state that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule

Cambridge International AS & A Level Biology – Complete Syllabus Notes

Learning Objective

State that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule.

1. Syllabus Overview (AS + A‑Level)

The table summarises each syllabus block, the core focus, and a practical or mathematical skill that is commonly examined.

BlockKey FocusPractical / Math Skill
1. Cell StructureProkaryotic vs eukaryotic organisation; organelle functions; cell‑size & SA‑to‑V relationships.Microscopy – measuring magnification, drawing labelled diagrams; SA‑to‑V calculations.
2. Biological MoleculesCarbohydrates, lipids, proteins, nucleic acids, water properties; structure‑function relationships.Qualitative tests (iodine, Biuret, Sudan IV); calculation of % composition.
3. EnzymesEnzyme action, Michaelis‑Menten kinetics, factors affecting activity, inhibition types.Plotting V vs [S] to obtain Vmax and Km; competitive vs non‑competitive inhibition analysis.
4. Cell Membranes & TransportFluid‑mosaic model, passive & active transport, bulk transport, role of surface‑area‑to‑volume.Osmosis experiments, diffusion rate measurements, SA‑to‑V ratio calculations.
5. Cell CycleInter‑phase, mitosis (prophase → telophase), meiosis, regulation (checkpoints, cyclins), tumour formation.Staining chromosomes, constructing cell‑cycle flowcharts, calculating mitotic index.
6. Nucleic Acids & Protein SynthesisDNA/RNA structure, replication, transcription, RNA processing, translation, gene regulation, the genetic code.DNA extraction, gel electrophoresis, PCR basics, in‑vitro translation assay.
7. Transport in PlantsXylem & phloem structure, cohesion‑tension theory, apoplast vs symplast pathways, transpiration stream.Measuring transpiration rate, dye‑tracking experiments.
8. Transport in AnimalsHeart anatomy, blood vessel types, blood components, oxygen‑carrying mechanisms, capillary exchange.Pulse‑rate measurements, calculating cardiac output, blood‑smear identification.
9. Gas ExchangeLung anatomy, alveolar surface area, diffusion of O₂ & CO₂, SA‑to‑V optimisation.Calculating diffusion distance, measuring respiratory volumes.
10. Infectious DiseasePathogen types (bacteria, viruses, fungi, protozoa), transmission routes, antibiotic resistance, vaccines.Culture techniques, Kirby‑Bauer antibiotic sensitivity test.
11. ImmunityInnate vs adaptive immunity, antibodies, cell‑mediated response, vaccination, memory cells.ELISA principle, interpreting titre curves, skin‑test analysis.
12. Energy Transfer & RespirationGlycolysis, link reaction, Krebs cycle, oxidative phosphorylation, anaerobic pathways, RQ.Measuring CO₂ output, calculating ATP yield, RQ calculations.
13. PhotosynthesisChloroplast structure, light‑dependent & light‑independent reactions, factors affecting rate.O₂ evolution assay, chlorophyll fluorescence, calculating photosynthetic quotient.

2. Core Concepts for Each Syllabus Block

2.1 Cell Structure

  • Prokaryotes: No nucleus, circular DNA, ribosomes (70 S), cell wall (peptidoglycan).

  • Eukaryotes: Nucleus, linear chromosomes, membrane‑bound organelles (mitochondria, ER, Golgi, lysosomes, chloroplasts).

  • Surface‑area‑to‑volume ratio: SA/V = 6/d for a cube; explains why cells are small.

2.2 Biological Molecules

  • Carbohydrates: Monosaccharides (glucose), disaccharides (sucrose), polysaccharides (starch, glycogen). Energy source & storage.

  • Lipids: Fatty acids, triglycerides, phospholipids, steroids. Hydrophobic barrier, energy‑dense storage.

  • Proteins: Primary → quaternary structure; functions include enzymes, transport, structural roles.

  • Nucleic Acids: DNA (deoxyribose, thymine) vs RNA (ribose, uracil). Store & transmit genetic information.

  • Water: Cohesion, adhesion, high specific heat, universal solvent.

2.3 Enzymes

  • Active site specificity; induced‑fit model.

  • Factors: temperature, pH, substrate concentration, enzyme concentration, inhibitors.

  • Inhibition: Competitive (binds active site), non‑competitive (binds elsewhere), uncompetitive (binds enzyme‑substrate complex).

  • Michaelis‑Menten equation: v = (Vmax[S])/(Km+[S]).

2.4 Cell Membranes & Transport

  • Fluid‑mosaic model: Phospholipid bilayer with embedded proteins (integral, peripheral).

  • Passive transport: diffusion, osmosis, facilitated diffusion.

  • Active transport: primary (ATP‑driven pumps) & secondary (co‑transport, anti‑port).

  • Bulk transport: endocytosis (phagocytosis, pinocytosis), exocytosis.

2.5 Cell Cycle

  • Inter‑phase: G₁ (growth), S (DNA synthesis), G₂ (pre‑mitosis).

  • Mitosis: Prophase, metaphase, anaphase, telophase → cytokinesis.

  • Meiosis: Meiosis I (reductional) & Meiosis II (equational); produces four genetically distinct haploid gametes.

  • Regulation: cyclins, CDKs, checkpoints (G₁, G₂/M, spindle).

  • Uncontrolled division → tumours (benign vs malignant).

2.6 Nucleic Acids & Protein Synthesis (Detailed)

2.6.1 What Is a Gene?

  • Definition: A gene is a defined linear sequence of nucleotides within a DNA molecule that contains the instructions for synthesising a functional product – usually a polypeptide (protein) or a functional RNA.

  • Genes include coding regions (exons) and regulatory regions (promoters, enhancers, operators) that control when, where and how much product is made.

2.6.2 DNA Structure Recap

  • Double‑helix of two antiparallel strands (5’→3’ and 3’→5’).

  • Backbone: deoxyribose + phosphate.

  • Bases: adenine (A), thymine (T), cytosine (C), guanine (G). Base‑pairing: A ↔ T, C ↔ G.

  • Genes are read in the 5’→3’ direction on the coding (sense) strand; the opposite strand serves as the template during transcription.

2.6.3 DNA Replication (Brief)

  1. Initiation: Origin of replication recognised; helicase unwinds DNA; single‑strand binding proteins stabilise strands.

  2. Priming: RNA primase synthesises short RNA primers.

  3. Elongation: DNA polymerase III adds nucleotides (5’→3’) to the 3’ end of the primer; leading strand continuous, lagging strand discontinuous (Okazaki fragments).

  4. Termination & Proof‑reading: DNA polymerase I replaces RNA primers with DNA; DNA ligase joins fragments; exonuclease activity corrects mismatches.

2.6.4 Transcription & RNA Processing (Eukaryotes)

StageKey Events
InitiationRNA polymerase II binds promoter (TATA box) with transcription factors; DNA unwinds.
ElongationRNA polymerase synthesises a complementary mRNA strand (5’→3’) using the template strand.
TerminationPoly‑A signal (AAUAAA) causes cleavage and release of the primary transcript.
Processing5’‑capping (7‑methylguanosine), splicing (removal of introns by spliceosome), poly‑A tail addition → mature mRNA.

2.6.5 Translation – From mRNA to Polypeptide

StageLocationKey MoleculesPrincipal Events
InitiationRibosome (cytoplasm or rough ER)Small ribosomal subunit, initiator Met‑tRNAi, eIFs, mRNA 5’ capStart codon (AUG) recognised; large subunit joins → functional ribosome.
ElongationSametRNAs, EF‑Tu, EF‑G, GTP, peptide‑transferase centretRNA anticodon pairs with codon at A‑site; peptide bond forms; ribosome translocates.
TerminationSameRelease factors (RF1, RF2), stop codons (UAA, UAG, UGA)Stop codon entered; release factors trigger hydrolysis, freeing the polypeptide.
Post‑Translational ModificationsRough ER, Golgi, cytoplasmKinases, glycosyltransferases, proteases, chaperonesPhosphorylation, glycosylation, cleavage, folding – determine activity, localisation, stability.

2.6.6 The Genetic Code

  • Read in triplets (codons) of mRNA nucleotides.

  • 64 codons → 20 standard amino acids + 3 stop signals.

  • Degenerate: most amino acids are specified by more than one codon.

  • Code is non‑overlapping and unambiguous (each codon specifies only one amino acid).

Standard Genetic Code (mRNA)
UUUPheUCUSer
UUCPheUCCSer
UUALeuUCASer
UUGLeuUCGSer
CUULeuCCUPro
CUCLeuCCCPro
CUALeuCCAPro
CUGLeuCCGPro
AUUIleACUThr
AUCIleACCThr
AUAIleACAThr
AUGMet (Start)ACGThr
GUUValGCUAla
GUCValGCCAla
GUAValGCAAla
GUGValGCGAla
… (remaining codons follow the same pattern) …
UAAStopUAGStop
UGAStop

2.6.7 Summary Statement

A gene is a specific sequence of nucleotides embedded in a DNA molecule. The sequence is transcribed into a messenger RNA (mRNA) which, after processing, is read by ribosomes in sets of three bases (codons). Each codon dictates the addition of a particular amino‑acid, and the ribosome links these amino‑acids together to form a polypeptide. Hence, the polypeptide is directly coded for by the gene.

3. Plant & Animal Transport

3.1 Transport in Plants

  • Xylem: dead, lignified vessels & tracheids; transports water & minerals from roots → shoots (cohesion‑tension theory).

  • Phloem: living sieve‑tube elements + companion cells; transports photosynthates (sucrose) from sources → sinks (pressure‑flow hypothesis).

  • Apoplast (cell‑wall continuum) vs. symplast (cytoplasmic continuum via plasmodesmata).

  • Transpiration pull, root pressure, and capillary action maintain upward flow.

3.2 Transport in Animals (Mammals)

  • Heart: four chambers, cardiac cycle (systole/diastole), cardiac output = HR × SV.

  • Blood vessels: arteries (elastic & muscular), veins (valves), capillaries (exchange).

  • Blood components: plasma, erythrocytes (haemoglobin), leukocytes, platelets.

  • Oxygen transport: ~98 % bound to haemoglobin (tetrameric protein), ~2 % dissolved.

  • Capillary exchange mechanisms: diffusion, bulk flow, pinocytosis.

4. Gas Exchange

  • Lung anatomy: trachea → bronchi → bronchioles → alveolar sacs; alveolar walls thin, surrounded by capillaries.

  • Partial pressure gradients drive diffusion of O₂ into blood and CO₂ out of blood.

  • Large surface area (~70 m²) and short diffusion distance (<0.5 µm) maximise exchange.

  • SA‑to‑V ratio is a key evolutionary optimisation for both lungs and gills.

5. Infectious Disease & Immunity

5.1 Infectious Disease

  • Pathogen categories: bacteria, viruses, fungi, protozoa, prions.

  • Transmission routes: direct contact, vectors, airborne, food‑borne, water‑borne.

  • Antibiotic resistance mechanisms: enzymatic degradation, target modification, efflux pumps.

  • Vaccination strategies: live‑attenuated, inactivated, subunit, toxoid, conjugate, mRNA vaccines.

5.2 Immunity

  • Innate immunity: physical barriers, phagocytes, natural killer cells, complement.

  • Adaptive immunity: B‑cell mediated (antibody) and T‑cell mediated (cell‑mediated) responses; clonal selection, memory.

  • Primary vs. secondary immune response – faster, larger titre on re‑exposure.

  • Immunisation: active (vaccines) vs. passive (maternal antibodies, immunoglobulin therapy).

6. Energy Transfer & Respiration

6.1 Cellular Respiration

  • Glycolysis (cytoplasm): glucose → 2 pyruvate + 2 ATP + 2 NADH.

  • Link reaction (mitochondrial matrix): pyruvate → acetyl‑CoA + CO₂ + NADH.

  • Krebs cycle: 2 acetyl‑CoA → 6 NADH + 2 FADH₂ + 2 ATP + 4 CO₂.

  • Oxidative phosphorylation: electron transport chain (ETC) creates proton gradient → ATP synthase produces ~34 ATP.

  • Overall: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + ~38 ATP (aerobic).

  • Anaerobic pathways: lactic acid fermentation (muscle) & alcoholic fermentation (yeast).

  • Respiratory Quotient (RQ): RQ = CO₂ produced / O₂ consumed (≈1 for carbs, 0.7 for fats).

6.2 Photosynthesis

  • Chloroplast structure: outer membrane, inner membrane, stroma, thylakoid membranes (grana).

  • Light‑dependent reactions (thylakoids): water splitting → O₂ + H⁺ + e⁻; NADP⁺ → NADPH; ATP synthesis via photophosphorylation.

  • Calvin‑Benson cycle (stroma): CO₂ fixation by Rubisco → 3‑PGA → G3P → regeneration of RuBP; net 3 CO₂ → 1 G3P (requires 9 ATP & 6 NADPH).

  • Factors affecting rate: light intensity, CO₂ concentration, temperature, water availability.

  • Photosynthetic quotient (PQ) ≈ O₂ evolved / CO₂ fixed (≈1.0).

7. Practical Skills Snapshot (Relevant to All Topics)

  • Microscopy: preparing wet mounts, staining, measuring magnification, drawing labelled diagrams.

  • Biochemical Tests: iodine (starch), Biuret (protein), Sudan IV (lipid), Benedict’s (reducing sugars).

  • DNA/RNA Work: extraction from fruit/cheek cells, agarose gel electrophoresis, PCR set‑up, spectrophotometric quantification (A₂₆₀).

  • Enzyme Kinetics: measuring reaction rates, constructing Michaelis‑Menten plots, analysing inhibition.

  • Transport Experiments: osmosis in potato cores, dye movement in plant stems, diffusion chambers.

  • Respiration & Photosynthesis Assays: CO₂ output (bubbling water), O₂ evolution (eudiometer), chlorophyll fluorescence.

  • Immunology: ELISA set‑up, antibody‑antigen precipitation, skin‑test interpretation.

  • Data Handling: plotting, calculating gradients, error analysis, using IMRaD format for lab reports.

8. Mathematical “Box” – Common Calculations

  • Surface‑area‑to‑volume ratio (cube): SA/V = 6/d where *d* = side length.

  • Respiratory Quotient (RQ): RQ = CO₂ produced / O₂ consumed.

  • Photosynthetic Quotient (PQ): PQ = O₂ evolved / CO₂ fixed.

  • Michaelis‑Menten equation: v = (Vmax[S])/(Km+[S]).

  • Cardiac output: CO = HR × SV (heart rate × stroke volume).

  • ATP yield (aerobic respiration): ≈ 38 ATP per glucose (adjusted to ~30–32 in eukaryotes due to shuttle costs).

  • Enzyme inhibition constants: Competitive → Ki = [ I ] × ( (Km’/Km) – 1 ).

9. Quick‑Reference Summary

A gene – a specific DNA nucleotide sequence – stores the instructions for a functional product. Through replication, transcription, RNA processing and translation, the sequence is ultimately expressed as a polypeptide. This central flow of information (DNA → RNA → protein) underpins every other biological process covered in the Cambridge AS & A‑Level syllabus, from cell structure to metabolism, transport, immunity and biotechnology.