Module 4 Section 3

Question 1

Three important characteristics of the genetic code

Answer 1

1. Degenerate
   - 64 unique ways to combine 4 different nucleotides in triplet sequence, but only 20 common AA
   - multiple codons encode the same AA
2. Robust
   - can absorb single nucleotide mutations with minimal consequences to the protein it encodes
   - due to degeneracy
3. Universal
   - All life on earth uses this genetic code

Question 2

Rules of the genetic code (5)

Answer 2

1. non-overlapping
2. does not contain pauses
3. code is read in triplets
4. it is read in a linear fashion
5. The code is resistant to mutations

Question 3

The reading frame

Answer 3

• Need a Met followed by a long stretch of w/o a stop codon
• if nucleotides randomly distributed, STOP codon would appear every 3/64 (~20) codons
• specific sequences in the mRNA also signal the start of translation

Question 4

Pauses in genetic code

Answer 4

• does NOT happen
• would make the reading frame immune to frameshift mutations, insertions and deletions would only alter one AA
• researchers used acridines (mutagens) to induce insertion/deletion mutations

Question 5

Translation is linear

Answer 5

• mRNA read 5’ to 3’ direction

- proteins synthesized from N-term to C-term

Question 6

Transition mutation

Answer 6

• most common mutation
• purine is replaced with purine or pyrimidine replaced with pyrimidine
• mutation in 3rd codon position rarely causes change
• mutation in 2nd codon tend to conserve chemical structure
• mutation in 1st position for most codons causes AA change, usually a chemically similar AA

Question 7

Silent Mutation

Answer 7

• no change in AA sequence

- non-deleterious

Question 8

Missense Mutation

Answer 8

• single-base substitution that leads to replacement of one AA with another
• can be deleterious or non-deleterious

Question 9

Nonsense Mutation

Answer 9

• codon change that results in a termination codon
• abort protein synthesis, resulting in an incomplete protein that is rarely functional
• usually very deleterious

Question 10

Non-Stop

Answer 10

• Results in the loss of a stop codon
• similar to nonsense, subject to degradation early in translation, no protein produced
• deleterious