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How are proteins synthesised?

There are two main steps involved in protein synthesis: transcription and translation. Transcription occurs in the nucleus of a cell whereas translation takes place in the cytoplasm.

Transcription begins when the hydrogen bonds between the bases of a DNA strand (A-T, C-G) break. This causes the strand to 'unzip' resulting in two single strands. In the nucleus are free, activated RNA nucleotides which line up to the complementary bases on each strand. A reaction occurs which causes the formation of the sugar-phosphate back bone between the RNA nucleotides. Nucleotides continue to be added until the stop codon is reached and the now mRNA (messenger RNA) can leave the nucleus via a nuclear pore.

In the cytoplasm, there are free amino acids as well as tRNA. tRNA has a specific amino acid binding site as well as an anticodon on the other end (three unpaired bases). An enzyme catalyses the reaction binding the amino acid to tRNA.

Meanwhile, the mRNA attaches to a ribosome in the cytoplasm by a specific subunit of rRNA.This subunit covers six bases which are now 'exposed'. The first three exposed bases (always AUG) attracts the tRNA with the anticodon UAC and the next exposed mRNA bases attract the complementary tRNA molecule so two are lined up against each other. As two amino acids are now in close proximity, the enzyme peptidyl transferase from the rRNA molecule catalyses the formation of a peptide bond. This allows the ribosome to move along the mRNA strand. Subsequently, the first three bases are released from the ribosome allowing the tRNA molecule to leave but leave its amino acid behind. As more bases are exposed, more tRNA is attracted and the process starts again. The polypeptide grows with each amino acid added until a stop codon is reached and the appropriate protein has been synthesised.

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