book 7 (control of prokaryotic gene expression)

Question 1

explain the effect of the absence of tryptophan on trp operon

Answer 1

1. must now synthesise more tryptophan since the tryptophan concentration in the cell is low, tryptophan is required for polypeptide synthesis in the cell during translation
2. trp repressor is constitutively expressed continuously in inactive, non-DNA binding form. thus it does not bind to the operator site
3. RNA polymerase recognises and binds to the promoter region of the trp operon and initiates transcription of the 5 structural genes
4. transcription now switched on
5. repressible enzymes are synthesised for the tryptophan biosynthesis

Question 2

explain the effect of the presence of tryptophan on trp operon

Answer 2

1. it is more cost efficient to stop the synthesis of proteins involved in the biosynthesis of tryptophan since tryptophan is present in abundance now
2. trp repressor is constitutively expressed and continuously synthesised in inactive, non DNA-binding form
3. tryptophan, acts as a co-repressor, binds to the allosteric site of the trp repressor and alters the 3D conformation at the DNA-binding site. trp repressor now active and binds to the operator
4. trp repressor at operator blocks and prevents RNA polymerase from accessing and transcribing the structural genes. there will be no initiation of transcription of structural genes
5. transcriptional of trp operon switched off
6. repressible enzymes are not synthesised

Question 3

state the structural genes of the lac operon, what each encodes for, and what are their functions

Answer 3

1. lac Z encodes for beta-galactosidase which catalyses the hydrolysis of lactose into glaactose and glucose. *it also converts lactose into allolactose
2. lac Y encodes for lactose permease which allows lactose entry into the bacteria
3. lac A encodes for lactose transacetylase which adds acetyl groups to lactose

Question 4

explain the effect of the absence of lactose on the lac operon

Answer 4

1. it is not cost-effective to synthesis the lac enzymes as no lactose is present
2. lac repressor is constitutively expressed and continuously synthesised in the active, DNA -binding form
3. lac repressor can recognise and bind to operator, to prevent RNA polymerase from accessing and transcribing the structural genes, there is no initiation of transcription of structural genes. transcription is switched off
4. beta-galactosidase, lactose permease, and lactose transacetylace not synthesised

Question 5

explain the effect of the presence of lactose on the lac operon

Answer 5

1. presence of lactose provides an alternative source of carbon so there is no need to quickly synthesise the enzymes that aid in the metabolism
2. lac repressor is constitutively expressed and continuously synthesised in the active, DNA-binding form
3. lactose from the surroundings can enter the cell via lactose permease. beta-galactosidase isomerises the lactose to form allolactose. this occurs as there is always low levels of permease and beta-galactosidase synthesis. allolactose functions as an inducer which binds to the allosteric site of the lac repressor, altering the 3D conformation of the DNA-binding site of the repressor, causing the inactivation of lac repressor
4. lac repressor no longer able to bind to DNA at the operator, allowing RNA polymerase to access and transcribe the structural genes, switching on the transcription of lac operon
5. beta-galactosidase, lactose permease and lactose transacetylase synthesised and used in the metabolism of lactose sugars

Question 6

explain the effects of the absence of glucose and presence of lactose

Answer 6

1. the absence of glucose means that bacteria must quickly switch to utilise lactose sugars as a carbon source to ensure it’s survival
2. adenylate cyclase is active when glucose concentration is low, leading to the increase in conversion of ATP to cAMP, increasing the concentration of cAMP
3. cAMP binds to the allosteric site of CAP and alters the conformation at DNA-binding site, CAP now active and binds to CAP-binding site, this facilitates more efficient positioning of RNA polymerase at the promoter
4. lactose enters via the qualifies leaky membrane and isomerises to form allolactose which functions as an inducer that binds to allosteric site of lac repressor and alters it’s 3D conformation at the DNA-binding site, inactivating lac repressor and is no longer able to bind to operator. operator exposed for the initiation of transcription by RNA polymerase. RNA polymerase can access and transcribe the structural genes
5. transcription is switched on with high rate of transcription by RNA polymerase
6. beta-galctosidase and lactose permease and lactose transacetylase synthesised at high levels

Question 7

explain the effect of the presence of lactose and presence of glucose on the lac operon

Answer 7

1. presence of glucose meant that the bacteria should utilise glucose first. only when glucose is used up, the bacteria will then utilise other sugars like lactose
2. there will be a decrease in the concentration of cAMP as it is being broken down by phosphodiesterases
3. CAP not bound by cAMP, CAP now inactive and does not bind to the CAP-binding site
4. RNA polymerase can bind to promoter and initiate transcription of structural genes but inefficiently
5. transcription switched on with low rates of transcription by RNA polymerase
6. beta-galactosidase, lactose permease and lactose transacetylase synthesised at low levels

Question 8

explain the effects of the presence of glucose and absence of lactose

Answer 8

1. in the presence of glucose, cAMP concentration decreases, CAP not bound to cAMP, inactivating CAP and it does not bind to the CAP-binding site
2. RNA polymerase binds to promoter ineffectively
3. the lac repressor is constitutively expressed and synthesised in active, DNA-binding form. it recognises and binds to the operator which prevents the RNA polymerase from accessing and transcribing the structural genes
4. no initiation of transcription of structural genes, switching off lac operon. beta-galactoidase, lac permease and lactose transacetylase not synthesised.

Question 9

explain the effect of absence of glucose and absence of lactose

Answer 9

1. in the absence of glucose, cAMP concentration increases. cAMP binds to allosteric site of CAP, altering its 3D conformation at the DNA-binding site and activating the CAP, allowing it to bind to the promoter
2. the lac repressor is constitutively expressed and synthesised in its active, DNA-binding form. it recognises and binds to the operator, preventing the RNA polymerase from accessing and transcribing the structural genes
3. no initiation of transcription of structural genes, switching off the lac operon off. beta-galactosidase, lac permease and lactose transacetylase not synthesised