Gene Control (CIE A Level Biology)

The enzyme lactase is an example of an inducible enzyme.

State why lactase can be described as an inducible enzyme.

[1]

Explain the importance of inducible enzymes.

[1]

In certain species of bacteria, such as Escherichia coli, transcription of the structural genes of the lac operon enables the production of lactase. This process only occurs in the presence of lactose, and is also affected by the presence or absence of glucose. 

In the absence of glucose, E.coli cells produce the molecule cAMP. cAMP binds to a catabolite activator protein (CAP), activating it and allowing it to bind to a region of the lac operon known as the CAP site. The presence of the CAP protein makes it easier for RNA polymerase to bind and greatly increases the transcription rate of the structural genes. RNA polymerase does not bind well to the promoter in the absence of CAP.

Fig. 1 illustrates the effect of CAP on the transcription rate of the structural genes.

Fig. 1

Explain why the structural genes are transcribed in the presence of lactose.

[3]

Suggest how the transcription of the structural genes would be affected if E.coli was grown in a medium that contained both glucose and lactose.

[3]

In the absence of lactose E. coli will not produce lactase.

State the role of the operator region in preventing the production of lactase.

Gibberellin is a plant hormone that plays an important role in controlling seed germination.

Describe how gibberellin influences gene expression in a germinating seed.

Scientists will often analyse the RNA that is produced when studying gene expression in cells.

Explain the importance of RNA analysis in the study of gene expression in an organism.

Suggest why it is important for gibberellin to control the germination of seeds.

Explain genetic control of protein production in a prokaryote using the lac operon.

The lac operon is a section of DNA present in the genome of Escherichia coli. The structural genes of the lac operon are only fully expressed when the bacteria are exposed to high lactose concentrations.

Fig.1 is a diagram showing the lac operon and a nearby region of the E. coli genome.

Fig. 1

(i)

Fig. 1 shows how the lac operon consists of structural genes and regulatory sequences.

Use Fig. 1 to identify two structural genes.

Complete Table 1 to name each structural gene and its product.

(ii)

Gene I is transcribed all the time to produce its protein. This is constitutive expression.

Explain why some genes show constitutive expression.

(iii)

Describe the effect of the product of gene I on the functioning of the lac operon.

If E. coli is put into a nutrient medium containing lactose, some new enzymes are synthesised. These are described as inducible enzymes.

(i)

Explain what is meant by an inducible enzyme.

(ii)

The structural genes of the lac operon are not expressed when lactose is absent.

Suggest one reason why this is beneficial to E. coli.

The tryptophan (trp) operon controls the synthesis of the amino acid tryptophan in many species of bacteria.

The synthesis of tryptophan occurs in five different steps, each requiring a different enzyme coded for by structural genes trpA, trpB, trpC, trpD, and trpE. The regulatory region of the trp operon includes the promoter (P), the operator (O), and a regulatory gene, trpR, which codes for a trp repressor protein.

The activation or repression of the trp operon is determined by the tryptophan levels in the environment of the bacteria. 

Fig. 1 shows the structure of the trp operon in the presence of high tryptophan levels.

Fig. 1

State the difference between a structural and a regulatory gene.

Using the information in Fig. 1, explain the effect of high tryptophan levels on the trp operon.

The enzymes produced by some structural genes can be classified as either inducible or repressible.

State whether the enzymes produced by the structural genes of the trp operon are inducible or repressible.

[1]

[1]

The trp operon can be regulated by another mechanism called attenuation; this involves a leader transcript, trpL. When tryptophan levels are high attenuation causes RNA polymerase to end the transcription of the structural genes prematurely.

Suggest how attenuation contributes to the regulation of tryptophan in bacteria.

The hormone oestrogen can diffuse into human body cells. Once inside a cell, oestrogen diffuses through the nuclear pores into the nucleus where it attaches to a transcription factor called oestrogen receptor (ER). This allows ER to combine with a cofactor, which in turn allows the cofactor to bind to the promoter region of a gene. This enables RNA polymerase to bind to the promoter region.

In certain types of breast cancer ER is no longer regulated by control mechanisms and may lead to the development of oestrogen induced tumours. Tamoxifen was one of the first effective treatments for these types of cancers and is still used with great success.

Fig. 1 compares the action of oestrogen and tamoxifen.

Fig. 1

Explain what is meant by the term transcription.

With reference to Fig. 1, suggest how tamoxifen is an effective treatment for breast cancer.

While ER enables RNA polymerase to bind to the promoter region, some transcription factors can prevent transcription from taking place.

Suggest two ways in which transcription factors might prevent transcription.

Transcription factors control the transcription of genes by binding to specific regions of DNA.

State the importance of controlling gene transcription for an organism.

Explain how transcription factors stimulate the expression of a gene.

Fig. 1 below shows the mechanism by which oestrogen initiates gene expression.

Fig. 1

Compare and contrast the mechanism of action shown in the diagram with the process of non-competitive enzyme inhibition.

Explain why oestrogen is unable to bind to the progesterone receptor in Fig. 1.

Totipotent cells from one organism are genetically identical, however, not all of these genes are ultimately expressed. This is known as differential gene expression.

Explain how differential gene expression forms an organism with specialised cells.