This animation shows how errors in DNA replication can lead to expanding sections of repeating nucleotides.

Some genes contain multiple repeats of three-nucleotide sequences, such as CAG. Increased numbers of these trinucleotide repeats are associated with several diseases, including Huntington’s disease and fragile X syndrome. As shown in the animation, trinucleotide repeats can expand due to slippage during DNA replication. The number of repeats can therefore increase with each cell division. Expanded repeats are also inherited and can grow longer over multiple generations.

This animation is a clip from a 2003 Holiday Lecture Series, Learning From Patients: The Science of Medicine. Depending on students’ background, it may be helpful to pause the animation at various points to discuss different structures or steps in the DNA replication process.