Gene+Mutations

Learning Objectives


 * Describe the 4 different types of gene/point mutation - deletions, additions, substitutions, inversions
 * Describe how each type of mutation affects the DNA and mRNA transcript
 * Define the term frame shift
 * Relate the different types of point mutation to the effects on the polypeptide coded for by the gene
 * describe examples of disorders that arise at a result of gene mutations



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__ Collaborative Notes __ A gene mutation is a change in the sequences of the bases in DNA. They can be base substitutions or inversions or they can be frame shifts (additions or deletions).
 * ** Inversion: ** two bases swap places or a sequence of bases "flips over"
 * ** Substitution: ** one base is replaced by another but the position doesn't change
 * ** Addition: ** an extra base is added into the sequence
 * ** Deletion: ** a base is removed from the sequence

Inversions and subsititions may or may not produce a functional protein as generally it would be only one amino acid which is affected or in some cases, there could be no effect at all due to the degeneracy of the genetic code. The gene inversion or substitution will only have an effect if the change in base sequences changes the amino acid for which a nucleotide codes. When the mRNA is produced from the section of DNA containing the mutation, the "wrong" bases are transribed to the mRNA and then translated to produce the polypeptide chain. If the new base sequence codes for the same amino acid then the mutation has no effect on the resulting protein but if there is a new amino acid in the sequence the protein may not be functional or might not work as it should.

Deletions and additions, on the other hand, are unlikely to produce a functional protein as they affect all the amino acids "downstream" of the change. This means that it is very unlikely that the intended protein is produced and can function properly. This is because these mutations are frame shifts - adding or deleting bases from the sequences causes the reading frame of the mRNA to change so that it is no longer reading the same 3-base triplets. This shift is continued along the DNA strand unless another mutation corrects it further along (ie. if a deletion occurs and then an addition occurs further along, the reading frame will be corrected after the addition and vice versa).

Example of frame shift: THE FAT CAT ATE THE RAT like this, it is easy to read the three letter "triplets" just like the mRNA reads the DNA nucleotides. If there was to be an extra letter added, the sentence would change and become THE FAT CAT //**S**//AT ETH ERA T this no longer says the same thing. When the reading frame is shifted, the triplets all change and the amino acids for which they code also change. This causes a very high chance of a non-functioning protein resulting from these frame shift mutations.