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The Genetic Code

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The Genetic Code Chromosomes contain DNA which through gene expression controls our characteristic phenotypes. Each DNA molecule is a chain of nucleotides, each of which contains one of four organic bases. The DNA of the genes in the nucleus is copied as messenger RNA (mRNA) by a process of transcription and it is actually the sequence of bases on mRNA that forms the genetic code. The mRNA genetic code is translated into a sequence of amino acids for a particular protein on the ribosome in the cell. Messenger RNA has a specific base sequence and each group of three bases codes for a single amino acid and is called a codon. Therefore, the genetic code of each gene is a sequence of codons which codes for the synthesis of a particular polypeptide chain of a protein or enzyme. Since there are 4 bases (Guanine, Adenine, Cytosine and Uracil), there are therefore 64 possible codons. ...read more.


Only those mutations which can pass into the gametes are clearly inherited. There can be changes to the genetic code sequence during the lifetime of individuals, for example due to exposure to radiation, the sun or certain chemicals. If a change to the DNA occurs, it is copied to all of the surrounding cells by mitosis and the mutated protein produced - for example the development of skin cancer due to sun exposure. These are called acquired mutations and clearly cannot be passed to the next generation. The genetic code is also non-overlapping - a specific 'start' codon (AUG) ensures that the ribosomes read the mRNA code in a unique way and there are also three specific 'stop' codons (UAG, UAA, UGA) to ensure that only that section of the mRNA which codes for the particular polypeptide is copied - therefore, they effectively define the reading frame of the mRNA sequence. ...read more.


It relies on the fact that the DNA replication process in all organisms is similar and that the resulting transgenic organism will express the particular codon sequence and hence produce a protein originally only found in the 'donor' organism. Examples this so called genetic engineering include the insertion of human genes into bacteria, which will then produce a human protein such as insulin, and genetically modified plants and animals where desirable genes (eg. conferring virus resistance) are transferred to the organism, which does not express this protein itself. Additionally, there is the potential for human gene therapy which involves inserting a 'normal' gene into a person to correct a genetic disorder - such trials are ongoing. The universal nature of the code across organisms and also the common transcription, translation and DNA replication systems offer a range of future possibilities to change the speed and path of evolution. These common features suggest that the genetic code was established very early in the history of life. ...read more.

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