Single gene Mutations

Single gene Mutations

(a) Mutations are random changes in the genome that can result in no protein or an altered protein being expressed.
(b) Single gene mutations involve the alteration of a DNA nucleotide sequence as a result of the substitution, insertion or deletion of nucleotides. Single-nucleotide substitutions include: missense, nonsense and splice-site mutations. Nucleotide insertions or deletions result in frame-shift mutations or an expansion of a nucleotide sequence repeat.

Homework Sheets 1_9

Gene mutations are changes in the order or number of nucleotides within a gene.
Any change in the base sequence alters the primary structure of the protein produced by the mutated gene.
Altered proteins are often non-functioning e.g. an enzyme with an active site into which the substrate will no longer fit.
Gene mutations fall into two categories :

• Point mutation – these involve a change in bases sequence , affect only one amino acid and may not affect organism drastically.
• Frame shift mutations – these affect the total number of bases and cause large sections of DNA to be misread. The resulting protein is usually non functional

Point Mutations
1. Substitutions – This is a point mutation where one base is replaced by a different base.

Example, Sickle Cell Anaemia

Sickle-cell anemia is a disease caused by the alteration of a single nucleotide in the gene for the hemoglobin protein (the oxygen-carrying protein that makes blood red). This single nucleotide change alters only one amino acid in the protein chain, but the results are devastating.Beta hemoglobin (beta globin) is a single chain of 147 amino acids, and in sickle-cell anemia, the sixth amino acid in the chain is valine, rather than glutamic acid.
Molecules of sickle-cell hemoglobin stick to one another, forming rigid rods. These rods cause a person’s red blood cells to take on a deformed, sickle-like shape, thus giving the disease its name. The rigid, misshapen blood cells do not carry oxygen well, and they also tend to clog capillaries, causing an affected person’s blood supply to be cut off to various tissues, including the brain and the heart. Therefore, when an afflicted individual exerts himself or herself even slightly, he or she often experiences terrible pain, and he or she might even undergo heart attack or stroke—all because of a single nucleotide mutation.

2. Inversion – This is a point mutation where a short section of bases breaks from the strand of DNA, turned 180 and replaced the wrong way round.

Frame Shift Mutations

1. Deletions – Base is lost. This is a frame shift mutation

2. Insertions – Additional bases is added to DNA. This is a frame shift mutation.

The mutation rate is measured as ‘mutations at gene locus/million gametes’ e.g a mutation rate of 20 results in 1 mutation in every 50,000 gametes.