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What is Genetic Code - Its Features and Degeneracy

BY: Amna Adnan | Category: Genetics | Submitted: 2010-07-28 20:50:15
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Article Summary: "Genetic code is the ordering of the nucleotide. All the genetic information is stored in the genetic code..."


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Introduction:-
Genetic code is the ordering of nucleotides and set of rules which is present inside the DNA molecule and is used to translate the genetic material into proteins. Nucleotides are the molecules joined together to form the structure of RNA and DNA. They play vital role in the cell's metabolism process. Genetic code always consists of three codons that are ATC, CGG etc. Chromosomes carry the genetic code and are made up of DNA.

Basic Information:-
There are 46 chromosomes in the human body. Each of the chromosomes contains many genes and these genes encode various proteins. The genes carry the genetic information which itself is built on four bases Adenine, Guanine, Cytosine and thymine. Adenine and Guanine are the purines and cytosine and thymine are the pyrimidines. DNA is a double stranded molecule. Both the strands are wound about each other in the form of double helix. The helix looks like a ladder and each rung of the ladder consists of two bases from each strand. These bases are joined together with hydrogen bonds. Usually Adenine pairs up with Thymine and Cytosine pairs up with guanine. In ribonucleic acids or RNAs, there is a difference of one base from DNA molecule that is Uracil replaces Thymine. RNA is usually single stranded unlike DNA and makes single stranded mRNA copy of one strand of the DNA is produced during transcription.

Information present in the Genetic Code and Its Transfer:-
In DNA molecule, a gene is made up of sequences of bases which act as storage molecules for the genetic information. Every cell in the body contains a complete set of genes which in return contain the complete genetic information for the cell. Not all the genes are expressed and produce proteins but only few. It does not mean that those genes are of no use. They are also of some importance.

When the gene starts functioning, a copy of genetic information transcribes the code from gene to codons present in messenger RNA. Messenger RNA is not a stable molecule and unlike DNA its life is very short. Messenger RNA is translated on the ribosome and becomes amino acid. Amino acids are the building blocks of proteins. Transfer RNA is another molecule of RNA which contains a specific region called the anticodon. Now anticodon region of tRNA pairs with the codon region of messenger RNA and synthesis of proteins occurs this way. tRNA carries the specific amino acid which is used in protein synthesis. When the tRNA carries the amino acid methionine and it has the anticodon UAC, this anticodon will pair up or bind to the codon of mRNA that is AUG. similarly the amino acid praline has the GGA anticodon.

Features of the Genetic Code:-

Start and Stop Codons:-
The process of translation always starts with the start codon but it is necessary that other sequence and initiation factors should also be present after the start codon. Otherwise there will be no production of proteins. Translation always starts with the AUG codon which codes for the amino acid methionine. If there will be no further sequences of codons after this starting codon, then there will be no translation of protein.

There are three stop codons which are known that is UAG, UGA, and UAA. UAG is called as the Amber, UGA is called as the opal and UAA is called as the ochre. They give signals when the translation of the proteins is finished.

Reading Frame Sequence:-
A codon starts from the initial nucleotide present in the sequence. From the initial point of the string translation process starts. If the example of the following string is taken that is AAACCCGGG, now if the reading starts from the initial nucleotide, there are codons, AAA, GGG, and CCC. If the nucleotides are read from the second position then the sequence of the codon will be AAG and vise versa. Each codon codes for a particular amino acid and it is always read in three reading frames. As DNA molecule is a double stranded molecule, so the sequence of the codons will be six that is three codons will be in the forward direction and three in the backward direction.

The translation of protein starts from the first codon and in the sequence first codon is always AUG in the messenger RNA sequence. It is the point in the protein translation that mutations occur usually frameshift mutations occur. They disrupt the actual function of the protein. In this type of mutation, the coding of the protein becomes short because they make the stop codon to be read early. There are less chances of frameshift mutations to be hereditary because if these mutations occur in the body of developing organism, they will cause the absence of the functional protein and as result the organism will die automatically even before birth.

Genetic code Degeneracy:-
It is the property of the genetic code that it is always has some function and codes for a particular amino acid. There is no uncertainty in the function of the codons. For example, there are amino acids which have different codon sequences but they code for the same amino acid. For example, GAA and GAG bath codons code for the same amino acid that is glutamic acid. Similarly leucine has codon sequences like UUG, UUA, CUA, CUU and CUC. All of these codon code for only one leucine amino acid. The codon position in the sequence is called as fourfold degenerate site if there are four codons which code for the same amino acid. For example, glycine amino acid codons have the fourfold degenerate site because all the codons at this position represent the same amino acid. The sequences of the glycine amino acid are GGG, GGA, GGU and GGC. Similarly if two codons at the same position represent the same amino acid, it is called as twofold degenerate site.

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