Consequences of Base Substitutions
- Imagine reading a sentence where a single letter is changed: "The dog ate the pie" becomes "The dog ate the pig."
- The meaning shifts, but the overall structure remains intact.
- Similarly, base substitutions in DNA can alter genetic instructions, with effects ranging from negligible to life-altering.
Definition
Base substitution
A base substitution is a gene mutation where one nucleotide in the DNA sequence is replaced by another.
Single-Nucleotide Polymorphisms (SNPs): Variations in the Genetic Code
- Single-nucleotide polymorphisms (SNPs) are the most common type of genetic variation in humans.
- They occur when a single nucleotide in the DNA sequence is altered.
- SNPs can be found in both coding and non-coding regions of the genome.
The genetic code is like a safety net: its redundancy helps protect against mutations.
Types of Base Substitution Mutations
1. Same-Sense Mutations
- These mutations change one codon into another that codes for the same amino acid.
- Because of the degeneracy of the genetic code, they have no effect on the protein.
- Don’t confuse mis-senseand nonsense mutations.
- Mis-sense mutations change one amino acid, while nonsense mutations create a stop codon.
A mutation changing CAG (glutamine) to TAG (stop codon) would truncate the protein.
Consequences of Base Substitutions
1. Neutral Mutations
- Many base substitutions have no effect on the organism,
- Especially if they occur in non-coding regions or result in same-sense mutations.
2. Harmful Mutations
- Mis-sense and nonsense mutations can disrupt protein function, leading to genetic disorders or diseases.
Cystic fibrosis is often caused by a nonsense mutation in the CFTR gene, resulting in a truncated, nonfunctional protein.
3. Beneficial Mutations
- Rarely, base substitutions improve protein function or confer an advantage, such as resistance to disease.
The CCR5-Δ32 mutation provides resistance to HIV by altering a receptor on immune cells.
Single-Nucleotide Polymorphisms (SNPs) and Genetic Diversity
- SNPs are a major source of genetic variation in populations.
- They can influence traits, susceptibility to diseases, and responses to medications.
SNPs in the BRCA1 gene are associated with an increased risk of breast and ovarian cancer.
Note- Not all SNPs have functional consequences.
- Many are neutral and serve as markers for studying genetic variation.
The Role of SNPs in Evolution
- While most mutations are neutral or harmful, beneficial mutations drive evolution by providing new traits that can be selected for in changing environments.
- How do scientists determine whether a mutation is beneficial, neutral, or harmful?
- Consider the role of the environment in shaping these outcomes.
Reflection and Review
- Base substitutions are a key source of genetic variation, with effects ranging from silent to severe.
- The degeneracy of the genetic code acts as a buffer, reducing the impact of many mutations.
- SNPs contribute to genetic diversity and are essential for understanding evolution and human health.
