4) hydrogen bonds.