Mutations

Mutationsmutations – errors in the DNA

can have a bad resultant effectcan have no effectcan have a positive resultant effect

Mutations are usually not an issue because

we have two copies of each gene.

Mutations

Changes in DNA result in:

1. silent mutations

2. missense mutations

3. nonsense mutations

Silent Mutationssilent mutation – has no effect on the function of the

cell; sequence of amino acids is not affected

often found in the non-coding (regulatory) regions

third NT of codons can be changed yet still code for the same amino acid

Missense Mutation

missense mutation – nucleotide sequence is changed such that a different amino acid is coded for

protein function may or may not be affected

Sickle cell anemia is a result of a missense mutation.

Sickle Cell Anemia

Nonsense Mutationsnonsense mutation – nucleotide sequence is

changed such that an amino acid is replaced with an early stop codon

short proteins are made; usually digested by proteases

nonsense mutations usually are lethal

Type of DNA Change:Point Mutationspoint mutation – nucleotide changes where one

base pair (bp) is replaced by another bp

Point mutations are also known as substitution mutations.

Type of DNA Change: Frameshift Mutationframeshift mutations – changes in the DNA

sequence which modify the translation reading frame

1.Deletions – removal of one or more NTs

2.Insertions – addition of one or more NTs

Type of DNA Change:Chromosomal Mutations

Sections of chromosomes can break off and reattach abnormally.

1.Translocation mutations

2.Inversion mutations

Chromosomal Mutationtranslocation mutations – large portions of DNA are

exchanged

Translocation

Down Syndrome

Chromosomal Mutationsinversion mutations – sections of a chromosome are

reversed

Crossing Over

Fixing Mutations

Enzymes are able to recognize when:nitrogenous bases are missingnitrogenous bases are fused together

These mutations are detected and fixed by DNA polymerase I.

Fixing MutationsEnzymes recognize when bases are incorrectly paired

together.

These mismatch mutations are fixed by DNA polymerase III.

How does the enzyme know which strand has the correct base pair?

The original template strand is methylated.

Causes of Mutations1. spontaneous errors due to enzymes

2. induced errors due to mutagenic agents UV radiation X-rays chemical (Agent Orange)

3. transposable elements

Spontaneous Errorsguanine may react with

water to form 8-oxo guanine

8-oxo G pairs up with adenine during replication

Transposable Elementssome DNA

sequences naturally move themselves around the genome - transposons