contains almost everything for the second exam. last chapter is slightly incomplete, but it is mainly an application thing in the exam, so not needed to study as such. everything else is a very comprehensible summary of the ppt used in class combined with logic from the textbook.
Chapter 14: Gene Regulation in Bacteria
Introduction:
- Gene regulation = the level of gene expression can vary under different conditions.
The benefits of regulating genes is that encoded proteins/RNAs will be produced only
when required, therefore the cell avoids wasting energy on materials it does not
need. It is useful for the following cellular processes:
1. Metabolism
2. Response to environmental stress
3. Cell division
4. Differentiation and Development
5. Several other reasons (e.g. cell fitness)
- Genes that are unregulated are called constitutive genes. They are constantly
expressed. Frequently, constitutive genes encode proteins that are continuously
necessary for the survival of the cell/organism (e.g. genes encoding structural
proteins like actin or tubulin). These are expressed in all cell types.
14.1. transcriptional regulation:
• The most common way to regulate gene expression in bacteria is at the transcription
initiation level -> The rate of mRNA production can be increased or decreased
• Transcriptional regulation involves the action of two main regulatory proteins:
1. Repressors (Bind directly or indirectly to DNA and inhibit transcription:
Negative control)
2. Activators (Bind directly or indirectly to DNA and increase transcription:
Positive control)
• Small effector molecules also affect gene transcription: these are Short peptides (e.g.
hormones), metabolites, drugs, sometimes amino acids, second messengers, etc and
bind to regulatory proteins and not to DNA directly.
• In some cases, the presence of a small effector molecule may increase transcription.
These molecules are termed inducers. They function in two ways:
- Bind activators and cause them to bind to DNA
- Bind repressors and prevent them from binding to DNA
Genes that are regulated in this manner are termed inducible genes
• In other cases, the presence of a small effector molecule may decrease transcription.
This can occur in two ways:
- Corepressors bind to repressors and cause them to bind to DNA (the
repressor here does not have the correct shape to bind the DNA on its own)
- Inhibitors bind to activators and prevent them from binding to DNA
Genes that are regulated in this manner are termed repressible genes
14.2. Regulation of the lac Operon
, • Enzyme Adaptation = A particular enzyme appears in the cell only after the cell has
been exposed to the enzyme’s substrate. If it has not been exposed to its substrate, it
won’t make the enzymes needed to metabolize the substance.
• François Jacob and Jacques Monod were interested in this phenomenon and focused
on lactose metabolism in E coli. They observed the following:
1. The exposure of bacterial cells to lactose increased the levels of lactose-
utilizing enzymes
2. Increase in these enzymes was due to the increased synthesis of the
proteins that form the enzyme
3. Removal of lactose abruptly terminated the synthesis of such enzymes
4. Analysis of the mutations in the lac operon showed that each protein
involved with lactose utilization is encoded by a separate gene
• Operon concept: a lot of genes are organized on operons. An operon is a regulatory
unit consisting of a few structural genes under the control of one promoter and one
terminator. This is more efficient as a lot of genes that are needed in the same
process can be regulated in one go. An operon encodes a polycistronic mRNA, which
is an RNA that contains the coding sequence for two or more structural proteins. This
allows a bacterium to coordinately regulate a group of genes that encode proteins
involved in the same process / function. NOT ALL GENES IN BACTERIA ARE IN
OPERONS!!
• The study of many operons revealed a general trend concerning inducible versus
repressible regulation:
1. Operons involved in catabolism (ie. breakdown of a substance) are typically
inducible (typically off until I need them) e.g. lac Operon
The substance to be broken down (or a related compound) acts as the
inducer
2. Operons involved in anabolism (ie. biosynthesis of a substance) are typically
repressible (on until I turn them off) e.g. trp Operon
The inhibitor or corepressor is the small molecule that is the product
of the operon
These two are important since the cell wants to remain at homestasis
Example 1: The lac operon:
• There are two distinct transcriptional units:
1. The actual lac operon: contains:
a. DNA elements: Involved in transcriptional regulation
• ‘Promoter’ Bound by RNA polymerase
• Operator Bound by the lac repressor protein (see later:
Operator sites)
• CAP site Bound by the Catabolite Activator Protein (CAP)
here a different enzyme may attach to regulate it
The CAP and operator are the ones involved in gene regulation
b. Structural genes: Contain the coding sequence for the enzymes
(these do the catabolism of lactose)
• lacZ Encodes b-galactosidase
• Enzymatically cleaves lactose and lactose analogues
• Also converts lactose into allolactose (an isomer)
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