Natural selection

Strains of bacteria with antibiotic resistance are common in hospitals.

Use your knowledge of selection to explain why.

Antibiotic resistance alleles are located in plasmids.

There are 11 different types of plasmid inside cells of the bacterium Bacillus thuringiensis.

A scientist determined the:

• length of each type of plasmid in kilobase pairs (kbp)

• mean plasmid copy number (mean number of each type of plasmid per cell).

Table 5 shows the scientist’s results for 5 of these plasmids.

The Spearman rank correlation coefficient (r_s) between plasmid length and mean plasmid copy number is determined using this equation.

where Σd² = the sum of all d² figures n = 5 (the highest rank number)

Use information in Table 5 and the formula to calculate the r_s value for this set of data.

Show your working.

The scientist determined that the r_s value between plasmid length and mean plasmid copy number for all 11 types of plasmid is –0.94

What can you conclude from this r_s value?

The length of the single circular (non-plasmid) DNA molecule in a B. thuringiensis cell is 5400 kbp

The total length of all plasmid DNA molecules in a B. thuringiensis cell is 65% greater than the length of the single circular DNA molecule.

Calculate the total number of kbp in all the plasmid molecules in a B. thuringiensis cell .

Show your working.

A growing population of B. thuringiensis cells includes a:

• germination phase

• growth phase

• dormant phase.

A scientist incubated a culture of B. thuringiensis cells at 20 °C

At intervals, the scientist:

• measured the cloudiness of the culture

• determined the mean plasmid B copy number per cell.

Cloudiness is a measure of the number of cells in a culture.

Figure 11 shows the scientist’s results.

Using information in Figure 11, what can you conclude about binary fission and the

replication of plasmid B in the different phases of a growing population of B. thuringiensis ?

The human disease, malaria, is caused by infection with a single-celled eukaryotic organism.

Figure 2 shows a diagram of Plasmodium vivax, one of the species that can cause malaria.

Other than the Golgi apparatus, name one structure in Figure 2 which shows that P. vivax is a eukaryote.

Describe two functions of the Golgi apparatus in a eukaryotic cell.

P. vivax evolved from a common ancestor in Africa. As humans migrated around the world, new strains of P. vivax evolved.

P. vivax is now extremely rare in Africa but there are several different strains of P. vivax in other parts of the world.

Figure 3 shows a phylogenetic diagram of the evolution of these different strains.

What does Figure 3 suggest is the order of human migration out of Africa?

Tick (✓) one box.

There are an estimated 229 million cases of human malaria worldwide per year. 94% of these cases are found in Africa, but are not caused by P. vivax. P. vivax does cause 61% of the cases of human malaria outside Africa.

Use this information to calculate the number of cases worldwide caused by P. vivax each year.

In Africa today, most of the human population are resistant to malaria caused by P. vivax.

Use your knowledge of natural selection to explain why this resistance is so common in Africa.

Clostridium difficile is a bacterial species that causes disease in humans.

Antibiotic-resistant strains of C. difficile have become a common cause of infection acquired when in hospital.

Explain how the use of antibiotics has led to antibiotic-resistant strains of bacteria becoming a common cause of infection acquired when in hospital.

Scientists suggested that factors, other than antibiotic use, led to the increase in antibiotic-resistant C. difficile infections. One suggested factor is people eating more trehalose in their diet.

Trehalose is a disaccharide formed from two glucose molecules.

Name another disaccharide formed from two glucose molecules.

Scientists investigated the effect of trehalose on the growth rate of C. difficile.

They grew populations of non-resistant and antibiotic-resistant C. difficile on separate agar plates with:

• no carbohydrate added

• trehalose added.

They measured the growth rate of the C. difficile.

Figure 2 shows the scientists’ results.

Describe how the scientists could use aseptic techniques to transfer 0.3 cm³ of C. difficile in liquid culture from a bottle onto an agar plate.

Use Figure 2 to evaluate whether more trehalose in the diet could be a factor in the increased number of antibiotic-resistant C. difficile infections.

Scientists investigated the effect of full sun and shade on the rate of photosynthesis in a species of shade-tolerant tree.

To estimate the rate of photosynthesis, the scientists measured uptake of carbon dioxide by trees in a forest. They measured uptake of carbon dioxide during two parts of the day:

• 08.30 – 09.40 hours

• 11.40 – 13.15 hours.

Figure 8 shows the scientists’ results.

Figure 8

Figure 8 not reproduced here due to third-party copyright restrictions

Calculate the total uptake of carbon dioxide between 11.40 and 13.15 hours in trees exposed to full sun in a forest that is 12 000 m² in area.

Give your answer in standard form. Show your working.

Figure 8 shows there is a small difference in the mean uptake of carbon dioxide between 08.30 and 09.40 hours by trees in full sun and by trees in the shade. When the scientists performed a statistical test on these data, they calculated P > 0.5

State what this P value tells you about this difference.

Explain your answer using the words probability and chance.

In this species of tree, very high light intensities can inhibit the release of electrons from chlorophyll.

Suggest how this could explain the results shown in Figure 8 for 11.40 to 13.15 hours.

Tomato plants grow best in high light intensities. To increase the yield of tomato plants, a farmer uses LED lightbulbs to provide additional light.

The increase in dry mass (D) produced when using additional light can be calculated using this equation.

Where L = light used in photosynthesis F = GPP to NPP conversion factor for tomato plants

Table 5 shows some of these values for LED lightbulbs.

Use the equation and Table 5 to calculate the increase in dry mass produced when using LED lightbulbs.

Give your answer in standard form and give the units.

Mature leaves from slow-growing, shade-tolerant plants produce poisonous chemicals that are a defence against being eaten by herbivores.

Suggest how this benefits slow-growing, shade-tolerant plants.

Scientists measured the concentration of poisonous chemicals produced by

shade-tolerant plant species in six taxa. They compared this with the mean concentration of poisonous chemicals produced by all plants and the phylogenetic relationships between the six taxa.

Figure 9 shows the scientists’ results.

A journalist published the following summary of these results.

‘The more recently a shade-tolerant plant species evolved, the greater the concentration of poisonous chemicals it produces.’

Do the data in Figure 9 support this summary? Justify your answer.

Describe how mRNA is formed by transcription in eukaryotes.

Describe how a polypeptide is formed by translation of mRNA.

Define ‘gene mutation’ and explain how a gene mutation can have:

• no effect on an individual

• a positive effect on an individual.

Write an essay on one of the topics below.

Either

The functions of enzymes and their importance in organisms.

or

The causes and importance of variation and diversity in organisms.

Scientists collected data on 800 000 human births. The data showed the mass of each baby at birth and whether the baby needed to be transferred to a special care unit for very ill babies.

Their results are shown in Figure 5.

Use Figure 5 to explain how human mass at birth is affected by stabilising selection.

The scientists studied 1500 more births. They recorded the mass at birth of each baby and the nature of the KIR gene in the mother’s genome.

Some of their results are shown in Table 2.

The scientists used a statistical test to test the following null hypothesis:

‘The presence of KIR2DS1 in the mother’s genome does not affect the frequency of births above 4500 g’

Tick (✓) one box that gives the name of the statistical test that the scientists should use with the data in Table 2 to test this null hypothesis.

The scientists calculated a P value of 0.03 when testing their null hypothesis.

What can you conclude from this result? Explain your answer.

Figure 6 shows a faulty form of meiosis that can occur in some plants.

Complete Figure 7 to show the chromosome content of the cells that would result from a normal meiotic division of the diploid parent cell shown in Figure 6.

The scientists tested their null hypothesis using the chi-squared statistical test. After 1 cycle their calculated chi-squared value was 350 The critical value at P=0.05 is 3.841

What does this result suggest about the difference between the observed and expected results and what can the scientists therefore conclude?

Use your knowledge of directional selection to explain the results shown in Table 3.

Scientists investigated treatment of a human bladder infection caused by a species of bacterium. This species of bacterium is often resistant to the antibiotics currently used for treatment.

They investigated the use of a new antibiotic to treat the bladder infection. The new antibiotic inhibits the bacterial ATP synthase enzyme.

Place a tick () in the appropriate box next to the equation which represents the reaction catalysed by ATP synthase.

The new antibiotic is safe to use in humans because it does not inhibit the ATP synthase found in human cells.

Suggest why human ATP synthase is not inhibited and bacterial synthase is inhibited.

The scientists tested the new antibiotic on mice with the same bladder infection. They divided these mice into three groups, C, R and A.

Group C was the control (untreated).

Group R was treated with an antibiotic currently used against this bladder infection.

Group A was treated with the new antibiotic.

They removed samples from the bladder of these mice after treatment and estimated the total number of bacteria in the bladder.

Their results are shown in Figure 2.

The antibiotics were given to the mice at a dose of 25 mg kg^–1 per day.

Calculate how much antibiotic would be given to a 30 g mouse each day.

Show your working.

Calculate the percentage difference in actual numbers of bacteria in group A compared with group R. The actual number of bacteria can be calculated from the log₁₀ value by using the 10^× function on a calculator.

Show your working.

The scientists suggested that people newly diagnosed with this bladder infection should be treated with both the current antibiotic and the new antibiotic.

Explain why the scientists made this suggestion.

Use information from Figure 2 and your knowledge of evolution of antibiotic resistance in bacteria in your answer.