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RELEVANT LEARNING OUTCOME:
(a) Explain what is meant by the terms gene mutation. For gene mutation, knowledge of how substitution, addition and deletion could change the amino acid sequence (including frameshift) is required.
👁️Mechanisms of gene mutation
👁️Mechanisms of gene mutation
Watch the video from 4:05 to 6:13, which will cover 3 types of gene mutations:
Substitution
one or more nucleotide is/are replaced
Addition
one or more nucleotide is/are added
Deletion
one or more nucleotide is/are removed
There is a fourth mechanism, which is inversion, where:
one or more nucleotides is/are inverted
Slide through the pictures on the right to review the four mechanisms of mutations.
Note that gene mutations can involved one or more nucleotide.
when there is only one nucleotide involved (e.g., deletion of one nucleotide), it is termed as a point mutation.
Q: Let's look at addition and deletion in terms of alphabet in a sentence.
Assuming words are only read in triplets, we have the following sentence:
CAT ATE THE RAT
How would the sentence read if the 'A' in CAT is deleted? (remember the rule that words must be read in triplets!)
A: CTA TET HER AT
The reading frame has been shifted due to the removal of one alphabet!
👁️Effects of gene mutations
👁️Effects of gene mutations
Watch the video from 4:16 - 7:43, which will cover the four effects of gene mutations:
Silent mutation
Mechanism: substitution
substitution of the 3rd nucleotide of the codon
due to the degeneracy of the genetic code, there is no change in amino acid coded for
Missense mutation
Mechanism: substitution and inversion
change in nucleotide sequence results in a different amino acid being coded for
the impact on the protein is significant, if the property of the changed amino acid is different (e.g., hydrophilic to hydrophobic)
Nonsense mutation
Mechanism: substitution, deletion, insertion and inversion
change in nucleotide sequences results in a premature STOP codon
the polypeptide is truncated (shorten), and usually the protein becomes non-functional.
Frameshift mutation
Mechanism: deletion and insertion that are not in multiples of three
shifts the downstream (sequence coming after the mutation) codon reading frame that results in changes in downstream amino acid coded for
the polypeptide chain will have a significantly different primary structure, which will cause it to fold differently to form a different 3D conformation and likely be non-functional.
frameshifts can also lead to nonsense and missense mutations
Q: Can you try linking which mechanism of mutation can give rise to which effect of mutations?
Substitution can lead to:
silent, missense and nonsense
Deletion and addition can lead to:
missense, nonsense and frameshift
Inversion can lead to:
missense and nonsense
Slide through the concept maps to note the keywords (in red) for each effect of mutations!
🕹️Simulation
🕹️Simulation
Using the following simulation, try to induce the different mechanism of mutations and see how it affects the amino acid sequence:
addition
deletion
substitution
Challenge! Can you induce a mutation that results in a nonsense mutation?
👁️Consequences of mutations
👁️Consequences of mutations
As seen above, effects of mutations can have affect the protein synthesised, thus the impact of the mutation is dependent on the cell type that the mutation occurred in, and also the role of the affected protein.
There are two main cell types in the body:
Germline cells
These are cells that give rise to gametes (sperm and eggs)
Somatic cells
These are all other cells in the body, apart from the germline cells
Watch the video to get an understanding on the difference between the impact of a mutation that occurs in the germline versus the somatic cell.
Look out for which of the mutation would be heritable (can be passed to offspring)
Think about which mutation would affect the individual, and which mutation would not have an impact on the individual?
Mutations to germline cells:
As these cells forms the gametes that would give rise to the next generation, mutations to germline cells are heritable.
As germline cells only contribute only to reproduction and not to the function of the body’s organs, mutations to germline cells do not affect the individual directly.
Mutations to somatic cells:
Genetic material in the somatic cells are not passed on to offspring, as such mutations to somatic cells are not heritable.
As somatic cells contribute to the function of the body's organs, mutations to somatic cells is likely to have an impact on the individual.
🖊️Conceptual Check!
🖊️Conceptual Check!
Substitution is the most common type of gene mutation, but is generally not as serious as addition or deletion. This is because base-pair substitution may lead to silent mutation or missense mutation, and base-pair addition or deletion may lead to frameshift mutation.
a) Explain why silent mutation may not lead to serious consequences.
Due to degeneracy of genetic code, where more than 1 codon can code for the same amino acid;
Same amino acid is coded for and sequence of polypeptide is the same;
Similar to the original protein, the resulting polypeptide fold into same 3D conformation and has the same function;
b) Explain why missense mutation may not lead to serious consequences.
The changed amino acid has R group with similar property;
The resulting protein will fold to have similar overall 3D conformation and thus similar protein function;
🖊️Learning Goals
🖊️Learning Goals
Check your understanding by attempting Qn 1 - 9 of the DNA Mutations Learning Goals.
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