16.3 Gene Control

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.

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.