6.1.1 (a) Gene mutations 

Mutations

MUTATION – A change in the sequence of bases in DNA.

If the change is within a gene coding for a protein, the protein made could be incorrect and non-functional.

Types of mutation 

•Point mutation

•Substitution

.Frameshift mutation

•Deletion

•Insertion

Substitution mutations

A mutation where one or more nucleotides in a DNA sequence are replaced with a different nucleotide or nucleotides.

When an insertion or deletion of a nucleotide or nucleotides moves the reading frame of a sequence of bases so each codon after the mutation now codes for an incorrect amino acid. 

Deletion mutations

Deletion mutation - One or more nucleotides are deleted and lost from a strand of DNA.

Insertion mutations

Chromosomal mutations

• Affect whole chromosomes or sections of chromosomes rather than individual genes.

• Can affect more than one gene.

• Can be caused by mutagens but usually occur during mitosis. 

Macrodeletions (aka chomosomal deletion) - a section of a chromosome breaks of and is lost within the cell. 

Duplication - regions get duplicated within a chromosome.

Translocation - a section of one chromosome breaks off and joins another non-homologous chromosome. 

Inversion - A section of chromosome breaks off, is reversed and joins back into the chromosome. 

An inversion occurs when part of your chromosome breaks off and then reattaches, but sometimes pieces along the edges are lost. Think of it as though you are reading the newspaper. Sometimes an internal page can get reversed. All the information is still there, but when you go to read the page, it doesn't make any sense. Therefore, having an inversion is an important complication.

Effects of mutations

Silent or neutral mutations. 

• Mutations occur but have no effect on the phenotype of the organism.

• No proteins are changed or regulated differently.

  • the protein synthesised could be normally functioning (due to the degenerate nature of the genetic code - more than one codon can code for one amino acid). 

  • the mutation may occur in the non-coding regions of DNA (introns).

  • The amino acid in the mutant primary sequence may not affect the overall shape or functions of the synthesised protein. 

Damaging mutations

• Mutations result in a phenotype that affects the organism in a negative way. 

• Proteins may not be synthesised or fully synthesised (if a stop codon is included early in the primary sequence) - aka a non-sense mutation. This protein therefore will not be functional in the organism. 

• Proteins may be synthesised, but with the incorrect primary structure due to the incorrect amino acid becoming incorporated in the polypeptide. If this polypeptide does not fold correctly and so will no longer be functional as it has an incorrect shape. 

Example of a damaging mutation - sickle cell anaemia

Beneficial mutations

• A mutation resulting in a protein being synthesised that results in a new and beneficial characteristic in the phenotype of the organism. 

Examples of beneficial mutations.

Resistance to infection by HIV.

• Some people have a mutation in a cell surface protein that usually binds to HIV and allow infection. If you have a mutation in this cell surface protein the HIV cannot 'dock' and so you are not susceptible to infection from HIV. 

The ability of human adults to digest lactose.

Ice fish with colourless blood

SIlent, non-sense and missense mutations.

Missense mutations - When the nucleotide change leads to a change in the primary sequence of the protein.

Non-sense mutations - Result in a codon that originally coded for an amino acid being replaced with a stop codon. 

Silent mutation - When a mutation does not result in the inclusion of a different amino acid. The example below is only showing the mRNA and polypeptide - remember that mutation occur in the DNA. 

PPQs