Differences between Lactose & Tryptophan Operons in Gene Regulation

1. Enumerate the differences between Lactose & tryptophan operons?

2. Gene Regulation In Eukaryotic Cells General features: In Eukaryotic Cells, it is more complicated and multifaceted. 1) The changes in environment;turn a set of genes ON or OFF (viral infection & heat shock). 2)Tissue specific regulation; specialization and organization of the cells in organs require activation of some genes & inactivation of others. 3)Operonsare not present in eukaryotic cells. 4) Specific regulatory sequence; for each gene. 5)House-keeping enzymes; are encoded by constitutive genes. 6)Temporal regulation mechanism, some genes are active only during certain period of life (inducible).

3. Levels of Gene Expression Control In Eukaryotic Cells • DNA or Chromosomal organization. • Gene transcription level. • Post-transcription level. • Translation control. • Post-translation control.

4. a) Gene expression control at level of DNA organization Methyl cytosine Specific protein • Formation multiple copies of genes. e.g.: rRNA & tRNA (the cell contains 150 – 450 transcription units). • Gene amplification (self –replication of some genes to increase their product). • Gene inactivation by: a) Change chromatin structure. formation a compact mass of inactive heterochromatin. e.g. x- chromosome form Barr body. b) DNA –methylation. - addition of methyl group to DNA-cytosine methyl cytosine. - Methy cytosine + specific protein methyl cytosine protein complex . Blocks transcription

5. b) Gene expression control at Transcription level • Control of the rate of transcription depends on: 1) Efficiency of promoter 2) Enhancer 3) Regulatory protein (transcription requires multiple regulatory proteins called general transcription machinery or regulatory protein complex). Promoter DNA Enhancer UPEs TATA -box Transcription initiation site 30 base pairs About 100 base pairs About several thousands of base pairs

6. 1)Promoter • Composition: • TATA –box : • Binding site of RNA –polymerase. • Formed of T & A. • About 30 base pairs in upstream from transcription initiation site. • UPEs (upstream promoter elements): • Each one about 8 – 12 base pairs • About 100 base pairs in upstream from initiation site. • Activity of promoter depends on the number & type of UPEs. • Weak promoter contains few UPEs. • Strong promoter contains several UPEs. • UPEs are required for accurate & efficient initiation of mRNA –synthesis.

7. 2) Enhancer • DNA –sequence • Increase rate of Gene transcription, when it interacts with transcription machinery complex in presence of the activator protein 3) Regulatory protein • May be activator or repressor protein. • Transcription requires multiple regulatory proteins, R. Protein complex. • Regulatory protein complex binds to TATA –box to facilitate the binding of RNA polymerase, that results in formation of transcription machinery complex. 4) Activator protein • Have 2 functional domains; one binds to Enhancer & the other connects the transcription machinery complex. • Increase rate of transcription & mRNA synthesis.

8. TATA –box Regulatory protein complex (multiple proteins) Initiation site enhancer DNA 1) R.P. complex is required for binding of RNA polymerase • No transcription, although RNA-polymerase & Transcription M.C. are bound to TATA-box. • ( Transcription is very low or not at all ) RNA-polymerase enhancer DNA 3) Activator protein facilitates interaction between Enhancer & T.M.C. to accelerate rate of transcription. Transcription machinery complex activator • DNA –loop allows interaction between: • - Activator protein, • Enhancer • Transcription .M.C. DNA Transcription ON

9. Gene Expression at Post-translation level 1) Proteolytic process: Pepsinogen (inactive) Pepsin (active) 2) Selective degradation: To maintain a suitable concentration of protein within the cell and removal the unuseful parts. 3) Chemical modification: removal or addition functional group to activate or inactivate some enzymes, such as phosphate group. Proteolytic enzymes