Aligning Sequences

Before scientists compare DNA sequences, they need to make sure that they are comparing stretches of DNA that are evolutionarily related, which are called homologous sequences.

The sequences then need to be aligned, or lined up against each other, so the related parts of the sequences can be compared.

Crossed out image of an apple and an orange on a balance.
Comparing homologous sequences is like comparing apples to apples. Comparing non-homologous sequences is like comparing apples to oranges.