The genetic code

• what is a gene?

  a section of DNA on a chromosome which codes for a specific polypeptide
• what is DNA?

  a store of genetic information, coded in the sequences of bases in the DNA, in thousand of sections along its length, called genes
• what does the base sequence do?

  it directs which amino acids join together - it therefore, determines which proteins are made, and because enzymes are proteins, it determines which reactions can take place in an organism
• what does DNA determine?

  the characteristsics of an organism
• what is the genetic code called?

  a triplet code
• what did biochemical experiments show about the genetic code?

  • a polynucleotide strand aways has 3x the number of bases as the amino acid chain it coded for
  • if 3 bases were removed from a polynucleotide chain, the polypeptide would have one fewer amino acid
  • if the polynucleotide had 3 extra bases, the polypeptide would have one more amino acid
• what is the genetic code?

  the genetic code consists of the sequences of the bases in DNA or RNA
• what did the biochemical experiments suggest?

  • that 3 bases coded for 1 amino acid-this result was supported by the logic of arithmetic(there are 4 different bases in DNA but 20 different amino acids occur in proteins)
  • if 1 base coded for 1 amino acid, only 4 amino acids could be mad-A, T, G and C would each code for 1 amino acid
  • if 2 bases coded for 1 amino acid, there would be(4x4)16 combinations, to make 16 amino acids-its not enough
  • if 3 bases coded for each amino acid, there would be (4x4x4)64 combinations-this would be more than enough to code for 20 amino acids
• how can you calculate the number of amino acids in a strand of DNA or mRNA?

  bases / 3 = amino acids
• how can you calculate the number of bases coding for the amino acids in DNA or mRNA?

  amino acids x 3 = bases
• what are the characteristics of the genetic code?

  • its a triplet code
  • there are 64 possible codes but only 20 amino acids are found in proteins-more than 1 triplet can encode each amino acid so the code is described as ’degenerate’ or ’redundant’
  • the code is punctuated-there are 3 triplet codes that don’t code for amino acids-in mRNA, they are called ‘stop‘ codons and mark the end of a portion to be translated
  • the code is universal-in all organisms known, the same triplet codes for the same amino acid
  • the code is non-overlapping-each base only occurs in 1 triplet
• why is the genetic code a triplet code?

  as 3 bases encode each amino acid so the code is a triplet code
• what is a codon?

  triplet of bases in mRNA that codes for a particular amino acid, or a punctuation sign
• why is it said that the genetic code is punctuated?

  as there are 3 triplet codes that don’t code for amino acids - in mRNA they are called ‘stop’ codons and mark the end of a portion to be translated - rather like a full stop at the end of a sentence
• what does describing the genetic code as ‘redundant’ or ‘degenerate’ mean?

  as a single amino acid may be coded for by more than 1 codon
• as the genetic code is universal, what is unlikely?

  its unlikely that the same code could have independently evolved twice, the inference is that all life that we know of comes from the same origin of the life event
• what is the genetic code sometimes quoted as?

  as DNA triplets, but it can also be quoted as RNA codons