Understanding:
• Codons of three bases on mRNA correspond to one amino acid in a polypeptide
• The amino acid sequence of polypeptides is determined by mRNA according to the genetic code
Codons
- The base sequence of an mRNA molecule encodes the production of a polypeptide
- The mRNA sequence is read by the ribosome in triplets of bases called codons
- Each codon codes for one amino acid with a polypeptide chain
- The order of the codons in an mRNA sequence determines the order of amino acids in a polypeptide chain
Genetic Code
- The genetic code is the set of rules by which information encoded within mRNA sequences is converted into amino acid sequences (polypeptides) by living cells
- The genetic code identifies the corresponding amino acid for each codon combination
- As there are four possible bases in a nucleotide sequence, and three bases per codon, there are 64 codon possibilities (43)
- The coding region of an mRNA sequence always begins with a START codon (AUG) and terminates with a STOP codon
Skill:
• Use a table of the genetic code to deduce which codon(s) correspond to which amino acid
Typically the genetic code shows the codon combinations expressed on an mRNA molecule
- Tables displaying the genetic code may occasionally show the sequence on the sense strand of DNA (non-coding strand)
- These sequences are identical to the mRNA codons with the exception of thymine (T) being present instead of uracil (U)
The Genetic Code (Wheel)
