Mutation is a process that creates genetic variation. mutation creates variations in protein-coding portions of genes that can affect the protein itself. more often, it creates variations in the switches that control when and where a protein is active, and how much protein is made. for example: lactase is an enzyme that infants break down lactose, a sugar in milk. normally the gene that codes for lactase is active in babies and then turned off at about age four. when people who do not make lactase consume milk, they experience discomfort. but some people have a variation in a genetic switch that keeps the lactase gene active and people who have it can keep milk in their diets even as adults.

mutations that occur in sex cells or gametes can be passed from one generation to the next. it is possible during gamete formation, meiosis, that there is an exchange of genes between homologous chromosomes, resulting in a change in the chromosome. this type of mutation is called crossing-over. mutations that occur in somatic or body cells, like a mutation causing skin cancer, will not be passed on.

most of the mutations in our dna are naturally-occurring. for example, when a cell divides, it makes a copy of its dna and sometimes the copy is not perfect. that small difference from the original dna sequence is a mutation. mutations can also be caused by exposure to specific chemicals or radiation, called mutagens.

there are many different ways that dna can be changed, resulting in different types of mutation.

substitution

a substitution is a mutation that exchanges one base for another. a substitution could:

~change a codon to one that encodes a different amino acid and cause a small change in the protein produced. for example, sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.

~change a codon to one that encodes the same amino acid and causes no change in the protein produced. these are called silent mutations.

~change an amino-acid-coding codon to a single "stop" codon and cause an incomplete protein. this can have serious effects since the incomplete protein probably won't function.

insertion

insertions are mutations in which extra base pairs are inserted into a new place in the dna.

deletion

deletions are mutations in which a section of dna is lost, or deleted.

how do mutations occur in the genetic code? (pick all correct answers)

a. this represents crossing over during meiosis.

b. during dna replication, the original dna strand gtc aca ggc atc is copied into complementary strand cag ccg tag. this is an example of deletion.

c. mutations can only occur when passed on thorough reproduction.

d. the majority of mutations are the result of exposure to mutagens.

e. a woman who has sickle cell anemia passes this disease on to her offspring. this means that the mutation for sickle cell anemia must occur in the woman’s gametes.

f. mutations can be silent and not affect the organism that has them.