12 Control of Gene Expression

• Control of Gene Expression in eukaryotes

  Regulation can occur at the chromatin level, transcriptional level, post-transcriptional level, translational level, and post-translational level. Basic molecular techniques allow scientists to study gene expression
• Gene expression refers to the transcription of a gene followed by translation to synthesize
• Control of Gene Expression in prokaryotes

  Operons like the trp and lac operons regulate gene expression using repressible and inducible systems. Regulatory genes encode proteins that control transcription of structural genes
• The control of gene expression determines the amount and timing of appearance of the functional gene product
• Cellular differentiation requires different cell types to synthesize different sets of proteins in order to be specialized
• Transcription in Prokaryotes

  Sigma factor binds to core RNA polymerase to form RNA polymerase holoenzyme. 2) RNA polymerase holoenzyme scans along the DNA, and its sigma factor recognizes and binds to the promoter elements at both the -10 and -35 sequences
• Regulation of gene expression can occur at different levels: Genome, Transcriptional, Post-transcriptional, Translational, Post-translational
• Genes which are constitutively expressed are often essential genes that code for products responsible for metabolic functions
• Transcription in Eukaryotes

  General/basal transcription factors recognize and bind to the TATA box of the promoter. 2) Recruitment of RNA polymerase. 3) Transcription initiation complex (TIC) is formed
• Differential gene expression


  Spatial and temporal regulation of gene expression resulting in different gene products produced
• Control of gene expression


  Refers to whether the gene is expressed or not, and how high or low the rate of expression is, at any given time
• Reasons for regulating gene expression

  • Some genes are constitutively expressed
  • Cells must be responsive to changes in the environment
  • Cellular differentiation - different cell types need to synthesize different sets of proteins
• In prokaryotes, enzymes involved in lactose metabolism are expressed by E. Coli only in the presence of lactose and absence of glucose
• Regulation of gene expression is important due to several reasons:
• Gene expression


  Transcription of a gene followed by translation to synthesize a functional product (e.g. RNA and/or protein)
• Cells must continuously "turn genes on and off" in response to circumstances and demands
• Each key stage in eukaryotic gene expression represents a potential control point where the expression of a particular gene can be regulated
• All cells within a multicellular organism have the same genome but express different genes at specific times or within specific tissues resulting in different gene products produced
• RNA polymerase holoenzyme


  Scans along the DNA and its sigma factor recognizes and binds to the promoter elements at both the -10 and -35 sequences
• Chromatin modification functions

  To control gene expression by making regions of DNA more or less accessible for transcription through the enzymatic addition or removal of chemical groups from chromatin
• Prokaryotes

  • Small ribosomal subunit, eukaryotic translation initiation factors and initiator-tRNA form a complex
  • Complex formed scans the mRNA to locate the start codon, AUG
  • Binding of the large ribosomal subunit completes the translation initiation complex
• Post transcriptional modification in Eukaryotes

  1. Addition of 5’cap to pre-mRNA

  2. Splicing - the process where introns are excised and exons are joined together

  3. Addition of poly A tail at 3’ end of pre-mRNA

  4. Formation of a mature mRNA
• Elongation and termination

  RNA polymerase unwinds and unzips the DNA double helix and transcribes the template strand from the start point of transcription to the termination sequence
• Post transcriptional modification in Prokaryotes

  1. Other forms of capping (e.g. NAD+)

  2. Splicing not frequent and usually on non-coding RNAs

  3. Addition of poly A tail at 3’ end of mRNA (no pre-mRNA)
• Prokaryotes


  • The ribosome continues to translate the remaining codons on the mRNA until a stop codon is reached (UAA, UGA, or UAG)
  • Termination occurs when a stop codon enters the Aminoacyl-tRNA (A) site, recognized by release factors triggering hydrolysis of the bond between the polypeptide and the tRNA
• Chromatin modification processes

  • Chromatin remodelling complex
  • DNA methylation
  • Histone acetylation / deacetylation
• Post transcriptional modification

  1. Eukaryotes

  2. Prokaryotes
• Mechanisms in prokaryotes are currently not as well-established
• Chromatin Modification


  Organization of DNA into chromatin helps to pack DNA into a compact form that fits inside the nucleus of a cell and regulate gene expression