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Regulation of Gene Expression (Bacteria often respond to environmental…
Regulation of Gene Expression
Bacteria often respond to environmental change by regulating transcription
The switch is a segment of DNA called an operator
The operator, the promoter, and the genes they control constitute an operon
The operon can be switched off by a protein called the trp repressor
The trp repressor is the protein product of a regulatory gene called trpR, which is located at
some distance from the operon it controls and has its own promoter
Tryptophan functions in the trp operon as a corepressor, a small molecule that cooperates
with a repressor protein to switch an operon off
There are two types of operons: repressible and inducible
The trp operon is an example of a repressible operon, one that is inhibited when a specific small molecule (tryptophan) binds allosterically to a regulatory protein
In contrast, an inducible operon is stimulated (induced) when a specific small molecule interacts with a regulatory protein
An inducer inactivates the repressor
When glucose levels are low, cyclic AMP (cAMP) accumulates in the cell
The regulatory protein catabolite activator protein (CAP) is an activator of transcription
Eukaryotic gene expression is regulated at many stages
The differences between cell types are due to differential gene expression, the expression of different genes by cells with the same genome
Chromatin modifications affect the availability of genes for transcription
Histone acetylation (addition of an acetyl group, —COCH3) and deacetylation of lysines in histone tails appear to play a direct role in the regulation of gene transcription
Inheritance of traits by mechanisms not directly involving the nucleotide sequence is called epigenetic inheritance
Transcription initiation is controlled by proteins that interact with DNA and with each other
Multiple control elements are associated with most eukaryotic genes
Distal control elements, grouped as enhancers, may be thousands of nucleotides away from the promoter or even downstream of the gene or within an intron
Noncoding RNAs play multiple roles in controlling gene expression
In the past few years, researchers have found small, single-stranded RNA molecules called microRNAs (miRNAs) that bind to complementary sequences in mRNA molecules
The phenomenon of inhibition of gene expression by RNA molecules is called RNA interference (RNAi)
This RNA interference is due to small interfering RNAs (siRNAs), similar in size and function to miRNAs and are generated by similar mechanisms in eukaryotic cells
Small RNAs can remodel chromatin and silence transcription
This RNA interference is due to small interfering RNAs (siRNAs), similar in size and function to miRNAs and are generated by similar mechanisms in eukaryotic cells
A program of differential gene expression leads to the different cell types in a multicellular organism
During development, cells become specialized in structure and function, undergoing cell differentiation
The physical processes that give an organism its shape constitute morphogenesis, the “creation of form.”
Maternal substances that influence the course of early development are called cytoplasmic determinants
These signals cause changes in the target cells, a process called induction
Molecular changes in the embryo drive the process, called determination, which leads to the observable differentiation of a cell
The molecular cues that control pattern formation, positional information, are provided by cytoplasmic determinants and inductive signals
These homeotic genes control pattern formation in the late embryo, larva, and adult
A maternal effect gene is a gene that, when mutant in the mother, results in a mutant phenotype in the offspring, regardless of the offspring’s own genotype
Cancer results from genetic changes that affect cell cycle control
Cancer-causing genes, oncogenes, were initially discovered in viruses
Normal versions of cellular genes, called proto-oncogenes, code for proteins that stimulate normal cell growth and division
The normal products of tumor-suppressor genes inhibit cell division
The Ras protein, the product of the ras gene, is a G protein that relays a growth signal from a growth factor receptor on the plasma membrane to a cascade of protein kinases