The Genetic Code

Question 1

Codon

Answer 1

Triplet of nucleotides in a gene or in the mRNA copy of a gene

• Codes for amino acid
• 64 possible codons
• Written in 5’ to 3’ direction
• 3 letter and single letter representations

Question 2

Nurenberg and Mathei’s Experiment

Answer 2

Cell-free system that deciphered the first letter of code

• All components necessary for protein synthesis in tube (RNA template, nucleotide, ribosome, amino acid, energy)
• Simple mRNA’s made of same/alternating nucleotide
• Repeating tract of U and C produced string of alternating amino acids (serine and leucine)… CONFIRMING 3 NUCLEOTIDES PER CODON

Question 3

AUG

Answer 3

START CODON –signals where protein synthesis should begin
* Only codon that codes for methionine
* First codon in every protein coding sequence!

Question 4

Redundancy of Genetic Code

Answer 4

• Multiple codons can specify the same amino acid
• Occurs due to there being more unique triplets (64) than amino acids (20)
• Provides buffer against harmful mutations, specifically against 3rd nucleotide
• CUU, CUA, CUG, etc. all code for Leu

Question 5

Translation STOP codons

Answer 5

UAA, UAG, UGA
* Do not code for amino acids
* Signals when translation is complete

Question 6

Open Reading Frame

Answer 6

Sequence of gene begnning from start codon to end codon.

• Coding region of a gene; Potential to code for a protein
• Determined by the first AUG
• Evidence when computers and researchers try to predict where genes are located in DNA.

Question 7

Reading Frames

Answer 7

• Nth reading frame: Translation of codons can occur from the nth nucleotide, where n = 1,2,3
• Creates different amino acids and produce alternative proteins
• 6 possible reading frames, 3 in one direction, 3 in the reverse direction
• Allows for precision and description when referring to genetic code

Question 8

Frame Shift Mutations

Answer 8

• The addition or subtraction of a nucleotide changes the reading frame
• Wrong amino acids added to ppt chain, resulting in faulty protein
• Does NOT occur when 3 nucleotides/1 codon is removed or added

Question 9

RNA World Hypothesis

Answer 9

• Some type of ancestral RNA molecule was the precursor to current
• Hypothetical stage in evolutionary history
• RNA was the main storage of genetic information and catalytic units
• Molecule responsible for the origin of life

REQUIRES FURTHER EVIDENCE

Question 10

Unambiguous Nature of Genetic Code

Answer 10

• Each of the codons in DNA/RNA codes for only one specific amino acid, or a start/stop signal
• Ensures precise protein synthesis (no confusion as to which amino acid is placed at each position)

Question 11

Exceptions to the Genetic Code

Answer 11

• Ciliates: UAA & UAG are not stop codons, code for glutamine
• Plastids, Mitochondria: have own DNA, whicih codes for proteins
  
  • CUA codes for threonine in yeast mitochon.
  • CUA codes for leucine in mammalian mitochon.

Question 12

Melanocortin Receptor: Case study on Mice

Answer 12

• Mice in forest have darker coat
• Mice in beach have lighter coat
• PPT chain associated melanocortin receptor differ by one amino acid, impacting phenotype
• This change is based on natural selection

Question 13

Retinoblastoma

Answer 13

A rare tumour of the eye affecting young children.

• Retinoblastoma protein is a tumour supressor – acts as a cell cycle break, stopping cell division
• Failure of Rb protein results in no cell cycle break, leading to cancer on retinas
• Treated using liposomes containing chemotherapy drugs

Question 14

Health conditions related to transcriptional errors

Answer 14

Diabetes, Alzheimer’s, Parkinson’s, Aging, Cancer, Down syndrome

Question 15

Direction of Codons

Answer 15

5’ to 3’