1. Explain the process of DNA replication. (8 marks)

DNA replication is semi-conservative. Helicase unwinds the double helix, DNA polymerase adds complementary nucleotides to each template strand. Leading strand is synthesized continuously while lagging strand forms Okazaki fragments. This ensures genetic information is accurately passed to daughter cells during mitosis.

2. Describe the role of natural selection in evolution. (6 marks)

Natural selection acts on phenotypic variation. Individuals with advantageous traits (e.g. antibiotic resistance in bacteria) survive and reproduce more successfully, increasing allele frequency in the population over generations. This leads to adaptation and speciation.

3. Compare and contrast aerobic and anaerobic respiration. (7 marks)

Aerobic respiration produces 38 ATP via glycolysis, Krebs cycle and electron transport chain using oxygen as final acceptor. Anaerobic respiration (fermentation) yields only 2 ATP, producing lactate or ethanol. Both begin with glycolysis in the cytoplasm.

4. Explain how the kidney maintains water balance. (6 marks)

ADH (vasopressin) increases aquaporin insertion in collecting duct cells, allowing more water reabsorption when plasma osmolarity is high. This osmoregulation prevents dehydration and maintains blood volume.

5. Discuss the impact of eutrophication on aquatic ecosystems. (5 marks)

Excess nutrients (nitrates/phosphates) cause algal bloom. Algae block light, leading to death of submerged plants. Decomposition by bacteria depletes dissolved oxygen, creating dead zones and reducing biodiversity.