Proteins are some of the most essential compounds on the planet. They perform a variety of tasks ranging from muscle contraction to fighting diseases. Over 50% of the dry weight of organisms are made of proteins; this is because things like your nails and hair are made of proteins. Proteins are also components of biological membranes, and they help regulate the passage of molecules through the membranes. This is all very important to the body, but the most important function, by far, is their use as enzymes to speed up the reactions in the body. Proteins contain the elements carbon, oxygen, hydrogen, nitrogen, and sometimes sulfur. Theses are all arranged into amino acids. The monomers of proteins which are held together by peptide bonds.
The process by which these all important protein molecules are made is a complicated one. It starts with the transcription of DNA into RNA. In this process the RNA uses the enzyme RNA polymerase, which shows the RNA where to start its transcription. As it moves along the strand of DNA it adds the complimentary nucleotides together, eventually the RNA polymerase will reach a termination signal and then the RNA will break off.
This can be one of three different types of RNA; the first could be Messenger RNA, which contains the code for the order of the amino acids in a protein. It also carries this information from the DNA of a structural gene to ribosomes, where the protein is made. The second type is Transfer RNA; these carry amino acids to the mRNA at ribosomes and fit them in their proper place in the protein chain. The last type is Ribosomal RNA it is a major component of ribosomes and that is most of what we know about it.
After the RNA is produced in the nucleus it is processed into mRNA by putting special caps and tails on the ends. The cap is used as a signal that will bind the mRNA to the ribosome and the tail is thought to extend the life of the mRNA by protecting it from the many mRNA digesting enzymes in the cytoplasm. Even after the cap and tail are placed onto the mRNA it is not yet ready, it needs the introns cut out. Introns are the useless sections of the DNA. After they are cut out what is left is a shorter, more mature mRNA. Then it heads off to the ribosome where the mRNA binds to the small ribosomal unit, then the initiation codon (AUG) pairs with the tRNA. This becomes the first amino acid in the peptide chain. Now the large ribosomal subunit binds to the small, which completes the complex.
The P and A sites are the two places where translation takes place in a ribosome. At this time the initiation codon is at the P site and the codon for the second amino acids at the A. A tRNA with a complimentary anticodon binds to the second mRNA codon, at the A site. The amino acid carried by the tRNA will become the second amino acid in the chain. Then an enzyme joins the two amino acids together by a peptide bond. Then the AUG leaves and empties the P site which is taken up by the second codon and the A site is filled with the third codon. This is called translocation. When the beginning of the mRNA comes out of the ribosome another can be there to start transcribing it. These steps repeat until they hit a termination codon. After this, the peptide chain is not yet a protein. Most of the time the first methonine must be removed and also some of its neighbors. Chemical groups can also be changed around, while sulfur bridges must be formed between parts of the molecule or even two separate chains to make up the larger protein.