(4) Types of mutation - deletion, insertion and substitution

A mutation causes a change in base sequence of the triplet codes for amino acids.

Deletion mutation:

A base might be deleted at random from the DNA base sequence. Usually just one base is deleted.

This will change the way in the nucleotide base sequence is read and will affect other bases further down the DNA strand.

The wrong triplets are recognised so the wrong amino acids will be coded for! So, in the diagram this section of base sequence changes from ...

... ATC GTT AGC CGA ... etc. to ... ATC TTA GCC GA. ... etc.

In other words an abnormal amino acid sequence is produced.

This will change how the base sequence of triplet codes is read in making RNA to code for a protein synthesis. The mutation will have knock on effects down the strand of DNA i.e. it may not now code for the correct sequence of amino acids to make the appropriate protein with its correct structure.

This might affect the protein's structure and inhibit is function.

The correct protein might not actually be made, with serious consequences.

 

Insertion mutation:

A new base may be inserted into the DNA base sequence into a position it should NOT occupy in a gene.

This will change the way the triplet codes are read i.e. it changes the amino acid code and code for the wrong amino acids.

In the diagram the original triplet codons are ... ATC GTT AGC CGA ... etc. but after the insertion of base T after the first triplet, the triplet codons now read quite differently ...

so this part of the base sequence becomes ... ATC TGT TAG CCG A.. ... etc.

Also, as a consequence, more than one amino acid triplet is changed because a whole sequence of bases can be affected. The wrong amino acids will be coded for.

Again, this mutation will change how the base sequence of triplet codes is read in making RNA to code for a protein synthesis. The mutation will have knock on effects down the strand of DNA and may not code for the correct sequence of amino acids to make the appropriate protein. This might affect the protein's structure and inhibit is function. The correct protein might not actually be made, with serious consequences.

One or bases may be inserted in a single mutation. If one or two bases are inserted the above applies.

BUT, if three bases are inserted, the original sequence before and beyond the insertion remains intact! Is the consequence an extra amino acid in the polypeptide-protein? Can the same functioning protein still be made?

 

Substitution mutation:

Another base in the DNA is substituted at random with a different base changing the base sequence.

Here there are two possible outcomes:

(i) there might not be any overall effect because some amino acids are coded for by more than one triplet and the substitution might make one of those other triplet codes.

e.g. in the diagram the 2nd triplet GTT mutated to ATT, but may still code for the same amino acid.

(ii) the sequence can't be read correctly because the code doesn't match the particular amino acid required.

The wrong amino acid will be coded for, or, it might not code for any amino acid at all.

Again, as with other types of mutation, how the base sequence of DNA triplet codes are read is changed, in making RNA to code for a protein synthesis. The mutation has knock on effects down the strand of DNA affecting the coding for amino acids to make the appropriate protein. This affects the protein's structure and inhibit is function and maybe the correct protein might not actually be made, with serious consequences.

See also the effects of non-coding DNA

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Summary of learning objectives and key words or phrases

Be able to interpret diagrams showing changes in the DNA triplet code for amino acids.

Know about the types of mutation using and interpreting diagrams to explain deletion mutation, insertion mutation and substitution mutation, all producing changes in base sequence of the nucleotide arrangement.

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