What Is the Structure of DNA?
Learn the structure of DNA for AQA GCSE Biology, including the double helix, nucleotides, sugar-phosphate backbone and complementary base pairing.
DNA is a polymer made from two strands that twist around each other to form a double helix. Each strand consists of repeating units called nucleotides.
This answer assumes AQA GCSE Biology (separate science), Higher tier. The complementary base-pairing detail is Higher-tier Biology content.
What is a nucleotide?
A nucleotide is the repeating unit from which DNA is built. Each DNA nucleotide contains:
- a sugar
- a phosphate group
- one of four bases
The four DNA bases are:
- adenine (A)
- cytosine (C)
- guanine (G)
- thymine (T)
The sugar and phosphate groups alternate to form the outside of each DNA strand. This is called the sugar-phosphate backbone. The bases are attached to the sugars and point towards the centre of the molecule.
A polymer is a large molecule made from many smaller, repeating units. Therefore, DNA is a polymer because it contains many nucleotides joined together.
Complementary base pairing
The bases on the two DNA strands pair in a particular way:
- adenine pairs with thymine
- cytosine pairs with guanine
The two strands are therefore complementary, meaning the base sequence on one strand determines the sequence on the other.
Worked example
If part of one DNA strand has the base sequence A-C-G-T, the complementary strand has the sequence T-G-C-A.
How DNA is organised
DNA is contained in chromosomes. A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids used to make a specific protein. The entire genetic material of an organism is called its genome.
Common exam mistake
Do not say that a nucleotide is just a base. A nucleotide contains a sugar, a phosphate group and a base. Also remember that DNA has two strands, not one, arranged as a double helix.
