bacteria: operons

Question 1

advantages of operons

Answer 1

1. allows bacteria to make economical use of energy and resources, as relevant genes are expressed only when necessary, preventing wastage and conserving resources
2. can be turned off or on according to changes/conditions of the environment
3. allows for functionally related proteins to be synthesised at the same time
4. provide a selective advantage to bacteria as they enable the bacteria to respond rapidly and appropriate to changes in environment

Question 2

what is a operon

Answer 2

cluster of functionally related genes which are under the control of the same promoter and operator, allowing for functionally related proteins to be synthesised together
includes a common promoter, an operator and one or more structural genes that are controlled as a unit
produces a polycistronic mRNA

Question 3

structure of lac operon

Answer 3

has 3 structural genes lacZ, lacY and lacA
promoter serves as both RNA polymerase and catabolic activator protein binding site
operator is the lac repressor binding site and overlaps with promoter

Question 4

function of lac operon structural genes

Answer 4

lacZ codes for beta-galactosidase: enzyme that hydrolyses lactose into glucose and galactose
lacY codes for permease: facilitates movement of
lactose from outside of cell to inside of cell
lacA codes for transacetylase: function remains unknown

Question 5

structure of trp operon

Answer 5

cluster of 5 structural genes: trpE, trpD, trpC, trpB and trpA
promoter serves as RNA polymerase binding site
operator serves as binding site for trp repressor complex with trp and is located within promoter

Question 6

regulatory gene: lac vs trp

Answer 6

lac operon: Lac l gene that codes for lac repressor, allosteric site binds to allolactose
trp operon: Trp R gene that codes for trp repressor, allosteric site binds to tryptophan

Question 7

type of operon: lac vs trp

Answer 7

lac is inducible, trp is repressible

Question 8

default state of operon: lac vs trp

Answer 8

lac is always off, trp is always on

Question 9

default state of repressor: lac vs trp

Answer 9

lac repressor is always active, trp repressor is always inactive

Question 10

effector: lac vs trp

Answer 10

lac operon has the inducer lactose which is the substrate
trp operon has the corepressor tryptophan which is the endproduct

Question 11

effect of effector on operon: lac vs trp

Answer 11

lac: turns on transcription of genes
trp: turns off transcription of genes

Question 12

type of metabolic pathway: lac vs trp

Answer 12

lac is a catabolic pathway that breaks down metabolites
trp is a anabolic pathway that synthesises metabolites

Question 13

describe negative regulation of lac operon in absence of lactose and glucose

Answer 13

1. regulatory lacL is constitutively transcribed, resulting in continued production of active lac repressor protein
2. lac repressor protein binds to operator by its DNA binding site
3. this prevents RNA polymerase from binding to the promoter
4. transcription of structural genes of the lac operon is prevented

Question 14

describe positive regulation of lac operon in the presence of lactose and glucose

Answer 14

1. when lactose is present, it enters the cell by permease
2. lactose is converted to allolactose by beta- galactosidase
3. Allolactose acts as an inducer and binds to allosteric site of lac repressor. This causes a
   change in confirmation of lac repressor and lac
   repressor becomes inactive
4. the DNA binding site of the inactive repressor is no longer
   complementary in shape and charge to the operator and can no longer bind it
5. Promoter site available for RNA polymerase to bind
6. transcription of strucural genes will occur and a polycistronic mRNA will be produced
7. however RNA polymerase has low affinity for promoter and the lac operon will not be fully activated

Question 15

describe positive regulation of lac operon in the presence of lactose but absence of glucose

Answer 15

1. When glucose is absent, high levels of cAMP will be present so cAMP binds to allosteric site of CAP to form a CAP-cAMP complex
2. formation of complex activates CAP and DNA binding site of activated CAP binds to promoter of lac operon
3. binding of activated CAP to operon increases the affinity of RNA polymerase to the promoter
4. Transcription frequency of structural genes lacZ,
   lacY and lacA to produce beta-galactosidase,
   permease and transacetylase respectively to breakdown lactose thuse increase

Question 16

why is there a time lag for lac operon genes to be expressed

Answer 16

it is the time taken for
transcription of genes and subsequent otranslation to form gene products

Question 17

why is the lac operon considered to be in dual control

Answer 17

it has negative regulation by the lac repressor and positive regulation by CAP

Question 18

describe trp operon in low levels of tryptophan

Answer 18

1. regulatory gene trpR is constitutively transcribed resulting in continued production of inactive trp repressor protein
2. inactive trp repressor is unable to bind to the operator
3. RNA polymerase is able to bind to the promoter
4. transcription of structural genes of trp operon can occur and produce a polycistronic mRNA

Question 19

describe trp operon in high lvels of tryptophan

Answer 19

1. When tryptophan is present in high concentrations in the
   cell
2. Tryptophan acts as a corepressor and binds to the
   allosteric site of trp repressor
3. This causes a change in conformation of trp repressor
   and trp repressor become active
4. The DNA binding site of the active repressor can bind to the operator
5. This prevents binding of RNA polymerase to promoter
6. And prevents transcription of structural genes &
   expression of operon
7. Synthesis of tryptophan is reduced/ stopped

Question 20

why are all the enzymes not present even in presence of lactose when the regulatory gene has a mutation?

Answer 20

operon is switched off

mutation in regulatory gene results in change in conformation of DNA binding site of repressor, so repressor binds irreversibly to or cannot dissociate from operator
OR
muation in regulatory gene causes a change in conformation of allosteric site of repressor, so the inducer cannot bind to the repressor and inactivate it, remaining bound to operator

Question 21

why are all the enzymes not present even in presence of lactose when the promoter has a mutation?

Answer 21

operon is switched off
RNA polymerase is unable to bind to promoter and no transcription can happen

Question 22

why are all the enzymes present even in absence of lactose when the regulatory gene has a mutation?

Answer 22

genes in operon is constitutively transcribed
1. mutation causes repressor to be truncated/non-functional/absent

2. mutation causes a conformational change in DNA binding site of repressor so it cannot bind to operator
3. (for trp operon) mutation causes a conformational change in allosteric site of repressor so repressor cannot bind to corepressor to form repressor complex and ultimately cannot bind to operator

Question 23

why are all the enzymes present even in absence of lactose when the operator has a mutation?

Answer 23

muation causes conformational change in DNA binding site in operator: Repressor unable to bind to operator so RNA polymerase can continue
to bind to promoter & transcribe structural genes

genes in operon are constitutively transcribed

Question 24

why are some enzymes absent at the end of the operon

Answer 24

one of the structural genes is mutated so the enzyme coded for is truncated, absent or non-functional
the other enzymes coded by the unaffected structural genes will still be produced: the enzymes are translated independently because each has its own start & stop codon on the polycistronic mRNA