The Genetic Code Flashcards
What does translation require?
Involves large, macromolecular machinery and accessory factors for imitation, elongation and termination
What does it mean for polymerization to be endergonic?
Driven by high-energy phosphianhydride bond cleavage
Why are several bases necessary?
Necessary to specify a single amino acid
Triplet code of 3 bases per codon is sufficient (64 different triplets of bases) and allows AA to be specified by more than one codon
who is the cutest?
you are. yes, you are. you dirty slut you x
What does it mean for genetic code to be non-overlapping?
Codons are read in sequence and in discrete triplets
What does it mean for genetic code to be degenerate?
AA can be coded for by more than one codon
3 AA (Arg/Leu/Ser) are specified by 6 different codons (synonymous)
Only Met and Trp are represented by a single codon
More than one tRNA may carry a specific AA (isoaccepting tRNAs)
Crick and Brenner
Discovered that a deletion of a nucleotide could abolish gene function
An insertion mutation could restore gene function as it suppresses other mutations
Insertion and deletions shift the reading frame = frameshift mutation
Two closely spaced deletions and insertions could not restore gene function, but three could due to the code being triplet
What does mRNA do?
Binds to molecules of tRNA which carry an AA
Each tRNA contains an anticodon complementary to the mRNA codon
When sequencing in the 1960s was not available, what was the alternative?
They utilized Aztobacter vinelandii polynucleotide phosphorylase which links nucleotides together without template
EXAMPLE: E. coli cells experiments
Broken open, centrifuged to remove cell walls to be left with mRNA, ribosomes and enzymes.
With ATP/GTP/AA = protein synthesized
DNase (removes DNA) = synthesis halted
Purified or synthetic mRNA = recovers resulting polypeptide
What is significant about synonyms?
Differ only in their third nucleotide
Changes in the 1st position specify similar AA
2nd position pyrimidines encode mostly hydrophobic amino acids
2nd position purines encode mostly polar amino acids
What are stop codons?
NONSENSE
UAG, UAA and UGA
Do not code for specific amino acids but signal translation termination
What are start codons?
AUG and GUG
The start point for polypeptide chain synthesis and can specify Met and Val
What is the structure of tRNAs?
Formed of 54 to 100 nucleotides arranged in a cloverleaf
5’ terminal phosphate group, 7-bp stem containing a 5’ terminal nucleotide
D-arm ends in a 7-nt loop containing dihydrouridine and the anticodon-arm has 5-bp stem ending in a loop containing the anticodon
The TXC arm has a 3’ CCA sequence with free 3-OH
What is the site of variability?
Variant arm containing 3 to 21 nucleotides:
tRNA has a complex tertiary structure, whilst individual chains are L-shaped with base pairs between stems with acceptors and T stems forming one left and D stems and anticodons forms the other
