DNA

Full Notes

DNA (Deoxyribonucleic Acid) is used as a way of encoding and storing genetic information in nature. It is a polymer of nucleotides, with the sequence of bases in the polymer providing the ‘code’ that is then used by an organism as a guide for making proteins.

Structure of DNA Nucleotides

The monomers that react to form DNA are called nucleotides and they have the same general structure.

Each nucleotide is made up of:

• A phosphate group (PO₄³⁻).
• A pentose sugar (2-deoxyribose).
• A nitrogenous base (Adenine, Cytosine, Guanine, Thymine).

DNA as a Polymer of Nucleotides

DNA 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.

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 complementary base pairing.

• Adenine (A) pairs with Thymine (T) via 2 hydrogen bonds.
• Cytosine (C) pairs with Guanine (G) via 3 hydrogen bonds.

Summary

• DNA is a polymer of nucleotides, each containing a phosphate group, 2-deoxyribose, and a nitrogenous base.
• Nucleotides join via phosphate–sugar links to form a sugar–phosphate backbone.
• DNA is double-stranded and forms a double helix.
• Complementary base pairing holds the strands together: A–T (2 H-bonds) and C–G (3 H-bonds).