Translation

Some RNAs will function as an RNA molecule (such as tRNA, rRNA, and miRNA), while messenger RNA (mRNA) is used as instructions to produce a protein. Once a gene has been transcribed and the mRNA processed, the next step is to use that mRNA as a set of instructions to build a protein. This process is called translation, and it takes place in the cytoplasm. During translation, the sequence of nucleotides in the mRNA is read in groups of three to determine the order of amino acids in a polypeptide chain. This chain then folds into a functional protein.

Codons

The mRNA sequence is read in sets of three nucleotides, called codons. Each codon corresponds to a specific amino acid. For example, the codon AUG codes for the amino acid methionine and also serves as the start codon, signaling where translation should begin. There are 64 possible codons, but only 20 amino acids, so more than one codon can code for the same amino acid. This redundancy minimizes the potential impact of mutations.

Some codons do not code for any amino acid. These are called stop codons, and they signal the end of translation. The three stop codons are UAA, UAG, and UGA.

tRNA and Charging

To match the codons in the mRNA to the correct amino acids, the cell uses molecules called transfer RNAs (tRNAs). Each tRNA has two important parts: one end carries a specific amino acid, and the other end contains a sequence of three bases called the anticodon, which is complementary to a specific codon on the mRNA.

Before translation can occur, each tRNA must be charged, meaning it must be attached to the correct amino acid. This process is carried out by enzymes called aminoacyl-tRNA synthetases. Each of these enzymes recognizes one amino acid and its matching tRNAs and links them together using energy from ATP. This ensures that the correct amino acid is added to the growing polypeptide chain as the mRNA is read.

Initiation

Translation begins when the small subunit of the ribosome binds to the mRNA and scans for the start codon, AUG. Once the start codon is found, a charged tRNA carrying methionine binds to it.

Elongation

During elongation, the ribosome moves along the mRNA, reading one codon at a time. For each codon, a tRNA with the matching anticodon binds and adds its amino acid to the growing chain. The ribosome helps form a peptide bond between the new amino acid and the previous one, extending the polypeptide. After the bond is formed, the ribosome shifts, the used tRNA exits, and the next codon is ready to be read. This process repeats, and the polypeptide chain grows longer with each new amino acid added.

Termination

Eventually, the ribosome reaches a stop codon on the mRNA. Since stop codons do not match any tRNA, no amino acid is added. Instead, a termination factor binds to the ribosome and causes the entire complex to fall apart. The newly made polypeptide is released into the cytoplasm, where it will fold into its final shape and begin carrying out its function.

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