Gene expression

• Transcription is the process by which the information in a DNA sequence is copied into a complementary RNA sequence.
• Both prokaryotes and eukaryotes alter their patterns of gene expression in response to changes in environmental conditions.
• Multicellular eukaryotes develop and maintain multiple cell types, each containing the same genome but expressing a different subset of genes.
• During development, gene expression must be carefully regulated to ensure that the right genes are expressed only at the correct time and in the correct place.
• Gene expression in eukaryotes and bacteria is often regulated at the transcription stage.
• Control of other levels of gene expression is also important.
• RNA molecules play many roles in regulating eukaryotic gene expressions.
• Disruptions in gene regulation may lead to cancer.
• Natural selection favors bacteria that express only those genes whose products are needed by the cell.
• Metabolic control occurs on two levels: cells can adjust the activity of enzymes already present and vary the number of specific enzyme molecules they make by regulating gene expression.
• The control of enzyme production occurs at the level of transcription, the synthesis of messenger RNA coding for these enzymes.
• Genes of the bacterial genome may be switched on or off by changes in the metabolic status of the cell.
• The basic mechanism for the control of gene expression in bacteria, known as the operon model.
• Papillomaviruses are associated with cancer of the cervix, and a virus called HTLV-1 causes a type of adult leukemia.
• Worldwide, viruses seem to play a role in about 15% of the cases of human cancer.
• Viruses can interfere with gene regulation in several ways if they integrate their genetic material into a cell’s DNA.
• Viral integration may donate an oncogene to the cell, disrupt a tumor-suppressor gene, or convert a proto-oncogene to an oncogene.
• Some viruses produce proteins that inactivate p53 and other tumor-suppressor proteins, making the cell more likely to become cancerous.
• Escherichia coli synthesizes tryptophan from a precursor molecule in a series of steps, with each reaction catalyzed by a specific enzyme.
• The five genes coding for the subunits of these enzymes are clustered together on the bacterial chromosome as a transcription unit, served by a single promoter.
• Transcription gives rise to one long mRNA molecule that codes for all five polypeptides in the tryptophan pathway.
• The mRNA is punctuated with start and stop codons that signal where the coding sequence for each polypeptide begins and ends.
• A key advantage of grouping genes with related functions into one transcription unit is that a single on-off switch can control a cluster of functionally related genes.
• In other words, these genes are coordinately controlled.
• When an E

  coli cell must make tryptophan for itself, all the enzymes are synthesized at one time.
• The switch is a segment of DNA called an operator.
• The operator, located within the promoter or between the promoter and the enzyme-coding genes, controls the access of RNA polymerase to the genes.
• The operator, the promoter, and the genes they control constitute an operon.
• The trp operon (trp for tryptophan) is one of many operons in the E

  coli genome.
• By itself, an operon is turned on: RNA polymerase can bind to the promoter and transcribe the genes of the operon.
• The operon can be switched off by a protein called the trp repressor.
• The repressor binds to the operator, blocks attachment of RNA polymerase to the promoter, and prevents transcription of the operon’s genes.
• Each repressor protein recognizes and binds only to the operator of a particular operon.
• 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.
• Regulatory genes are transcribed continuously at slow rates, and a few trp repressor molecules are always present in an E

  coli cell.
• The complete initiation complex must be assembled before the polymerase can begin to move along the DNA template strand to produce a complementary strand of RNA.
• Protein degradation is a process where a cell marks a protein for destruction by attaching a small protein called ubiquitin to it.
• Proteasomes are giant protein complexes that recognize and degrade tagged proteins.
• The interaction of general transcription factors and RNA polymerase II with a promoter usually leads to only a slow rate of initiation and the production of few RNA transcripts.
• Mutations making specific cell cycle proteins impervious to proteasome degradation can lead to cancer.