topic 5: regulation of gene expression Flashcards

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1
Q

when do prokaryotes and eukaryotes change their gene expression?

A

in response to environmental changes

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2
Q

what roles does gene regulation play in multicellular eukaryotes?

A
  • regulates development
  • responsible for differences between different cells types (cell specialization)
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3
Q

how do bacteria respond to environmental change?

A

by regulating transcription (produce only the products that they need)

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4
Q

what are two ways bacteria regulate metabolic pathways?

A
  1. regulation of enzyme activity by feedback inhibition: controlled by allosteric regulation
  2. regulation of enzyme production by gene expression regulation: controlled by the operons
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5
Q

in the example of regulation of amino acid tryptophan in E.coli, what are the different methods to regulate gene expression?

A

[in the presence of tryptophan in the environment, there is no need to produce it]
1. regulation of enzyme activity (rapid response): tryptophan binds to Enzyme 1 and inhibits its activity

  1. regulation of enzyme production (long-term response: the production of enzymes is inhibited to prevent the production of genes and using unnecessary cell resources
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6
Q

what is an operon? what does it consist of?

A
  • a prokaryotic DNA segment that includes:
  • the operator
  • the promoter
  • a group of functionally related genes
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7
Q

what is the operator?

A
  • a segment of DNA which works as a regulatory switch (on-off) that controls a cluster of functionally related genes
  • controls if transcription will take place
  • consists of a specific sequence within the promoter of these genes
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8
Q

what is the promoter?

A

a sequence where the RNA polymerase binds (the enzyme involved in transcription)

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9
Q

what is the use of the group of functionally related genes?

A

the genes will be transcribed into different polypeptides, which form the enzymes that, for example, produce tryptophan

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10
Q

what are other molecules/proteins that bacteria have that are involved in blocking or activating the transcription process?

A
  • repressors
  • co-repressors
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11
Q

what are repressors?

A
  • a protein that switches off the operon
  • produced by a separate regulatory gene
  • prevents gene transcription by binding to the operator and blocking DNA polymerase binding
  • can be in an active or inactive form, depending on the presence of other molecules (ex: tryptophan or lactose)
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12
Q

what is a co-repressor?

A
  • a molecule that cooperates with a repressor protein to switch an operon off (operon inactivation)
  • ex: tryptophan
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13
Q

what is negative gene regulation? what are the two examples of it?

A
  • negative gene regulation: operons are switched off by the active form of the repressor
    two types of such operons:
  • repressible operons
  • inducible operons
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14
Q

describe repressible operons

A
  • usually active
  • usually regulate gene expression of enzymes involved in anabolic pathways
  • their synthesis is repressed by high levels of the end product (co-repressor) which activates the repressor
  • example: trp operon
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15
Q

describe inducible operons

A
  • usually inactive (INducible = INactive)
  • usually regulate gene expression of enzymes involved in catabolic pathways
  • their synthesis is induced by a chemical signal (inducer) which inactivates the repressor
  • example: lac operon
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16
Q

what does the trp operon contain?

A

it contains genes of enzymes involved in tryptophan synthesis

17
Q

‘transcription is normally on but can be inhibited when a small molecule binds allosterically to a regulatory protein’ the following describes?

A

repressible operon

18
Q

describe the process when each occurs:
(1) tryptophan is absent
(2) tryptophan is present

A
  • (1) the repressor is inactive –> cannot bind to operator –> operon is ON –> RNA polymerase transcribes genes –> production of enzymes involved in tryptophan production –> tryptophan is synthesized
  • (2) trp (a co-repressor) is present –> binds to the trp repressor protein –> repressor is activated –> binds to operator –> operon is OFF –> no production of tryptophan
19
Q

what does the lac operon contain?

A

it contains genes of enzymes used in lactose metabolism (lactose hydrolysis)

20
Q

‘transcription is normally off but can be activated when a small molecule binds allosterically to a regulatory protein’ the following describes?

A

inducible operon

21
Q

describe the process when each occurs:
(1) lactose is absent
(2) lactose is present

A
  • (1) lac repressor is active by itself –> binds to operon –> operon is OFF –> lactose hydrolysis stops
  • (2) allolactose (an inducer) is present –> binds to repressor and inactivates it –> lac operon is ON –> lactose hydrolysis is activated
22
Q

what is allolactose?

A
  • a disaccharide similar to lactose (isomer)
  • consists of the monosaccharides D-galactose and D-glucose linked through a β1-6 glycosidic linkage instead of the β1-4 linkage of lactose
23
Q

if both glucose and lactose are present, what do cells prefer and why?

A

cells always prefer glucose as it is a better energy source

24
Q

what is positive gene regulation?

A

when operons are switched on by the active form of the activator

25
Q

what is an activator? what is an example?

A
  • a stimulatory protein (that stimulates transcription)
  • example: catabolite activator protein (CAP) in E.coli which enhances transcription of the lac operon
26
Q

explain positive gene regulation in E.coli

A

(1) when glucose and lactose are both present: E.coli will prefer to use glucose –> there will be low quantities of the enzymes which are needed for lactose break down (low transcription)

(2) when only lactose is present and glucose is short in supply: E.coli will use lactose as an energy source –> sufficient quantities of enzymes are synthesized for lactose breakdown (enhanced transcription)

27
Q

what is the effect of having glucose absence?

A

enhanced transcription of enzymes that breakdown lactose (stimulated lactose hydrolysis)

28
Q

in what operon does both positive and negative gene regulation take place?

A

lac operon

29
Q

describe the process when each occurs:
(1) low glucose levels
(2) high glucose levels

A

(1) low glucose levels:
- increase in levels of cAMP (cyclic AMP)
- CAP is activated by binding to cAMP
- activated CAP attaches to the promoter of the lac operon, increases affinity of RNA polymerase, accelerates transcription of the lac operon
- hydrolysis of lactose to glucose & galactose

(2) high glucose levels:
- decrease in levels of cAMP
- CAP detaches from the lac operon
- decreased affinity of RNA polymerase, decreased transcription of the lac operon

30
Q

explain how active CAP can increase RNA polymerase affinity

A

active CAP binds before RNA polymerase, helps it attach better (on the promoter), therefore increasing its affinity, and transcribing at a faster rate

31
Q

a typical human cell expresses how much of its genes at a given time?

A

20%

32
Q

only ___% of DNA codes for proteins, the rest codes for what?

A
  • 1.5
  • RNA products (rRNAs and tRNAs) or is not transcribed at all
33
Q

abnormalities in gene expression lead to?

A

cancer

34
Q

what are the stages eukaryotic gene expression can be regulated at?

A
  • regulation of chromatin structure
  • regulation of transcription initiation
  • post-translational regulation
35
Q
A