Wize AP Biology Textbook > DNA Replication & Repair

Types of DNA Mutations

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Types of DNA Mutations
Mutations in DNA occur when there is a change in the normal DNA sequence. 

Point Mutations or Base Substitutions
The term point mutation means that only one base pair is mutated. Point mutations can arise due to:
Errors in DNA replication.
Environmental exposure to damaging agents like UV light, radiation, etc.
Harmful chemicals/ toxins.
Byproducts of normal cell metabolism (e.g. lipid oxidation).

There are 3 types of point mutations:
Nonsense mutation: a premature stop codon is introduced into a sequence 
Example: UGC (Cysteine) --> UGA (stop)
Missense mutation: a change to the amino acid sequence 
Example: UGC (Cysteine) --> UGG (Tryptophan)
Silent mutation: no change to the amino acid sequence 
Example: UGC (Cyteine) --> UGU (Cysteine)
Wize Concept
Mutations can be:
BENEFICIAL: improves the fitness of an organism (e.g. resistance to a virus, lactose tolerance)
BENIGN: does not change the overall fitness of an organism (e.g. change in eye color or silent mutations)
HARMFUL:  negatively impacts the fitness of an organism (e.g. development of diseases like cancer and birth defects) 


Photo by Jonsta247 / CC BY


Exam Tip
To remember that nonsense mutations cause stop codons, just remember the mnemonic: STOP the NONSENSE!


When a nucleotide is replaced by another this is called a base substitution. Most commonly, point mutations are base substitution.
There are two different kinds of substitutions: transitions and transversions.
Transitions: purine for purine, pyrimidine for pyrimidine.
Example: A for G, C for T.
Transversion: purine for pyrimidine and vice-versa.
Example: A for T, C for G.

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Frameshift Mutations
Other mutations have the potential of causing changes in the reading frame of the nucleotide sequence. These are called frameshift mutations. They can be caused by mutations such as insertions or deletions of fragments of DNA.



Wize Concept
You may not have covered what "reading frame" means yet in your biology course, so here's a quick explanation. 
The DNA sequence codes for proteins. 
Every 3 nucleotides codes for one amino acid. 
Therefore, when nucleic acids are translated into proteins, the triplets of nucleotides that are read can make a huge difference in the outcome of a protein.

Example: consider the following sequence: TGTGAA. Reading in triplets, TGT codes for cysteine, while GAA is glutamic acid. So this sequence of amino acids is a cysteine linked to a glutamic acid.

Now, consider I insert a random nucleotide in the middle of this sequence: TGTTGAA. Again, reading in triplets from left to right, now this codes for TGT = still cysteine, but the next triplet will be TGA = STOP. Turns out TGA is a stop codon, which tells the cellular machinery to STOP making this protein. 
Due to this single base insertion, the rest of the protein is now missing. 
This is an example of an insertion causing a shift in reading frame and a premature stop codon.

Practice: DNA Mutations
You are studying a DNA sequence and find a spontaneous mutation that has occurred, resulting in a nonsense mutation. Which of the following is true?

A) This mutation could not have resulted due to an error in replication by DNA polymerase.

B) This mutation will result in a premature stop codon in the amino acid sequence.

C) This mutation will not change affect the folding structure of the encoded protein.

D) This mutation will result in the substitution of an amino acid in the sequence.

E) This mutation must have arisen from the failure to repair a G-T mismatch.

I don't know

Practice: Type of Mutation
a) What type of point mutation occurred in the DNA sequence below, substitution, insertion, or deletion? 

Initial sequence:      3’ ATGCGATTATCTAGTATT 5’
Mutated sequence: 3’ ATGCGATTATCTAGGTATT 5’

b) Does the mutation cause a frameshift, synonymous, missense, or nonsense mutation in the amino acid sequence?

I don't know