In-depth summary for content covered in the Gene Expression topic of A-level AQA Biology, including content on mutation, stem cells, epigenetics, cancer and more.
This will still be applicable to other exam boards, but take caution when looking at key-words and the order of specific processes.
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Any change to the quantity or structure of DNA is called a mutation
-> however more specifically, a change in base sequence is a gene mutation
This can occur in a few different ways
Substitution –
This involves a nucleotide in DNA being replaced by one with a different base
-> depending on what / where base this is, the effects can vary
If one of the three stop codons are formed, production of the polypeptide will stop early
Forming a shorter chain that likely differs in shape
And likely lacks the same functionality
The formation of a codon for a different amino acids can also occur
This causes a different primary polypeptide structure
This will impact the tertiary structure as well therefore
However the impacts of this depend on how different / how severely
the structure is changed
The structure of a non-important section could also be changed
-> e.g. for an enzyme if the active site is still in shape, then functionality will likely persist
The formation of a different codon that codes for the same amino acid as before is also
possible
Since the code is degenerate
This results in no change to the polypeptide structure
A mutation in an intron will likely have no effect on polypeptide structure also
Since this is a non-coding region which is spliced from the mRNA
Deletion –
This involves the loss of a nucleotide base from a DNA sequence
-> which has major effects to the base sequence
Since bases are read in triplets
The deletion of one causes a frame shift
This alters all of the triplets after the deletion site
Leading to a very high probability of a non-functional enzyme
Figure 1 – reproduced from [1]
, Oliver Dyson
Other types of gene mutation:
There are a few other ways in which base sequences of genes can be changed
-> including:
Addition of bases
This has a similar effect to deletion, causing a frame shift, however the shift occurs to the
right rather than the left
Duplication of bases
This involves the repetition of one or more bases, causing a frame shift to the right
Inversion of bases
When a group of bases becomes separated from the sequences
But attaches back but the wrong way around, reversing the base sequence of this section
Translocation of bases
A group of bases becomes separated from the DNA sequence
And inserts into the DNA sequence of another chromosome / gene
These all cause a large impact in organisms, leading to non-functional proteins / enzymes
Causes of mutations:
Mutations can arise spontaneously and randomly
-> via processes such as DNA replication involving small errors
Despite this being random it can be predicted using probabilities, and this is called the
natural mutation rate and this varies from organism to organism, at around 1-2 per 100,000
genes
Mutagenic agents are substances / factors which increase the probability of mutation
-> and come in many forms:
Ionising radiation – for example UV, X-rays, gamma rays all disrupt DNA structure
Alpha and beta particles also have an effect
Chemicals – for example nitrogen dioxide, alter DNA structure and interfere with transcription
-> these each have their own impacts to cells / DNA
Nitrous acids / nitrous oxides – can remove -NH 2 groups from cytosine in DNA, changing it to
uracil
Benzopyrene – adds a large group to guanine that makes it unable to pair with cytosine
This causes DNA polymerase to insert another base besides guanine
Ionising radiation – produce reactive ‘free radicals’ in cells which alter the shape of DNA,
impacting DNA polymerase action
UV in particular affects thymine, causing it to from bonds with it’s neighbouring nucleotides
(disrupting replication)
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