Proteins are polymers (polypeptides – aka monomers joined by peptide bonds) of amino acids, of which there are 20 which occur naturally.
They are synthesised in the cytoplasm on ribosomes which decode the mRNA in the 5′–>3′ direction.
Most proteins contain between 100 and 400 amino acids, and as the order of amino acids per protein can be different there are 20^100 to 20^400 possible stuctures.
The average amino acid has a molecular mass of 110, so using this we can estimate the mass of different proteins by multiplying the average mass by the number of amino acids in the protein – eg. a 400 amino acid protein has an estimated molecular weight of 44000.
It is estimated that there are 10^7 or 10^8 different proteins in nature.
– Amino Acids
The 20 different amino acids are:
| Single Letter Code | Short Name | Name |
| A | Ala | Alanine |
| C | Cys | Cysteine |
| D | Asp | Aspartic Acid |
| E | Glu | Glutamic Acid |
| F | Phe | Phenylalanine |
| G | Gly | Glycine |
| H | His | Histidine |
| I | Ile | Isoleucine |
| K | Lys | Lysine |
| L | Leu | Leucine |
| M | Met | Methionine (START*) |
| N | Asn | Asparagine |
| P | Pro | Proline |
| Q | Gln | Glutamine |
| R | Arg | Arginine |
| S | Ser | Serine |
| T | Thr | Threonine |
| V | Val | Valine |
| W | Trp | Tryptophan |
| Y | Tyr | Tyrosine |
*Met (Methionine) is also a start signal in translation for Eukaryotic cells. When the codon for Met is read (AUG), translation begins. Met is often removed or altered once translation has been completed. The START codon is different in Prokaryotes, possibly GUG – valine.
Each protein is coded for by 3 bases – called a triplet. Since there are 4 bases in total, of which 3 can be chosen there are 4^3 possible combinations – 64.
Here’s the triplet codes for each Amino Acid in most cells:
If you’d like the file this screenshot came from: Amino Acid Codes
These codes are almost universal, with the exception of a few types of cell. These include Human Mitochondria, where there are several triplet changes – such as UGA coding for Trp rather than STOP and AUA coding for Met instead of Ile.
– tRNA and Codon Triplets
- Amino acids are linked to an adapter molecule of tRNA. This forms an anticodon which will match a codon on the mRNA.
- The amino acid is bonded to the 3′ end of the complementary tRNA strand.
- Essentially, anticodons come in when they match the mRNA strand and are then removed, leaving an amino acid completemtary to the codon.
- This is repeated over and over to form a chain of amino acids until a stop codon is reached (UAA, UAG or UGA) and the completed polypeptide chain is released.
To explain this better I’ve found this animation. This is not my work, rather that of the American Society for Microbiology. If found it on their page here. Click here to watch the translation in bacterial cells video.
– Mutations caused by Errors
- Wild Type = Normal Sequence
- Miss-sense = One base changed, resulting in the sequence coding for a different Amino Acid.
- Non-sense = One or more bases changed, resulting in termination of chain.
- Silent = One of more base changes but the same amino acid coded for.
- Frameshift Base Deletion = One base removed, resulting in the change of most of the following amino acids.