Gene Expression and Regulation

I. Gene Regulation in Eukaryotes Transcriptional Controls 1. Gene amplification 2. Chemical Modification Post-transcriptional Controls Translational Controls

A. Transcriptional Control 1. Gene Amplification: A gene sequence can be repeated many times on a DNA sequence…resulting in multiple mRNA transcripts, therefore more gene expression.

Different gene regions of a DNA molecule TRANSCRIPTION Different gene regions of a DNA molecule mRNA rRNA tRNA Transcript processing proteins mature mRNA subunits for ribosomes mature tRNA convergence of RNAs TRANSLATION Pools of amino acids, tRNAs, and ribosomal subunits in the cytoplasm synthesis of polypeptide chain on the platform of an intact ribosome FINAL PROTEIN Fig. 15.5, p. 245 for use in cell or for export

A. Transcriptional Control 2. Chemical Modification: A DNA segment may be wrapped tightly with histones, preventing gene expression. In mammalian females, the extra “X” is tightly bound, resulting in the presence of a Barr Body in the nucleus.

Mosiac Effect and X inactivation

B. Post Transcription Splicesomes: Enzymes that rearrange segements of introns, creating new combinations of proteins.

C. Translational Controls mRNA transcript will be digested when it reaches the cytoplasm (a good thing, or once a gene were turned on it would be forever expressed). A transcript has a cap added to the 5’ end and a poly-A tail added to the 3’ end. If these tails/caps are long, it will take the enzymes in the cytoplasm a greater amount of time to digest the coding region of the transcript.

unit of transcription in a DNA strand exon intron exon intron exon 3’ 5’ transcription into pre-mRNA poly-A tail cap 5’ 3’ (snipped out) (snipped out) 5’ 3’ mature mRNA transcript Fig. 14.9, p. 229

II. Gene Regulation in Prokaryotes Prokaryotes have only one DNA molecule (circular and not protected by nuclear envelope) and this DNA molecule is not bound up with histones. Thus, gene regulation in prokaryotes is unique. One of the best known pathways of gene recognition is the lac Operon, a regulatory pathway by which bacteria are able to produce the enzyme to digest lactose only when necessary (when lactose is present in the environment).

A. Operon and DNA Operon is a regulatory system that controls DNA transcription in prokaryotes. Operon contains a promoter (the specific nucleotide sequence that tells a cell to begin or start transcription), an operator (a segment of DNA that can be used to turn gene expression on or off) and more than one gene.

B. Actors in Lac Operon Regulatory or Repressor Protein – Binds with operator (segment of DNA) to prevent a gene from being transcribed. Substrate/Inducer – Interacts with protein to prevent it from adhering to DNA. Operator – Section of DNA Promoter – Section of DNA. Operon – The entire system.

(codes for trans-acetylase) regulator gene gene 1 (codes for b-galactosidase) gene 2 (codes for premease) gene 3 (codes for trans-acetylase) transcription, translation promoter (binding site for RNA polymerase) operator (binding site for repressor) repressor protein lactose operon Fig. 15.4a, p. 243

translation into three polypeptide chains for three different enzymes RNA polymerase mRNA transcript translation into three polypeptide chains for three different enzymes lactose b-galactosidase permease trans-acetylase Fig. 15.4b, p. 243

Without Lactose: Describe the chain of events that occurs in a bacterial colony when no lactose is present.

With Lactose: Describe the chain of events that occur when lactose is present in a bacterial culture:

Positive Control: Glucose Digestion Bacterial cells digest glucose before lactose. If glucose is present, cAMP is converted to ATP. cAMP, when present, bonds to promoter region and helps RNA polymerase begin transcription of lactose digestion genes.