Mutations Flashcards
How many protein coding genes are there in the human genome?
~22,000
What % of the human genome do protein coding genes constitute?
~1-2% of all DNA
What is the result of only a small percentage of the human genome coding for proteins?
Mutations in much of the genome are of little/no consequence
What happens if a mutation is within/close to gene?
They are likely to cause disease
What determines the consequences of a mutation?
The type of mutation and its location within a gene
What is the most common form of sequence variation?
Single-base substitutions
How often do single base substitutions occur?
About every 300bp
What is a single-base substitution also known as?
A single nucleotide polymorphism (SNP)
What are ~2/3 SNPs?
C→ T changes
What are more common, transitions or transversions?
Transitions
How many amino acids are specified by the genetic code?
20
How many codon triplets are there per amino acid?
Can be 1, 2, 3, 4 or 6
How many stop codons are there?
3
What normally happens where there is a single base change at the 3rd codon position?
Does not normally result in amino acid change
What is it called when a single base change doesn’t result in an amino acid change?
A silent substitution
Are silent substitutions always truly silent?
No
What do most amino acid changes result fmor?
Changes at codon positions 1 and 2
What are the possible mutations that changes amino acids?
- Missense mutations
- Silent mutations
- Nonsense mutations
- Frameshift mutations
- Promoter, start codon and stop codon mutations
- Intron splice site mutations
What happens in a missense mutation?
One amino acid is substituted by another, normally by a single base change
What happens in a silent mutation?
Single base substitution that does not change the amino acid
Why can some silent mutations affect amino acids?
Can disrupt RNA splicing, and therefore still cause heritable disease
What happens in a nonsense mutation?
An amino acid codon is changed to a stop codon by change of a single base
What happens in a frameshift mutation?
The reading frame of mRNA is altered in some way
How can the reading frame of mRNA be altered?
- Insertions
- Deletions
- Splice-site mutations
What are often found in the alternate reading frame?
Stop codons
How many alternate reading frames are there following a frameshift mutation?
2
What is the result in a stop codon being in a alternative reading frame?
Causes the ribosome to terminate translation prematurely
What is meant by a conservative missense mutation?
Some amino acid substitutions are better tolerated that others
Give an example of a fairly well tolerated missense mutation?
Substitution of valine by alanine might be well tolerated in non-critical regions of proteins
Why may a valine to alanine substitution be well tolerated?
They have similar properties
What has resulted from the tolerance of valine to alanine substitutions?
They are adjacent in the genetic code, by evolution
What is the effect of small insertions/duplications?
Either maintain or disrupt the reading frame
What results in the maintenance of the reading frame?
Gain or loss of multiples of 3bp
Why does the gain/loss of multiples of 3 bp result in the maintenance of the reading frame?
As this is the gain/loss of a whole amino acid(s)
What do gains/losses of non-multiple of 3’s cause?
Frameshifts
What can frameshifts produce?
Premature termination codons (PTCs)
What happens to mRNAs that contain PTCs?
They are degraded by ‘nonsenses mediated decay’ (NMD), so little/no protein produced
Why is NMD essential?
To mop up mutated mRNA
What is NMD?
An evolutionary protective mechanism
Why is NMD advantageous?
It’s better to have produced reduced amounts of protein than mutated proteins
What do mutations at intron splice sites usually result in?
Skipping of the exon immediately adjacent to the mutation
What happens in the skipped exon is a multiple of 3bp?
The mRNA is shortened, but remains in frame
What happens if the skipped exon is not a multiple of 3bp?
The mRNA is shortened, and will contain a frameshift and a PTC, leading to NMD
Why may it better if the mutation is not a multiple of 3?
Because the protein will be broken down, so no mutated protein made
What causes base changes?
- Sequence changes during DNA replication
- Chemicals can induce mutation
- Exposure to different types of radiation
How can sequence changes during DNA replication cause mutation?
NAME?
How can chemicals induce mutation?
NAME?
What kinds of radiation can cause mutation?
NAME?
What happens in tautomeric shift?
A proton briefly changes position, and then jumps back to where it should be
Where does tautomeric shift occur?
In the 4 bases of DNA
What are the rare forms?
The forms of the bases where the proton is in the alternative position
What is different about the rare forms?
They have altered base pairing properties
How do the rare forms behave?
As an altered template base during DNA replication
What do the rare forms cause?
Bonding to another base
What anomalous base pairing is caused by tautomeric forms?
- A - C ( with 2 H bonds)
- T - G (with 3 H bonds)
What happens if the C base is in it’s rare tautomeric form?
DNA polymerase will recognise it as a T base, and an A will be inserted into the new strand.
What happens in slippage during replication?
The newly synthesised strand loops out, or the template strand loops out
What is the result of the newly synthesised strand looping out?
The addition of one nucleotide to the new stand, to maintain base pairing
What is the result of the template stand looping out?
The omission of one of the nucleotides on the new strand, so the newly synthesised strand is one short of what it should be
Is slippage during replication common?
Fairly
When does slippage during replication occur?
When several of the same bases occur one after the other
Give two chemical mutagens that cause base changes
- Nitrous acid
- Ethyl methane sulphonate (EMS)
What does nitrous acid do?
Replace amino groups with keto groups
What is the result of the replacement of amino groups with keto groups?
Changes the properties of the bases
What are the bases changed to when amino groups are substituted for keto groups?
- C → U; base pairs with A
- A → H; base pairs with C
- G → X; base pairs with C
What does EMS cause?
Removal of purine rings
What happens when a purine ring is removed?
The purine backbone stays the same, but the ring is removed, so no template base there
What can apurinic sites be paired with during replication?
Any base
What is the result of apurinic sites pairing with any base during replication?
3/4 times, it pairs with the wrong base
What is2-amino-3-methylimidazo[4,5-f]quinoline(IQ)?
A heterocyclic aromatic amine food mutagen?
Where is IQ identified?
Cooked meats and cigarette smoke condensates
What does IQ do?
Disrupts the packing of bases
How doesIQ disrupt the packing of bases?
The planar molecule slips into the DNA double helix
What does IQ cause?
Mostly single base deletions at G-C base pairs, as it interferes with separation of bases in the 2 strands
How does IQ act?
- Intercalation of a molecule of IQ forces the bases further part on one DNA strand
- This leads to misreading by the DNA polymerase, and deletion of a single base
Other that IQ, what else intercalates into DNA?
Ethidium bromide
Where is ethidium bromide used?
In electrophoresis to obtain DNA
What does ionising radiation produce?
Ions during interactions with cellular molecules