The Genetic Code

The Genetic Code Overview This mental faculty will examine how selective information is en edictd in DNA, and how that information is interpreted to shoot down just about changes in jail cells and tissues. Objectives 1. Understand the triplet nature of the ancestral regulation, and have it away the meaning of the term codon. 2. spot that the computer statute is degenerate, and what that means. 3. Know that the grave is unambiguous, and what that means. 4. Know the identities of the absorb and stop codons, and understand how they work. The Genetic Code It has been mentioned in a variety of modules that DNA stores inheritable information.That much was clear from theexperimentsof Avery, Macleod, and McCarty and Hershey and Chase. However, these experiments did non rationalizehowDNA stores genetic information. Elucidation of the structure of DNA by Watson and rick did not offer an obvious explanation of how the information might be stored. DNA was constructed from subst ructures containing only quartette workable bases (A, G, C, and T). The big question was how do you code for whole of the traits of an organism using only a four garner original rudiment? Recall thecentral dogma of molecular biology.The information stored in DNA is ultimately transferred to protein, which is what gives cells and tissues their note properties. Proteins atomic number 18 linear set up of aminic group group blisterys, and there argon 20 aminic acids found in proteins. So the real question becomes how does a four letter alphabet code for all possible combinations of 20 amino acids? By constructing multi-letter oral communication out of the four garner in the alphabet, it is possible to code for all of the amino acids. Specifically, it is possible to make 64 diametrical three letter words from just the four letters of the genetic alphabet, which covers the 20 amino acids easily.This kind of reasoning led to the proposal of a triplet genetic code. Experimen ts involvingin vitrotranslation of short synthetic RNAs eventually confirmed that the genetic code is indeed a triplet code. The three-letter words of the genetic code are known ascodons. This experimental approach was also use to work out the relationship between individual codons and the various amino acids. After this cracking of the genetic code, several properties of the genetic code became unmistakable * The genetic code is composed of nucleotide triplets.In other words, three nucleotides in mRNA (a codon) specify one amino acid in a protein. * The code is non-overlapping. This means that successive triplets are realize in order. for each one nucleotide is part of only one triplet codon. * The genetic code is unambiguous. Each codon specifies a particular amino acid, and only one amino acid. In other words, the codon ACG codes for the amino acid threonine, andonlythreonine. * The genetic code is degenerate. In contrast, each amino acid can be specify bymorethan one codon. * The code is nearly universal.Almost all organisms in nature (from bacteria to humans) use exactly the said(prenominal) genetic code. The high-minded yetions include more or less changes in the code in mitochondria, and in a few protozoan species. * A Non-overlapping Code * The genetic code is read in groups (or words) of three nucleotides. After instruction one triplet, the indicant frame shifts over three letters, not just one or two. In the followers example, the code wouldnotbe read GAC, ACU, CUG, UGA * * Rather, the code would be read GAC, UGA, CUG, ACU * * Degeneracy of the Genetic Code There are 64 different triplet codons, and only 20 amino acids. Unless both(prenominal) amino acids are stipulate by more than one codon, some codons would be completely meaningless. Therefore, some redundancy is built into the system some amino acids are coded for by multiple codons. In some cases, the tautologic codons are related to each other by sequence for example, leucine is specify by the codons CUU, CUA, CUC, and CUG. Note how the codons are the same except for the third nucleotide military post. This third position is known as the wobble position of the codon.This is because in a number of cases, the identity of the base at the third position can wobble, and the same amino acid will still be specified. This property allows some protection against mutation if a mutation occurs at the third position of a codon, there is a good vista that the amino acid specified in the encoded protein wont change. * Reading Frames * If you think about it, because the genetic code is triplet based, there are three possible ways a particular message can be read, as shown in the following figure * * Clearly, each of these would yield completely different results.To illustrate the point using an analogy, consider the following set of letters * thered trickatethehotdog * If this string of letters is read three letters at a time, there is one reading frame that works * th e red fox ate the hot dog * and two reading frames that produce tripe * t her edf oxa tet heh otd og * th ere dfo xat eth eho tdo g * Genetic messages work much the same way there is one reading frame that makes sense, and two reading frames that are nonsense. * So how is the reading frame chosen for a particularmRNA? The answer is found in the genetic code itself.The code contains signals for starting and stopping translation of the code. Thestart codonisAUG. AUG also codes for the amino acid methionine, but the first AUG encountered signals for translation to begin. The start codon sets the reading frame AUG is the first triplet, and later(prenominal) triplets are read in the same reading frame. supplanting continues until astop codonis encountered. There are three stop codonsUAA,UAG, andUGA. To be recognized as a stop codon, the tripletmustbe in the same reading frame as the start codon. A reading frame between a start codon and an in-frame stop codon is called an brusk readin g frame.Lets see how a sequence would be translated by considering the following sequence 5-GUCCCGUGAUGCCGAGUUGGAGUCGAUAACUCAGAAU-3 First, the code is read in a5 to 3 direction. The first AUG read in that direction sets the reading frame, and subsequent codons are read in frame, until the stop codon, UAA, is encountered. Note that there are three nucleotides, UAG (indicated by asterisks) that would otherwise constitute a stop codon, except that the codon is out of frame and is not recognized as a stop. In this sequence, there are nucleotides at either demolition that are remote of the open reading frame.Because they are outside of the open reading frame, these nucleotides are not used to code for amino acids. This is a common situation in mRNA molecules. The region at the 5 end that is not translated is called the5 untranslated region, or5 UTR. The region at the 3 end is called the3 UTR. These sequences, even though they do not encode some(prenominal) polypeptide sequence, are no t wasted in eukaryotes these regions typically contain regulative sequences that can affect when a message gets translated, where in a cell an mRNA is localized, and how long an mRNA lasts in a cell ahead it is destroyed.A detailed examination of these sequences is beyond the scope of this course. The Genetic Code thick of Key Points * The genetic code is a triplet code, with codons of three bases cryptanalysis for specific amino acids. Each triplet codon specifies only one amino acid, but an individual amino acid may be specified by more than one codon. * A start codon, AUG, sets the reading frame, and signals the start of translation of the genetic code. Translation continues in a non-overlapping fashion until a stop codon (UAA, UAG, or UGA) is encountered in frame. The nucleotides between the start and stop codons exist an open reading frame.