Copying billions of DNA bases is not perfect. In Genes Make Proteins, Proteins Make Traits you saw that a gene's bases set a protein's amino acids. So what happens when a base changes?
Key idea
A mutation is a change in DNA. It can be harmful, helpful or neutral (no effect), depending on how it changes the protein and on the environment the organism lives in.
Why it works
There are 64 codons but only 20 amino acids, so many amino acids have several codons. A change from UUU to UUC still gives Phe: the protein is the same, so the mutation has no effect. A change that swaps an amino acid may change the protein's shape a little or a lot. A change that makes a stop codon cuts the protein short. Most mutations land in DNA that does not code for proteins, or change nothing important.
Worked example: Same or different?
A codon GGC (Gly) mutates to GGA. Use the codon table. Does the protein change?
- Look up GGA: Gly.
- Gly is still Gly.
Answer: No. It is a silent mutation.
Worked example: A helpful one
Most mammals stop making lactase, the enzyme that digests milk sugar, after they are weaned. Many adults keep making it. How?
- A change near the lactase gene keeps the gene switched on into adulthood.
- In places where people kept dairy animals, adults with this change could use milk as food.
Answer: A mutation that keeps the lactase gene on, which helped where milk was a food.
Common mistake
Students think all mutations are harmful and create monsters. Most have little or no effect, a few are harmful, and some are helpful, like the change that lets many adults digest milk sugar. Whether a mutation helps can depend on the environment.