GENETIC CODE

Question 1

POINT mutations that alter GC

Answer 1

Single Base Substitutions:

1.Silent mutations: No amino acid change (due to code degeneracy or introns).

2. Neutral mutations: Amino acid change, but protein remains functional (due to similar R groups).
3. Missense mutations: Amino acid change resulting in a nonfunctional protein (e.g., GAG → AAG: Glu to Lys).
4. Nonsense mutations: Amino acid codon changes to a stop codon (e.g., UGG → UAG: Trp to STOP).

Question 2

FRAME SHIFT MUTATIONS

Answer 2

Insertion: Causes the reading frame to shift to the right.

Deletion: Causes the reading frame to shift to the left.

Question 3

Genetic code features

Answer 3

Codon is a triplet: Three nucleotides specify one amino acid.

Non-overlapping code: A single base change affects only one codon, not multiple.

Degenerate: Multiple codons can code for the same amino acid.

Question 4

restoring gene function

Answer 4

True Revertant: The original DNA sequence is restored.

Pseudo Revertant: A second mutation suppresses the first.
- Intragenic: Suppressor mutation within the same gene (e.g., frameshift restoration).
- Intergenic: Suppressor mutation in a different gene (e.g., suppression of a nonsense mutation).

Question 5

intragenic frame shift suppressors

Answer 5

Crick and Brenner proved the genetic code is a triplet.

Mutation in the r2 locus caused a frameshift.

Restored function when three bases were inserted, fixing the frameshift.

Question 6

start and stop codons in prokaryotes

Answer 6

Start Codon: AUG (methionine).

Stop Codons: UGA, UAA, UAG (terminate the peptide chain).

Question 7

supressor tRNA mechanism

Answer 7

Suppressor tRNA can read through stop codons, leading to extended polypeptides.

Cells prevent this by:
1. Duplicating tRNA genes to ensure correct translation.
2. Suppressor tRNAs competing with release factors for STOP codons.