The Nature of the Genetic Code

The Nature of the Genetic Code
?One of the most remarkable facts of life is that each cell in an organism contains all the information required to determine all the characteristics of that whole organism?1. This information is an organism?s genome, and is made up of the genetic code and is found in its dna. The code determines the sequence of amino acids in a protein, the manufacture of which is one of the cell?s most important roles. All the proteins in an organism are determined by four bases: Adenine, Cytosine, Guanine and Thymine. Three bases (or a triplet) code for one amino acid; there are about 20 amino acids, which, when arranged in a specific order (containing several hundred amino acids) will form a protein. The dna in a nucleus does not directly synthesise these proteins. Firstly, messenger rna (mRNA) is made on a dna template in the nucleus, in a process called transcription. Here the dna, via the enzyme dna polymerase breaking up the hydrogen bonds between bases, unzips itself exposing its nucleotide bases, with which mRNA forms by complementary base pairing: A,T,C and G bases on the dna are replicated as U,A,G and C bases respectively on the mRNA.
The rules of complementarity allow genes to be successfully transferred from one medium (dna) to another (mRNA). The mRNA strand then enters the cytoplasm, where it will combine with a ribosome on its small subunit which contains an mRNA binding site. The ribosome also has a transfer rna (tRNA) binding site, and allows a triplet on the mRNA (known as a codon) to bind onto the complementary anticodon of a tRNA molecule, which carries with it an amino acid. As the ribosome moves along the mRNA molecule (always from the 5? à3?direction), the chain of amino acids held together by peptide bonds grows longer, until the desired polypeptide?s primary structure has been synthesised. The polypeptide will become the final protein after it has been folded to form the secondary, tertiary and quaternary structures ? thus, gene expression is said to have taken place. This is the universal process of how the genetic code within the nucleus is able to turn into one of the millions of proteins in our body. The process is universal as the same codons specify the same amino acids in almost all organisms. [image][image] It is known that ?a sequence of three nucleotides codes for one amino acid?2, due to the work done by a team led by Francis Crick in 1961 who proved the triplet code. Using enzymes he added or deleted nucleotide bases in the dna of a bacteria-killing virus (a phage). When one or two bases were added or deleted, the viruses would not infect the bacteria, suggesting a frameshift mutation, which is when ?everything downstream of the mutation will be translated in a different reading frame?3. However when three bases were added or deleted, the virus was able to infect the bacteria meaning the gene had only been partially affected meaning that each triplet specifies one amino acid. The results also proved that the genetic code is non-overlapping, which means that each base is part of only one triplet, and therefore each base is only involved in one amino acid. This illustrates how the arrangement of the genetic code?s codons end up as a protein. Another structural characteristic of the genetic code are its punctuation codons. The start and end of a code of dna which codes for a specific polypeptide (a cistron), is determined by the punctuation codons. The start is signalled by tac , and the end is signalled by att, atc and act on dna. This allows the enzyme dna polymerase to attach itself at the particular base sequence, known as the promoter site enabling the transcription of dna to begin. Seeing as there are 64 possible codons, but only 20 amino acids, the code is said to be degenerate. Therefore it means that ?most amino acids are coded for by several triplet codes?4. A triplet code (three bases) would provide 64 possible combinations which is more than enough for all 20 amino acids; on the other hand a two base code would only have 16 possible combinations, which is not enough. In order to fully understand the nature of the genetic code, scientists had to work out which of the 64 codons determined each amino acid. To do this they made synthetic mRNA molecules, like uuu-uuu-uuu (called poly-U), and added to a cell free environment, containing only isolated ribsomes and enzymes needed for polypeptide synthesis. The polypeptides that were synthesised were studied to determine their amino acid sequence. Poly-U synthesised phenylalanine (used for growth in infants), meaning that the codon uuu codes for phenylalanine. Now the specific codons of any amino acid are known, giving us more knowledge over which bases of the genetic code creates which genes. Now the codons for all amino acids are known, we can apply the genetic code in many different ways, for example ?bacteria can be genetically engineered to make human proteins?5. This is all thanks to the fact that the genetic code is universal. The same bases (A,T,C and G) can be found in all organisms, so codons, and thus genes can be transferred from organism to organism via the use of recombinant dna technology. Hence, bacteria like E coli can be made to produce insulin for diabetics

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