Basis of Genetic Code (SL IB Biology)

Genetic code

• DNA molecules carry the genetic code as a sequence of nitrogenous bases in the nucleotides
  • These bases are adenine, guanine, cytosine and thymine
• One of the strands of a DNA molecule will carry the base sequence that will be read by enzymes
  • This strand is known as the coding strand 
• The sequence of bases that form genes on the coding strand will determine the order of amino acids in the proteins that are synthesised 
• The code is read as a triplet of bases, called a codon, with each sequence of three bases coding for one amino acid
  • Remember that there are 20 different amino acids that could be coded for
• The sequence of amino acids will determine the shape and function of the protein that is synthesised from the code

From gene to protein diagram

The sequence of DNA bases in the genes codes for the production of a specific protein molecule

The genetic code is universal

• The genetic code is universal, meaning that almost every organism uses the same code (there are a few rare and minor exceptions)
• The same triplet codes code for the same amino acids in all living things (meaning that genetic information is transferable between species)
  • The universal nature of the genetic code is why genetic engineering is possible
• This provides evidence for a universal common ancestor from which all living organisms on Earth evolved
• Over time, mutations have led to changes in some of the base sequences of organisms
  • These base sequences form the genome of an organism
  • Some base sequences form part of regions that code for proteins, called coding sequences, while others are located in regions that do not code for proteins (non-coding sequences)
• Many of these coding and non-coding sequences have remained unchanged in all organisms and are known as conserved sequences
• Highly conserved sequences are usually found in the genes that code for proteins involved with transcription and translation, as well as histone proteins which help to package DNA tightly into the nucleus
• The similarity in these sequences indicate that living organisms share a universal ancestry