The information molecules
Nucleic acids are large molecules that store and carry genetic information. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). They are made of carbon, hydrogen, oxygen, nitrogen and phosphorus.
Nucleotides
The building block of a nucleic acid is a nucleotide. Each nucleotide has three parts:
- a pentose (5-carbon) sugar — deoxyribose in DNA, ribose in RNA;
- a phosphate group;
- a nitrogenous base.
Many nucleotides join to form a long chain, with the sugar and phosphate groups making a backbone and the bases pointing inward. In DNA the bases are adenine (A), thymine (T), guanine (G) and cytosine (C). In RNA, uracil (U) replaces thymine.
Key idea
DNA is a double helix of two strands held by base pairs: A pairs with T and G pairs with C. RNA is usually a single strand and uses A–U.
DNA and RNA
DNA is a double helix — two strands twisted together and held by complementary base pairs. It stores the instructions for making proteins and is passed from parents to offspring, so it carries the information for inherited characteristics. RNA is usually single-stranded and helps to build proteins from the DNA instructions. The order of bases along DNA is the genetic code that determines the sequence of amino acids in a protein, and because a protein's shape and function depend on that sequence, DNA ultimately controls the activities of the whole cell. When a cell divides, the two DNA strands separate and each acts as a template so that an exact copy can be made.
Example
If one DNA strand reads A-G-C-T, the matching strand must read T-C-G-A, because A pairs with T and G pairs with C.
Remember
Nucleotide = sugar + phosphate + base. DNA double helix, A–T and G–C. RNA single strand, A–U.