Reality → Life → Genetics → Protein biosynthesis
Mediated by RNA molecules, protein-building amino acids are assembled according to the genetic information stored in DNA. When a cell, at anytime during its cycle, needs a new protein, the corresponding gene is transcribed to a complementary RNA segment [1] . This segment (a 'messenger RNA') then leaves the cell nucleus and feeds into a 'ribosome', a complex compound of protein and RNA in the cytoplasm [2] . Short other RNA molecules ('transfer RNA') carry a specific amino acid on one side, while bearing the corresponding anticodon on the opposite side. In the ribosome, the transfer RNA detects the corresponding codon of the messenger RNA. A ribosome-catalyzed translation is initiated by which a chain of amino acids (a polypeptide) is assembled in accordance with the code that was transcribed to the messenger RNA [3] . Immediately after its synthesis, the polypeptide folds in a highly complex and little understood process, giving the protein its functionality. Cells regulate all steps of the synthesis through a multitude of special proteins and RNAs [4] . Sometimes the repression or activation of a gene becomes heritable [5] .
The transcription of the genetic information from DNA to RNA is enabled by the enzyme RNA polymerase. The hydrogen bonds of the double helix are loosened locally and its two strands separate. The RNA polymerase slides along one strand, using it as a template for the synthesis of a complementary strand of messenger RNA (with three differences to DNA replication: (a) only one strand is 'copied'; (b) thymine is replaced by uracil; and (c) the process extends only over a small segment of DNA). After transcription, the two DNA strands zip up again. Initiation and termination of the transcription depend on the type of protein to be synthesized.
The rough endoplasmic reticulum (see also Cell components) is studded with thousands to millions of ribosomes. Discovery of the amazing structure and function of the ribosome is the culmination of groundbreaking research awarded with a trilogy of Nobel prizes (1962 prize in medicine, 2006 and 2009 prizes in chemistry).
The translation from the messenger RNA's code into synthesis of a sequence of amino acids starts at a start codon and ends at a stop codon. After the first codon is translated, the corresponding amino acid is retained by the ribosome and the transfer RNA is released. A new transfer RNA specific to the next codon of the messenger RNA produces the next amino acid, which is then joined with the preceding acid via a peptide bond. The product is again retained and the transfer RNA released. The process (see video) is repeated until the stop codon is encountered. The ribosome then releases the new polypeptide and the messenger RNA.
Gene expression may be affected by: nucleotide sequences that mark the point of initiation of transcription; enhancer molecules that control the rate of transcription; chemical alteration of chromosomal DNA and protein; restriction enzymes that cut the nucleotide strand; elimination of some non-coding segments through splicing; modification of the product protein; and control of RNA transport, translation process, and degradation of messenger RNA.
The study of heritable changes not rooted in a changed nucleotide sequence is the subject of epigenetics. One mechanism that regulates gene expression and sometimes causes heritable changes is the process of DNA methylation.