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Summary Article: protein synthesis
from The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Series of chemical reactions in the cell by which amino acids, lined up in the order specified by the corresponding gene, are connected to form a molecular chain, the polypeptide, which will then adopt the three-dimensional shape characteristic of a protein (known as protein folding).

Before a protein can be made, the corresponding gene (encoded in DNA) needs to be transcribed into RNA (see transcription). The resulting messenger RNA (mRNA) may be edited before its translation into an amino acid sequence. Protein biosynthesis as such occurs on the ribosome, a highly complex particle of RNA and proteins found in all organisms. There are separate mechanisms for the beginning (initiation), continuation (elongation), and termination of protein synthesis, each requiring specific sets of protein factors – the initiation, elongation, and release factors respectively. The ensemble of the ribosome with all associated factors and RNAs is called the translational apparatus.

A set of 20 standard amino acids is used in the synthesis of virtually all proteins. Additional amino acids, such as selenocysteine, can be incorporated with the help of special mechanisms.

Amino acids can only take part in protein synthesis if they are attached to a specific transfer RNA, or tRNA, by a specific set of enzymes, the tRNA synthetases. The tRNA is at the heart of the translation of the three-letter code of the nucleic acids DNA and RNA into the amino acid sequence of a protein, as specified by the genetic code. It has an L-shaped structure binding the amino acid at one arm and recognizing the corresponding three-letter codon with the other.

The ribosome has three binding sites for tRNAs. Each tRNA charged with an amino acid first comes into the A (acceptor site), while the tRNA bound to the last amino acid that has been added to the chain is located in the P (peptidyl) site. The third site is the E (exit) site.

In the course of the elongation cycle, the ribosome transfers the growing polypeptide chain from the P-bound tRNA onto the new amino acid in the A site, thus creating a new polypeptide bond. This is the only catalytic activity of the ribosome. Then, the now empty tRNA in the P site moves on to the E site, and the tRNA carrying the extended chain moves into the P site, so the cycle can be repeated.

The growing polypeptide chain is threaded through a tunnel in the ribosome before it emerges.

In bacteria and Archaea, transcription, translation, and protein folding can occur in a production-line style, such that a messenger RNA still being synthesized can already be translated, and a polypeptide chain emerging from the ribosome can already undergo folding and interact with molecular chaperones assisting their folding and assembly.

In eukaryotes, however, transcription is located in the nucleus, yet protein synthesis occurs in the cytoplasm and on the membranes of the endoplasmatic reticulum. Thus, messenger RNA needs to be exported from the nucleus and there is a time lag between transcription and translation.

All proteins and some shorter peptides are produced on ribosomes as described here. However, some short peptides are produced by multi-enzyme complexes that are completely independent of the ribosome.

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RNA and protein synthesis

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