Mutations are changes in DNA sequence, and they are a major source of genetic variation. Some mutations have little or no effect, while others change proteins, disrupt cell function, or contribute to disease. Understanding mutation types helps explain inheritance, evolution, cancer, antibiotic resistance, and genetic disorders.
In a gene sequence, even one changed base can matter because DNA is read in three-base codons.
Understanding Biology: Types of Mutations
Many mutations begin during DNA copying. Before a cell divides, enzymes unzip the DNA and build a matching strand. DNA polymerase usually selects the correct base, then proofreads its work.
Repair enzymes can remove many remaining errors. This system is very accurate, but it is not perfect. DNA can be damaged by ultraviolet light, tobacco smoke, some chemicals, and radiation.
A damaged base may pair incorrectly during the next round of copying. Cells repair damage every day, which helps prevent harmful changes from becoming permanent.
The effect of a small sequence change depends on where it occurs. A change in a protein coding region can swap one amino acid for another. This is called a missense mutation.
Some amino acid swaps barely affect the protein because the new amino acid has similar properties. Others change the protein's shape, stability, or active site. A protein is like a folded tool.
If one important part changes, it may no longer bind its usual molecule or carry out its job. Changes in noncoding DNA can matter too. They may alter instructions that control when, where, or how much of a gene is used.
Reading groups of three bases gives cells a fixed starting pattern. When a base is added or lost in the wrong amount, every group after that location can be regrouped. The resulting protein often has many incorrect amino acids and may stop soon afterward.
By contrast, adding or removing three bases keeps the grouping pattern, though it adds or removes one amino acid. Whether this is serious depends on the protein region involved.
A loss in a flexible region may have little effect. A loss in a region needed for folding or binding can be damaging.
Chromosomal mutations affect much larger pieces of DNA. A deletion can remove many genes. A duplication can create extra copies of genes.
Inversions reverse a DNA segment, while translocations move a segment to a different chromosome. Cells need a balanced amount of many gene products. Extra or missing chromosome material can disrupt that balance.
Errors during meiosis can give an egg or sperm an extra chromosome or leave one out. This can lead to conditions such as Down syndrome, which usually involves an extra copy of chromosome 21.
It is important to separate inherited mutations from mutations that arise later in the body. A mutation in an egg, sperm, or early embryo can be present in many cells and may be passed to children. A mutation in one skin, lung, or blood cell usually stays within that cell's descendants.
Cancer can develop when body cells collect mutations in genes that control division, DNA repair, or cell death. When studying mutation diagrams, track the original sequence carefully, keep the reading groups fixed, and compare the protein before and after the change. The location and cell type often matter as much as the mutation name.
Key Facts
- Substitution: one DNA base is replaced by another, such as GAG becoming GTG.
- Insertion: one or more bases are added to a DNA sequence, such as ATG CCA becoming ATG ACC A.
- Deletion: one or more bases are removed from a DNA sequence, such as ATG CCA becoming ATC CA.
- Frameshift mutation: an insertion or deletion not in multiples of 3 changes the codon reading frame.
- Silent mutation: a codon changes but the amino acid stays the same, such as GAA and GAG both coding for glutamic acid.
- Nonsense mutation: a codon changes into a stop codon, causing early termination of translation.
Vocabulary
- Mutation
- A mutation is a change in the nucleotide sequence of DNA.
- Codon
- A codon is a group of three mRNA bases that specifies an amino acid or a stop signal during translation.
- Frameshift
- A frameshift is a change in the reading frame caused by an insertion or deletion that is not a multiple of three bases.
- Point mutation
- A point mutation is a change affecting a single nucleotide or base pair in DNA.
- Chromosomal mutation
- A chromosomal mutation is a large-scale change in chromosome structure or number.
Common Mistakes to Avoid
- Calling every substitution a frameshift is wrong because a substitution changes one codon but usually does not shift the grouping of all later codons.
- Assuming every mutation is harmful is wrong because many mutations are neutral, some are silent, and a few can be beneficial in certain environments.
- Forgetting that codons are read in groups of three is wrong because insertion or deletion effects depend on whether the number of bases changed is a multiple of 3.
- Confusing gene mutations with chromosomal mutations is wrong because gene mutations affect a DNA sequence within a gene, while chromosomal mutations affect large chromosome segments or whole chromosomes.
Practice Questions
- 1 A coding DNA strand reads ATG GAA TTT CCG. A substitution changes GAA to GTA. How many codons are changed directly, and is this a substitution, insertion, or deletion?
- 2 A DNA sequence has 30 bases. A deletion removes 2 bases near the beginning of the sequence. How many complete codons could be read from the remaining bases, and would the mutation cause a frameshift?
- 3 A mutation changes a codon from one amino acid codon to a stop codon near the middle of a gene. Explain how this affects the protein and why it is usually more serious than a silent mutation.