A codon is a sequence of three DNA or RNA nucleotides that corresponds with a specific amino acid or stop signal during protein synthesis.
Codons are fundamental to the process of translation, where the genetic code carried by messenger RNA (mRNA) is decoded to produce a specific sequence of amino acids in a polypeptide chain. Here’s a more detailed breakdown:
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Definition: A codon is a trinucleotide sequence (a sequence of three nucleotides) found in either DNA or RNA. Most commonly, the term is used in the context of mRNA.
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Function: Codons serve two primary functions:
- Encoding Amino Acids: The majority of the 64 codons specify which of the 20 amino acids will be added to a growing polypeptide chain during protein synthesis. Each codon "codes" for a specific amino acid.
- Signaling Termination: Three codons (UAA, UAG, and UGA) do not code for amino acids; instead, they act as "stop" signals, indicating the end of the protein sequence. These are known as stop codons or termination codons.
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The Genetic Code: The relationship between each codon and its corresponding amino acid (or stop signal) is known as the genetic code. This code is nearly universal across all organisms, providing strong evidence for a common origin of life.
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Number of Codons: There are 64 possible codons, derived from the four nucleotide bases (Adenine (A), Guanine (G), Cytosine (C), and Uracil (U) in RNA or Thymine (T) in DNA) arranged in triplets (4 x 4 x 4 = 64).
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Redundancy (Degeneracy): The genetic code is degenerate, meaning that multiple codons can code for the same amino acid. This redundancy helps minimize the impact of mutations. For example, several different codons might code for the amino acid leucine.
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Start Codon: The codon AUG serves as the "start" codon. It signals the beginning of protein synthesis and also codes for the amino acid methionine.
Example:
Consider the mRNA sequence: 5'-AUG-GGC-UAU-UAA-3'
- AUG: Start codon (methionine)
- GGC: Glycine
- UAU: Tyrosine
- UAA: Stop codon
This sequence would result in the production of a short polypeptide chain consisting of methionine-glycine-tyrosine. The process would terminate at the UAA stop codon.
In summary, a codon is a crucial element in the central dogma of molecular biology, linking the genetic information stored in DNA (and transcribed into RNA) to the synthesis of proteins, the workhorses of the cell.
