The way in which the code is used to synthesise protein can be broken down into two key stages. These are transcription and translation.
This is the first stage in protein synthesis. The genetic information required for protein synthesis is contained in DNA, which remains in the nucleus. Protein synthesis however, occurs at the ribosomes in the cytoplasm (often they are on the rough endoplasmic reticulum). Therefore, a messenger RNA molecule (mRNA) is made, which carries the required genetic information from the nucleus to the ribosomes. The process by which mRNA is made is called transcription.
A section of the DNA double helix unwinds and unzips in that part which carries the code for the required protein (the gene). One of the strands then acts as a template (pattern) for the production of a molecule of mRNA.
mRNA synthesis occurs in the following stages:
- The start of each gene on DNA is marked by a special sequence of bases.
- The RNA molecule is built up from the four RNA nucleotides (each containing a different organic base A, C, G and U), these are found in the nucleoplasm.
- The nucleotides attach themselves to the bases on the DNA by complementary base pairing, just as in DNA replication. The only difference is that the RNA base U attaches to the DNA base A
- Only one strand of DNA has a mRNA molecule formed against it. The other DNA strand does not form a molecule of mRNA
- The new nucleotides are joined to each other by covalent bonds by the enzyme RNA polymerase
- The initial mRNA contains some regions that are not part of the protein code. These are called introns. The introns are cut out by enzymes
- The final mRNA molecule diffuses out of the nucleus through a nuclear pore into the cytoplasm to the ribosomes.
This process leads to a molecule of mRNA with an identical genetic message to the gene, but in a complementary nucleotide sequence. The information carried by mRNA must now be decoded to form a protein.
This is the process by which the genetic information in mRNA directs the synthesis of a polypeptide by controlling the order of insertion of amino acids into the growing polypeptide (i.e.. it determines the protein's primary structure). It occurs in four stages:
This involves a molecule of RNA called transfer RNA (tRNA). Its function is to translate the information carried by mRNA by correctly positioning amino acids into the growing polypeptide chain according to the sequence of nucleotides in mRNA. Each cell contains about 60 different types of tRNA it is mainly single stranded, 70-90 nucleotides long, and some portions of the molecule are double stranded giving the whole molecule a clover-leaf shape.
Each tRNA molecule possesses two important features:
- An anticodon site, which consists of a triplet of unpaired bases. The sequence of bases in this site varies from molecule to molecule and there is an anticodon sequence that is complementary to each codon sequence found on mRNA.
- An amino acid binding site at the free end of the molecule that can bind a specific amino acid.
The particular amino acid that binds to each tRNA molecule is somehow determined by the anticodon sequence. The actual amino acid is that which would be specified by the nucleotide sequence complementary to the anticodon, i.e. the codon on mRNA.
E.g.. the codon UCU specifies the amino acid serine. Thus the tRNA molecule that could recognise and bind serine would carry the anticodon AGA.
However, before the correct amino acid can be bound to tRNA, it must first undergo an initial activation step. Twenty amino acids are commonly involved in protein synthesis. Each amino acid of the common 20, has its own specific activation enzyme which forms a complex with its specific amino acid. Energy is required for this and is provided by ATP. The activated amino acid is then accepted by a specific tRNA molecule to form a complex called an amino acyl tRNA.
Consider a sequence of mRNA to be translated:
A U G A A A C G G U U A mRNA
codons *met lys arg leu
for amino acids:
- *met = methionine (initiation)
- lys = lysine
- arg = arginine
- leu = leucine
An mRNA molecule attaches to a ribosome. A tRNA molecule with an attached a.a. bearing the anticodon to the codon AUG and a modified amino acid (a modified molecule of the amino acid methionine) is positioned on the ribosome. This marks the initiation of the polypeptide chain. (also energy is required.) The ribosome is now ready to receive the tRNA with attached a.a. specified by the next codon, AAA, i.e.. the tRNA with attached a.a bearing the anticodon UUU.
A peptide linkage is now formed between the two adjacent amino acids. The amino acid becomes detached from its tRNA, and a dipeptide is now attached to the tRNA which is still at the ribosome.
The next tRNA molecule (bearing the anticodon GCC and the amino acid arginine) is positioned. Energy is required for the selection and binding of tRNA molecules. Once again, a peptide linkage is formed between the amino acids and in this way, the peptide increases in length.
These steps are repeated until the entire sequence of codons on mRNA have been "read". Termination of protein synthesis occurs when a termination codon (UAA,UAG or UGA) is reached and the newly synthesised polypeptide is released from the final tRNA molecule.
Following completion, the polypeptide may undergo modification before it becomes a functional protein
In eukaryotic cells, the polypeptide is released into cavity of the RER for transport through the cell. The final modification of the protein will often take place in the Golgi apparatus.
Often several ribosomes "read" simultaneously along the same messenger so that several polypeptides can be made from the same mRNA. A chain of ribosomes attached to the same mRNA is called a polysome.
The role of nucleic acids in the storage and transfer of genetic information can be summarised as follows:
DNA - transcription - RNA - translation - Proteins - Metabolic reactions and their regulation
The whole process of protein synthesis is summarised by the diagram below