genes and triplet code

• What is a gene?

  base sequence of DNA that codes for :

  • amino acid sequence of a polypeptide
  • functional RNA
• How do we know amino acids are coded in triplets?

  • there are 20 common amino acids in proteins
  • each amino acid must have a code in DNA
  • DNA has only 4 bases
  • 4^2 is less than 20 but 4^3 is greater than 20
  • therefore 64 possible triplets for 20 amino acids meaning some codes code for the same amino acid
• Features of the genetic code

  • triplets are read in one direction
  • the start of a DNA sequence is always the same triplet, if this amino acid is not needed it is later removed
  • 3 triplets are stop codes, not coding for an amino acid
  • code is degenerate
  • code is non-overlapping
  • code is universal
• Why is the genetic code degenerate?

  Most amino acids are coded for by more than one triplet
• Why is the genetic code universal?

  The same codon makes the same amino acid in different organisms.