The sequence of amino acids in a polypeptide is determined by the sequence of codons in the messenger RNA (mRNA) molecule that is translated during protein synthesis.
The Central Role of mRNA
mRNA acts as the intermediary between the genetic information encoded in DNA and the protein that is ultimately produced. The process unfolds as follows:
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Transcription: DNA serves as a template for the creation of mRNA. The sequence of nucleotide bases in the DNA dictates the sequence of nucleotide bases in the mRNA.
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Translation: The mRNA molecule then travels from the nucleus to the ribosome in the cytoplasm. The ribosome "reads" the mRNA sequence in triplets called codons. Each codon corresponds to a specific amino acid, according to the genetic code.
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Amino Acid Assembly: Transfer RNA (tRNA) molecules, each carrying a specific amino acid, recognize and bind to the corresponding codon on the mRNA. The ribosome catalyzes the formation of peptide bonds between the amino acids, building the polypeptide chain.
The Genetic Code: A Key to Translation
The genetic code is a set of rules that defines how each codon is translated into an amino acid.
| Feature | Description |
|---|---|
| Codon Size | Each codon consists of three nucleotide bases (e.g., AUG, GCA, UUU). |
| Redundancy | Most amino acids are encoded by more than one codon. |
| Start Codon | AUG, which codes for methionine (Met), also serves as the start signal for translation. |
| Stop Codons | UAA, UAG, and UGA are stop codons that signal the termination of translation. |
| Universality | The genetic code is nearly universal across all organisms, providing evidence for a common ancestor. |
Example:
If a segment of mRNA has the sequence AUG-GCA-UUU, the resulting polypeptide would begin with Methionine (Met), followed by Alanine (Ala), and then Phenylalanine (Phe).
In Summary
The amino acid sequence of a polypeptide is a direct consequence of the mRNA sequence, which is itself derived from the DNA sequence. The genetic code is the translator that links the nucleotide sequence of mRNA to the amino acid sequence of the polypeptide.
