11.5 Mutations

Mutations and Their Effects

Mutations are permanent changes in the DNA nucleotide sequence of an organism. They're the raw material for genetic diversity and evolution, but they can also disrupt protein function and lead to disease. This section covers the major types of mutations, their effects on proteins, how cells repair DNA damage, and how scientists detect mutagens.

Point vs. Frameshift Mutations

Point mutations involve a change to a single nucleotide. They come in three forms:

• Substitution: One nucleotide is replaced by another. This may or may not change the resulting amino acid.
• Insertion: A single nucleotide is added into the sequence.
• Deletion: A single nucleotide is removed from the sequence.

Because the genetic code is read in triplets (codons), a single-nucleotide insertion or deletion can shift the entire reading frame downstream of the mutation. However, a single substitution only affects one codon.

Frameshift mutations occur when the number of nucleotides inserted or deleted is not divisible by three. This shifts the reading frame from the mutation point onward, changing every downstream codon. The result is usually a completely altered amino acid sequence and, in most cases, a nonfunctional or truncated protein. Frameshifts tend to be far more damaging than simple substitutions.

Effects of Mutation Types

Once you know the type of mutation, you can predict its effect on the protein:

• Silent mutation: A substitution that does not change the amino acid. This happens because of codon degeneracy (multiple codons code for the same amino acid). For example, both GCU and GCC code for alanine, so swapping U for C at the third position has no effect on the protein.
• Missense mutation: A substitution that results in a different amino acid being incorporated. The impact ranges from harmless to severe depending on how different the new amino acid is and where it sits in the protein. Sickle cell disease, for instance, results from a single missense mutation in the hemoglobin gene.
• Nonsense mutation: A substitution that creates a premature stop codon (UAA, UAG, or UGA). Translation ends early, producing a truncated protein that is typically nonfunctional.

Light vs. Dark Repair Mechanisms

Cells have evolved repair systems to fix DNA damage before it becomes a permanent mutation.

Light repair (photoreactivation) specifically fixes UV-induced thymine dimers (covalent bonds that form between adjacent thymine bases on the same strand). The enzyme photolyase binds to the dimer and uses energy from visible light to break those abnormal bonds, restoring the original bases. This mechanism only works when light is available and only targets pyrimidine dimers.

Dark repair (excision repair) does not require light and can fix a wider range of DNA damage. It works in three steps:

1. Recognition and excision: Enzymes detect the damaged segment and cut it out of the strand.
2. Resynthesis: DNA polymerase fills in the gap using the undamaged complementary strand as a template.
3. Ligation: DNA ligase seals the nick, joining the new segment to the existing strand.

These repair mechanisms are critical for maintaining genetic stability. When repair systems fail, mutations accumulate, which can contribute to cancer and other genetic disorders.

Mutagens and Carcinogen Detection

Mutagens and DNA Damage

A mutagen is any agent that increases the mutation rate above the spontaneous background level. Mutagens fall into three categories:

Physical mutagens:

• Ionizing radiation (X-rays, gamma rays) has enough energy to cause double-strand breaks and oxidative damage to DNA.
• Non-ionizing radiation (UV light) doesn't break the backbone but induces pyrimidine dimers, especially thymine dimers, by creating covalent links between adjacent pyrimidines on the same strand.

Chemical mutagens:

• Alkylating agents (e.g., ethylmethanesulfonate, nitrosoguanidine) add alkyl groups to DNA bases, which causes mispairing during replication.
• Base analogs (e.g., 5-bromouracil) are structurally similar to normal bases and get incorporated into DNA during replication. Once in place, they mispair, leading to point mutations.
• Intercalating agents (e.g., ethidium bromide, acridine orange) wedge themselves between stacked base pairs, distorting the helix. During replication, this causes insertions or deletions, resulting in frameshift mutations.

Biological mutagens:

• Viruses (e.g., human papillomavirus) can integrate their DNA into the host genome, disrupting normal gene function or regulation.
• Transposons (e.g., Tn elements) are mobile genetic elements that jump to new locations in the genome, potentially inserting into and disrupting genes.

The Ames Test

The Ames test is a widely used bacterial assay that screens chemicals for mutagenic (and therefore potentially carcinogenic) activity. It relies on a simple principle: if a chemical causes mutations, it can reverse a pre-existing mutation in a bacterial gene.

How it works:

The test uses specially engineered strains of Salmonella typhimurium that carry mutations in the histidine biosynthesis operon. These bacteria cannot synthesize histidine on their own, so they cannot grow on media lacking histidine.

1. Expose the mutant Salmonella to the test chemical. (A liver enzyme extract is often added to simulate mammalian metabolism, since some chemicals only become mutagenic after metabolic activation.)
2. Plate the bacteria on histidine-deficient media.
3. Incubate and count the colonies that grow. Each colony represents a revertant, a bacterium in which a new mutation restored the ability to make histidine.
4. Compare the number of revertant colonies to a negative control (no chemical exposure). A few spontaneous revertants are normal.

A significant increase in revertant colonies compared to the control indicates the substance is mutagenic. Because most carcinogens are also mutagens, a positive Ames test flags the chemical as a potential carcinogen.

Classifying Mutations from Sequence Data

To identify a mutation type, compare the mutated sequence to the wild-type sequence:

• If a single nucleotide is swapped for another, it's a substitution (point mutation).
• If one nucleotide has been added or removed, it's a single-nucleotide insertion or deletion. If this shifts the reading frame, it's also a frameshift.
• If the number of nucleotides added or removed is not divisible by three, it's a frameshift mutation.

Then determine the effect on the protein by translating both sequences using the codon table:

• Amino acid unchanged → silent mutation
• Different amino acid → missense mutation
• Premature stop codon → nonsense mutation

Genetic Variation and Heredity

Mutations are the ultimate source of new genetic variation in populations. Without them, all individuals in a species would carry identical DNA, and natural selection would have no raw material to act on.

Most mutations are neutral or harmful, but occasionally one confers a survival or reproductive advantage in a given environment. Over generations, beneficial mutations can spread through a population via natural selection, driving adaptation and evolution. Mutations are also heritable: if they occur in germ-line cells (cells that produce gametes), they can be transmitted from parents to offspring.