AQA A-Level Biology 3.4 Genetic information, variation and relationships between organisms: complete overview

What section 3.4 actually demands

Section 3.4 is the genetics backbone of AQA A-Level Biology (specification 7402). It threads a single idea through six dot points: genetic information is stored as a base sequence, that sequence is expressed into proteins, it is altered by mutation, reshuffled by meiosis, sorted by natural selection, and finally used to classify the diversity of life. Examiners reward students who can move fluently along that chain rather than treating the six topics as separate.

Two skills are tested in parallel. The first is precise recall of named processes and molecules, especially the ordered steps of transcription and translation and the two sources of variation in meiosis. The second is application: reading selection graphs, calculating gamete combinations, and explaining real cases such as antibiotic resistance. Strong candidates drill both.

The molecular foundation: DNA, genes and chromosomes

Genetic information lives in the base sequence of DNA. Where that DNA sits, and how it is packaged, differs between cell types.

In prokaryotes, the DNA is short, circular and not associated with histone proteins, lying free in the cytoplasm with smaller plasmids alongside. In the eukaryotic nucleus, the DNA is very long, linear and wound around histone proteins to form chromosomes. Mitochondrial and chloroplast DNA resembles prokaryotic DNA: short, circular and histone-free.

The vocabulary must be exact. A gene is a base sequence coding for one polypeptide or a functional RNA. Its locus is its fixed position on a chromosome. An allele is a version of that gene. The genome is every gene in a cell; the proteome is every protein the cell can make. The proteome can exceed the gene count because of alternative splicing and post-translational modification.

Eukaryotic genes are interrupted: coding exons are separated by non-coding introns, and the genome carries large amounts of other non-coding DNA, including multiple repeated sequences between genes. This is why genome size does not track organism complexity.

From sequence to protein: the genetic code in action

A triplet of bases codes for one amino acid. Three properties of the code recur in the exam:

• Universal - the same triplet means the same amino acid in nearly all organisms (the basis of genetic engineering across species).
• Non-overlapping - each base belongs to one triplet only.
• Degenerate - most amino acids have several triplets, because 64 triplets code for about 20 amino acids.

Transcription copies one gene. The DNA unwinds, and RNA polymerase uses the template (antisense) strand to assemble complementary RNA nucleotides, with uracil replacing thymine, building pre-mRNA. In eukaryotes, splicing then removes the introns and joins the exons to form mature mRNA, which leaves the nucleus through a pore. Prokaryotes have no introns and skip splicing.

Translation happens at a ribosome. The mRNA is read in codons; tRNA molecules with complementary anticodons deliver specific amino acids; peptide bonds join them (using ATP) until a stop codon ends the chain. The order of codons fixes the order of amino acids, which fixes the way the polypeptide folds and therefore its function.

Mutation: the source of new alleles

A gene mutation is a change in the DNA base sequence, arising spontaneously in replication and accelerated by mutagens such as UV light, ionising radiation and chemicals in tobacco smoke.

A substitution swaps one base, altering at most one triplet. It may be silent (degenerate code gives the same amino acid), missense (one amino acid changes), or nonsense (a stop codon appears). A deletion removes a base and causes a frameshift, changing every triplet downstream, so its effect is usually far greater.

Mutations matter because they are the ultimate source of new alleles. Meiosis and sexual reproduction only reshuffle existing alleles; mutation creates them. Most are neutral or harmful, but the occasional beneficial allele provides the raw material on which natural selection acts.

Meiosis: shuffling the genetic deck

Meiosis turns one diploid cell into four haploid gametes through two divisions, halving the chromosome number so fertilisation can restore it. It generates variation in two examinable ways.

Independent segregation (metaphase I): homologous pairs line up randomly at the equator, so each gamete gets a random mix of maternal and paternal chromosomes. The number of combinations is $2^n$, where $n$ is the haploid number; in humans that is $2^{23}$, over eight million.

Crossing over (prophase I): homologous chromosomes pair and exchange sections of chromatid at chiasmata, producing new allele combinations on a chromosome.

Random fertilisation then multiplies the variation: any genetically distinct sperm can fuse with any genetically distinct egg, giving $2^{2n}$ chromosome combinations from independent segregation alone, before crossing over is counted.

Natural selection: sorting the variation

A population shares a gene pool, and its genetic diversity is the number of different alleles in that pool. Selection changes allele frequencies in a fixed sequence: mutation generates variation, a selection pressure means some phenotypes survive and reproduce better, advantageous alleles are passed on, and over generations their frequency rises.

AQA names two patterns and expects you to recognise a third from data:

• Stabilising selection favours the modal phenotype, narrows the range and holds the mean constant (human birth mass).
• Directional selection favours one extreme and shifts the mean (antibiotic resistance, peppered moths).
• Disruptive selection favours both extremes and splits the population.

Selection produces adaptations, classified as anatomical (structural, such as streamlining), physiological (functional, such as venom production), or behavioural (such as migration or courtship displays).

Taxonomy: organising the diversity selection produces

A species is a group of similar organisms that interbreed to produce fertile offspring, named by the binomial system (genus capitalised, species lower case, both italicised). Horses and donkeys are separate species because their offspring, the mule, is infertile.

Modern classification is phylogenetic: organisms are grouped by evolutionary relationships into a non-overlapping hierarchy of taxa, from Domain through Kingdom, Phylum, Class, Order, Family and Genus to Species. The three-domain system (Bacteria, Archaea, Eukarya), based on ribosomal RNA, replaced the five-kingdom system. Relationships are increasingly judged from DNA base sequences, mRNA, amino acid sequences and immunological comparisons, which are more objective than appearance.

Courtship behaviour ensures species recognition (avoiding infertile hybrids), identifies a mature and receptive mate of the opposite sex, and synchronises mating. Because it is often species specific, its similarity is also used as evidence in classification.

How section 3.4 is examined

A typical AQA profile across the papers:

• Multiple choice. Code properties, identifying the stage of meiosis, classifying a type of selection or an adaptation.
• Short structured (2 to 5 marks). Ordered steps of transcription or translation, mutation effects, gamete number calculations, defining species or genome.
• Application and data (4 to 6 marks). Reading a selection graph, explaining antibiotic resistance, interpreting DNA-similarity tables to infer relationships.

A revision sequence that works

1. Diagrams first. Draw transcription and translation with molecules and locations labelled, and the meiosis sequence marking where crossing over and independent segregation occur.
2. Then the calculations. Drill $2^n$ gamete problems until the haploid-number rule is automatic.
3. Then the named cases. Memorise one worked example for each selection type and each adaptation type.
4. Finally, synoptic links. Connect mutation to protein structure (3.1), and natural selection to enzymes and disease, ready for Paper 3.