Book : Molecular Biology
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Posted by: CHELSEA
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Writer CHELSEA

Genetic Code and Rules

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Codons

Scientists main problem lay in the fact that while there were only 4 nitrogen bases (nucleotides), there were 20 amino acids for which those nucleotides had to code. If adenine, thymine, guanine, and cytosine each coded for a particular amino acid, then the DNA/mRNA information system would only be able to code for 4 amino acids. If, however, groups of two nucleotides coded for a single amino acid, the story is somewhat different. Given four nucleotides looked at in groups of two, there are sixteen possible combinations (AA, AT, AG, AC, TA, TT, TG, TC, GA, GT, GG, GC, CA, CT, CG, CC); that sixteen is still not enough to code for twenty amino acids. But if the nucleotides code for amino acids in groups of three then there are sixty-four possible combinations. Scientific experiments have verified that nucleotides code for amino acids in successive groups of threes. These groups of threes are called codons.

Degeneracy of the Genetic Code

As we know, since the genetic code is read in triplets and there are four possible bases that can occupy each position, the number of possible codons is 4 X 4 X 4, or 64 codons. However, there are only 20 known amino acids. Experiments have shown that three codons function also function stop codons, acting as termination signals in translation. Yet that brings the count up to only twenty-three necessary codons. The vast difference between possible codon variations and needed codon variations means, as seen in the figure below, that each amino acid is specified by more than one codon. Because the genetic code therefore does not code to its capacity, it is called "degenerate".

Rules of the Genetic Code Overview

In addition to the principles we have already discussed, there are basic rules that govern the translation of the genetic code into a protein. There are three principle rules we will discuss: The sequence of bases in a codon must follow the direction of translation. The code is non-over-lapping. The code is read in a fixed reading frame.

Rule 1

The first rule is somewhat basic. It says that since mRNA is translated in the 5' to 3' direction, the codon sequences must occur in a similar orientation so that they will be properly translated. This simply means that the first base of a codon must be located at the 5'-most end of the codon. Codons must always be read from 5' to 3'.

Rule 2

The second rule means that any one nucleotide can only be a part of one codon. It cannot be a part of two different codons. Therefore, successive codons are composed of adjacent, not over-lapping, trinucleotides. For example, given the code AACT, AAC could be a codon with T starting a new codon or ACT could be a codon with the first A the last letter of a previous codon. But AAC and ACT cannot both be codons at the same time.

Rule 3

The final rule states that once you begin reading the code from a specific nucleotide, you continue reading it by threes until the end. The beginning of an amino acid sequence is specified by a start codon located somewhere in the mRNA sequence, this is usually an AUG, but can also be a GUG. The end of a sequence is specified by one of three stop codons: UAA, UAG, or UGA. A consequence of this rule is that the genetic code can be read in three different reading frames depending on which base one begins with. For example, the sequence: ACGACGACGACGACG can be read in the three following ways. 1. ACG ACG ACG ACG ACG (each codon specifies the amino acid threonine) 2. A CGA CGA CGA CGA CG (each codon specifies the amino acid arginine)


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