Genetic Code Chart
o There are four different nucleotides in RNA that together must form an unambiguous code for the 20 common amino acids.
o The code must be a combination of any three nucleotides.
o However, any three nucleotides gives 4^3 = 64 possible combinations.
o These are more possibilities than there are amino acids.
o Thus more than one series of three nucleotides may code for any amino acid; The code is degenerative.
o The code is also unambiguous (clear), one codon codes for only one amino acid and never two.
o In addition the code is almost universal (mitochondria present an exception), nearly every living organism uses the same code.
Genetic Code Chart
o Redundancy is usually expressed in the third letter of the codon (3’ position).
o Three consecutive nucleotides on a strand of mRNA represent a codon.
o All but three possible codons code for amino acids, so there are only 61 codes for amino acids.
o The remaining codons UAA, UGA, and UAG are stop codons, which signal an end to the protein synthesis.
o AUG is a start codon and it also codes for amino acid methionine.
o Codon changes have been opposed by the most intense selective pressure during evolution.
o By convention, a sequence of RNA nucleotides is written 5’ → 3’ We must understand probabilities.
o For instance, you must be able to figure out how many possible codons exist.
o As discussed above, four possible nucleotides can be placed in 3 spots giving 4^3 = 64.
o What about this? “A polypeptide contains 100 amino acids. How many possible amino acid sequences are there for this polypeptide? ”
o Well first we know that there are 20 amino acids and from the information we just received we know that there are 100 spots to place them. So it must be 20^100 possible sequences.
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