The Structure of DNA

DNA (deoxyribonucleic acid) is an important biological molecule. On this page, we will just focus on the structure of DNA (DNA's function and the different types of DNA molecules will be covered later).

DNA is made up of nucleotides

DNA is made up of molecules called nucleotides. A single DNA molecule is usually made up of many hundreds or thousands of nucleotides.

A nucleotide is made up of a sugar, a phosphate group and a base

A DNA nucleotide is made up of three components bonded together:

• A sugar (called deoxyribose)
• A phosphate group
• A chemical called a base

The phosphate group and the base are both bonded to the sugar.

The structure of a nucleotide.

There are four different bases that can be present in DNA nucleotides. They are called adenine, thymine, guanine and cytosine - however, they are often simply referred to by their first letters: A, T, G and C. Each DNA nucleotide contains one of these four bases.

Each DNA nucleotide contains one of four bases: A, T, G or C.

Many nucleotides bond together to form a strand of DNA

Nucleotides can bond to each other. The phosphate group of one nucleotide forms a bond to the sugar of another nucleotide.

Two nucleotides can join together by forming a bond between the phosphate group of one nucleotide and the sugar of the other nucleotide.

Many nucleotides can bond together in this way to form a long chain. This chain is called a strand of DNA.

A DNA strand has a long line of alternating sugar molecules and phosphate groups, called the sugar-phosphate backbone. The bases are attached to the sugar molecules, so they stick off to the side of the sugar-phosphate backbone.

The sequence of bases varies between different DNA molecules.

Many nucleotides joined together to form a DNA strand. The sugar and phosphate groups form the sugar-phosphate backbone. The bases stick out from this backbone.

DNA is usually double-stranded

Sometimes, DNA just exists as a single strand, as shown above. However, what usually happens is that two strands of DNA come together to form double-stranded DNA. This is shown in the diagram below.

The two strands run in opposite directions (you can look at the arrangements of the sugar and phosphate groups in the diagram to see this more clearly).

The bases on the two strands line up with each other to form base pairs. This pairing up of bases happens according to the following rules:

• A and T always pair up together.
• G and C always pair up together.

Double-stranded DNA. A and T always pair up together, and G and C always pair up together.

Double-stranded DNA coils into a double-helix

In the diagram above, double-stranded DNA was shown as a straight line, like a ladder with the base pairs as the rungs and the two sugar-phosphate backbones as the two sides of the ladder.

However, in reality, double-stranded DNA is not straight but is coiled. The two strands twist around each other to form a shape called a double-helix. The word 'helix' means a coil, and it is a double-helix because there are two coils wrapped around each other.

In double-stranded DNA, the two strands coil around each other to form a shape called a double-helix.

3D animation of the structure of double-stranded DNA. Image: Zephyris (Richard Wheeler) via Wikimedia Commons, License: https://commons.wikimedia.org/wiki/Commons:GNU_Free_Documentation_License,_version_1.2

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.