A2 Biology and Physics — Synoptic Application

A2 Biology synoptic questions frequently connect gene expression with homeostasis and physiological control. Gene expression regulation: transcription factors (proteins that bind to DNA regulatory regions — promoters and enhancers) control whether RNA polymerase can transcribe a gene. Steroid hormones (e.g., oestrogen) can pass through cell membranes and bind to intracellular receptors, which then act as transcription factors — a direct link between hormone signalling and gene expression. In contrast, protein hormones (insulin) bind to membrane receptors and use second messenger systems (cAMP) that activate intracellular enzymes without entering the nucleus. Epigenetic regulation: DNA methylation silences genes without changing the DNA sequence; histone acetylation relaxes chromatin structure, allowing transcription. Both are heritable epigenetic changes that explain how identical DNA can produce different cell types. Homeostasis synoptic connections: the insulin-glucagon system for blood glucose regulation connects to: membrane receptor structure (protein hormone receptors), enzyme inhibition (glycogen phosphorylase activation), and feedback loops. Negative feedback maintains set point: deviation from the norm triggers a corrective response that reverses the deviation. Positive feedback amplifies a signal (e.g., oxytocin during labour, action potential depolarisation). The nervous system: myelinated neurones show saltatory conduction — the action potential jumps between Nodes of Ranvier, dramatically increasing conduction velocity (up to 120 m/s) compared to unmyelinated fibres (0.5–2 m/s). This links to multiple sclerosis pathology (demyelination slows or blocks conduction).