Regulation of Gene Expression--AP Biology
Prokaryotes
A bacterium can adjust its metabolism to the changing environment and available food sources
This metabolic control occurs on two levels:
Increasing or decreasing the activity of metabolic enzymes
Regulating genes that code for metabolic enzymes
Operons: The Basic Concept
In bacteria, genes are often clustered into operons, composed of
An operator--the “on-off” switch
A promoter
Genes for metabolic enzymes
An operon can be switched off by a protein called a repressor
A co-repressor is a small molecule that works with a repressor to switch an operon off
Repressible and Inducible Operons
A repressible operon is one that is usually on; binding of a repressor to the operator shuts off
transcription (ex: trp operon)
Mutations in repressible operons cause the genes to be continuously transcribed and expressed
An inducible operon is one that is usually off (ex: lac operon); a molecule called an inducer inactivates
the repressor and turns on transcription
Inducible operon
lac operon--contains genes coding for enzymes in hydrolysis and metabolism of lactose
If lactose is present in the bacterial cells the gene is turned on to make the enzymes that metabolize
lactose; the pathway is switched on by a form of lactose
Lactose absent, repressor active, operon off
Lactose present, repressor inactive, operon on
Inducible enzymes usually function in catabolic pathways
Repressible enzymes usually function in anabolic pathways
Regulation of the trp and lac operons involves negative control of genes because operons are switched
off by the active form of the repressor
Control of eukaryotic gene expression
Typical eukaryotic genome is much larger than that of a prokaryotic cell
Gene expression— genes that are available for transcription and translation in a cell
Leads to cell differentiation —cells becoming physically different and able to do different functions even
though they all have the same DNA
DNA organization
Proteins called histones are responsible for the first level of DNA packing in chromatin
Each “bead” is a nucleosome, the basic unit of DNA packing
Nucleosomes supercoil then loop and condense to form chromosomes during cell division
Interphase chromatin is usually much less condensed than that of mitotic chromosomes and have:
highly condensed areas called heterochromatin, which doesn’t get transcribed and
less compacted areas, called euchromatin which do get transcribed
Gene expression in eukaryotes
All organisms must regulate which genes are expressed at any given time
Many key stages of gene expression can be regulated in eukaryotic cells including transcription,
translation, and post-translation
Is a complex processes that involves
Regulatory genes and elements
Transcription factors
Chromatin modifications
Histone acetylation —promotes the initiation of transcription
DNA methylation --reduces transcription or causes long-term inactivation of genes in cellular
differentiation
Regulatory genes and sequences
Regulatory gene: sequence of DNA that codes for a regulatory protein or RNA molecule
Regulatory sequence: stretches of DNA that interact with regulatory proteins to control transcription
Small regulatory RNA’s —small pieces of RNA that can bind to mRNA to block it’s translation
Transcription factors
Segments of noncoding DNA that help regulate transcription by binding certain proteins called
transcription factors
Promoters—site where RNA polymerase binds to start transcription
Proximal control elements are located close to the promoter
Distal control elements, groups of which are called enhancers, may be far away from a gene or even in
an intron
An activator is a protein that binds to an enhancer and stimulates transcription of a gene
Repressors inhibit expression of a particular gene
Interactions of all these factors determine how much of the gene product will be produced
Gene regulation accounts for some of the phenotypic difference in organisms with similar genomes
Mechanisms of Post-Transcriptional Regulation
Many types of regulatory mechanisms operate at various stages after transcription
Such mechanisms allow a cell to fine-tune gene expression rapidly in response to environmental
changes
Ex: alternative RNA splicing, different mRNA molecules are produced from the same primary transcript,
depending on which RNA segments become exons and which become introns