Students should be able to describe DNA as a polymer made from four…

The idea

DNA is a polymer, meaning it is a large molecule built from many repeating smaller units joined together. The repeating unit of DNA is the nucleotide. Each nucleotide has three parts: a common sugar called deoxyribose, a phosphate group, and one of four different bases. Because there are four bases, there are four different nucleotides, but all four share the same sugar and phosphate. The nucleotides join in a long chain, and the chain of sugar and phosphate groups forms the backbone, while the bases stick out from it. Two of these chains twist around each other to form the double helix. When describing DNA, always state that it is a polymer, name the monomer as a nucleotide, and state that there are four different nucleotides.

What earns the marks

• DNA is a polymer, a large molecule made from many repeating smaller units.
• The monomer of DNA is called a nucleotide.
• There are four different nucleotides in DNA, differing in their base.
• Each nucleotide contains a common sugar and phosphate group plus one base.
• The nucleotides join together in a chain to form the DNA polymer.
• The sugar and phosphate groups form the backbone of the DNA strand.

Common mistakes and how to put them right

• Calling the monomer an amino acid.

  Put it right: The monomer of DNA is a nucleotide, while amino acids are the monomers of proteins.

• Saying DNA is made of four bases only.

  Put it right: DNA is a polymer made from four different nucleotides, each containing a base.

• Describing DNA as a single molecule with no repeating units.

  Put it right: DNA is a polymer with many nucleotides joined in a chain.

Exam tips

• Use the word 'polymer' and the term 'nucleotide' in the same sentence to secure both marking points.
• State clearly that there are four different nucleotides, not four bases alone.
• If asked to describe structure, mention the sugar-phosphate backbone and the sequence of bases.