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What Are mRNA Codons?

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  • Written By: E.A. Sanker
  • Edited By: John Allen
  • Last Modified Date: 07 November 2016
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The genetic information of an organism is expressed through a system known as the genetic code, in which messenger ribonucleic acid (mRNA) codons play an important role. MRNA codons are sets of nucleotides that act as a template for protein synthesis. This template is created through transcription from deoxyribonucleic acid (DNA). MRNA later interacts with transfer RNA (tRNA) during translation, forming a polypeptide chain of amino acids. Each mRNA codon consists of three bases that correspond with matching bases on a tRNA anticodon, which in turn is attached to a specific amino acid.

DNA and RNA strands consist of chains of nucleotides that are connected to each other via complementary base pairing. The four DNA nucleobases, which are the key components of the nucleotide molecules, are adenine (A), thymine (T), guanine (G), and cytosine (C). In RNA, uracil (U) replaces thymine. Adenine pairs with thymine or uracil, while guanine pairs with cytosine.

MRNA is a template created from DNA through a process known as transcription. The enzyme RNA polymerase splits the DNA double helix and pairs the single strands of DNA with complementary RNA bases. For example, a DNA set of bases reading AATCAG will create an mRNA set reading UUAGUC. The mRNA strand then breaks off for further processing.

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Organelles called ribosomes are the site of translation, the process by which mRNA is decoded into a corresponding protein. In translation, mRNA is “read” as a series of nucleotide triplets known as mRNA codons. Using the example from the previous paragraph, the mRNA codons we have are UUA and GUC. The translation process pairs each of these mRNA codons with a complementary tRNA anticodon. UUA will pair with tRNA anticodon AAU, and GUC will pair with CAG.

Each tRNA molecule contains an anticodon site, which binds to mRNA, and a terminal site, which attaches to a specific amino acid. The tRNA molecule carries its amino acid to the site of translation. As the tRNA molecules bind to the complementary mRNA codons, these amino acids form a growing polypeptide chain. The set of amino acids in the polypeptide chain determines the structure and function of the protein being synthesized. In this way, the original DNA information is finally expressed as a specific protein.

To continue with our example, suppose we have mRNA codons UUA and GUC. UUA codes for the amino acid leucine, and GUC codes for valine, so the polypeptide chain at this point would consist of leucine followed by valine. Several mRNA codons correspond to each amino acid. Another codon that codes for leucine, for instance, is UUG.

Some mRNA codons do not code for an amino acid at all, and instead function as “stop” codons. These triplets signal the end of translation and bind to proteins called release factors, which cause the polypeptide chain to be released. MRNA stop codons are UGA, UAG, and UAA. A corresponding start codon also exists, which signals the start of translation. The usual start codon is AUG, which codes for the amino acid methionine.

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