Proteins, Macromolecular Polymers, Built Up Of Chains
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Introduction Proteins are macromolecular polymers, built up of chains of repeating units called amino acids. There are twenty different naturally occurring amino acids, all of which have the vital amino (-NH₂) and carboxylic acid (-COOH) groups bonded to the central carbon atom, as well as an “R” group specific to that amino acid. Proteins are versatile and useful molecules, they can be used in many different roles in the body. For example, the catalysis of biological reactions, the transmission of messages around the body and the transportation of molecules (eg. the transportation of oxygen by haemoglobin). Proteins are coded for by the sequence of nucleotide bases in the DNA sequence of genes specific to the protein molecule in question. This sequence of bases is copied in the process of transcription which takes place in the nucleus of the cell. The copied sequence then undergoes post-transcriptional modification. This allows the introns (which are of no use in translation) to be removed from the sequence and the exons to be organised into the correct sequence to code for a specific protein. This modified sequence is then used in the process of translation to code for the amino acid sequence of the protein. Translation takes place in the ribosomes of the cell and is the process which results in the final formation of the linear polypeptide chain of the protein. This polypeptide chain is the basic primary structure of the protein. Proteins have four basic levels of