Common Misconceptions
in A Level Biology

The central dogma of molecular biology describes a two-step process: DNA → mRNA (transcription), then mRNA → protein (translation). A gene is a sequence of DNA base pairs that, when transcribed and translated, produces a specific polypeptide chain. That polypeptide may be a structural protein, an enzyme, a receptor, a signalling molecule, or a hormone — and it is this protein that then influences phenotype, often indirectly and through multiple biochemical steps.

Consider eye colour. The genes involved do not "make" a colour — they code for enzymes in the melanin synthesis pathway. One enzyme converts a precursor into melanin; the amount and type of melanin deposited in the iris determines the colour perceived. Disrupt one enzyme and less melanin is produced, producing a lighter colour. The gene is several biochemical steps removed from the trait itself. A student who writes "the gene for blue eyes makes the eyes blue" is describing a process that simply does not exist.

This distinction becomes especially important in exam questions on gene expression, the effects of mutations, and why the same genotype can produce different phenotypes in different environments. Gene–environment interaction only makes sense if you understand that the gene is not the trait — it is the instruction for making a protein, which then operates within a particular environmental context.

The terms dominant and recessive describe which allele's phenotypic effect is expressed when two different alleles are present in the same individual. They say nothing whatsoever about how common the allele is in the population. A dominant allele can be extraordinarily rare — and a recessive allele can be the most frequent version in a population.

Huntington's disease provides the clearest demonstration. It is caused by a dominant allele — yet it is very rare in the population. Conversely, many recessive alleles (including those for certain blood types) are highly prevalent. Allele frequency is governed by evolutionary forces — natural selection, genetic drift, mutation pressure, and migration — not by dominance relationships. Dominance is about expression in individuals; frequency is about distribution in populations. These are entirely separate questions.

The Hardy-Weinberg equilibrium provides the mathematical framework for calculating allele and genotype frequencies in a population. It demonstrates that allele frequencies remain constant across generations provided: no mutation occurs, the population is large enough that chance does not matter, mating is random, there is no migration, and there is no natural selection. When any of these conditions is violated, allele frequencies shift — regardless of which allele happens to be dominant.

A mutation is any change in the base sequence of DNA. Most mutations occur in non-coding regions of the genome — introns, regulatory sequences, or the large stretches between genes — and have no effect on protein structure whatsoever. They are neutral. Even point mutations within a coding region may be silent: the degeneracy of the genetic code means that multiple codons can specify the same amino acid, so the protein produced may be completely unchanged despite the altered base sequence.

Some mutations are beneficial. The mutation conferring partial resistance to malaria in carriers of the sickle-cell allele is the most cited example in biology — in a malaria-endemic environment, the heterozygous condition is selectively advantageous, and this is why the allele persists at relatively high frequencies in affected populations. Antibiotic resistance in bacteria arises from mutations that are highly beneficial to the bacterium. The entire diversity of life on Earth traces back to accumulated mutations over billions of years of evolution — without them, adaptation is impossible.

Whether a mutation is harmful, neutral, or beneficial also depends critically on the environment. A mutation that reduces melanin production is neutral in a low-UV environment and harmful in a high-UV environment. Evolution does not produce mutations that are inherently good or bad — only mutations that may or may not be favoured by the current selective pressure.

An enzyme works by binding its substrate at the active site — a region whose three-dimensional shape is complementary to the substrate molecule. This binding forms the enzyme-substrate complex, lowers the activation energy of the reaction, and allows the conversion of substrate to product. Once the product is released, the enzyme's active site is free and structurally unchanged. It can bind a new substrate molecule immediately. The enzyme is not consumed — this is the defining property of a catalyst.

This is why enzyme concentration affects reaction rate — more enzyme molecules means more active sites available simultaneously — while the enzyme itself does not appear in the balanced equation for the reaction it catalyses. Enzymes are permanently altered by denaturation — but this is destruction of tertiary structure, not consumption. A denatured enzyme no longer catalyses the reaction because its active site has changed shape irreversibly, not because it was incorporated into the product.

This misconception frames photosynthesis as existing to serve animals — which inverts the biological reality. Plants photosynthesise to meet their own energy needs. The fundamental purpose is the synthesis of glucose, which provides the chemical energy (via cellular respiration) and the carbon skeleton for all of the plant's organic molecules — its cellulose, its amino acids, its lipids.

The light-dependent reactions occur in the thylakoid membranes. Photons are absorbed by photosynthetic pigments and their energy is used to split water molecules — a process called photolysis: 2H₂O → 4H⁺ + 4e⁻ + O₂. The oxygen released here is genuinely a waste product. The protons and electrons released from water are what the plant actually needs — they are used to reduce NADP⁺ and to generate a proton gradient that drives ATP synthesis.

These products — NADPH and ATP — then drive the Calvin cycle in the stroma, where CO₂ is fixed via RuBisCO into G3P and ultimately into glucose. The oxygen in the atmosphere today is an evolutionary by-product of billions of years of photosynthesis — of enormous consequence for life on Earth — but from the individual plant's metabolic perspective, it is simply what is left over after extracting the hydrogen from water.

The fluid mosaic model, proposed by Singer and Nicolson in 1972, describes the membrane as a phospholipid bilayer in which proteins are embedded like tiles in a mosaic — but unlike tiles in a wall, everything moves. The word fluid is load-bearing: individual phospholipid molecules rotate on their axis and move laterally within their leaflet at physiological temperatures. The membrane is not a fixed structure — it is a two-dimensional liquid.

Embedded proteins are not stationary either. Integral proteins that span the bilayer can drift laterally. Channel proteins open and close. Carrier proteins change conformation to transport molecules. Receptor proteins bind signalling molecules and trigger conformational changes downstream. The entire composition of the membrane changes continuously as vesicles fuse with it during exocytosis and bud off from it during endocytosis.

A common secondary misconception concerns cholesterol. Students often write that cholesterol "makes the membrane more fluid." This is imprecise. At high temperatures, cholesterol fits between phospholipid tails and restricts their movement, reducing fluidity. At low temperatures, it prevents the tails from packing too closely, preventing solidification. Cholesterol acts as a fluidity buffer — it moderates the membrane's response to temperature in both directions.