2.3.6 The Genetic Code

Question 1

Nature of the Genetic Code

Answer 1

A gene is a sequence of nucleotides that forms part of a DNA molecule (over DNA molecule contains many genes)
This sequence of nucleotides (the gene) codes for the production of a specific polypeptide (protein)
Protein molecules are made up of a series of amino acids bonded together
The shape and behaviour of a protein molecule depends on the exact sequence of these amino acids (the initial sequence of amino acids is known as the primary structure of the protein molecule)
The genes in DNA molecules, therefore, control protein structure (and as a result, protein function) as they determine the exact sequence in which the amino acids join together when proteins are synthesised in a cell

Question 2

The Triplet Code

Answer 2

The sequence of DNA nucleotide bases found within a gene is determined by a triple (three-letter) code
Each sequence of the 3 bases (ie.e each triplet of based) in a gene codes for 1 amino acid
These triplets codes for different amino acids
- there are 20 different amino acids that cells use to make up different proteins
For example:
- CAG codes for the amino acid valine
- TTC codes for the amino acid lysine
- GAC codes for the amino acid leucine
- CCG codes for the amino acid glycine
Some of these triplets of bases code for start (TAC - methionine) and stops signals
These start and stop signals tell the cell where individual genes start and stop
As a result, the cell reads the DNA correctly and produces the correct sequences of amino acids (and therefore the correct protein molecules) that it requires to function properly
The genetic code is non-overlapping
- each base is only read once in which codon it is part of)
There are 4 base, so there are 64 different codons (triplets) possible (4 cubed = 64), yet there are only 20 amino acids that commonly occur in biological proteins
- this is why the code is said to be degenerate: multiple codons can code for the same amino acids
- the degenerate nature of the genetic code can limit the effect of mutations
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 genetic information is transferable between species)
- the universal nature of the genetic code is why genetic engineering (the transfer of genes from one species to another) is possible

Question 3

Codons and Anticodons

Answer 3

Once mRNA had been formed and left the nucleus, it moves to the ribosomes where it can act as a template for protein synthesis
Each triplet within mRNA code is describes as a codon
The tRNA molecules that transfer amino acids possess anticodons which are complementary to the codons on mRNA