Changes

[[Genes]] are segments of [[DNA]] that contain the information required to make a [[protein]].  Gene expression refers to all the processes involved in converting genetic information from a [[DNA]] sequence, or [[protein]].  In [[prokaryotes]], there are just two processes required: [[transcription]] and [[translation]].  In [[eukaryotes]], there is an additional step [[RNA]] processing (splicing), which intervenes.

[[Genes]] are segments of [[DNA]] that contain the information required to make a [[protein]].  Gene expression refers to all the processes involved in converting genetic information from a [[DNA]] sequence, or [[protein]].  In [[prokaryotes]], there are just two processes required: [[transcription]] and [[translation]].  In [[eukaryotes]], there is an additional step [[RNA]] processing (splicing), which intervenes.

'''Transcription'''

==Transcription==

:The synthesis of a single-stranded RNA molecule using DNA as a template is referred to as transcription.  The enzyme that catalyzes this reaction is known as RNA polymerase.  Although the subunit structure and details of the process differ significantly in prokaryotes and eukaryotes, the chemical reaction is identical.   

:The synthesis of a single-stranded RNA molecule using DNA as a template is referred to as transcription.  The enzyme that catalyzes this reaction is known as RNA polymerase.  Although the subunit structure and details of the process differ significantly in prokaryotes and eukaryotes, the chemical reaction is identical.   

:'''Prokaryotic Transcription'''[[Image:Image009.gif|right|thumb|]]

===Prokaryotic Transcription===

[[Image:Image009.gif|right|thumb|]]

::Bacterial RNA polymerases are composed of multiple subunits.  The core enzyme consists of 2 alpha, beta and beta prime subunits.  This enzyme contains the catalytic activity for RNA synthesis.  Association of the core enzyme with the sigma subunit is required for initiation of transcription.  This complex is referred to as the holoenzyme.  The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence.  In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35.  The sigma subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site).  This complex is referred to as the closed complex.

::Bacterial RNA polymerases are composed of multiple subunits.  The core enzyme consists of 2 alpha, beta and beta prime subunits.  This enzyme contains the catalytic activity for RNA synthesis.  Association of the core enzyme with the sigma subunit is required for initiation of transcription.  This complex is referred to as the holoenzyme.  The holoenzyme binds to DNA nonspecifically and scans for a promoter sequence.  In E. coli, the most common type of promoter contains two conserved sequences centered at -10 and – 35.  The sigma subunit interacts specifically with these sequences in the double stranded DNA, positioning the active site at +1 (transcription start site).  This complex is referred to as the closed complex.

::Transcription continues until a termination signal is reached.  The current model is that the RNA dissociates from the DNA – RNA hybrid to form a stem-loop structure.  The remaining AU base pairs are not strong enough to keep RNA hybridized with the DNA.  Dissociation of the RNA destabilizes RNA polymerases and transcription terminates.

::Transcription continues until a termination signal is reached.  The current model is that the RNA dissociates from the DNA – RNA hybrid to form a stem-loop structure.  The remaining AU base pairs are not strong enough to keep RNA hybridized with the DNA.  Dissociation of the RNA destabilizes RNA polymerases and transcription terminates.

:'''Eukaryotic Transcription'''

===Eukaryotic Transcription===

::Another subunit of TFIIH contains a kinase activity responsible for phosphorylating the CTD region of Pol II.  Although this activity can be influenced by many factors, in simple systems it is believed to correlate with the initiation of transcription and that movement of RNA polymerase away from the promoter.

::Another subunit of TFIIH contains a kinase activity responsible for phosphorylating the CTD region of Pol II.  Although this activity can be influenced by many factors, in simple systems it is believed to correlate with the initiation of transcription and that movement of RNA polymerase away from the promoter.

:'''Summery of Transcription'''

===Summery of Transcription===

:Although euckaryotic transcription is much more complex than prokaryotic transcription, initiation involves the same basic steps in both:

:Although euckaryotic transcription is much more complex than prokaryotic transcription, initiation involves the same basic steps in both: