Why are start and stop codons important?
Why are stop and start condons important?
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Start and stop codons are important because they tell the cell machinery where to begin and end translation, the process of making a protein. The start codon marks the site at which translation into protein sequence begins. The start codon is the first codon of a messenger RNA (mRNA) transcript translated by a ribosome. The start codon always codes for methionine in eukaryotes and a modified Met (fMet) in prokaryotes. The start codon is often preceded by a 5' untranslated region (5' UTR). In most organisms, the only start codon is ATG, a triplet made up of the DNA bases adenine, guanine and thymine. ATG also codes for the amino acid methionine when found in the middle of a gene. In the mRNA template, ATG is replaced by AUG because the base uracil always appears in place of thymine in RNA. The stop codon ( or termination codon ) marks the site at which translation ends. These are codons in a molecule of messenger RNA that do not code for an amino acid and thereby signal the termination of the synthesis of a protein. The three stop codons are: UAA, UAG, and UGA where, U is uracil, A is adenine, and G is guanine. When a ribosome reaches the codon UAG, UAA, or UGA on an mRNA strand then protein synthesis is terminated. A protein known as a release factor binds to the ribosome and adds a water molecule to the end of the amino acid chain. This reaction hydrolyzes the polypeptide chain from the tRNA, allowing the protein to exit the ribosome. Without the stop codon, protein synthesis would simply stop without releasing the completed protein ( or defective protein )...
- 18 Dec 2017 @ 12:34