Gene expression is the process by which DNA is used to make proteins, which then go on to perform various important functions in the body. The protein could be an enzyme, hormone or receptor, for example.
The process of gene expression includes the following steps:
- Transcription - Transcription is the process by which a segment of DNA is used to generate an RNA template. The DNA segment is “read” by an enzyme called RNA polymerase, which produces a strand of RNA that is complimentary to the DNA. In this complementary RNA strand, all thymine bases are replaced by uracil.
- Processing - This primary RNA transcript is then modified to convert it into mature messenger RNA (mRNA) that can be used in translation. The mRNA undergoes splicing to remove the non-coding parts of the transcript (introns) so that only the coding sections (exons) remain.
- Non-coding RNA maturation - Non-coding regions of RNA (ncRNA) are transcribed as precursors which are then processed further. For example, these regions may be transcribed as pre-ribosomal RNA (pre-rRNA) which then undergoes cleavage to become ribosomal RNA (rRNA).
- RNA export - The majority of mature RNA is then transported from the nucleus to the cytoplasm. Although some RNAs function in the nucleus, most are carried through pores in the nucleus into the cytosol, including all RNAs involved in protein synthesis.
- Translation - The final mRNA carries the information needed to code for proteins. Every three base pairs on the mRNA corresponds to a binding site for a transfer RNA (tRNA) which carries an amino acid. The amino acids are then linked together in a chain by a ribosome to create a rudimentary protein chain.
- Protein folding - The long chain of amino acids folds to form a three-dimensional structures using enzymes called chaperones. This three-dimensional structure is the final, functional form of the protein.