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Home > Genetic code

The genetic code is a set of rules, which maps DNA sequences to proteins in the living cell through the mechanism of protein synthesis. Nearly all living things use the same genetic code, called the standard genetic code, and all use small variations of it.

The code is represented by an immensely long polymeric molecule, Deoxyribonucleic Acid ( DNA), and each functional portion of this molecule is referred to as a gene. Each gene is transcribed into a short template for protein synthesis, Ribonucleic Acid ( RNA), which is in turn translated into an amino acid chain ( polypeptide). The gene sequence is composed of units called codons, each coding for an amino acid, hence the phrase ‘genetic code’. The polypeptide is ultimately folded into a 3-dimensional protein structure, which will go on to perform some specific function in the cell such as an enzyme subunit or cell membrane component.

This chain of events involving RNA transcription, and polypeptide translation is referred to as gene expression. Some genes encode other elements such as ribosomal RNAs and transfer RNAs, both of which are involved in protein synthesis.

Both DNA and RNA are comprised of 4 nucleotideA nucleotide is an organic molecule consisting of a nitrogenous heterocyclic base (a purine or a pyrimidine), a pentose sugar ( deoxyribose in DNA or ribose in RNA), and a phosphate or polyphosphate group. A nucleoside is similar, except that it contains bases:

DNA: adenineAdenine is one of the two purine bases used in forming nucleotides of the nucleic acids DNA and RNA. In DNA, adenine (A) binds to thymine (T) to assist in stabilizing the nucleic acid structures. In RNA, adenine binds to uracil (U). Adenine forms adenosin, guanineGuanine (2- amino-6- oxy purine) is one of the four main nitrogenous bases found in nucleic acids (e. DNA and RNA). Guanine is a purine derivative and in Watson-Crick base pairing forms hydrogen bonds with cytosine. The nucleoside is called guanosine., cytosineCytosine is one of the 5 main nitrogenous bases used in storing and transporting genetic information within a cell. It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached (an amine group at position 4 and a keto gro and thymineThymine (CHNO, 2- oxy-4- oxy-5- methyl pyrimidine, 2,4-di oxy-5- methyl pyrimidine, 5-methyluracil) is one of the bases of the nucleic acid found in DNA. It can base pair with adenine. Thymine combined with deoxyribose creates the nucleoside thymidine.

RNA: adenine, guanine, cytosine and uracilUracil is one of the four RNA bases, replacing thymine as found in DNA. Just like thymine, uracil can form a base pair with adenine via two hydrogen bonds, but it lacks the methyl group present in thymine. Uracil, in comparison to thymine, will more readi

Codons are non-overlapping groups of the three bases. There are 4³ = 64 codons. For example, the RNA sequence UUUAAACCC contains the codons UUU, AAA and CCC, each of which specifies one amino acid. So, this RNA sequence represents a protein sequence, three amino acids long. ( DNA is also a sequence of nucleotide bases, but there thymine takes the place of uracil.)

The standard genetic code is shown in the following tables. Table 1 shows what amino acid each of the 64 codons specifies. Table 2 shows what codons specify each of the 20 standard amino acids involved in translation. These are called forward and reverse codon tables, respectively. For example, the codon AAU represents the amino acid asparagine (Asp), and cysteine (Cys) is represented by UGU and by UGC.

1 Table 1: Codon table

¹The codon AUG both codes for methionine and serves as an initiation site: the first AUG in an mRNA's coding region is where translation into protein begins.
²This is a start codon for prokaryotes only.

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Topics: Genetic Code Amino Codons Acid Codon Acids Codes Table Aug...

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