Regulation of Gene Expression (A Level only) (AQA A Level Biology)

A group of researchers investigated the relationship between the percentage of red meat in the diet and the number of male deaths from colon cancer in 14 different countries.

They plotted the data from each country on the graph in Figure 1 below.

Figure 1

A website reported the results of this investigation using the headline ‘Red meat causes cancer’. Explain why the researchers could not support this view based on the results of their investigation alone.

Explain why the death rate from colon cancer in Figure 1 is given per 100,000 men and not given as the total number of deaths.

Human cells contain certain genes that control their growth and division, one of these genes codes for a protein that stops cell division. There are substances found in red meat that could potentially lead to genetic mutation.

Describe and explain how these substances might increase the chance of getting colon cancer.

Name the type of tumour that causes cancer.

Some rat mothers spend significant amounts of time licking, grooming, and nursing their pups. Others spend much less time doing this, seemingly ignoring their pups. Highly nurtured rat pups usually grow up to be calm adults, while rat pups who receive less attention usually grows up to be anxious. Scientists have discovered that the difference between a calm and an anxious rat is epigenetic, rather than genetic. 

Define what is meant by the term epigenetics.

The rat pups in part (a) behave differently because of a change in a glucocorticoid receptor (GR) gene during development. The gene is highly methylated and inactive at birth. If a rat mother is attentive towards her pups, the pups’ GR gene demethylates, making the gene increasingly active. These pups will be more relaxed in response to stress. The pups that were not given attention, and do not express the GR gene, became anxious more easily. 

What is methylation and how does it make genes inactive?

Name two environmental factors that can affect methylation of DNA in humans.

In certain cases, increased methylation could lead to cancer. Explain how.

RNA interference (RNAi) has been shown to be effective against the hepatitis B virus (HBV). Scientists used small interfering RNAs (siRNAs) to silence one of the genes involved in the replication of HBV.

What are siRNAs?

Describe how siRNAs can silence the genes involved in the replication of HBV.

In order to function, an siRNA must be taken up by a protein complex in the cytoplasm and its two RNA strands separated. The reaction between the protein complex and the siRNA it takes up involves the hydrolysis of ATP.

Suggest how the hydrolysis of ATP helps this reaction to occur.

Table 1 below shows how the bases from the hepatitis B virus gene being silenced in part (a) match up with the bases in the corresponding mRNA and siRNA molecules.

Complete Table 1.

Table 1

A patient with a spinal cord injury was healed using stem cell therapy by the following process:

1. Stem cells collected from patient’s own tissues.
2. Stem cells cultured in laboratory for one month to increase their numbers.
3. Stem cells injected into injury site of patient.
4. Stem cells develop into nerve cells and replace damaged nerve tissue.

Give two general properties of stem cells.

Explain how the stem cells in part (a) developed into nerve cells.

Explain why the stem cells used in part (a) were taken from the patient and not from a stem cell donor.

Although it would also be possible to use embryonic stem cells from embryos created by IVF to treat the patient in part (a), this is a controversial method.

Explain why.

There are two main forms of tumours. One of these is benign tumours.

Describe two ways in which both types of tumour may cause harm to the body.

Melanoma, also known as malignant melanoma, is a type of skin cancer that develops from pigment-producing cells in the skin known as melanocytes.

Some people who spend too much time sunbathing develop melanoma. Explain why.

A type of gene, known as tumour suppressor genes, control cell division.

Distinguish between totipotent, multipotent and pluripotent cells

Pluripotent stem cells extracted from the early embryo can be used to treat a variety of diseases including muscular dystrophy, osteoporosis and macular degeneration. However, in 2006, scientists in Japan found that adult stem cells could become induced pluripotent stem cells. Some believe that the use of induced pluripotent stem cells is a more appropriate method for treatment of patients than pluripotent stem cells.

Evaluate the use of pluripotent and induced pluripotent stem cells in treatment of life changing diseases such as those mentioned above.

In plants, totipotent cells are located in the growing shoot and root tips. Scientists used a plant culture technique to investigate the effect of the plant age on the level of totipotency.  Young plants were divided into five groups with 5 in each group. Each group was grown for a specific time period:

Group A was grown for 5 days

Group B was grown for 10 days

Group C was grown for 15 days

Group D was grown for 20 days

Group E was grown for 25 days

A plant tissue culture technique was used, the method was carried out as follows:

1. Cut off a 5 mm shoot tip from each of the 5 plants in groups A-E, this is the explant.
2. Place each explant on a nutrient agar medium inoculated with growth hormones and glucose.
3. Cover the explant samples to prevent contamination and grow for 2 weeks in a suitable environment.   

The results are shown in Table 1.

Table 1

The scientists concluded that explants grown from older plants exhibited less totipotency than those from younger plants.

Evaluate this statement.

Describe the observations that would be made by the scientists to determine the percentage totipotency of the plants in this investigation.

Explain how transcription factors stimulate the expression of a gene.

Figure 1 shows the mechanism by which oestrogen initiates gene expression.

 Figure 1

Compare and contrast the mechanism of action shown in Figure 1 with the process of non-competitive enzyme inhibition.

Plant cells use transcription factors in a similar way to animal cells. One transcription factor found in plant cells is called PIF. When activated, PIF binds to the promoter region which triggers transcription of the amylase gene. PIF is inhibited by the presence of a repressor protein called DELLA. The plant hormone gibberellin, stimulates the breakdown of DELLA to activate PIF. This process is shown in Figure 2.

 Figure 2

Using the information in Figure 2, suggest how the application of gibberellin hormone could facilitate germination of seeds.

Totipotent cells from one organism are genetically identical, however not all of these genes are ultimately expressed.

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

In the 1800s Jean Baptiste Lamarck published his theory of inheritance of acquired characteristics which suggested that an organism can inherit characteristics that had been acquired during its parents lifetime. Lamarck’s theory has been much debated in the scientific world over the years, but more recently, advances in the understanding of epigenetics has led scientists to reconsider Lamarck’s ideas.

Outline how the field of epigenetics supports the theory of inheritance of acquired characteristics.

Explain how changes to DNA methylation and acetylation can activate gene expression.

Epigenetic therapy could be used to treat diseases resulting from epigenetic changes by reversing changes to the acetylation of histones and methylation of DNA.

Suggest how knowledge of epigenetics could lead to the development of more effective treatments for diseases such as cancer.

Another method to treat diseases is to silence genes after they have already been transcribed.

Describe how gene expression can be inhibited at the point of translation.

State and explain three reasons why malignant tumours are considered more harmful than benign tumours.

Oestrogen is a hormone which is thought to play a role in the increased risk of breast cancer in post-menopausal women. Oestrogen only affects target cells, mainly those in the breast tissue.

Explain how an increase in oestrogen levels may lead to the development of breast cancer.

Tamoxifen is a drug that has been used to help prevent and treat breast cancer by blocking the effects of oestrogen in the breast tissue. Scientists developing treatment for breast cancer completed a trial on mice to compare an alternative drug called Anastrozole with Tamoxifen.

Anastrazole works by inhibiting an enzyme called aromatase which is responsible for the production of oestrogen in post-menopausal women.

The scientists first removed the ovaries from the mice, then they injected them with tumour causing chemicals found in tobacco twice a week for 3 weeks. After week 3, the mice were given the drugs for a further 10 weeks.

The mice were then randomly allocated to 1 of 3 groups, each with 20 mice:

• Group A was given a placebo
• Group B was given the drug anastrozole
• Group C was given the drug tamoxifen

The scientists removed the ovaries from the mice and injected them daily with Aromatase. Explain why these stages were necessary.

The results of the investigation can be seen in Figure 1.

Figure 1

The scientists concluded that Anastrozole was the most effective at controlling tumour growth due to oestrogen.

Use the data shown in Figure 1 to evaluate this claim.

Ras is a proto-oncogene which, when switched on, is involved in stimulating cell growth and division.

Describe and explain how a mutation in the Ras gene could lead to the production of a cancerous tumour.

Suggest why surgical removal of a benign tumour is usually enough to prevent the tumour reoccuring without a need for chemotherapy or radiotherapy.

Haemoglobin is a globular protein made up of 4 polypeptide chains each known as a globulin. There are three types of globulin protein, alpha, beta and gamma globulin, each coded for by a different gene. The globulin present is determined by which gene is expressed and the expression changes throughout the first 90 weeks after conception, as shown in Figure 1. It is the specific globulin combination in fetal haemoglobin which means it has a higher affinity for oxygen compared to adult haemoglobin.

Describe the changes in gene expression throughout the period of time represented in Figure 1 and suggest a reason for those changes with reference to oxygen dissociation.

 Figure 1

Using the data in Figure 1 calculate the ratio of beta to gamma globulin at 24 weeks after conception