Nucleic Acids
Quick Notes
- Two types of nucleic acid: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).
- Composed of nucleotides: phosphate group, sugar (ribose or deoxyribose), and nitrogenous base.
- DNA is double-stranded and stores genetic information.
- RNA is single-stranded and involved in protein synthesis.
- Nucleic acids are called polynucleotides due to repeated nucleotide units.
Full Notes
Nucleic acids are biomolecules responsible for the storage and transmission of genetic information. They control all cellular activities by coding for the synthesis of proteins.
Chemical Composition of Nucleic Acids
Nucleic acids are polymers of nucleotides, and each nucleotide consists of:

- A nitrogenous base
- Purines: Adenine (A), Guanine (G)
- Pyrimidines: Cytosine (C), Thymine (T in DNA), Uracil (U in RNA)
- A pentose sugar
- Deoxyribose in DNA
- Ribose in RNA
- A phosphate group
A nucleotide = Nitrogen base + Sugar + Phosphate
A nucleoside = Nitrogen base + Sugar
The nucleotides are joined by phosphodiester bonds between 3′- and 5′- positions of adjacent sugar molecules.
Structure of DNA
DNA (Deoxyribonucleic Acid) is a polymer made of repeating nucleotide units.

Nucleotides are linked by covalent bonds between the phosphate of one nucleotide and the deoxyribose of another.
These phosphodiester linkages occur between carbon atoms 5 and 3 on each pentose sugar. This forms a sugar-phosphate backbone with bases attached to the sugars.
DNA as a Double Helix
DNA consists of two complementary strands, forming a double helix.

Hydrogen bonding occurs between specific base pairs on two strands of DNA.

The specific base pairing occurs because of the hydrogen bonding that can form between paired bases. This is called complimentary base pairing.
- Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
- Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds.
RNA (Ribonucleic Acid)
- Single-stranded
- Sugar is ribose instead of deoxyribose.
- Thymine is replaced by Uracil (U).
- Involved in protein synthesis via transcription and translation.
Biological Functions of Nucleic Acids
Nucleic acids perform vital biological functions related to heredity and protein synthesis.
DNA
- Stores genetic information.
- Guides synthesis of RNA.
- Passes genetic material from one generation to the next.
RNA
- Involved in decoding and executing the genetic instructions stored in DNA.
- Types:
- mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.
- tRNA (transfer RNA): Brings amino acids for protein synthesis.
- rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.
Summary
- Nucleic acids are polynucleotides built from nucleotides containing a base, a sugar, and phosphate.
- DNA is double-stranded with complementary base pairing and stores genetic information.
- RNA is single-stranded and functions in protein synthesis.
- Phosphodiester bonds link nucleotides between the 3′ and 5′ positions of sugars.