Genetic Mutations: Classification, Mechanisms, and Functional Effects
Introduction to Genetic Anomalies: Lou Gehrig’s Disease (ALS)
Historical Context: The study of DNA mutations often begins with Lou Gehrig, a famous baseball player diagnosed with Amyotrophic Lateral Sclerosis (ALS), which subsequently became known as Lou Gehrig’s disease. The high visibility of famous individuals getting specific disorders often triggers a massive increase in research funding and scientific interest.
Nature of ALS: ALS is characterized as a muscle-wasting disease.
The Genomic Cause: ALS is caused by a specific type of mutation known as a repeat expansion.
Repeat Sequences: Within the genome, there are areas inside genes that consist of repeated sequences of DNA.
Normal vs. Critical States: It is standard to have a small number of these repeats. However, if these sequences expand, they can reach a critical threshold where they destroy or alter the function of the protein they reside within.
Mechanism of Expansion: Strand Slippage:
This mutation typically occurs due to mistakes during DNA replication.
In regions of high repeats, the DNA template and the newly synthesized daughter strand may undergo folding or bending.
This leads to strand slippage, where the DNA polymerase replicates a specific region of the genome multiple times, resulting in the expansion of those repeats.
Defining Mutation and Heritability
Verbatim Definition: A mutation is defined as a "change to the nucleotide sequence that may or may not result in the change of an amino acid sequence."
Primary Criteria: To be considered a mutation, there must be a heritable change in the genetic information—a change to the nucleotide sequence capable of being passed down through cellular reproduction.
Epigenetics vs. Mutation: Under current scientific definitions, epigenetic modifications are not considered mutations because they do not change the underlying DNA sequence. This definition may shift in the future as researchers realize some epigenetic changes can be inherited; however, for now, a sequence change is mandatory.
Modes of Inheritance:
Mitosis: Mutations occurring in somatic (body) cells are passed down to daughter cells via mitosis within the individual organism.
Meiosis: Mutations occurring in the gonads (gamete-producing cells) can be passed down to the next generation through meiosis and gamete formation.
Evolutionary and Functional Implications of Mutations
Raw Material of Evolution: Mutations are the fundamental source of genetic variation. They change the proteins produced by genes, creating different phenotypes that environmental factors can then select for or against.
Impact on Survival: Variations in phenotype produced by mutations can make an individual more or more capable at survival and reproduction.
Disease and Disorders: Mutations are frequently linked to diseases because if a mutation affects a protein, it can destroy normal metabolic or catabolic processes.
Beneficial Mutations: Not all mutations are deleterious. Many are beneficial or neutral.
Example: Lactase Persistence: Most humans today can tolerate lactose, but this is actually a mutant state. The "normal" or ancestral state in humans is to shut down the lactase gene after weaning. The mutation in the regulatory region of the lactase gene allowed continued transcription of lactase, providing access to more nutrients. This beneficial mutation was heavily selected for in certain human populations.
Somatic versus Germline Mutations
Somatic Mutations:
These occur in non-reproductive body cells.
They affect the specific organism in which they occur but are not passed to offspring.
Most cancers are the result of somatic mutations that produce disease within specific tissues of the organism.
Germline Mutations:
These occur in the cells intended for gamete production.
Inheritance Patterns: Because humans are diploid and a mutation is a single event, it is typically not matched by a mutation on the homologous chromosome. Therefore, a germline mutation will be passed down to of an individual’s offspring.
Systemic Presence: If an offspring receives a germline mutation, that mutation will be present in every single cell of their body, including their own germline, because the organism developed from a single mutant gamete.
Medical Precautions: The vulnerability of the germline to radiation is why lead blankets are used to cover the gonad area during X-rays; X-rays cause massive DNA damage that could be passed to future generations.
Scales of Mutation: Gene versus Chromosomal
Gene Mutation: A single mutation occurring in an individual gene within an individual cell. These are usually small-scale, such as point mutations.
Chromosomal Mutation: Large-scale changes such as aneuploidy, inversions, and large-scale translocations.
Cancer as Evolution: Cancer is often defined as "evolution on a cellular level." Mutant oncogenic cells are better at survival and reproduction than surrounding cells, producing more of themselves until they damage surrounding tissues.
Single Nucleotide Polymorphisms (SNPs) and Human Variation
SNP Definition: A single base substitution or a change in one nucleotide.
Polymorphism: Refers to "many forms" (many morphs). It is the primary form of variation among individuals of the same species.
Haplotypes: Collections or groupings of individual SNPs. Genomic services like ancestry.com or 23andMe use haplotypes to identify ethnic or geographical groupings. For example, possession of a specific collection of SNPs can indicate ancestry from a specific region.
Human Genomic Similarity: Humans are extraordinarily non-variable compared to other species. The similarity across the entire human genome is . There is only genetic variation among the entire species of homo sapiens. (Other less variable species, like cheetahs, are often on extinction watch lists due to lack of diversity).
Types of Base Substitutions: Transitions and Transversions
Transitions: The replacement of a purine with another purine () or a pyrimidine with another pyrimidine ().
These maintain the proper diameter of the DNA double helix.
They are harder for repair mechanisms to identify because the shape of the molecule is largely unchanged; only improper hydrogen bonding serves as a signal.
Transversions: The replacement of a purine with a pyrimidine or vice versa.
These alter the shape of the double helix, making it too wide or too narrow.
These are easier for repair proteins to identify due to the physical distortion of the DNA diameter.
Insertions, Deletions, and Frameshifts
Indels: A shorthand term for insertion or deletion mutations.
Frameshift Mutations: The addition or removal of one or two nucleotides shifts the entire reading frame of the ribosome.
Every amino acid following the mutation point will be changed.
This often results in truncated proteins or extremely long polypeptide chains if the mutation causes the ribosome to miss the natural stop codon and continue through the poly A tail.
Three-Nucleotide Indels: Adding or removing segments in multiples of three results in the addition or loss of exactly one codon (and thus one amino acid). These do not cause a frameshift and typically have the least drastic effect on the protein compared to single or double base indels.
Causes: These often occur via polymerase slippage, where the enzyme accidentally skips a base or replicates it twice.
Categories of Functional Mutations
Missense Mutation: A base substitution that results in a different amino acid being incorporated into the protein.
Neutral Missense Mutation: The new amino acid has similar chemical properties (e.g., replacing one hydrophobic amino acid with another). This usually results in minimal to no change in protein function.
Non-neutral: Replacing a polar amino acid with a non-polar one, or a positively charged one with a negatively charged one, significantly alters protein shape and function.
Nonsense Mutation: A mutation that converts a codon for an amino acid into a premature stop codon, resulting in a truncated, often non-functional protein.
Silent Mutation: A change in the DNA sequence that, due to the degeneracy of the genetic code (often at the third position of a codon), produces the exact same amino acid. There is no change in the phenotype, but these are useful for tracking evolutionary relationships between closely related species.
Functional Allele Interactions: Gain vs. Loss of Function
Loss of Function (Null) Mutations:
These destroy the protein’s ability to function.
Recessiveness: They are often recessive because a single functional copy of the gene can usually produce enough protein to maintain a normal phenotype.
Phenotypes: These can manifest as incomplete dominance (e.g., pink flowers instead of red) when one functional copy only produces of the required pigment.
Gain of Function Mutations:
These increase the amount of protein produced or enhance its functional ability.
Dominance: They are often dominant because the overactive or abnormal protein cannot be "hidden" by the presence of a normally functioning allele.
Oncogenes: These are typical gain-of-function mutations associated with cancer (oncogenesis), pushing cells through the cell cycle or preventing apoptosis.
Questions & Discussion
Question regarding DNA Repair: If a mutation occurs, is it always because proofreading failed?
Response: Proofreading by DNA polymerase does not catch of mistakes. Additionally, mutations can be caused by external environmental damage rather than internal replication errors. In some cases, like a "peeling sunburn," the damage is so extensive (mass cell death) that the organism does not have enough repair proteins available to fix the volume of damage occurring simultaneously.
iClicker Question: If a germline mutation is passed to the next generation, where will it be found?
Answer: It will be in all of the cells of the progeny (both somatic and germline cells) because the offspring originated from that single mutant cell. Consequently, they will pass the mutation to of their own offspring.