Ch. 39: The Genetic Code

translation

where transcript (RNA) is converted to polypeptides (proteins)

the genetic code

set of rules by which genetic information (nucleotide) is translated to protein (amino acid)

codons

three nucleotides that specify a particular amino acid

reading frame

each potential starting point for interpreting the 3 letter code

directionality

always 5’ to 3’

non-overlapping code

each nucleotide is part of only one codon. a single nucleotide mutation will only change one codon

features of the genetic code

non-overlapping, unambiguous (each codon corresponds to just one AA), degenerate (multiple codes for most AAs), first two bases often enough to specify which AA codon relates to

synonymous codons

codons specifying the same amino acid

termination codon

UAA, UGA, UAG

initiation codon

Methionine codon (AUG)

frameshift mutation

an addition or deletion that knocks the protein off of it’s reading frame

point mutations

change in 1 base

silent mutation

mutation in 3rd base of codon, same as it was supposed to be

nonsense mutation

codes for STOP on accident

missense mutations

still have amino acid put in, but it’s the wrong one

conservative missense mutation

amino acid put in has similar properties to what was supposed to be there

nonconservative missense mutations

mutated amino acid is not similar to what was supposed to be there

tRNA acceptor stem

the site where the amino acid is covalently attached to the tRNA on the adenosine (of CCA) at the 3’ end of this stem

anticodon arm

contains 3 base sequence that binds to the complementary codon in the mRNA

inosine

a deamination of guanine that can base pair with any nucleotide, typically found in the anticodon arm, used to stabilize tRNA

wobble

allows one tRNA to recognize several codons due to flexible pairing at the 3rd nucleotide of codon position (1st nucleotide of anticodon on tRNA), allows us not to have 64 tRNAs for 64 different codons

aminoacyl tRNA

amino acid linked to terminal adenosine of CCA and tRNA via ester linkage

aminoacyl tRNA synthetase

1 of these per amino acid, this binds ATP and the correct amino acid (based on properties) with correct tRNA

specificity of tRNA synthetase

binding pocket matches target AA, has specific ions to bind, have activation and editing site that check the shape and size