(a) Chlamydia is a sexually transmitted infection (STI) caused by a bacterium.
Fig. 2.1 shows the number of chlamydia cases in a country in 2018.
(a)(i) Calculate the percentage decrease in chlamydia cases between age groups 20–24 and 25–29.
Give your answer to two significant figures.
(a)(ii) Describe how the spread of STIs, such as chlamydia, can be controlled.
(b) Chlamydia can damage the reproductive system.
(b)(i) State the part of the female reproductive system that produces oestrogen.
(b)(ii) Describe the role of oestrogen at puberty.
(c) Complete the sentences about drug resistance and genetic variation:
Bacteria can be killed by drugs called …………………………………. . The development of resistant strains is an example of …………………………………. selection.
Resistant strains have gene mutations caused by random changes in the …………………………………. of DNA bases, forming new …………………………………. . Mutation rates increase due to …………………………………. radiation and chemicals.
In animals and plants, another source of genetic variation is nuclear division called …………………………………. .
▶️ Answer/Explanation
(a)(i) 59% decrease (to 2 s.f.).
Calculation: \(\frac{\text{Change}}{\text{Original}} \times 100 = \frac{110-45}{110} \times 100 \approx 59\%\).
(a)(ii) Control methods include:
• Condom use (barrier method).
• Regular screening/testing and contact tracing.
• Education on transmission/prevention.
• Antibiotic treatment for infected individuals.
(b)(i) Ovary produces oestrogen.
(b)(ii) Oestrogen at puberty:
• Triggers secondary sexual characteristics (e.g., breast growth).
• Initiates the menstrual cycle.
(c) Completed sentences:
• Antibiotics; natural selection.
• Sequence; alleles; ionising.
• Meiosis (source of genetic variation).