Chapter 15
Regulation of Gene Expression
Concept 15.1 Bacteria Respond through Transcription
● Natural selection has favored bacteria that express only
those genes whose products, one gene-one polypeptide, are needed.
● Bacteria achieve metabolic control by either:
○ adjusting enzyme activity (feedback inhibition)
○ adjusting the production level of certain enzymes (transcription of genes).
● The operon or unit of genetic function consists of:
○ Promoter - specific DNA sequence that marks a transcription unit, includes the operator
○ Operator - DNA sequence at the start of an operon that controls access of RNA polymerase to the genes
● Regulatory genes code for repressor proteins that interact with the operator of an operon, genes far away from operon.
● Operons can be repressible, always on, or inducible, always off,
Concept 15.1 Negative Gene Regulation- repressible
● E. coli builds tryptophan in a multistep pathway, each step involves a different enzyme.
● The genes that code for these enzymes are clustered together in the trp operon.
● The trp operon is always on so RNA polymerase has free access to the promoter for transcription.
● If tryptophan is present it acts as a corepressor, binding to a
regulatory protein or the repressor, which binds to the operator making RNA polymerase unable to bind for transcription.
Concept 15.1 Negative Gene Regulation- inducible
● Lactose metabolism or hydrolysis in E. coli is a reaction catalyzed by the enzyme
beta-galactosidase.
● The gene for beta-galactosidase is part of the lac operon which is composed of 2 other genes
involved in lactose digestion.
● The lac operon is inducible, the operator is always off meaning that a regulatory protein or repressor is bound to it.
● When lactose is present and its isomer allolactose is bound to the repressor it becomes inactivated and unable to bind to the operator- transcription is on!
● Repressible enzymes, as in the tryptophan
production, are usually anabolic whereas inducible enzymes, as in lactose digestion, are usually
catabolic.
Concept 15.1 Positive Gene Regulation
● E. coli prefers to use glucose when both
glucose and lactose are present and will only use lactose when glucose is in short supply.
● How does E. coli sense glucose levels and relay this information to transcription?
● Cyclic AMP or cAMP accumulates when
glucose levels are scarce and interacts with a regulatory protein called CAP, an activator, that binds to DNA to stimulate transcription.
● If glucose levels increase in the cell, cAMP falls and CAP detaches from the lac operon.
● The lac operon is under dual control: negative control by the lac repressor through transcription and positive control by CAP through
transcription rate.
Concept 15.2 Eukaryotic Gene Expression
● Eukaryotic cells express different genes of the same genome or differential gene expression.
● In histone acetylation acetyl groups -COCH3 are
attached to histone tails that allow transcription proteins easier access to genes for transcription.
● In DNA methylation methyl groups -CH3 added to certain bases block transcription.
● In epigenetic inheritance traits are not passed on by nucleotide sequences but by mechanisms of chromatin modifications that are not fully understood but can be reversible in some cases.
● Transcription factors bind to control elements, noncoding DNA sequences that serve to regulate transcription.
● Enhancers or distal control elements that serve specific genes.
Concept 15.3 Noncoding RNA and Gene Expression
● Research suggests that a significant amount of the genome may be transcribed into non-protein coding RNAs or ncRNA.
● microRNA or miRNAs are single-stranded RNA molecules capable of binding to complementary sequences in mRNA.
● These miRNAs are part of larger folded RNA, that along with a 7-8 nucleotide miRNA sequence, attaches to
mRNA to block translation or degrades it.
● Small interfering RNAs or siRNAs are similar to
miRNA but they combine with other siRNAs and double stranded RNA that can interfere with translation.
● piwi-associated RNAs or piRNAs are small ncRNAs that induce the formation of heterochromatin that can block expression of some transposons or jumping
genes.
Concept 15. 4 Researching Gene Expression
● Genomic libraries are complete sets of plasmid-containing bacterial cell clones that carry a particular DNA segment from the original genome.
● Bacteriophages have been used as cloning vectors for making genomic libraries.
● Bacterial artificial chromosomes or BACs are used as vectors, they are large plasmids carrying just the genes of interest made of up 100-300kb.
● Clones are stored in multi-welled plates, one clone per well, ie plasmid with cloned beta-globin gene from the
hummingbird.
● Complementary DNA or cDNA is another type of DNA library created from fully processed mRNA and uses reverse transcriptase, DNA polymerase, and DNA ligase.
● Once cDNA is created it is modified by restriction enzymes and inserted into a DNA vector forming a cDNA library.
Concept 15.4 Researching Gene Expression
● How do scientists detect the presence of a gene of interest in plasmids?
● Nucleic Acid hybridization is a process of complementary base pairing between the DNA of the gene of interest and its complementary single stranded molecule of either DNA or RNA that is called a nucleic acid probe, made of Each nucleic acid probe is made of radioactive isotopes.
● Once the gene of interest is found in a E. coli clone, those cells can be grown in large quantities for further study.
● Due to the complexity of eukaryotic gene expression, often scientists will often use an expression vector, which is a cloning vector with a highly active promoter upstream of the restriction site where the eukaryotic gene will be inserted.
● cDNA is used when introns are present in eukaryotic genes.
● Yeast, single-celled eukaryotic fungi are often used as cloning vectors because they contain plasmids, are eukaryotic, and are easily grown.
● DNA electroporation is another method to introduce recombinant DNA into eukaryotic cells, creating pores in the membrane for DNA delivery.
