Mutagenesis Flashcards
(50 cards)
When does a mutation occur?
-When there is a change in the nucleotide sequence resulting in a triplet codon differing in the a’a it should represent, thus causing a different a’a to be translated
What is a silent substitution?
-A substitution which occurs at the third position of a codon which does not change the a’a produced
What are the three types of point mutations?
- Missense
- Silent
- Nonsense
What is a missense mutation?
-One a’a is substituted for another (may interrupt the tertiary structure by disrupting to bonding)
What is an example of a missense mutation?
-Sickle cell anaemia
When can silent mutations cause a problem?
- When the mutation disrupts RNA splicing
- The nucleotide sequence may change the splice sites in a way that the specific nuclease may no longer recognise the splice sequence or mat introduce a splice sequence
What is a nonsense mutation?
- When an a’a codon is mutated to produce a stop codon
- Produces a short non-functoning peptide
What is a frameshift mutation?
-Alterations in the reading frame of mRNA caused by insertions, deletions and splice site mutations
What is a common result of frame shift mutations?
-The introduction of stop codons causing the ribosome to terminate translation prematurely
Can a gain or loss of 3bp cause a frameshift?
-No the reading frame will be maintained
What happens to the mRNA when a gain/loss introduces a premature stop codon?
Why does this happen?
- mRNA is degraded by nonsense mediated decay and little or no protein is produced
- Happens as a protective mechanism
What happens when there is a mutation in an intron splice site?
-The exon is skipped immediately next to the mutation
In which two ways can sequence changes occur during DNA replication?
- Tautomeric Shift
- Slippage
What is tautomeric shift?
-Each of the 4 bases in DNA can undergo tautomeric shift where by a proton briefly changes position
How does tautomeric shift introduce a mutation?
-The rare tautomeric form of a nucleotide has altered base-pairing properties and behaves as an altered base template during replication
Which two bases undergo tautomeric shift and what does it cause them to behave like?
- C->A
- T->G
What is slippage during DNA relication?
-One of the nucleotides slips out of sequence above the rest of the mRNA molecule
How does slippage introduce a mutation?
- Slippage on the newly synthesised strand and the base is paired again introducing an extra nucleotide
- Slippage on the template strand and nucleotide is missed out on the new strand
How does nitrous acid cause mutations in DNA replication?
-Replaces amino groups with keto groups causing:
C->U
A->H
G->X
How does Ethyl Methane Sulphonate (EMS) cause mutations in DNA replication?
-Causes removal of purines and apurinic sites can be paired with any base during replication
What is a transition in DNA replication?
-Where the mutation occurs within the same group of purines or pyrimidines eg A->G or C->T
What is a transversion in DNA replication?
-When there is a mutation which changes the nucleotide group, ie purine pyrimidine
Name a base-stacking mutagen
- IQ is a food mutagen
- Ethidium Bromide
How do base-stacking mutagens lead to mutations?
-Disrupts the packing of DNA bases by intercalation and mostly causes single base deletions by forcing the bases further apart on one DNA strand so that DNA polymerase misreads leading to a deletion
