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Questions and Answers
What is the predominant site for control of gene expression in prokaryotes?
Which term describes the sequences that interact with proteins to regulate the accessibility of promoter regions in prokaryotic DNA?
How are the regulatory proteins that affect RNA polymerase activity classified?
What is the role of the lac operator in prokaryotic gene regulation?
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Who were the first to elucidate a transcriptionally regulated system, such as the lac operon?
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Which term describes a genetic arrangement in bacteria where a polycistronic structural gene is regulated by a common promoter and regulatory genes?
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What specific type of molecule is lac, in the context of the lac operon?
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Which of the following is NOT a common example of an operon in bacteria?
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What type of proteins bind to specific sequences at the operator region to regulate transcription?
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What distinguishes activators from repressors in gene regulation?
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Study Notes
Chromatin Structure
- Chromatin requires additional proteins, known as Non-histone Chromosomal (NHC) proteins, for higher-level packaging.
- Loosely packed chromatin regions are called euchromatin, which stains light, and are transcriptionally active.
- Densely packed chromatin regions are called heterochromatin, which stains dark, and are inactive.
The Search for Genetic Material
- Despite the discovery of nuclein and the principles of inheritance, it took time to discover that DNA is the genetic material.
- By 1926, scientists had narrowed the search to chromosomes in the nucleus, but the question of what molecule was the genetic material remained unanswered.
Transforming Principle
- Frederick Griffith's experiments with Streptococcus pneumoniae in 1928 led to the discovery of a transforming principle.
- In these experiments, a living organism changed its physical form.
DNA Organisation
- In prokaryotes, DNA is organised in large loops held by proteins in the nucleoid region.
- In eukaryotes, DNA is organised in a more complex structure involving histones.
- Histones are basic proteins rich in lysine and arginine, which carry positive charges.
- Histone octamers are formed by eight histone molecules, around which negatively charged DNA is wrapped to form a nucleosome.
- Nucleosomes contain 200 bp of DNA helix and form the repeating unit of chromatin.
- Chromatin appears as a 'beads-on-string' structure under an electron microscope.
Gene Expression
- In prokaryotes, control of transcriptional initiation is the primary site for controlling gene expression.
- Regulatory proteins can act as activators or repressors to regulate RNA polymerase activity at promoters.
- Accessibility of promoter regions is regulated by protein-operator interactions.
- Each operon has a specific operator and repressor.
The Lac Operon
- The lac operon is a transcriptionally regulated system elucidated by Francois Jacob and Jacque Monod.
- It consists of a polycistronic structural gene regulated by a common promoter and regulatory genes.
- The lac operon is an example of an operon, a common arrangement in bacteria.
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Description
Explore the concept of chromatin packaging at a higher level which requires Non-histone Chromosomal (NHC) proteins. Learn about euchromatin and heterochromatin, their characteristics, and transcriptional activity in a typical nucleus.
