Control of Gene Expression

Anmerkungen:

• Control can occur at the level of transcription or at the level of translation.

1. Level at Transcription

   Anmerkungen:

   • The energy required to generate genes is not efficient, hence it only expressed when needed.
   1. Structure and Method of Replication

      Anlagen:

      • Transcription
   2. Inducible: lac

      Anmerkungen:

      • Binding of proteins to DNA. The binding of proteins to DNA determines whether a gene is expressed or not. Small molecules determined if the protein interacts with the DNA.
      1. lac

         Anmerkungen:

         • The lactose transported into the the cell by the lactose transporter, it then break down by p-galactosidase into glucose and galactose.
      2. Negative control

         Anmerkungen:

         • The genes are coordinately controlled. If there's presence of lactose, it is broken down to a galactoside which interact with a  the protein so to unbind it.
      3. CAP

         Anmerkungen:

         • Catabolite Activator Protein. This complexes with cAMP to bind with RNA polymerase to initiate transcription. cAMP and CAP complex is a positive effector.
      4. Mutations

         Anmerkungen:

         • Two types are: High level expression: 1. i- inactive repressor, hence the gene transcription/translation is unhindered due to the inability of the repressor to bind to the operator. 2. o- mutation to operator to disable the repressor to bind. No induction at all: 1. is: Inducer cannot bind to the repressor hence the repressor permanently binds to the operator 2. p- mutation to the promoter to prevent RNA polymerase to bind.
   3. Repressible: Trp
      1. Mechanism

         Anmerkungen:

         • Repressor is generated by the i DNA sequence. RNA polymerase will bind if the repressor isn't bound to the protein. If there are levels of tryptophan detected, the tryptophan then bound to the repressor to bind to the
      2. Attenuation

         Anmerkungen:

         • Depending on binding, the site may change to bind with either the 2nd or 4th. If bound with tryptophan, shorter mRNA is generated, i.e 2 will bind with 3 where if there are no tryptophan mRNA generated will be long. 
2. Eukaryotic Gene Expression

   Anmerkungen:

   • Difference in the proaryotic and eukaryotic gene expression differs as so. More positive regulatory genes Larger more complex multimeric regulatory proteins Restricted by chromatin structure Transcription separated by time in space from translation in the cytoplasm.
   1. Level of Control

      Anmerkungen:

      • level of control can occur at different stages 7 possible stages: Transcription Posttranscriptional processing Translation Postranslational processing Protein degradation mRNA degradation Protein targeting and transport.
   2. Typical Sequence of Genes

      Anmerkungen:

      • TATA Box GC box CAAT box TO increase the level of transcription, there's a enhancer normally consisting of Hormone Response Element (hormone receptor protein plus hormone) They loop back to the the start zone to bind with a RNA Polymerase complex.
   3. Transcription Factors

      Anmerkungen:

      • Many transcription factors are needed and if all the necessary factors are present, the transcription can take place. Most important one is TFIIH or (Transcription Factor, Interacting with RNA polymerase II, H)
      1. Action

         Anmerkungen:

         • 2 actions: Helicase action and Signalling initiation of transcription TFIIH helicase activity, then COO domain of RNA polymerase II is phosphorylated by TFIIH, after termination, the RNA polymerase II is released and dephosphorylated and recycled.
   4. Chromatin

      Anmerkungen:

      • Chromatin acts negatively on gene expression, condensed chromatin and nucleosome prevent transcription by steric hindrance. Hence nucleosome needs to be removed or altered. Mostly by acetylation of histones where chromatin is activated for transcription by removing one nucleosome to allow gene expression 
   5. Translational Control

      Anmerkungen:

      • E.g control in the translation factors