BackMutations and DNA Repair Mechanisms: Oct 31
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Mutations & DNA Repair Mechanisms
Introduction to Mutations
Mutations are permanent changes in the DNA sequence of an organism. They are a fundamental source of genetic variation, which underlies evolution and can also lead to disease. Mutations may arise spontaneously or be induced by external factors, and their effects depend on their nature and location within the genome.
Genetic Variation: Mutations introduce new alleles into populations, providing material for natural selection.
Disease Causation: Some mutations disrupt normal gene function, leading to genetic disorders or cancer.
Examples: Mutations in the BRCA1 gene increase the risk of breast and ovarian cancer.
Classifying Mutations by Molecular Change
Mutations can be classified based on the type of molecular change in DNA:
Substitutions: Replacement of one nucleotide with another.
Insertions and Deletions: Addition or removal of nucleotides, which may cause frameshifts.
Transition: Purine-to-purine (A↔G) or pyrimidine-to-pyrimidine (C↔T) substitution.
Transversion: Purine-to-pyrimidine or vice versa (A/G ↔ C/T).
Types of Point Mutations and Their Outcomes
Missense Mutation: Changes one amino acid to another, potentially altering protein function.
Nonsense Mutation: Converts a codon to a stop codon, resulting in premature termination of translation.
Silent Mutation: Alters a nucleotide without changing the amino acid due to genetic code redundancy.
Synonymous vs. Non-synonymous: Synonymous mutations do not change the amino acid; non-synonymous do.
Example: The codon GAT (Asp) can mutate to TAT (Tyr), TAG (Stop), or TTT (Phe), each with different consequences for the protein.
Point Mutations (Substitutions)
Point mutations involve the substitution of a single nucleotide. The effect depends on the specific change and its position within the gene.
Missense: GAT (Asp) → TAT (Tyr)
Nonsense: GAT (Asp) → TAG (Stop)
Silent: TAT (Tyr) → TAC (Tyr)
Example: A mutation in the coding region of BRCA1 that introduces a premature stop codon can lead to loss of tumor suppressor function.
Learning Objectives
Explain the implications of the location of mutations for inheritance.
Identify intrinsic and extrinsic origins of DNA damage and mutations.
Describe normal biological processes that can lead to DNA damage.
Explain how intrinsic and extrinsic processes lead to mutations.
Identify the types of mutations created by each process.
Explain the role of mutation in biological, ecological, and disease processes.
Identify the different processes by which DNA is repaired.
Explain how these processes can lead to mutations.
Genetic Code Table
The genetic code translates nucleotide triplets (codons) into amino acids. Some codons signal initiation or termination of translation.
First Position | Second Position | Third Position | Amino Acid |
|---|---|---|---|
U | U | U | Phe |
U | A | A | Stop |
C | G | G | Arg |
A | U | G | Met (Start) |
G | A | A | Glu |
U | G | A | Stop |
U | A | G | Stop |
C | A | U | His |
A | A | U | Asn |
G | U | U | Val |
G | G | G | Gly |
U | C | U | Ser |
A | G | G | Arg |
C | U | U | Leu |
A | C | U | Thr |
G | C | U | Ala |
U | G | G | Trp |
C | G | U | Arg |
A | U | U | Ile |
G | A | G | Glu |
U | C | C | Ser |
C | A | C | His |
A | A | C | Asn |
G | U | C | Val |
G | G | C | Gly |
U | C | A | Ser |
A | G | A | Arg |
C | U | A | Leu |
A | C | A | Thr |
G | C | A | Ala |
Additional info: Table entries are representative; refer to a full codon table for all possible codons.
Examples of Mutation Effects
Vivipary in Strawberries: Mutations can cause developmental changes, such as vivipary (premature germination of seeds).
BRCA1 Gene: Sequencing can reveal mutations that may increase cancer risk; nonsense mutations are particularly concerning.
