The genetic code
A gene is a short section of DNA that codes for a specific polypeptide. The sequence of bases in a gene determines the sequence of amino acids in a polypeptide (its primary structure), according to the genetic code. The genetic code is a triplet code: three bases in DNA codes for one specific amino acid. It is also non-overlapping, degenerate and almost universal.
Feature | Significance |
Triplet code | A sequence of 3 bases in DNA (or mRNA) codes for one specific amino acid. There are 43=64 possible triplets, enough to code for the 20 amino acids. |
Non-overlapping | 3 bases codes for 1 amino acid, the next 3 bases codes for the next amino acid. Each base is part of 1 triplet. Order of triplets in DNA (or mRNA) therefore determines the order of amino acids in a polypeptide. |
Degenerate | Genetic code is redundant (more triplet codes than amino acids) therefore most amino acids are coded for by more than 1 triplet code. Helps reduce the potential harmful effects of point mutations. |
Universal | Almost all organisms use the same genetic code. Very few exceptions (including mammalian mitochondria). Allows gene transfer between organisms (genetic engineering). Provides evidence for a common ancestor. |
Start and stop codons | AUG start codon tells the ribosome where to begin translation. 3 different stop codons (UAA, UGA, UAG) indicate where the ribosome stops translation and release polypeptide. |
Due to the degenerate nature of the genetic code, a change in the base sequence will only sometimes cause a change in the amino acid sequence in the polypeptide.
The DNA template (antisense) strand directs the synthesis of the mRNA. The mRNA is complementary in sequence to the DNA template strand and has the same base sequence as the DNA coding (sense) strand (except thymine is replaced by uracil). It is the sequence of bases in the DNA coding strand and mRNA that codes for the sequence of amino acids in a polypeptide.
Gene expression
Protein synthesis involves two stages: transcription and translation. In transcription, a single stranded messenger RNA (mRNA) copy of a gene is synthesised by RNA polymerase in the nucleus, using one DNA strand as a template. The mRNA then leaves the nucleus via a nuclear pore. In translation, the order of bases in mRNA determines the order in which amino acids are joined together at a ribosome, in the cytoplasm, to form a polypeptide.