What is a gene/ point mutation? What is the effect of it?
- a random change in the DNA base sequence of a gene
- this produces a new allele
When do gene/ point mutations occur? How does this affect how your answers should be worded?
- during DNA replication
- this means that when talking about gene/ point mutations, you should first refer to change in DNA base sequence.
- the change first happens to the DNA triplets found in the replicated DNA, which will most likely be part of a new genetically identical daughter cell, at the end of mitosis.
- this means that the mutation will only manifest itself, when the daughter cell containing the wrongly replicated DNA produces a faulty protein during protein synthesis.
EXAM STYLE QUESTION AO1
How does a mutation cause a non functional protein?
- change in the DNA base sequence
- change the sequence of amino acids in the polypeptide chain/ primary structure
- change the position of ionic and disulfide bonds
- change the tertiary structure and therefore shape of the protein
- the protein is non functional: APPLY THIS SPECIFICALLY TO THE QUESTION
What are the 6 types of gene/ point mutations?
- substitution
- deletion
- addition
- duplication
- inversion
- translocation
(theoretically, inversion and translocation mutations are chromosomal mutations and occur in meiosis, but AQA leave this detail out).
Describe a substitution mutation and it’s effects. (3 points)
- one (nucleotide) base is incorrectly copied
- this would cause a change of only one amino acid
- the amino acid sequence may not necessarily change: genetic code is DEGENERATE
Describe a deletion mutation and it’s effects. ( points)
- one (nucleotide) base isn’t copied from the original DNA base sequence
- frameshift of all DNA bases downstream of the mutation
- all DNA triplets downstream of the mutation are changed
- all amino acids downstream of the mutation are changed
Describe an addition mutation. (4 points).
- additional (nucleotide) base inserted into the new DNA
- frameshift of all DNA bases downstream of the mutation
- all DNA triplets downstream of the mutation are changed
- all amino acids downstream of the mutation are changed
Describe a duplication mutation. (2 points)
- sequence of bases is coped multiple times
- sequence of amino acids is duplicated
(important for VNTR in 8.4)
A LEVEL SPEC ONLY
Describe an inversion mutation. (2 points)
- sequence of bases is inverted
- sequence of amino acids is reversed
A LEVEL SPEC ONLY
Describe a translocation mutation. (1 points)
- sequence of bases is transferred to a different chromosome
BEYOND A LEVEL SPEC
Understanding how an inversion mutation occurs.
- During prophase I of meiosis, as chromatin condenses into chromosomes, two chromosomes may get tangled up.
- this process sometimes leads to crossing over
- however in this case, enzymes are able to repair the chromosomes and DNA, so that each chromosome has it’s original DNA, but the sequence of bases which ‘broke off’ is simply reattached in a reversed order to the original one.
BEYOND A LEVEL SPEC
Understanding how a translocation mutation occurs.
- During prophase I of meiosis, as chromatin condenses into chromosomes, two chromosomes may get tangled up.
- this process sometimes leads to crossing over
- However, sometimes this process is messed up and enzymes attach part of one chromosome to the other chromosome with which it go tangled up with.
-Therefore, the sequence of bases is transferred to a different chromosome. - This is not the same as crossing over: the chromosome which lost some of it’s DNA base sequence doesn’t get anything in return.
What are the two mutations which could result in a shorter polypeptide chain? Explain both.
DELETION: one less DNA triplet is found in the code as a base is missing, and therefore one less amino acid is found in the primary structure
SUBSTITUTION: the new triplet code created by the gene mutation could code for a STOP codon.
What are the three cases that could lead to a silent mutation? Explain each.
The genetic code is DEGENERATE.
- a substitution mutation may still result in the same amino acid being produced
A mutation can occur in the INTRON.
- Introns are removed from pre-mRNA, and therefore are not translated.
- However, if the mutation happens in a promotor region (which is also an intron) then the changes could be profound: the transcription factor may no longer be complementary to the promotor region and therefore may not bind to it. The gene is not transcribed and not expressed.
The mutation affects the NON-ACTIVE SITE of an enzyme
- The active site of the enzyme still has the same DNA base sequence, the same amino acid sequence, the same bonds can form in the same place, and it’s shape is still complementary to it’s substrate. The enzyme is still fully functional.
