Proteins are the end product of many metabolic processes. A typical cell requires thousands of different protein molecules at any given moment. So, the protein requirements of a particular cell has to be satisfied by synthesizing them at site or has to be transferred from where it is being synthesized.
This is in accordance with the central dogma of molecular genetics, which postulates that the genetic information flows from nucleic acid to proteins. In Eucaryotic cells, protein synthesis requires the participation of over 70 ribosomal proteins; 20 or more enzymes to activate amino acid precursors, etc. And about 100 additional enzymes for the final processing of different kinds of proteins and 40 or more types of transfer and ribosomal RNAs. In total, 300 different macromolecules are required to synthesize polypeptides or proteins.Many of these macromolecules are organized into the complex 3D structure of the ribosomes to carry out stepwise trans location of the mRNA as the polypeptide is assembled.
Steps in protein synthesis
Transcription The first step in transcription is the partial unwinding of the DNA molecule so that the portion of DNA that codes for the needed protein can be transcribed. Once the DNA molecule is unwound at the correct location, an enzyme called RNA polymerase helps line up nucleotides to create a complementary strand of mRNA. Since mRNA is a single-stranded molecule, only one of the two strands of DNA is used as a template for the new RNA strand.
After transcription, the new RNA strand is released and the two unzipped DNA strands bind together again to form the double helix. Because the DNA template remains unchanged after transcription, it is possible to transcribe another identical molecule of RNA immediately after the first one is complete. A single gene on a DNA strand can produce enough RNA to make thousands of copies of the same protein in a very short time.
In translation, mRNA is sent to the cytoplasm, where it bonds with ribosomes, the sites of protein synthesis. Ribosomes have three important binding sites: one for mRNA and two for tRNA. The two tRNA sites are labeled the A site and P site.
Once the mRNA is in place, tRNA molecules, each associated with specific amino acids, bind to the ribosome in a sequence defined by the mRNA code. tRNA molecules can perform this function because of their special structure. tRNA is made up of many nucleotides that bend into the shape of a cloverleaf. At its tail end, tRNA has an acceptor stem that attaches to a specific amino acid. At its head, tRNA has three nucleotides that make up an anticodon.
An anticodon pairs complementary nitrogenous bases with mRNA. For example if mRNA has a codon AUC, it will pair with tRNA’s anticodon sequence UAG. tRNA molecules with the same anticodon sequence will always carry the same amino acids, ensuring the consistency of the proteins coded for in DNA.
Translation begins with the binding of the mRNA chain to the ribosome. The first codon, which is always the start codon methionine, fills the P site and the second codon fills the A site. The tRNA molecule whose anticodon is complementary to the mRNA forms a temporary base pair with the mRNA in the A site. A peptide bond is formed between the amino acid attached to the tRNA in the A site and the methionine in the P site.
The ribosome now slides down the mRNA, so that the tRNA in the A site moves over to the P site, and a new codon fills the A site. (One way to remember this is that the A site brings new amino acids to the growing polypeptide at the P site.) The appropriate tRNA carrying the appropriate amino acid pairs bases with this new codon in the A site. A peptide bond is formed between the two adjacent amino acids held by tRNA molecules, forming the first two links of a chain.
The ribosome slides again. The tRNA that was in the P site is let go into the cytoplasm, where it will eventually bind with another amino acid. Another tRNA comes to bind with the new codon in the A site, and a peptide bond is formed between the new amino acid to the growing peptide chain.