Any type of DNA mutation is a permanent change in the genetic material.
All DNA mutations are copied during DNA replication and transmitted from parent to descendant cells. So, all types of DNA mutations are heritable. Nevertheless, not all mutations will be passed on from one generation of living organisms to another.
Germ cell mutations vs somatic mutations
To be passed on to offspring, mutations have to affect the genetic information of germ cells.
Germ cells are the reproductive cells of an organism and mutations that change DNA of these cells called germ cell mutations.
Different types of DNA mutations that appear during a lifetime and occur in cells other than germ cells are called somatic mutations. Somatic mutations are not inherited by future generations.
Gene mutations vs chromosomal mutations
Depending on changes, that occur with DNA, all types of DNA mutations can be divided into two main groups:
- gene mutation;
- chromosomal mutation.
The main difference between gene and chromosomal mutations in the level at which changes occur.
Gene mutations are small changes affecting the nucleotide sequence of one gene. The two main types of gene mutations are point mutations and frameshift mutations.
Chromosome mutations are substantial alterations of the genetic material of an organism. They change either the number or the structure of chromosomes. Types of chromosomal mutations are deletions, duplications, insertions, inversions, and translocations.
Three types of point mutations
A point mutation is a chemical change in the nucleotide sequence that alters just one or a few nucleotides.
Three types of point mutations that occur within the protein coding region of a gene are classified by the effect on the resulting protein:
Silent mutations do not have any effect on the cell’s metabolism because the change in codon's nucleotides does not lead to the change of protein's amino acids.
Missense mutations code for different amino acids. The result of these type of gene mutations can be altered but still functional protein.
Nonsense mutations code for premature stop codons which lead to producing non-functional shortened proteins.
A frameshift mutation
A frameshift mutation is the insertion or deletion of one or two nucleotides within a sequence of codons.
5 types of chromosomal mutations
Chromosomal mutations involve rearrangements of genetic hereditary material that affect multiple genes located on the same or separate chromosomes.
The five types of chromosomal mutations include:
- 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 DNA Mutations (physical and chemical mutagens)
Causes of DNA mutation can be divided into two types:
- spontaneous mutations;
- induced mutations.
Spontaneous mutations are a result of the molecular interactions which take place naturally within the cell.
Induced mutations are caused by agents outside the cell.
Some substances or events that increase the rate of mutation in an organism are called mutagens.
Two general categories of mutagens are physical mutagens and chemical 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)
Benzene increases the risk of cancer
The Ames test procedure
The Ames test is a test that measures a mutagenic potential of a chemical.
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.