Edexcel A Level Biology A Topic 6: Immunity, Infection and Forensics Exam Questions

About Edexcel A Level Biology A Topic 6: Immunity, Infection and Forensics

Topic 6 of Edexcel A Level Biology A (Salters-Nuffield) uses forensic science and infectious disease as its contexts. You learn how DNA profiling uses PCR and gel electrophoresis to identify individuals and establish relationships, and how body temperature, rigor mortis, decomposition, forensic entomology and succession are used to estimate time of death. The topic then compares bacterial and viral structure, the infection cycles of TB and HIV, and non-specific and specific immune responses. You also study active and passive immunity, the evolutionary race between pathogens and hosts, bactericidal and bacteriostatic antibiotics, hospital acquired infections and aseptic technique. It is examined in Paper 1 and Paper 3.

What you need to know

• DNA profiling Non-coding DNA contains short tandem repeats that vary between people. PCR amplifies the DNA, restriction enzymes may cut it, and gel electrophoresis separates fragments by size.
• Time of death Body temperature falls after death, rigor mortis develops and then passes, decomposition stages follow, and insect species (e.g. blowfly larvae) colonise in a predictable succession.
• Bacteria and viruses Bacteria have a cell wall, plasmids and 70S ribosomes; viruses have nucleic acid in a protein capsid. TB infects lung macrophages; HIV infects T helper cells, leading to AIDS.
• Non-specific responses Inflammation, lysozyme action, interferon and phagocytosis by neutrophils and macrophages respond to any pathogen.
• Specific responses Antigen-presenting cells activate T helper cells, which activate B cells and T killer cells. B cells divide to form plasma cells (antibodies) and memory cells. Immunity can be natural or artificial, active or passive.
• Antibiotics and infection control Bactericidal antibiotics kill bacteria, bacteriostatic antibiotics stop them growing. Hospital acquired infections are reduced by hand washing, isolation and careful antibiotic prescribing. Aseptic technique prevents contamination in practical work.

Common mistakes in Topic 6: Immunity, Infection and Forensics

• Saying DNA profiling uses coding DNA; it uses non-coding short tandem repeats.
• Writing that HIV kills the patient directly; HIV destroys T helper cells, so opportunistic infections cause death.
• Confusing bactericidal with bacteriostatic antibiotics.
• Stating that body temperature is useful at any time after death; it is only reliable for about the first 24 hours.
• Describing T killer cells as producing antibodies; only plasma cells (from B cells) secrete antibodies.

Try three Topic 6: Immunity, Infection and Forensics exam-style questions