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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.

Diagram of the structure of a nucleotide. At the top left is a blue circle, labelled "Phosphate Group". Joined to this, on its right, is a red pentagon, labelled "Sugar (Deoxyribose)". Joined to this, on its right, is a purple rectangle labelled "Base".

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.

Diagram showing four DNA nucleotides, laid out in a 2 by 2 grid. In each one, the base (which is represented by a rectangle) is a different colour and has a different letter written in it. Top left has the letter A on a dark blue background. Top right has the letter T on a purple background. Bottom left has the letter G on a green background. Bottom right has the letter C on a yellow background.

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.

Diagram showing two DNA nucleotides joining together. At the top of the diagram there are two DNA nucleotides, one above the other, but not joined together. Then there is an arrow pointing away from them to a drawing of the same two nucleotides but now with a bond between the phosphate group of the first nucleotide and the sugar of the second 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.

Diagram of a DNA strand. There are many nucleotides stacked on top of each other. The sugar of each nucleotide is joined by a line to the phosphate group of the nucleotide below. This line represents a chemical bond. The stack of sugar and phosphate groups has a box drawn around it and is labelled "Sugar-phosphate backbone". The bases stick out from this backbone. They are labelled "Bases". All four DNA bases are present at some point in the sequence of nucleotides.

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.
Diagram of double-stranded DNA. There are two strands running in opposite directions. Each strand has a sugar-phosphate backbone, which runs vertically in the diagram, and bases sticking off of the backbone. The bases of the two strands pair up, making them like rungs of a ladder (with the sugar-phosphate backbones being like the two sides of the ladder). A and T are always paired up together, and G and C are always paired 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.

Diagram of double stranded DNA. The double-helix shape is shown, consisting of two sugar-phosphate backbones (shown as red lines) coiled around each other, with pairs of bases (shown as coloured rectangles) between them, like rungs on a twisted ladder. The sugar-phosphate backbones are both labelled. One base pair is also labelled.

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

Animated gif of a double-stranded DNA molecule rotating anticlockwise. The sugar-phosphate backbones of the two strands are coiled around each other. The base pairs run between then like rungs of a ladder.

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.

1/6

What is DNA made of?

2/6

What is the structure of a nucleotide?

3/6

What are the four bases found in DNA nucleotides (first letters only)?

4/6

What is the structure of a strand of DNA?

5/6

What is the structure of double-stranded DNA?

6/6

What are the DNA base pairing rules?

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