[Maximum number: 4]

Stem cells in the bone marrow can be forced into blood vessels in a process called mobilization. Mobilization of stem cells from the bone marrow into the blood vessels represents the basis for modern bone marrow transplantation procedures.

To test the effect of light on the mobilization of stem cells, mice were subjected to a simulated "jet lag" by advancing the light-dark cycle by 12 hours. This was done by subjecting mice to a 24 -hour light period before the results shown in the graph were recorded. The results were compared to the stem cells in control mice under normal conditions of 12 hours of light ( □ ) and 12 hours of darkness ( dark box) and 12 hours of darkness ( dark box ).

(a)

One important chemical in the mobilization of stem cells is a protein, CXCL12, which maintains the stem cells inside the bone marrow. The breakdown of CXCL12 causes the mobilization of stem cells to the blood vessels.

The graph below shows the mobilization of stem cells and the production of mRNA for CXCL12 when the bone marrow is treated with two different chemicals (isoprenaline and clenbuterol).

Explain how the amount of mRNA for CXCL12 gives an indication of the amount of protein CXCL12 produced.

[ 1 ]

(b)

Compare the effect of isoprenaline and clenbuterol with the normal release of stem cells and the production of mRNA for CXCL12.

[ 3 ]

[Maximum number: 2]

The image of a nucleosome was obtained using molecular visualization software.

(a)

Explain how methylation of nucleosomes affects DNA transcription.

[ 2 ]

[Maximum number: 4]

Psoriasis is a disorder characterized by inflammation of the skin. It causes the formation of red patches (lesional skin), which often have flaky, silver-white surfaces (skin plaques).

Some drugs are available to treat psoriasis. During treatment, the changes in psoriasis severity can be measured by taking daily photographs of the affected region(s) of the body.

In one study, five males of a similar age and with similar psoriasis severity were provided with the same dose of a medicinal drug for seven days. The skin maps show the change in psoriasis severity on the back of one of the males during this study.

(a)

One drug commonly used to treat psoriasis is called guselkumab. It reduces inflammation in affected regions of skin.

Tissue samples of lesional skin were taken from people with psoriasis before treatment with guselkumab (Day 0) and after one week of treatment (Day 7). Tissue samples of unaffected skin were also taken from these people before treatment with guselkumab. The numbers of samples were different for the three groups.

Skin cells called fibroblasts were extracted from all tissue samples. The scientists measured the expression of two genes that are known to cause inflammation, WNT5A and IL24. They calculated the percentage of fibroblasts that overexpress (express more than normal) both the WNT5A and IL24 genes. The graph shows the data. The crosses on the graph represent mean percentages.

Statistical significance levels

Statistical significance levels

State the highest percentage of fibroblasts that overexpress WNT5A and IL24 in the lesional tissue sample at Day 0.

[ 1 ]

(b)

Calculate the decrease in the mean percentage of fibroblasts in lesional tissue that overexpress WNT5A and IL24 after treatment with guselkumab.

[ 1 ]

(c)

Explain the conclusions that can be drawn from the data in the graph.

[ 2 ]

[Maximum number: 2]

Diabetes is often associated with the failure of the $\beta$ (beta) cells in the pancreas, but it is unclear what actually causes this failure. FoxO1 is a protein which acts as a transcription factor to regulate the expression of genes involved in cell growth. FoxO1 also regulates increase in number and differentiation in cells such as pancreatic $\beta$ cells.

A study was conducted using mice lacking the gene for FoxO1 in $\beta$ cells (IKO) as well as normal (control) mice. Blood glucose levels after fasting were compared for four groups of mice: young ( 3 months old) male mice, young ( 3 months old) female mice, older females (who have had several pregnancies) and aging males (16-20 months).

(a)

When there are high blood glucose levels, more FoxO1 is found in the nucleus of the cell than in the cytoplasm. Suggest a role of FoxO1 considering this and the data.

[ 2 ]

[Maximum number: 2]

The Chinese soft-shelled turtle, Pelodiscus sinensis, lives in salt water marshes. The turtle can live under water and out of water.

These turtles have fully developed lungs and kidneys, however, many microvilli have been discovered in the mouth of P. sinensis. A study was undertaken to test the hypothesis that oxygen uptake and urea excretion can simultaneously occur in the mouth.

Initial experiments involved collecting nitrogen excretion data from P. sinensis. The turtle urinates both in water and out of water. When in water it allows waste products to be washed out of its mouth. When out of water it regularly dips its head into shallow water to wash its mouth. The table shows the mean rates of ammonia and urea excretion from the mouth and kidney over six days.

(a)

Expression of the urea transporter gene by cells in the turtle's mouth was assessed by measuring mRNA activity. Turtles were kept out of water for 24 hours and then injected with either a salt solution that matched the salt concentration of the turtle, dissolved ammonia or urea, followed by another 24 hours out of water.

[ 2 ]

(i)

Suggest a reason for the greater expression of the gene for the urea transporter after an injection with dissolved ammonia than an injection of urea.

[ 2 ]

[Maximum number: 3]

Arabidopsis is a small flowering plant in the mustard family (Brassicaceae) that is widely used in basic research. It has a short life cycle, flowers quickly producing a large number of seeds and is easy to cultivate. It forms a circle of leaves known as a rosette that lies close to the soil. Flowers form at the end of short stems.

A study was carried out of differences in development between Arabidopsis plants grown in long days ( 16 hours light, 8 hours dark) or short days ( 8 hours light, 16 hours dark). The sixth leaf (L6) to emerge in the rosette of each plant was used in all investigations.

New leaves are initiated by the meristem and go through four stages as they develop.
- Stage 1 (S1) - rapid cell division
- Stage 2 (S2) - cell division has ceased, cell expansion continues
- Stage 3 (S3) - decreasing cell expansion rate
- Stage 4 (S4) - leaf growth complete

The start of each stage of leaf development for plants grown in long days and short days is shown above the first graph.

(a)

Using the data in the bar chart, discuss the evidence for Arabidopsis plants adapting to different daylight regimes by changing the pattern of gene expression.

[ 3 ]

[Maximum number: 3]

Very soon after fertilization, parental epigenetic methylation is reversed in the DNA. Later, tissue-specific epigenetic modifications are made to the embryonic DNA. The graph follows the degree of methylation from different sources during embryonic development.

According to the graph, what are the changes in DNA methylation during embryonic development?

Only the paternal DNA becomes demethylated.

The maternal DNA becomes demethylated first.

The methylation patterns of the parents' DNA are erased before fertilization.

The methylation patterns of both parents are erased after fertilization.

[Maximum number: 1]

What is a proteome?

The genes that code for all the proteins in the ribosome

The group of proteins that generate a proton gradient in mitochondria

The entire genome of a prokaryote

The entire set of proteins expressed by an organism at a certain time

[Maximum number: 1]

Which statement correctly describes genome and proteome?

Only the genome but not the proteome can be analysed using gel electrophoresis.

The genome and the proteome are the same in all tissues in an organism.

In cells of different tissues, the genome is the same while the proteome varies.

Only mutations in the proteome but not in the genome cause any variability.

[Maximum number: 1]

What is the proteome of an individual?

The amino acids unique to an individual making up the proteins in cells

The way in which an individual's polypeptides are folded into a three-dimensional structure

The proteins synthesized as an expression of an individual's genes

All possible combinations of amino acids an individual contains