Genetics

How Gene Expression Created Proteins



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Gene expression is an important natural process where the inheritable information provided by a gene on a DNA strand in a cell is turned into a gene product with some function for the organism. There are several stages to the process and the end product of gene expression will be either a protein or an RNA strand.




In overview the process of gene expression can be described in terms of two types of process known transcription and translation. In the case of an RNA product of gene expression only transcription occurs, as this is simply the process by which RNA is made from DNA. But for producing a protein, which is what we're interested in here, both processes, transcription and translation are required.




The process of transcription involves several steps. To begin with is the unwinding of the DNA double helix. Then comes the use of one of these strands to act as a template for transcription. RNA molecules are then synthesised using the RNA polymerase enzyme. Finally, you get the ending of the synthesis on reaching a terminator signal. The fully transcribed RNA then leaves the vicinity of the DNA, leaving the nucleus and into the cytoplasm, in the case of eukaryotic cells, for example. Once finished with the two DNA strands then reanneal.




The second of the processes of gene expression is translation. During this process the RNA sequence is used to create new polypeptide chains after it reaches cell organelles called ribosomes. There are several steps involved. Firstly, you get the attachment of mRNA to the ribosomes. Then comes the passage of the RNA through the ribosome. As it passes through the ribosome the reading of the RNA occurs.




During this process the correct amino acids are brought to the site and are then joined appropriately to other amino acids by way of peptide bonding. The result of this process is the creation of a polypeptide chain. The whole process terminates when a stop codon is found by the ribosome (which could be UAA, UAG, or UGA).




These unbranched polypeptide chains are just the primary structure of proteins though. Secondary structure emerges as extra bonds form between the polypeptides. One common form is the alpha-helix shape caused when hydrogen bonds hold the chains together. Further, even more complex, structure can also appear through folding of the protein. This gives a 3 dimensional, tertiary structure. The globular proteins that result can become enzymes and antibodies, for example.

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