Mutations: DNA Changes That Help, Harm or Do Nothing

0:00What happens when the DNA in a cell changes?

0:04DNA stores instructions as a sequence of chemical units called bases, written as the letters A, T, G and C.

0:15A mutation is a change in the DNA sequence. Here, one base has changed from A to T.

0:23A mutation can be neutral, harmful or helpful.

0:28Most mutations are neutral: they have no effect on the organism.

0:33Harmful: this change is in a gene for hemoglobin, the protein that carries oxygen in red blood cells.

0:40Each version of a gene is called an allele. Two copies of this sickle cell allele cause sickle cell disease.

0:48Helpful: lactase persistence. Some mutations let adults keep making lactase, the enzyme that digests the sugar in milk.

0:58The same change can be both. One copy of the sickle cell allele gives some protection against malaria.

1:05In animals, only mutations in sex cells, sperm and eggs, are passed on to offspring.

1:13Mutations make new alleles, the raw material for evolution.

1:18Over many generations, helpful alleles, like those for lactase persistence, can become more common.

1:25Mutations happen at random. They do not happen because an organism needs them.

1:31Mutations are random changes in DNA that can help, harm or do nothing.