14 - The Genetic Code

Question 1

Degenerate def

Answer 1

Multiple codons can encode the same amino acid
- as there are 64 possible codons that can code for 20 amino acids

Question 2

Sidney brenner - 1957 info

Answer 2

Disproved Gamow and his diamond code hypothesis of amino acids
- theorised that genetic code is written in a non-overlapping triplet style

Question 3

Non-overlapping genetic code def

Answer 3

One codon codes for only one amino acid in the chain of nucleotides

Question 4

Seymour Benzer work from 1959-1960

Answer 4

• T4 can infect both E. coli B and K strains
  Rll - rapid lysis locus gene in T4
• T4 bacteriophage can grow in E.Coli B strains, but not E.coli K strains
• bred mutant T4 bacteriophages multiple times until found mutants that could grow in E. coli K
• was able to map the genes responsible for mutations that allowed T4 to survive in E. coli K
• found 2000 mutations in the rll gene
• found two coding regions (genes) - A and B
• found gene B could tolerate some mutations
• so was a non-essential part of gene B

Question 5

Crick et. al 1961 - info

Answer 5

Determined experimentally that the genetic code is made up of triplets
- by isolating, and combining frame shifts and mutations in the rll and r+ genes in T4 bacteriophages using work from Seymour Benzer

Question 6

1961 - Marshall Nirenberg work

Answer 6

• Took polynucleotide phosphorylase and UDP and generated poly(U)RNA
• added this to 20 tubes containing E. coli extract
• added 19 amino acids to each tube
• one tube contained a radio-labelled amino acid
• incubated and acid-precipitated each tube to look for incorporation of the radioactive amino acid into protein
• revealed that UUU is a codon that encodes phenylalanine
• showed genetic code is made of triplets of bases - codons

Question 7

Gobind Khorana and his work - and the rest of the slides

Question 8

Cricks adaptor hypothesis - what can read the RNA code to synthesise the protein

Answer 8

• An adapter molecule - an intermediate between mRNA and the protein
• would translate info into protein
• would have an anti-codon that can read the mRNA via base pairing
• so adaptor molecule should be a nucleic acid
• would have an anti-codon
• so the molecule was tRNA

Question 9

Molecule that’s adapts mRNA strand into protein

Answer 9

tRNA molecule - translation

Question 10

tRNA structure

Question 11

Crick - wobble hypothesis - 1966

Question 12

Charging tRNA mechanism

Question 13

Energy required for tRNA charging

Question 14

Summary: Characteristics of the genetic code

Answer 14

• codons are triplets of bases
• codons do not overlap
• the genetic code is degenerate - each amino acid can be coded for by one or more proteins
• code includes punctuation in the form of three ‘STOP’ codons that dont code for an amino acid
• initiation code is AUG (methionine)
• the mRNA strand is read from the 5’ to 3’ end
• If there are any mutationsor errors in the DNA, the message may be changed and incorrect protein formation results
• highly specific tRNA: amino acid combinations translate the code, matching codons with specific amino acids