Mod 7 Regulating Gene Expression
How is gene expression controlled in prokaryotes?
-it can be controlled at the transcriptional level by inhibiting transcription so that RNAs are not produced
-or by inhibiting translations so that proteins aren't produced
How do bacteria control gene expression?
-bacteria is single-celled & precise coordination of gene expression is needed for cells to respond quickly to their environment
-bacteria usually respond to their environmental change by regulating expression at the transcription
What are sigma factors?
-in prokaryotes, sigma factors play a major role in the recognition & binding of RNA polymerase to the promoter to start transcription
Why do bacteria produce sigma factors?
-to recognize different promoters allowing control of gene expression under different conditions
-ex during heat shock in E. coli, certain sigma factors identify promoters to produce proteins to protect the cell when exposed to heat
-these sigma factors are inactive until a certain temperature is reached
What are operons?
-a cluster of genes controlled by one promoter with a single 'on-off' switch called an operator
-a regulatory gene produces a protein, which when activated binds to the operator region, blocking transcription
-this means that all genes are either transcribed or none at all
What is the Trp Operon?
-is one of the many operons in the E. Coli genome
-E. coli needs the amino acid tryptophan (Trp) to survive like humans
-trp acts as a co-repressor, a small molecule that cooperates with a repressor to switch an Operon off
-the trp regulatory gene synthesizes mRNA that codes for an inactive repressor
What happens if there is tryptophan in the environment?
-when tryptophan is present
-represses activate
-operon off
-binds to the operator & genes are turned off
What happens if there is no tryptophan in the environment?
-if tryptophan is absent
-repressor inactive
-operon on
-allows genes of the operon to be transcribed
What is the Lac Operon?
-inducible operon because it's always turned off unless induced
-the genes of the lac operon code for proteins that import & metabolize lactose sugar for energy
-the lac regulatory gene synthesizes mRNA that codes for an active repressor
-the active repressor binds at the operator and blocks transcription of the genes to use lactose for energy
What does it mean to have a positive control of the lac operon?
-e.coli interacts with an activator, a positive regulatory protein
-it can sense the glucose concentration & rely on this information to the genome to control the lac operon
-the positive regulatory protein is called the cAMP receptor protein
What does it mean to have a negative control of the lac operon?
-involves the lac repressor protein blocking transcription when lactose is absent
What is chromatin?
-DNA is wrapped around histone proteins to form the protein -DNA complex chromatin
-the condensed nature of chromatin helps it regulate gene expression as RNA polymerase cannot access DNA when it is condensed
What is the process of chromatin modification?
-DNA can be methylated
-usually associated with reduced gene transcription
-DNA methylation can cause long-term activation of genes in cellular differentiation
What is epigenetic inheritance?
-patterns of inheritance that are not due to differences in gene sequence
-chemical modifications on histones & DNA vary in cell types
-environmental factors (Diet, smoking) can change some of these patterns
-daughter cells inherit the changed patterns during mitosis, & some through meiosis