A mutation represents a change in the DNA, which is the genetic material found in all living organisms. DNA contains the instructions for building and operating an organism, and these instructions are organized into genes. These alterations can occur spontaneously during normal cellular processes or be induced by environmental factors like radiation or certain chemicals. Such changes can range from small modifications in a single building block of DNA to large-scale rearrangements of genetic information.
Changes Within Genes
Mutations occurring within genes involve changes to the DNA sequence at a smaller scale, often affecting individual nucleotides or short stretches of DNA. These are frequently referred to as gene mutations or point mutations, where a single nucleotide base is changed, inserted, or deleted. These changes can have varied effects on the protein produced from the gene, ranging from no impact to a complete loss of function.
One common type of gene mutation is a substitution, where one DNA base is exchanged for another. Depending on the specific change, a substitution can result in a “silent” mutation if the new codon still codes for the same amino acid, meaning there is no change in the protein. Conversely, a “missense” mutation occurs when the substitution leads to a different amino acid being incorporated into the protein, potentially altering its function. A “nonsense” mutation is another outcome, where a substitution creates a premature stop signal, resulting in a shortened, often non-functional protein.
Another significant category of gene mutations includes insertions and deletions, particularly those that are not in multiples of three nucleotides. These are known as frameshift mutations because they shift the “reading frame” of the genetic code. Since DNA is read in groups of three bases (codons), adding or removing one or two nucleotides can cause all subsequent codons to be misread. This typically leads to a completely different sequence of amino acids and often results in a non-functional protein.
Changes in Chromosomes
Beyond changes within individual genes, mutations can also involve large-scale alterations in chromosomes, which are the organized structures containing DNA. These chromosomal mutations can affect the structure of chromosomes or their total number. Such changes often involve significant amounts of genetic material, leading to more pronounced effects than gene mutations.
Structural chromosomal mutations involve rearrangements of segments within or between chromosomes. These include:
- Deletion: A segment of a chromosome is lost.
- Duplication: A chromosomal segment is repeated, resulting in extra gene copies.
- Inversion: A segment breaks off, reverses its orientation, and reattaches to the same chromosome.
- Translocation: A segment moves from one chromosome to a non-homologous chromosome, or segments are exchanged between different chromosomes.
Changes in the number of chromosomes are another type of large-scale mutation, referred to as aneuploidy. This condition means a cell has an abnormal number of chromosomes, either more or fewer than the usual complete set. For humans, this typically means having a number other than 46 chromosomes in a somatic cell. Examples include monosomy, where a single chromosome is missing, or trisomy, where there is an extra copy of a chromosome. These numerical changes arise from errors during cell division, where chromosomes fail to separate properly.