Cells

2016-10-17T14:15:48Z

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The human body contains millions of cells.

Gene expression

2015-12-01T12:55:16Z

150179931:

[[DNA|DNA]] is transcribed into [[MRNA|mRNA]] which translates into protein. [[Gene|Gene]] expression uses the information from a [[Gene|gene]] to produce a [[Protein|protein]]. It is believed that [[Eukaryotes|eukaryotes]], [[Prokaryotes|prokaryotes]] and archae undergo gene expression, with speculation still going on about viruses.  Bacterial transcription is very reliant on promoter regions at the beginning of each gene. Promoter regions act as the DNA regulatory elements through which transcription is initiated and controlled.  

== Basics about Gene expression ==

Gene activity is mainly controlled at the [[Transcription|transcription]] level. Genes that are always being expressed are said to undergo constitutive 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]], DNA binding proteins and regulatory proteins. Both eukaryotes and prokaryotes use [[RNA polymerase|RNA polymerase]] to initiate [[Transcription|transcription]] which binds to specific binding sites on [[DNA|DNA]] called [[Promoter|promoter sequences]]. Regulatory proteins either repress or activate [[Transcription|transcription]]. RNA Polymerase stops transcribing at the terminator sequences, two forms of termination exist. 

== Gene expression in Prokaryotes ==

More information is available on how prokaryotes carry out cell reactions, with the model organism being ''[[Escherichia coli|E. coli]]. An organism ''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|β-galactosidase]] molecules were present, whereas with [[Lactose|lactose]], 1000s 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|operon]] <ref>Berg,J.M, Tymoczko,J.L, Stryer, L, 2007, Biochemistry, 6th ed., W.H. Freeman p 897</ref>. More specifically, the [[The Lac Operon|lac operon]] is made up of a regulator gene which codes for the repressor, an operator site and structural genes z, y and a.

[[Image:Lac operon - basic.JPG|left|400px|Lac operon - basic.JPG]]

[[Helicase|Helicase]] is used to unwind double stranded [[DNA|DNA]] into single stranded DNA, to which [[RNA polymerase|RNA polymerase]] and [[RNTP|rNTPs]] are added.

=== Initiation ===

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 sequence|consensus sequences]] as they produce maximum efficiency transcription. [[RNA polymerase|RNA polymerase]] is Mg2+ 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 σ70 to form a [[Holoenzyme|holoenzyme]], with the sigma factor makeing sure that [[RNA polymerase|RNA polymerase]] binds to correct [[DNA|DNA]] sequence.

== References ==

<references />

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Cells

Gene expression