Core idea Molecular biology explains how information is stored in DNA, copied, repaired, expressed as RNA and protein, and regulated in cells. Clinical medicine depends on these processes: replication errors cause mutations, repair defects cause cancer susceptibility, transcription defects cause disease, and altered proteins change physiology.
Learning outcomes
Describe DNA structure and replication; explain polymerase fidelity and repair; compare transcription and RNA processing in prokaryotes and eukaryotes; trace translation and protein folding; and connect molecular defects to laboratory diagnosis and disease.
1. DNA structure and information storage
DNA is a double helix of antiparallel polynucleotide strands. Each nucleotide contains deoxyribose, phosphate and a nitrogenous base. Adenine pairs with thymine through two hydrogen bonds; guanine pairs with cytosine through three. The sequence carries information, while the sugar-phosphate backbone provides chemical stability.
DNA is packaged around histone proteins into nucleosomes and higher-order chromatin. Euchromatin is relatively accessible for transcription; heterochromatin is more condensed. Chromatin state can change without altering the DNA sequence through methylation, histone modification, nucleosome repositioning and non-coding RNA.
2. Semiconservative DNA replication
Replication begins at origins where helicase separates the strands. Single-strand binding proteins stabilise the template; topoisomerases relieve torsional stress; primase lays RNA primers; and DNA polymerase extends a new strand only in the 5′→3′ direction. The leading strand is continuous, while the lagging strand is made as Okazaki fragments that are joined by ligase.
Replication is accurate because polymerases select complementary bases, proofread newly added nucleotides and cooperate with post-replication repair. Telomeres protect chromosome ends, and telomerase extends telomeric repeats in germ cells, stem cells and many cancers.
3. DNA damage and repair
| Damage | Repair pathway | Clinical relevance |
|---|---|---|
| Small base mismatch after replication | Mismatch repair | Defects cause microsatellite instability and Lynch syndrome. |
| Bulky ultraviolet-induced lesion | Nucleotide-excision repair | Defects cause photosensitivity and xeroderma pigmentosum. |
| Deaminated or oxidised base | Base-excision repair | Protects against endogenous metabolic damage. |
| Double-strand break | Homologous recombination or non-homologous end joining | Repair failure causes chromosomal instability and cancer susceptibility. |
| Interstrand crosslink | Fanconi-associated repair networks | Defects cause marrow failure, congenital anomalies and cancer risk. |
4. Transcription: DNA to RNA
RNA polymerase reads a DNA template and synthesises RNA 5′→3′. Promoters recruit transcription factors and polymerase; enhancers can act at a distance through chromatin looping. The primary transcript of a eukaryotic protein-coding gene receives a 5′ cap, introns are removed by spliceosomes and a 3′ poly-A tail is added.
Alternative splicing allows one gene to produce different transcripts. Splicing errors can cause exon skipping, intron retention or abnormal reading frames. A molecular test should therefore consider whether the variant affects a splice enhancer, branch point or canonical donor/acceptor site.
5. RNA types and post-transcriptional control
Messenger RNA carries coding information; transfer RNA brings amino acids; ribosomal RNA forms the catalytic core of ribosomes. MicroRNAs and long non-coding RNAs regulate stability, translation and chromatin. RNA editing and selective degradation alter the amount and form of transcript available for translation.
6. Translation and protein fate
Ribosomes read mRNA codons from 5′ to 3′. Translation begins at a start codon, proceeds through peptide-bond formation and ends at a stop codon. Protein folding, disulfide-bond formation, glycosylation, phosphorylation, cleavage and trafficking determine whether the protein reaches the correct compartment and functions properly.
A variant may produce a stable but inactive protein, a misfolded protein retained in the endoplasmic reticulum, a toxic aggregate, or a protein with abnormal location. These mechanisms explain why the same DNA-level category can produce different clinical phenotypes.
7. Mutations and molecular consequences
Substitutions may be synonymous, missense, nonsense or affect splicing. Insertions and deletions can cause a frameshift; repeat expansions may become unstable; structural variants alter gene dosage or regulation. A mutation can be pathogenic through loss of function, gain of function, dominant-negative interference or dosage imbalance.
8. Laboratory applications
PCR
Amplifies a defined DNA region through denaturation, primer annealing and extension. Used for pathogen detection, genotyping and confirmation, but vulnerable to contamination and primer mismatch.
Reverse-transcription PCR
Converts RNA to cDNA before amplification and measures transcript abundance or splice products.
Electrophoresis
Separates nucleic acids by size and charge, allowing product-size comparison and fragment analysis.
Sequencing
Reads nucleotide order. Sanger sequencing is targeted; next-generation sequencing examines many genes but requires coverage, quality control and interpretation.
9. Molecular biology in clinical reasoning
When selecting a test, start with the phenotype and suspected mechanism. A karyotype or microarray is more useful for chromosome-level change; a targeted PCR or Sanger test suits a known variant; a panel, exome or genome may be used for genetically heterogeneous disorders. A negative result means the method did not identify a reportable change; it does not exclude every molecular mechanism.
Interpret the whole chain: DNA variant → RNA effect → protein effect → cellular pathway → phenotype. A laboratory report is not a diagnosis until the molecular finding fits the clinical picture and test limitations.
10. Examination summary
DNA replication is semiconservative and 5′→3′; transcription produces RNA that is capped, spliced and polyadenylated; translation converts codons into protein; repair pathways maintain genome stability. Molecular disease may result from sequence, copy-number, regulatory, splicing or epigenetic change. PCR, electrophoresis and sequencing reveal different parts of the pathway and must be chosen according to the clinical question.
