Bacterial Gene regulation (23/24)

Question 1

Distinguish between constitutive and regulated gene expression

Answer 1

Constitutive gene
a gene that is transcribed continually as opposed to a facultative gene, which is only transcribed when needed
Regulated gene expression
a gene that codes for a protein, such as a repressor, that controls the transcription of another gene or group of genes

Question 2

Describe polycistronic RNA and explain why it is possible for several different polypeptides to be translated from one mRNA molecule in prokaryotic

Answer 2

a mRNA that encodes several proteins and is characteristic of many bacterial and chloroplast mRNAs. Polycistronic mRNAs consist of a leader sequence which precedes the first gene.
Because Translation and transcription occurs in the same compartment of the bacterial call

Question 3

Explain the concept of a repressible operon, including the functions of a promoter, operator, repressor and co-repressor

Answer 3

A repressor blocks the attachment of RNA polymerase to the promoter, thus preventing transcription of the genes into messenger RNA.
Promoter
DNA sequences that define where transcription of a gene by RNA polymerase begins
Operator
a sequence of nucleotides near the start of an operon to which an active repressor can attach
Co-repressor
a small molecule that binds to a bacterial repressor protein and changes the protein’s shape, allowing it to bind to the operator and switch an operon off

Question 4

Explain how the inducible lac operon is regulated in response to the availability of lactose and glucose

Answer 4

When no glucose is present but lactose is still present, CAP is bound to the activator, cAMP provided an alternative energy source to activate the activator once CAP is bound to it. Allolactose is bound to lac repressor, this prevents the lac repressor from binding to the operator. RNA polymerase then binds to the operon and transcriptase occurs down the operon.
When glucose and lactose is present, again allolactose is bound to the lac operon to prevent it from binding to the operon. However because there is insufficient amount of cAMP, it won’t bind to the activator with the CAP bound to it, there is less transcription occurring on the operon itself.
The same process of RNA polymerase binding to the operon occurs therefore transcription can occur on the lac operon itself and metabolise and absorb glucose.

Question 5

Predict the effects of mutations in operons and in regulatory genes

Answer 5

Mutations in regulator genes can affect the expression of all structural genes in an operon

Question 6

Distinguish between inducible and repressible of gene expression

Answer 6

Inducible - default expression is OFF. The effector molecule that binds to the regulatory protein is an inducer (switches gene expression ON)
Repressible - default expression is ON. The effector molecule that binds to the regulatory protein is a (co-repressor) ( switches gene expression OFF)

Question 7

Distinguish between positive and negative regulation of gene expression

Answer 7

Positive
The regulatory protein is an activator (has a positive effect on gene expression when it binds to the DNA)
Negative
The regulatory protein is a repressor (has a negative effect on gene expression when it binds to the DNA)