Continuity and Change

The lac operon is the model system for understanding how genes get switched on and off. It is inducible, meaning its default state is OFF and lactose (via allolactose) triggers it ON by inactivating the repressor bound to the operator. A second layer, CAP bound to cAMP, gives positive control that boosts transcription only when glucose is scarce — this is why lactose alone produces a much stronger response than lactose plus glucose. In eukaryotes, transcription factors and alternative splicing add further layers, letting one gene generate multiple distinct proteins.

Homeostasis keeps internal conditions stable using a receptor to detect deviation from a set point, a control centre to process it, and an effector to correct it. Negative feedback dominates — it counteracts and reduces the original deviation, restoring the set point — while positive feedback amplifies change and is rare and short-lived, as in childbirth. Graphs of homeostatic variables typically oscillate around the set point, and examiners often ask you to read amplitude or lag time directly from such data.

Mitosis produces two genetically identical diploid daughter cells for growth, repair and asexual reproduction, while meiosis involves two divisions to produce four genetically unique haploid gametes. Variation in meiosis comes from two named sources you must cite explicitly: crossing over in prophase I and independent assortment at metaphase I. Checkpoints at G1, G2 and metaphase halt the cycle if DNA damage or spindle errors are detected, and failure here is linked to cancer.

Asexual reproduction (one parent, mitosis-based) produces genetically identical clones fast but with no variation, while sexual reproduction (meiosis + fertilisation) is slower but supplies the genetic variation natural selection needs. This choice of strategy connects directly to population growth: populations grow exponentially while resources are abundant, then slow as density-dependent factors bite, levelling off at carrying capacity $K$. Overshoot beyond $K$ triggers a population crash if resources can't regenerate fast enough — the sustainability failure mode examiners like to test.

Monohybrid and dihybrid crosses use Punnett squares to predict ratios, with unlinked dihybrid crosses giving the classic 9:3:3:1 ratio. Linked genes on the same chromosome don't assort independently, skewing ratios away from 9:3:3:1, with recombinants arising only from crossing over. At HL, the chi-squared test compares observed offspring ratios to the expected Mendelian ratio to judge whether a deviation is due to chance or a real biological effect such as linkage.