Chapter 8

Section 8.7: Mutations

Objectives

• SWBAT distinguish between different types of mutations.

• SWBAT explain why mutations may or may not affect phenotype.

• SWBAT list factors that cause mutations.

Vocabulary

• Mutation • Point mutation• Frameshift mutation• Mutagen

Mutations

• Mutation is a change in an organism’s DNA.• There are many different types of mutations.– Mutations that can affect a single gene occur

during replication. – Mutations that affect a group of genes or an

entire chromosome happen during meiosis.

Mutations

• Base substitution: the replacement of one base by another.

• The insertion or deletion of single bases. Such mutations involve a “frame shift” in the process of translation.

• Inversion of a section of DNA. • Duplication or deletion of a section of DNA

Inversion Mutation

Mutations

• Most mutations that alter fitness, in an evolutionary sense, will lower it.

• However, from time to time, a mutation will be beneficial and its fitness will allow it to increase in a population.

Types of Gene Mutations

• Point Mutation – a mutation in which one nucleotide is substituted for another.– An incorrect nucleotide is put in the place of the

correct one.– Usually this type of error is caught by DNA

Polymerase’s proofreading but sometimes it will slip through.

– A point mutation can sometimes, though not always, be very significant (ex. sickle cell anemia).

Types of Gene Mutations

• Frameshift Mutation – involves the insertion or deletion of a nucleotide in the DNA sequence. – This usually has greater effects because a

frameshift mutation shifts the entire sequence of base pairs which follow it.

– An example of how it works: • You start with the sequence THE CAT ATE THE RAT. • If you remove the first E, then everything shifts to the

left and you read THC ATA TET HER AT. The meaning of the sequence has been altered.

Any time the reading frame is shifted by either a deletion or insertion, a whole nucleotide sequence loses its meaning.

mutatedbase

Which type of mutation do the two diagrams represent? Which type of mutation is going to have the bigger impact?

Chromosomal Mutations

• During meiosis, homologous chromosomes exchange DNA segments via crossing over. – On occasion, homologous chromosomes do not

align properly and segments of different sizes are swapped.

– This is called gene duplication because one chromosome may have two copies of a gene or genes while the other may have no copies of the gene or genes.

Chromosomal Mutations

Chromosomal Mutations• Translocation – a piece of one chromosome

moves to a non-homologous chromosome.

Mutations Effects

• Chromosomal mutations affect lots of genes and tend to have a big effect on an organism.

• A mutation may break up a gene causing the gene not to work, or it could make a new hybrid gene with a new function (which might turn out to be adaptive – or not).

• Translocated genes may also come under the control of new promoters making the genes more or less active than usual.

Mutations Effects

• Gene mutations can do a number of things such as:– Change amino acid sequences – Change how proteins are folded – thus changing

their function– Alter the active site of an enzyme so the enzyme

ceases to be a catalyst– A gene substitution could also insert a premature

stop codon into a forming polypeptide chain.

Mutations Effects

• Some gene mutations do not affect an organism’s phenotype. – Recall that many codons code for the same amino

acid – thus some substitutions have no effect.– For example, is AAG changes to AAA, the resulting

protein still has the correct amino acid, lysine. – A mutation that does not affect the resulting

protein is called a silent mutation.

Mutations and Natural Selection

• Mutations occur in both body and germ cells but they can be passed on to offspring only in germ cells. – Mutations occurring in germ cells are the

underlying source of genetic variation, which is the basis for natural selection!

– Mutations in the germ line affect offspring-phenotype and are often very harmful.

– Typically the phenotypes that result are less adaptive, though not always.

Mutations and Natural Selection

– Natural selection typically removes the mutant alleles (mutant genes) from the population.

– Occasionally, a mutation results in a more adaptive phenotype, which are favored by natural selection and increase in a population.

Factors Causing Mutation

• Mutagens – agents in the environment that can change DNA.– They speed up the rate of replication errors and

can even break DNA strands. – Some are natural (ex. ultraviolet rays from the

sun) while others are human caused (toxic pollution, agricultural herbicides and pesticides, industrial chemicals, radioactivity, etc.).

– Mutagens are used to treat cancer (a positive)

Silent Spring