Nucleic Acids: DNA and RNA

This lesson covers the structure and function of DNA and RNA — the two types of nucleic acid — as required by the Edexcel A-Level Biology B specification (9BI0), Topic 1: Biological Molecules. You need to understand the structure of nucleotides, the formation of polynucleotides, the double helix structure of DNA, complementary base pairing, and the differences between DNA and RNA.

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What Are Nucleic Acids?

Nucleic acids are large biological molecules that store and transmit genetic information. They are polymers of nucleotides.

There are two types:

Nucleic Acid	Full Name	Location	Function
DNA	Deoxyribonucleic acid	Nucleus (and mitochondria/chloroplasts)	Stores the genetic code — the instructions for making proteins
RNA	Ribonucleic acid	Nucleus and cytoplasm	Involved in protein synthesis — mRNA, tRNA and rRNA

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Nucleotide Structure

A nucleotide is the monomer of nucleic acids. Each nucleotide consists of three components:

1. A pentose sugar — either deoxyribose (in DNA) or ribose (in RNA).
2. A phosphate group (PO₄³⁻).
3. A nitrogenous base — one of five possible bases.

The Five Nitrogenous Bases