Exam Notes Sample: Cell Biology and Genetics Revision Notes

Model Answer

1. Cell Structure and Organisation

• All living organisms are built from cells, the basic unit of life; cells are either prokaryotic (no true nucleus, e.g. bacteria) or eukaryotic (membrane-bound nucleus and organelles, e.g. animal, plant and fungal cells).
• Prokaryotic cells are smaller (roughly 1–5 µm), lack membrane-bound organelles, and carry DNA as a single circular chromosome plus small circular plasmids that can carry, for example, antibiotic-resistance genes.
• Eukaryotic cells (roughly 10–100 µm) compartmentalise function into membrane-bound organelles, which allows specialised, efficient biochemistry and separates incompatible reactions (e.g. DNA replication from protein synthesis).
• Plant cells differ from animal cells in three key respects: a rigid cellulose cell wall outside the plasma membrane, a large permanent central vacuole that maintains turgor pressure, and chloroplasts that carry out photosynthesis.
• The cytoskeleton (microfilaments, microtubules, intermediate filaments) gives cells shape, enables organelle movement, and forms the spindle apparatus used during cell division.
• Cell size is limited by the surface area to volume ratio: as a cell grows, volume increases faster than surface area, eventually limiting the rate of diffusion needed to supply the cell and remove waste.

2. Cell Membrane and Transport

• The plasma membrane follows the fluid mosaic model: a phospholipid bilayer with embedded proteins, cholesterol and glycoproteins, all able to move laterally within the layer, giving the membrane its selective, dynamic character.
• Diffusion: the passive net movement of molecules from a region of high concentration to low concentration, down a concentration gradient, requiring no metabolic energy.
• Facilitated diffusion: passive movement of large or charged molecules (e.g. glucose, ions) through specific channel or carrier proteins, still moving down the gradient but too large or polar to cross the bilayer unaided.
• Osmosis: a special case of diffusion — the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
• Active transport: movement of molecules or ions against a concentration gradient, using ATP and carrier proteins, e.g. the sodium-potassium pump that maintains resting membrane potential in neurones.
• Endocytosis and exocytosis: bulk transport of large particles, fluid or macromolecules via vesicle formation (endocytosis) or vesicle fusion with the membrane (exocytosis), used for example in hormone secretion.
• Cells placed in hypertonic, hypotonic or isotonic solutions will lose water, gain water, or remain unchanged respectively — a common practical/data-response question in this area.

3. Cell Division: Mitosis and Meiosis

• The cell cycle comprises interphase (G1 — growth; S — DNA replication; G2 — further growth and preparation) followed by the division phase (mitosis or meiosis) and cytokinesis (division of the cytoplasm).
• Mitosis produces two genetically identical diploid daughter cells for growth, repair and asexual reproduction, in four named stages: prophase (chromosomes condense, spindle forms), metaphase (chromosomes align at the equator), anaphase (sister chromatids pulled to opposite poles), and telophase (nuclear envelopes reform).
• Meiosis is a two-round division producing four genetically distinct haploid gametes, introducing variation through crossing over between homologous chromosomes (prophase I) and independent assortment of chromosomes (metaphase I).
• Errors in chromosome segregation (non-disjunction) during meiosis can cause aneuploidy, e.g. an extra copy of chromosome 21 resulting in Down syndrome.
• The cell cycle checkpoints (G1, G2 and metaphase checkpoints) ensure damaged or incomplete DNA is not passed to daughter cells; loss of checkpoint control is strongly linked to cancer.

4. DNA Structure and Replication

• DNA is a double helix of two antiparallel strands joined by hydrogen bonds between complementary base pairs: adenine–thymine (2 bonds) and cytosine–guanine (3 bonds), which is why GC-rich DNA is more thermally stable.
• Each strand has a sugar-phosphate backbone (deoxyribose plus phosphate groups) running 5′ to 3′, with the nitrogenous bases projecting inward to pair across the two strands.
• Replication is semi-conservative: each new double helix retains one original (template) strand and one newly synthesised strand, as demonstrated classically by the Meselson-Stahl experiment.
• Key enzymes: helicase unwinds and separates the double helix; DNA polymerase adds complementary nucleotides in the 5′ to 3′ direction only; ligase joins the short Okazaki fragments produced on the lagging strand.
• Because polymerase only works 5′ to 3′, the leading strand is synthesised continuously while the lagging strand is synthesised discontinuously — a distinction commonly tested in written papers.

5. Genetics: Mendelian Inheritance

• A gene is a length of DNA coding for a specific protein or characteristic; different versions of a gene are called alleles, which may be dominant or recessive with respect to one another.
• Genotype is the allele combination an organism carries (e.g. Bb); phenotype is the observable characteristic that results from the genotype and, often, the environment.
• A monohybrid cross tracks the inheritance of one gene; a heterozygous x heterozygous cross (Bb x Bb) produces a 3:1 phenotypic ratio in the offspring (1 BB : 2 Bb : 1 bb genotypically).
• A dihybrid cross tracks two genes on different chromosomes simultaneously; independent assortment during meiosis gives a characteristic 9:3:3:1 phenotypic ratio in the offspring.
• Codominance occurs when both alleles are fully and separately expressed in the heterozygote (e.g. the AB blood group); incomplete dominance instead produces a blended, intermediate phenotype.
• Sex-linked inheritance: genes carried on the X chromosome (e.g. haemophilia, red-green colour blindness) show characteristic inheritance patterns because males are hemizygous (only one X).

6. Gene Expression: Transcription and Translation

• Transcription (in the nucleus): RNA polymerase binds a promoter, unwinds the DNA, and reads the template strand 3′ to 5′, building a complementary mRNA strand in which uracil replaces thymine.
• Pre-mRNA is processed before leaving the nucleus: introns (non-coding sequences) are spliced out, exons (coding sequences) are joined together, and a protective 5′ cap and poly-A tail are added.
• Translation (at the ribosome): mRNA codons (triplets of bases) are read in the 5′ to 3′ direction; tRNA molecules, each with a matching anticodon, deliver the correct amino acid, which is joined by a peptide bond to the growing polypeptide chain.
• The genetic code is degenerate (several different codons can code for the same amino acid, reducing the impact of some mutations) and near-universal across almost all organisms.
• Translation begins at a start codon (AUG, coding for methionine) and ends at one of three stop codons (UAA, UAG, UGA), which do not themselves code for an amino acid.

7. Mutation and Genetic Variation

• A gene mutation is a random change in the base sequence of DNA; mutations can be spontaneous (replication errors) or induced by mutagens such as UV light, ionising radiation or certain chemicals.
• Point mutations: substitution (one base swapped for another), insertion or deletion of a single base. Insertions and deletions cause a frameshift, altering every codon downstream and usually producing a non-functional protein.
• A silent mutation does not change the amino acid produced (due to code degeneracy); a missense mutation changes one amino acid (e.g. sickle-cell anaemia); a nonsense mutation creates a premature stop codon, truncating the protein.
• Genetic variation also arises from crossing over, independent assortment and random fertilisation during sexual reproduction, providing the raw material on which natural selection acts.

8. Enzymes in Cell Biology

• Enzymes are globular proteins that act as biological catalysts, lowering the activation energy of a reaction without being consumed, via an active site that is complementary in shape and charge to a specific substrate.
• The induced-fit model describes the active site changing shape slightly as the substrate binds, improving the fit and catalytic efficiency compared with a rigid lock-and-key.
• Enzyme activity is affected by temperature (rate rises to an optimum, then falls sharply as the enzyme denatures), pH (each enzyme has an optimum pH outside which it denatures), and substrate or enzyme concentration.
• Competitive inhibitors resemble the substrate and bind the active site directly, so their effect can be reduced by raising substrate concentration; non-competitive inhibitors bind elsewhere and change the active site’s shape, and their effect cannot be overcome in the same way.

Exam Tips

• Learn the organelle table cold — structure-to-function questions are near-guaranteed in short-answer sections.
• Practise drawing and labelling a Punnett square under timed conditions; examiners award marks for correct ratios, not just the final phenotype.
• Always state whether a process is passive (diffusion, osmosis, facilitated diffusion) or active (active transport, endo/exocytosis) — this distinction is frequently examined.
• When comparing mitosis and meiosis, lead with the number of divisions and the genetic outcome; these are the two facts most commonly asked for.
• Use precise terminology: “gene” and “allele” are not interchangeable, and examiners penalise loose use of “DNA” versus “chromosome” versus “gene”.
• For mutation questions, name the mutation type (silent, missense, nonsense, frameshift) before explaining its effect on the protein — naming the type is usually a mark on its own.