There are, for example, non-coding multiple repeats of base sequences…

The idea

Between genes, eukaryotic DNA contains short base sequences repeated many times, known as variable number tandem repeats (VNTRs). The repeated sequence, for example AGAT, is common to everyone, but the number of repeats at a given position varies between individuals and is inherited. Because these repeats lie between genes and are non-coding, a change in repeat number often has no effect on a polypeptide, allowing variation to accumulate. This variation is exploited in genetic fingerprinting: DNA is cut with restriction endonucleases at specific recognition sequences, or amplified by PCR. The fragments are then separated by gel electrophoresis according to their length or size, moving towards the positive electrode due to DNA's negative charge.

What earns the marks

• The repeated sequences are non-coding and lie between genes, so variation in repeat number often does not alter a polypeptide.
• The number of repeats varies between individuals while the repeated sequence itself is common to all.
• Fragments differ in length, size or mass, which allows them to be separated by gel electrophoresis.
• DNA fragments carry a negative charge, causing them to move towards the positive electrode.
• Restriction endonucleases cut DNA at specific recognition sequences, not at random positions.

Common mistakes and how to put them right

• Giving 'they are short' on its own as the reason fragments separate.

  Put it right: Link separation to differences in length, size or mass.

• Stating that DNA fragments are positively charged.

  Put it right: DNA is negatively charged and moves to the positive electrode.

• Claiming the repeats code for a protein.

  Put it right: They are non-coding sequences located between genes.

• Saying restriction enzymes cut DNA at random positions.

  Put it right: They cut at specific recognition sites.

Exam tips

• When explaining separation in gel electrophoresis, clearly distinguish that negative charge causes movement, while differences in length or size cause the actual separation.
• State early in any fingerprinting answer that the repeats are non-coding.
• Name restriction endonucleases and recognition sites rather than writing 'cutting enzymes'.