Mutations

MUTATIONS

We have talked a little bit about mutations. These are changes in the nucleotide sequence of a gene in a genome. This issue can happen because of errors in DNA replication, carcinogen exposure, or exposure to radiation. Not all mutations are noticeable but these are linked to things like cancer, evolution, and immune functioning.

Mutations may involve the duplication of very big sections of DNA because of genetic recombination. Changes in chromosome number, as is seen in polyploidy and aneuploidy, can cause large mutations, some of which aren t compatible with life. In other cases, it can result in a new species. Transposons are segments of DNA that are able to move within the genome. Certain mutations may be favorable to the individual having the mutation. Neutral mutations are those that do not affect the organism s fitness but can lead to genetic drift.

Small-scale mutations just affect one or a couple of nucleotides. Point mutations affect one nucleotide only. Insertions happen when one or more nucleotides are added to the genome. Deletions happen when one or more nucleotides are removed from the genome. Substitution mutations happen when one nucleotide gets exchanged for another. If a mutation adds or subtracts just three nucleotides, the effect is not as great as if more or less than three nucleotides are added or subtracted. This leads to a frameshift mutation, where the reading of the rest of the gene is incorrect. This is because codons happen in groups of three.

Large-scale mutations affect large sections of the chromosome. Chromosome translocations happen when there is exchange of genetic material from one type of chromosome to another. Inversion happens when the direction of the genome is reversed.

A mutation can inactivate a protein. This usually is a recessive trait. Gain of function mutations will increase the activity of a protein. Dominant negative mutations make an altered gene product that is antagonistic to the cell. Lethal mutations will kill the cell. Deleterious mutations can decrease the organism s fitness, while advantageous mutations increase the organism s fitness.

Examples of beneficial mutations include one for HIV resistance in some Europeans, who may also have had resistance to the bubonic plague in the 14th century. Malaria resistance is conferred by people who are heterozygous for sickle-cell anemia. The ability to digest lactose is a beneficial mutation in humans. Antibiotic resistance in bacteria is beneficial to the organism.