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Questions and Answers
What is the primary function of bacterial promoters?
What is the primary function of bacterial promoters?
Which sequences are part of the consensus sequence in bacterial promoters?
Which sequences are part of the consensus sequence in bacterial promoters?
How does a down-mutation affect promoter efficiency?
How does a down-mutation affect promoter efficiency?
What is the significance of the Pribnow box in bacterial promoters?
What is the significance of the Pribnow box in bacterial promoters?
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What type of mutation is an example of a mechanism for promoting adaptation in bacteria?
What type of mutation is an example of a mechanism for promoting adaptation in bacteria?
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What does the '+1' symbol indicate in the context of bacterial promoters?
What does the '+1' symbol indicate in the context of bacterial promoters?
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What might be a consequence of increased conformance to the consensus sequence?
What might be a consequence of increased conformance to the consensus sequence?
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Which of the following phenotypes is NOT mentioned as part of the adaptation in Escherichia coli?
Which of the following phenotypes is NOT mentioned as part of the adaptation in Escherichia coli?
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What type of genes are classified as housekeeping genes and are expressed continuously?
What type of genes are classified as housekeeping genes and are expressed continuously?
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What is the primary function of the lac operon in E. coli?
What is the primary function of the lac operon in E. coli?
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Which of the following describes the structure of operons in bacteria?
Which of the following describes the structure of operons in bacteria?
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Which environmental changes can trigger the activation of regulated genes?
Which environmental changes can trigger the activation of regulated genes?
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What is the role of lacA in the lac operon?
What is the role of lacA in the lac operon?
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What best describes polycistronic RNA in bacterial transcription?
What best describes polycistronic RNA in bacterial transcription?
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Which protein is NOT encoded by the lac operon?
Which protein is NOT encoded by the lac operon?
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How does transcriptional control primarily regulate protein expression in bacteria?
How does transcriptional control primarily regulate protein expression in bacteria?
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Study Notes
Prokaryotic Transcription
- Prokaryotic transcription is the process of creating RNA from DNA in prokaryotes.
- It involves three main steps: initiation, elongation, and termination.
Initiation
- RNA polymerase binds to the promoter region of DNA.
- The promoter region contains consensus sequences, such as the -35 and -10 regions, that are recognized by RNA polymerase.
- The sigma (σ) factor plays a crucial role in recognizing promoter sequences.
- Unwinding of DNA occurs at the promoter region.
- Transcription initiates at the +1 site.
Elongation
- RNA polymerase moves along the template DNA strand during elongation.
- It synthesizes a complementary RNA strand by adding nucleotides that pair with the template strand.
- The RNA polymerase synthesizes mRNA using only one DNA strand as a template.
- These mRNA molecules are also known as transcripts.
- Operons do not have introns, which are non-coding regions of DNA.
- There are no post-transcriptional modifications involved.
Termination
- RNA polymerase stops when it encounters termination sequences in DNA.
- These sequences signal the end of a gene, triggering RNA polymerase to detach from the DNA molecule.
- There are two main types of termination:
- Intrinsic terminators: rely on a specific DNA sequence that forms a hairpin structure in the newly formed RNA.
- Rho-dependent terminators: require a protein called Rho factor to detach from the DNA. Allolactose is one example of those factors in the lac operon.
Transcriptional Regulation
- Transcription is regulated by regulatory proteins.
- Negative regulation involves repressors which bind to operator sites, blocking RNA polymerase from binding to the promoter.
- Positive regulation involves activators that enhance RNA polymerase binding to the promoter.
The lac Operon
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The lac operon is a classical example of gene regulation in bacteria, specifically E.coli.
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It contains the genes for lactose metabolism.
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The lac operon is regulated by both negative and positive mechanisms.
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Negative regulation is controlled by the lac repressor, which binds to the operator region of DNA and prevents transcription of the structural genes.
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Positive regulation involves CAP (Catabolite Activator Protein) which binds to a specific CAP-binding site. Activation happens when cAMP binds to CAP, which helps initiate transcription.
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The lac operon is regulated in response to the presence or absence of glucose and lactose.
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When lactose is absent the lac repressor is produced and binds to the operator. Transcription is blocked.
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When lactose is present, it is converted to allolactose, which binds to the lac repressor, causing it to detach from the operator, allowing transcription.
Bacterial Promoters
- Promoters are DNA sequences that signal the beginning of a gene.
- Promoters are recognized by RNA polymerase, and their sequence determines transcription efficiency.
- Consensus sequences at positions -35 and -10 relative to the start site of transcription are crucial for promoter recognition in bacteria.
Bacterial RNA Polymerase
- RNA polymerase is the enzyme responsible for transcribing DNA into RNA.
- Prokaryotic RNA polymerase is a holoenzyme, composed of core enzyme and sigma (σ) factor.
- The core enzyme (containing the subunits β, β') is responsible for the incorporation of nucleotides into the nascent RNA molecule.
- The sigma subunit helps in recognizing promoters, and the exact sigma factor used in prokaryotic transcription varies according to the gene.
Alternative Sigma Factors
- Sigma factors play crucial roles in regulating gene expression, often responding to changing environmental conditions or cellular stress.
- Alternative sigma factors promote transcription of specific genes relevant to the cellular stress.
Regulon
- A regulon is a set of genes or groups of genes that are regulated by a common regulatory mechanism (sigma factor), often in response to environmental conditions.
- Repressor protein in the lac operon influences the regulated gene transcription.
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Description
This quiz covers the key processes of prokaryotic transcription, including the steps of initiation, elongation, and termination. Understand how RNA polymerase interacts with DNA and the significance of the promoter region and sigma factor in transcription initiation.
