amazon.co.ukA-Level Biology revision on DNA, genes and protein synthesis: nucleic acid structure, semi-conservative replication, the genetic code, transcription, translation and gene mutations, with multi-board exam practice and mark-scheme logic.
A DNA nucleotide = deoxyribose (pentose sugar) + phosphate group + nitrogenous base (A, T, C or G). Nucleotides join by condensation between the phosphate of one and the sugar of the next, forming a sugar-phosphate backbone linked by phosphodiester bonds. Two polynucleotide strands run antiparallel (one 5'→3', one 3'→5') and twist into a double helix, held by hydrogen bonds between complementary bases: A-T (2 hydrogen bonds), G-C (3 hydrogen bonds). Complementary base pairing plus the constant width of paired bases makes DNA stable yet able to unzip. RNA contrasts in four ways: ribose sugar (not deoxyribose), uracil replaces thymine, single-stranded, shorter. mRNA (carries the code from nucleus to ribosome), tRNA (cloverleaf, anticodon, amino-acid binding site) and rRNA (ribosomal structure) all feature in mark schemes.
Know the structure-function links examiners award: the double helix stores large amounts of information in a compact stable form; hydrogen bonds are weak enough to allow unzipping for replication and transcription; complementary base pairing enables accurate copying; phosphodiester bonds give backbone strength; the sequence of bases codes the amino-acid sequence of polypeptides. ATP is a nucleotide derivative (adenine + ribose + three phosphates) - expect a link question between DNA nucleotides and ATP structure.
Base classification matters for exam precision: purines (adenine, guanine - double-ring, larger) pair with pyrimidines (thymine, cytosine - single-ring, smaller), keeping the double helix a constant width; hydrogen bonding numbers (A-T 2, G-C 3) explain why G-C rich DNA is more stable and melts at higher temperatures. In eukaryotes, DNA wraps around histone proteins to form nucleosomes, then coils into chromosomes - this packaging is what epigenetic modification (histone acetylation/methylation) adjusts to switch genes on or off. Prokaryotic DNA is naked (no histones) and circular. Also note that most eukaryotic DNA is non-coding: introns, regulatory sequences and repeats; only a small fraction codes for polypeptides, and the coding regions are the genes at their loci on homologous chromosomes.
The full sequence earns marks in order: (1) DNA helicase unwinds the double helix by breaking the hydrogen bonds between complementary bases; (2) the two exposed antiparallel strands each act as a template; (3) free DNA nucleotides are attracted to their complementary exposed bases (A-T, G-C) and hydrogen-bond into place; (4) DNA polymerase joins adjacent nucleotides with phosphodiester bonds, synthesising each new strand 5'→3'; (5) because the strands are antiparallel, the leading strand is synthesised continuously while the lagging strand is made in Okazaki fragments joined by DNA ligase; (6) each daughter molecule contains one conserved parental strand and one newly built strand - hence semi-conservative. Evidence: the Meselson-Stahl experiment grew E. coli in heavy nitrogen (¹⁵N), then transferred to light (¹⁴N); after one generation all DNA was intermediate density, after two, half intermediate and half light - only semi-conservative replication predicts this. Exam questions often test why the experiment excludes conservative and dispersive models, so know the predicted banding patterns.
Add the conditions and error-control detail: replication happens during S phase of interphase before cell division, requiring free nucleotides, ATP, helicase, polymerase and ligase. Complementary base pairing makes replication highly accurate: A always pairs with T and G with C, and DNA polymerase proof-reads newly added nucleotides, excising mismatches - mutations persist only when errors escape repair. When asked to evaluate the Meselson-Stahl evidence, state the three predictions: a conservative model predicts one heavy and one light band after one generation; a dispersive model predicts intermediate DNA at every generation that gets progressively lighter; only the observed pattern (intermediate after one generation, half intermediate and half light after two) fits semi-conservative replication. DNA replication in prokaryotes is similar but starts from a single origin and proceeds in both directions around the circular chromosome.
A gene is a base sequence coding for a polypeptide or functional RNA. Eukaryotic genes contain introns (non-coding) and exons (coding); prokaryotic DNA has no introns. The genetic code is: triplet (three bases = one codon = one amino acid), non-overlapping (each base belongs to only one codon), degenerate (most amino acids have multiple codons - so some substitutions are silent), and universal (same codons code the same amino acids in all organisms - the basis of genetic engineering). A gene on a chromosome sits at a locus; homologous chromosomes carry the same genes at the same loci but may carry different alleles.
Transcription: (1) RNA polymerase binds the promoter region and unwinds/unzips the DNA; (2) free RNA nucleotides pair with the exposed template strand (uracil pairs with adenine); (3) RNA polymerase joins them with phosphodiester bonds to make pre-mRNA; (4) in eukaryotes the pre-mRNA is spliced to remove introns, and modified with a 5' cap and poly-A tail, producing mature mRNA; (5) the mRNA leaves via a nuclear pore. Only the template strand is copied because the code must match the sense strand when translated. Prokaryotes produce mRNA directly (no splicing) and translation can begin immediately - a comparison the exam uses. Alternative splicing of exons lets one gene code several polypeptides, increasing diversity of gene products in humans.
Two strand roles examiners test: only the template strand is transcribed (the sense strand carries the same sequence as the mRNA, in T/U terms); RNA polymerase moves along the template in the 3' to 5' direction, synthesising mRNA 5' to 3'. The promoter is the DNA sequence RNA polymerase binds to start transcription - transcription factors (see gene-expression topic) control this access, so a gene's expression depends on which factors are active, not just the gene's presence. Contrast eukaryotic and prokaryotic protein synthesis in one line each if asked: eukaryotes transcribe in the nucleus, splice the pre-mRNA, then translate in the cytoplasm; prokaryotes have no nucleus, produce mRNA directly with no splicing, and translation can begin before transcription finishes - this speed underlies rapid bacterial responses to environmental change.
Translation occurs on the ribosome in the cytoplasm (or on rough ER for secreted proteins): (1) the ribosome attaches to the start codon (AUG) of mRNA; (2) tRNA molecules, each carrying a specific amino acid, have anticodons complementary to mRNA codons; (3) the ribosome moves along the mRNA, and two tRNAs occupy the ribosome at once so the ribosome can catalyse peptide bond formation between adjacent amino acids (using ATP to attach amino acids to tRNA first); (4) tRNA leaves, emptied, and is reused; (5) translation stops at a stop codon (UAA, UAG, UGA), releasing the polypeptide, which folds into its tertiary structure (assisted by chaperones in some cases). Codon-anticodon pairing by complementary base pairing guarantees the amino-acid sequence matches the gene. The polypeptide may then be modified in the Golgi (e.g. glycosylation) - link this back to cell-structure content for synthesis-and-secretion questions.
tRNA earns its own marks: a single strand of ~80 nucleotides folded into a cloverleaf shape held by hydrogen bonds between complementary bases, with an anticodon at one end and an amino-acid binding site at the other; ATP and activating enzymes attach each amino acid to its specific tRNA - the energy in that bond later supplies peptide-bond formation. Ribosomes consist of a large and a small subunit of rRNA and protein: the small subunit binds mRNA, the large subunit holds the tRNAs and catalyses peptide bonds. Several ribosomes can translate one mRNA simultaneously (a polysome), producing many copies of the polypeptide quickly. The completed polypeptide is often modified after translation - folded, with sugars added in the Golgi, or cleaved into active fragments (e.g. insulin from pro-insulin) - before becoming a functional protein.
Gene mutations change the base sequence of a chromosome. They arise spontaneously during DNA replication (base pairing errors) or are induced by mutagens - UV radiation, ionising radiation, and chemicals such as tar in tobacco smoke. Types and their consequences: substitution - one base replaced: silent if the new codon codes the same amino acid (degenerate code), missense if a different amino acid is inserted (may alter tertiary structure, e.g. sickle-cell anaemia from a single substitution in haemoglobin), nonsense if a stop codon is created (truncated, non-functional protein); insertion or deletion of bases not in multiples of three - frameshift: the reading frame shifts so every codon downstream changes, inserting wrong amino acids and often creating a premature stop codon; duplication - a base sequence repeated; inversion - a sequence reversed. Chromosome mutations include deletion, duplication, inversion and translocation of whole sections, and non-disjunction producing aneuploidy (e.g. trisomy 21).
Mutations in gametes are inherited and fuel genetic diversity and evolution; mutations in somatic cells (not inherited) can produce cancer if they affect genes controlling cell division (proto-oncogenes, tumour-suppressor genes - see gene-expression topic). Because the code is degenerate and non-overlapping, substitutions are often silent while insertions/deletions are catastrophic - a classic comparison question. Beneficial mutations (e.g. antibiotic resistance in bacteria, lactose tolerance in humans) show mutation as the source of new alleles on which natural selection acts, linking this sub-topic to genetics, evolution and ecosystems.
Mutagens increase the mutation rate: ultraviolet light causes thymine dimers in DNA; ionising radiation (X-rays, gamma) breaks chromosomes and damages bases; chemicals such as tar in tobacco smoke and mustard gas are alkylating agents. Cells possess DNA repair mechanisms (excision repair enzymes) that correct most damage, but unrepaired mutations become permanent and are copied in every future replication. Repeat-expansion mutations (e.g. Huntington's disease - extra CAG repeats) show that inserting multiples of three also mutates proteins by inserting extra amino acids without a frameshift. Distinguish somatic mutations (body cells - not inherited, can cause cancer) from germ-line mutations (gametes - inherited, source of new alleles on which selection acts). Rates vary: most base pairs replicate faithfully, but with billions of bases per cell division, spontaneous mutations occur constantly - the raw material of genetic diversity.