Genetic code

Characteristics of Genetic Code

Linear, codons, Degenerate, nonsense codons, No punctuation, Not all codons require own tRNA, Almost universal

Linear - genetic code

mRNA read 5’→3’, starts at AUG (alternative start codon rare), special tRNAchemically modified methionine (N-formyl methionine - bacteria) starts -

Codons - genetic code

set of 3 nucleotides, each codon specifies an AA

Degenerate - genetic code

4 base (4³) = 64 different codons, 61 codons for 20 AA (1-6 codons per AA), codon bias

Codon bias

prefere codon for AA, influence amount of tRNA in cell - change ratio, could lead to complete loss of tRNA, still same genetic code

Codon optimization

switch out codon for one favoured by expression, or add plasmid if missing tRNA

Nonsense codons - genetic code

UAA/ UAG/ UGA, doesn’t specify AA, signal end of protein synthesis, occasionally specify unusual AA if has additional signals

No punctuation - genetic code

Nothing separates consecutive codons (lines up at start and assume rest in same), nucleotide addition/deletion changes reading frame, change AA seq, likely result in pre-mature stop codon, Frameshift mutation

Not own tRNA - genetic code

most genomes have 30-50 tRNA, more than 1 codon can bp with anticodon from tRNA - Wobble

Wobble Position

loose base pairing between 3’ base of mRNA and 5’ base of anticodon (non waston crick), cell doesn’t need new tRNA for each codon, mediated by U and I

Hypoxanthine (I)

Deaminated adenine, found in some tRNA in the anticodon loop (5’ end), base pair with C/ U/ A

How wobble works

C and A only waston crick base pair, I is modified nt that allows lots of base pairing (C/U/A), uracil main reason becuase weak bp affinity - doesn’t hinder

Almost universal Genetic code

few exception in mitochondria and chloroplast - adapt to lifewithin cells, rare exceptions in free living organisms (green algae and protozoa)

Use of universal genetic code

use bacterial system to express protein found in plants/animals, recombinant human gene cloning

Exception to universal code

controlled by tRNA, most relate to stop codons (stop → AA or AA → stop)