A mutation - a permanent change in the genetic material of an organism.
All mutations are heritable in that they will be copied during DNA replication.
Not all mutations will be passed on to future generations, however.
Only changes that affect the genetic information contained in the reproductive cells of an organism, called germ cell mutations, will be passed on to offspring.
Mutations that arise in the other cells of an organism during its lifetime are called somatic cell mutations.
Somatic mutations are not inherited by future generations.
Different types of DNA mutations
Three types of point mutations
A chemical change that affects just one or a few nucleotides is called a point mutation.
Point mutations that occur within the protein coding region of a gene are classified by the effect on the resulting protein:
- Silent mutations: which code for the same amino acid (no effect on the cell’s metabolism).
- Missense mutations: which code for a different amino acid (a slightly altered but still functional protein product).
- Nonsense mutations: which code for a premature stop codon (the gene unable to code for any functional polypeptide product).
A frameshift mutation
A frameshift mutation is the insertion or deletion of one or two nucleotides within a sequence of codons.
Types of chromosomal mutations
Chromosomal Mutations - mutations that involve rearrangements of genetic material that affect multiple genes, including genes located on separate chromosomes.
The five types of chromosomal mutations are deletions, duplications, insertions, inversions, and translocations:
- Deletions: removal of a large chromosomal region, leading to loss of the genes within that region.
- Duplications (or amplifications): lead to multiple copies of a chromosomal region, increasing the number of the genes located within that region. Some genes may be duplicated in their entirety.
- Insertions: the addition of material from one chromosome to a nonhomologous chromosome.
- Inversions: reversing the orientation of a chromosomal segment.
- Translocations: interchange of genetic material between nonhomologous chromosomes.
Transposons - are short strands of DNA capable of moving from one location to another within a cell’s genetic material.
Causes of Mutations (physical and chemical mutagens)
Spontaneous mutations - mutations as a result of the molecular interactions that take place naturally within the cell.
Mutations that are caused by agents outside the cell are said to be induced.
A substance or event that increases the rate of mutation in an organism is called a mutagen.
There are two general categories of mutagens:
- Physical Mutagens
Ionizing radiations (X-rays, gamma rays and alpha particles cause DNA breakage)
Ultraviolet radiations (Wavelength above 260 nm can be absorbed by nitrogenous bases of DNA, producing pyrimidine dimers, which can cause replication errors.)
- Chemical Mutagens
a molecule that can enter the cell nucleus and induce mutations:
Reactive oxygen species (ROS)
Aromatic amines and amides may cause carcinogenesis
Benzene increases the risk of cancer
The Ames test for mutagenicity
The Ames test is a simple test that measures the potential for a chemical to be mutagenic.
This test uses a mutant strain of bacterium that is unable to synthesize the amino acid histidine.
A suspension of the mutant bacteria is treated with the chemical and then spread onto a medium that lacks histidine.
Only those bacteria that have undergone a reverse mutation — that is, a second mutation that restores their ability to synthesize histidine — will be able to grow. The more mutations induced by the chemical, the more likely it is that a reverse mutation will occur.
Therefore, the number of colonies of bacteria that appear on the histidine-free medium indicates how strong a mutagen the chemical is.
Most chemical mutagens are carcinogenic.