gene mutation

gene mutation

change in DNA base sequence

what causes mutation [two ways]

-can occur spontaneously (naturally, without any external factor) e.g. errors during DNA replication

-can be caused by mutagens (external factors)

→e.g. UV radiation, ionising radiation (X- rays, gamma rays) and chemicals

types of mutations (3) [definitions]

-substitution: one base is replaced by another

-deletion: one or more bases are removed from the sequence

-insertion: one or more bases are added into the sequence

types of substitution [definitions]

nonsense ( codon becomes a stop codon) [tip: no = stop]

missense (codon codes for a diff amino acid)

silent (codon still codes for the same amino acid)

effect of nonsense mutation (3)

-codon becomes a stop codon

-translation is stopped early

-produces a shorter polypeptide chain

effect of missense mutation (2)

-codon codes for a different amino acid
-this change in primary structure may change tertiary structure (3D shape) of protein

effect of silent mutation

-has no effect

-there will be no change to the primary protein structure

-the codon still codes for the same amino acid

why does silent mutation have no effect on the primary protein structure

due to the degenerate nature of the code

insertion and deletion mutation causes..

frameshift mutation

how does deletion/ insertion affect protein structure and function? (4)

-an insertion / deletion will cause frame shift

-this shifts the reading frame so all triplets downstream of the mutation will be different

-the protein will have a different sequence of amino acids (downstream of the mutation)

-the tertiary structure will be different

why is deletion/ insertion mutation more harmful than substitution?

-an insertion / deletion will cause frame shift

-this means more DNA triplets are changed

-greater change to protein structure

[substitution will only change the amino acid for the triplet in which the mutation occurs]

tip: when insertion or deletion mentioned in q→ add frameshift in answer [if relevant]

What number of bases will not cause a frameshift, and why?

if the number of bases added or removed is a multiples of three (3,6 9 etc), it wont cause a frameshift. This is bcs the genetic code is read in triplets. The reading frame is unchanged so the triplets will still be read correctly.

for an insertion mutation- where is the frame shift to?

for a deletion mutation- where is the frame shift to?

the right

the left

frameshift mutation near the end of a gene affects fewer amino acids than frameshift near the start

Discuss the possible effects that mutation can have on the structure and function of a protein (3 marks)

-mutation changes tertiary structure of protein

-Deletion mutations can lead to shorter protein

-protein can be unchanged due to silent mutation

-mutation may result in a loss of function or may make function of a protein better.

explain why a DNA mutation may not produce a change in the function of protein

-silent mutation might code for the same amino acid

-genetic code is degenerate

-also mutation may occur in non-coding DNA (intron) so there is no change to the amino acid sequence

types of effects of the mutations

some mutations can have a:

neutral effect [no effect] e.g. silent mutation

harmful [protein becomes non-functional]

beneficial [has positive effect on an organism, protein works better→ increases its chance of survival]