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 regions (introns) If introns in mRNA are spliced out, then the mutation

never surfacesSome amino acids can be coded for by multiple

codons. (referred to as redundancy in the genetic code It is possible that one change in the base pair will still

produce 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

In the DNA, themutant templatestrand has an A where the wild-type template has a T.

The mutant mRNA has a U instead of an A in one codon.

The mutant (sickle-cell) hemoglobin has a valine (Val) instead of a glutamic acid (Glu).

Mutant hemoglobin DNAWild-type hemoglobin DNA

mRNA mRNA

Normal hemoglobin Sickle-cell hemoglobin

Glu Val

C T T C A T

G A A G U A

3 5 3 5

5 35 3

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

Mutation

DNA: 5' - ATG ACT CAC CGA GCG CGA AGC TGA - 3‘ 3' - TAC TGA GTG GCT CGC GCT TCG ACT - 5'

mRNA: 5' - AUG ACU CAC CGA GCG CGA AGC UGA - 3' Protein: Met Thr His Arg Ala Arg Ser Stop

DNA: 5' - ATG ACT CAC TGA GCG CGA AGC TGA - 3‘ 3' - TAC TGA GTG ACT CGC GCT TCG ACT - 5‘

mRNA: 5' - AUG ACU CAC UGA GCG CGU AGC UGA - 3‘Protein: Met Thr His Stop

Template Strand

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.

Types of DNA Change1. Point Mutation (Base-Pair Substitution)2. Frameshift Mutation3. Chromosomal Mutation

Silent mutation

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

Chromosomal Mutationsinversion mutations – sections of a chromosome are

reversed

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

Classwork/HomeworkPg. 263 #1-8