The sequence of nucleotides in messenger RNA (mRNA) encodes the information necessary for the synthesis of proteins through a process called translation, which occurs after transcription. Here’s how this encoding works:
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Nucleotide Structure: mRNA is composed of four types of nucleotides, which are made up of three parts: a nitrogenous base (adenine (A), uracil (U), cytosine (C), or guanine (G)), a ribose sugar, and a phosphate group. The sequence of these nucleotides determines the genetic information carried by the mRNA.
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Codons: The information in mRNA is read in groups of three nucleotides, known as codons. Each codon corresponds to a specific amino acid or a signal during the translation process. For example:
- AUG - Methionine (also serves as the start codon)
- UUU - Phenylalanine
- UAA, UAG, UGA - Stop codons (which signal the termination of protein synthesis)
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Genetic Code: The genetic code is a set of rules that defines how the sequence of codons is translated into corresponding amino acids. There are 64 possible codons (4^3 combinations of three nucleotides), which code for 20 standard amino acids and include stop signals. This redundancy in the genetic code means that some amino acids are specified by more than one codon.
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Translation Process: The process of translation occurs in the ribosome and involves several key components:
- Ribosomal RNA (rRNA): Together with proteins, rRNA makes up the ribosomes, which are the sites of protein synthesis.
- Transfer RNA (tRNA): Each tRNA molecule carries a specific amino acid and has an anticodon region that is complementary to a corresponding codon on the mRNA strand. For example, if the codon on the mRNA is AUG, the tRNA with the anticodon UAC will deliver Methionine to the ribosome.
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Initiation: Translation begins when the small ribosomal subunit binds to the mRNA at the start codon (typically AUG). The corresponding tRNA then pairs with the start codon, and the large ribosomal subunit joins to form a complete ribosome.
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Elongation: The ribosome continues to move along the mRNA, facilitating the sequential pairing of tRNAs to codons, and catalyzing the formation of peptide bonds between the amino acids they carry, thereby elongating the growing polypeptide chain.
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Termination: The translation process continues until a stop codon is reached. At this point, the ribosome recognizes the stop codon, and the newly synthesized polypeptide chain is released.
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Post-Translational Modifications: After translation, the polypeptide chain may undergo various modifications (like phosphorylation, glycosylation, etc.) that are essential for its proper functioning as a protein.
In summary, the nucleotide sequence of mRNA encodes information for building proteins by defining a sequence of codons that correspond to specific amino acids. This information is translated into a polypeptide chain, ultimately folding into a functional protein that carries out various roles in the cell.