DNA is transcribed into mRNA which translates into protein. Gene expression uses the information from a gene to produce a protein. It is believed that eukaryotes, prokaryotes and archae undergo gene expression, with speculation still going on about viruses.
Basics about Gene expression
Gene activity is mainly controlled at the transcription level. Genes that are always being expressed are siad to undergo constituve expression and those only expressed conditionally undergo regulated expression.<ref>Berg,J.M, Tymoczko,J.L, Stryer,L,2007, Biochemistry, 6th ed.,W.H Freeman</ref> Gene expression control involves the interactions between DNA, DNA binding proteins and regulatory proteins. Both eukaryotes and prokaryotes use RNA polymerase to initiate transcription, which binds to specific binding sites on DNA called promoter sequences . Regulatory proteins either repress or activatetranscription.
Gene expression in Prokaryotes
More information is available on how prokaryotes carry out cell reactions, with the model organism beingE.coli which has its whole genome sequenced,is easy to grow and can be genetically modified. Bacteria use lactose as an energy source when glucose is in limited amounts. Researchers used X-gal, a coloured compund, to observe that in the presence of glucose, a few β-galactosidase molecules were present, whereas with lactose, 1000's of β-galactosidase were presnt. This suggested that levels of enzyme expression changed with changes in environment and his was co-ordinated by an expression unit called an operon.<ref>Berg,J.M, Tymoczko,J.L, Stryer, L, 2007, Biochemistry, 6th ed., W.H. Freeman p 897</ref> More specifically, thelac operon is made up of a regulator gene which codes for the repressor, an operator site and structural genes z, y and a.
The RNA polymerase binds onto the promoter which consitutes of a -35 sequence TTGACA, a -10 sequence TATAAT , both upstream of the start site ( TG/ AT). The sequences are known as consensus sequences as they produce maximum efficiency transcription. RNA polymerase is Mg<sup>2+</sup> dependent and has a core made of 2 α, 1β, 1 β' and 1 ω subunits.<ref>Andersen-Lykke,J, Christiansen,J, 1998,Nucleic Acid Research,Oxford Journals, [online] 26 (24) Available at: naroxfordjournals.org/content/26/24/5630.full [accesses 2 December 2011]</ref> The core combines with a σ<sup>70</sup> to form a holoenzyme, with the sigma factor makeing sure that RNA polymerase binds to correct DNA sequence.<sub></sub>