6.2 - Structure of DNA

The double helix

• DNA (deoxyribonucleic acid) is the chemical that all of an organism's genetic material is made from.
• It carries the coded instructions that determine an organism's inherited characteristics.
• In animal and plant cells, DNA is found in the nucleus, packaged into long structures called chromosomes.

DNA is a polymer with a double-helix shape

• DNA is a polymer - a very large molecule built from many small repeating units joined into long chains.
• These repeating units are called nucleotides.
• A DNA molecule is made of two long strands.
• The two strands are coiled around each other in a spiral shape called a double helix (a double-stranded spiral).
• This twisted, two-stranded shape is the form DNA normally takes inside the cell.
• The strands contain four different compounds called bases.
• The order of the bases running down the molecule stores the coded information, so a single molecule can carry an enormous amount of instruction.

Nucleotides and the sugar-phosphate backbone

• Each of the two strands of a DNA molecule is a polymer, built from many small repeating units called nucleotides joined in a long chain.
• A DNA molecule is made from four different nucleotides.

The three parts of a nucleotide

1. Phosphate group - The same in every DNA nucleotide
2. Sugar (deoxyribose) - The same in every DNA nucleotide
3. Base - The only part that varies, attached to the sugar

There are four possible bases:

1. Adenine (A)
2. Thymine (T)
3. Cytosine (C)
4. Guanine (G)

Because the base is the only part that changes, there are effectively four different nucleotides, one for each base.

The sugar-phosphate backbone

• The nucleotides join so that the phosphate of one is linked to the sugar of the next, repeating for thousands of nucleotides.
• The sugars and phosphates alternate all the way along each strand, forming a continuous chain called the sugar-phosphate backbone.
• A base is attached to each sugar and sticks out to the side from the backbone.
• Only one strand is shown here. In the whole molecule the bases on the two strands face inwards towards each other, in the middle.