Molecular Biology
1. What this chapter covers, and how NEET PG actually tests it
Molecular biology is rarely tested as pure mechanism.
It is tested through three lenses: a repair pathway that has failed, a drug or toxin that blocks a specific step, and a laboratory technique chosen for a specific question.
Each lens has a small, closed list.
There are five DNA repair pathways, and each has a named disease. There are three eukaryotic RNA polymerases, and specific inhibitors of each. Antibiotics that block translation divide cleanly by ribosomal subunit. Three blots detect three different molecules.
Learning these as matched pairs rather than as separate facts is what makes the topic manageable, because the exam almost always supplies one member of a pair and asks for the other.
This chapter covers replication and DNA repair, transcription and RNA processing, translation and the genetic code, and the laboratory techniques.
| In scope here | Deliberately out of scope |
|---|---|
| Replication machinery, telomerase, the five repair pathways | Cancer biology and oncogenes (see Pathology) |
| RNA polymerases, splicing, post-transcriptional processing | Detailed virology replication strategies (see Microbiology) |
| Translation steps, genetic code, mutation types, protein synthesis inhibitors | Antibiotic spectra and clinical use (see Pharmacology) |
| Blotting, PCR, FISH, sequencing, epigenetic mechanisms | Statistical interpretation of test results (see PSM) |
2. Replication and DNA repair
2.1 The replication machinery
Replication is semiconservative, each daughter molecule retaining one parental strand — the finding of the Meselson-Stahl experiment.
DNA polymerase can only synthesise in the 5' to 3' direction and cannot start a chain from nothing.
Those two constraints generate everything else about the process.
Because a chain cannot be started de novo, primase must lay down a short RNA primer, later removed and replaced.
Because synthesis runs only one way, the two template strands cannot be copied identically. The leading strand is made continuously, while the lagging strand is made in short Okazaki fragments that are subsequently joined by DNA ligase.
Helicase unwinds the double helix, single-strand binding proteins prevent reannealing, and topoisomerase relieves the supercoiling that unwinding creates ahead of the fork.
Topoisomerase is the most clinically important of these, because it is a drug target twice over.
Bacterial DNA gyrase is inhibited by fluoroquinolones. Human topoisomerase I is inhibited by irinotecan and topoisomerase II by etoposide, both used as cytotoxics.
DNA polymerase also proofreads, using a 3' to 5' exonuclease activity to excise a mismatched base immediately after inserting it.
Proofreading and mismatch repair are frequently confused but are distinct in timing and in machinery. Proofreading is the polymerase correcting itself at the moment of insertion; mismatch repair is a separate system that scans the finished strand afterwards for errors that escaped.
In prokaryotes, DNA polymerase III performs the bulk of synthesis while DNA polymerase I removes the RNA primers using a 5' to 3' exonuclease activity and fills the gaps.
2.2 Telomerase
The lagging strand problem means the extreme end of a linear chromosome cannot be fully replicated, so chromosomes shorten with each division.
Telomerase solves this, and it is a reverse transcriptase — an RNA-dependent DNA polymerase carrying its own RNA template — that extends the telomeric repeats.
It is active in germ cells and stem cells but silenced in most somatic cells, which is why somatic cells have a finite division limit.
Reactivation of telomerase is found in the great majority of cancers, and is one of the mechanisms by which they achieve replicative immortality.
2.3 Five repair pathways, five diseases
Each pathway handles a different kind of damage, and each has a characteristic disease when it fails.
| Pathway | Damage repaired | Disease when defective |
|---|---|---|
| Nucleotide excision repair | Bulky lesions, UV pyrimidine dimers | Xeroderma pigmentosum |
| Base excision repair | Single damaged bases from deamination or oxidation | — |
| Mismatch repair | Replication errors escaping proofreading | Lynch syndrome, with microsatellite instability |
| Homologous recombination | Double-strand breaks, using the sister chromatid | BRCA1 and BRCA2, Fanconi anaemia |
| Non-homologous end joining | Double-strand breaks, without a template | Ataxia telangiectasia |
The pairings are logical once the damage type is clear.
Xeroderma pigmentosum patients develop severe photosensitivity and skin cancers in childhood, because ultraviolet light is the specific insult their pathway cannot handle.
Lynch syndrome causes microsatellite instability because microsatellites are short repeated sequences where polymerase slips most easily, so uncorrected errors accumulate there first and are detectable as changed repeat lengths.
Homologous recombination is accurate because it copies from the sister chromatid, which requires S or G2 phase. Non-homologous end joining simply ligates broken ends together at any point in the cycle, and is therefore error-prone — the trade-off being that it is always available.
3. Transcription and RNA processing
3.1 Three polymerases, three products, three inhibitors
Eukaryotes use three RNA polymerases, and the numbering follows the product alphabetically.
| Polymerase | Product | Location | Inhibitor |
|---|---|---|---|
| I | rRNA | Nucleolus | — |
| II | mRNA and snRNA | Nucleoplasm | Alpha-amanitin |
| III | tRNA and 5S rRNA | Nucleoplasm | — |
Alpha-amanitin is the toxin of the death cap mushroom, and its selective inhibition of RNA polymerase II explains the clinical course: messenger RNA synthesis stops, existing protein is gradually depleted, and hepatic failure develops over days rather than hours.
Prokaryotes have only one RNA polymerase, which needs no primer and is inhibited by rifampicin — the basis of that drug's selectivity.
3.2 Processing the transcript
Three modifications convert the primary transcript into mature messenger RNA.
A 7-methylguanosine cap is added at the 5' end, protecting the transcript and directing ribosomal binding.
A poly-A tail is added at the 3' end following an AAUAAA signal, and its length influences transcript stability.
Splicing removes introns, performed by the spliceosome, an assembly of small nuclear ribonucleoproteins.
These snRNPs are clinically significant beyond their function, because they are autoantigens.
Anti-Smith antibodies target snRNPs and are highly specific for systemic lupus erythematosus, while anti-U1 ribonucleoprotein antibodies characterise mixed connective tissue disease.
Splicing errors themselves cause disease. Many beta-thalassaemia mutations are splice-site mutations rather than mutations in the coding sequence, producing an abnormally processed transcript from an otherwise intact gene.
4. Translation and the genetic code
4.1 Properties of the code
The genetic code has four properties worth stating precisely, because questions test them individually.
It is degenerate, meaning most amino acids have more than one codon. It is unambiguous, meaning each codon specifies only one amino acid. It is non-overlapping, and it is near-universal, with mitochondria being the notable exception.
Degeneracy is concentrated in the third base, which is described as the wobble position because base pairing there is less stringent.
That is why silent mutations occur disproportionately at the third position — a change there frequently specifies the same amino acid.
4.2 Mutation types and their consequences
| Mutation | Effect | Note |
|---|---|---|
| Silent | No amino acid change | Usually third-base |
| Missense | One amino acid substituted | Sickle cell disease is the classic example |
| Nonsense | Premature stop codon | Truncated, usually non-functional protein |
| Frameshift | Insertion or deletion not a multiple of three | Usually most severe, since everything downstream is misread |
| Splice site | Abnormal transcript processing | Many beta-thalassaemias |
A frameshift is generally more damaging than a nonsense mutation occurring at the same position, because it corrupts every subsequent codon rather than simply stopping translation.
An insertion or deletion of three bases, by contrast, adds or removes one amino acid and preserves the reading frame — which is why the multiple-of-three rule matters.
4.3 The ribosome and where drugs act
Translation proceeds through initiation, elongation and termination, with three ribosomal sites: A for the incoming aminoacyl-tRNA, P for the growing peptidyl chain, and E for exit.
Peptide bond formation is catalysed by peptidyl transferase, which is a ribozyme — the catalytic activity resides in ribosomal RNA rather than in protein, which is a directly examinable point.
Prokaryotic ribosomes are 70S, made of 30S and 50S subunits; eukaryotic ribosomes are 80S, made of 40S and 60S. This difference is the entire basis of antibiotic selectivity.
| Subunit | Drugs |
|---|---|
| 30S | Aminoglycosides (block initiation, cause misreading), tetracyclines (block the A site) |
| 50S | Chloramphenicol (peptidyl transferase), macrolides and clindamycin (translocation), linezolid (initiation) |
Two toxins act on translation rather than antibiotics.
Diphtheria toxin and Pseudomonas exotoxin A both ADP-ribosylate elongation factor 2, halting translocation and therefore protein synthesis entirely.
Shiga toxin and Shiga-like toxin inactivate the 60S ribosomal subunit, which is why they affect human cells rather than bacterial ones.
5. Laboratory techniques
5.1 The three blots
The naming is arbitrary and therefore worth fixing with a hook.
| Technique | Detects |
|---|---|
| Southern | DNA |
| Northern | RNA |
| Western | Protein |
The conventional hook is that the sequence SNoW corresponds to DRoP.
Southern blotting was named after its inventor, and the other two were named as jokes following the geographical pattern, which is why no logic connects the names to the molecules.
Western blotting has the widest clinical use, historically as the confirmatory test in HIV diagnosis.
Southern blotting retains a niche role in detecting large repeat expansions, such as those of fragile X syndrome, which are too long for standard amplification methods to handle reliably.
5.2 Amplification and localisation
The polymerase chain reaction amplifies a target sequence through repeated cycles of three steps: denaturation by heating, annealing of primers, and extension by a heat-stable polymerase, classically Taq.
The requirement for a heat-stable enzyme is the whole reason the technique works as a cycle, since an ordinary polymerase would be denatured in the first heating step.
Fluorescence in situ hybridisation uses labelled probes to detect specific sequences on chromosomes.
Its value is resolution. A standard karyotype detects only large abnormalities, whereas FISH detects submicroscopic deletions such as the 22q11.2 deletion of DiGeorge syndrome, which a karyotype would report as normal.
ELISA detects an antigen or an antibody using an enzyme-linked reporter, and is the workhorse screening assay in serology.
5.3 Epigenetic regulation
Gene expression can be altered without changing the sequence, by two mechanisms that operate in opposite directions.
Methylation of cytosine in CpG islands silences transcription. Heavy promoter methylation switches a gene off, and this is one route by which tumour suppressor genes are inactivated in cancer without any mutation.
Histone acetylation activates transcription, by neutralising the positive charge on histones and loosening their grip on DNA.
Deacetylation therefore silences, which is why histone deacetylase inhibitors are used as anticancer agents — they restore expression of silenced genes.
A simple way to hold the direction is that methylation mutes and acetylation activates.
Genomic imprinting and X-inactivation both operate through methylation, which is why they produce stable, heritable silencing without any change in sequence.
5.4 Sequencing and gene editing
Sanger sequencing uses chain-terminating dideoxynucleotides, which lack the 3' hydroxyl group needed to add the next base.
Because termination occurs randomly at each position, the reaction generates fragments of every possible length, and separating them by size reads the sequence directly.
Next-generation sequencing parallelises this massively, sequencing millions of fragments at once, which is what makes whole-exome and whole-genome testing clinically affordable.
The practical distinction is that Sanger sequencing interrogates one known gene accurately, while next-generation sequencing searches many genes at once — so the choice depends on whether the clinical suspicion is specific or broad.
CRISPR-Cas9 edits sequence directly. A guide RNA base-pairs with the target site and the Cas9 nuclease cuts both strands there.
What happens next depends on which repair pathway the cell uses, which links this technique back to section 2.
Non-homologous end joining introduces small errors and typically disrupts the gene, which is how a gene is knocked out. Supplying a template instead allows homologous recombination to insert a corrected sequence, which is how a gene is repaired.
Worked clinical vignettes
Q1. A child develops severe sunburn after minimal exposure and has multiple skin cancers by age eight. Which DNA repair pathway is defective?
Pick an option to check your answer.
Show explanation
Solution. Ultraviolet light produces pyrimidine dimers, which are bulky helix-distorting lesions.
Nucleotide excision repair is the pathway that removes bulky lesions, and its failure causes xeroderma pigmentosum.
(a) Mismatch repair failure causes Lynch syndrome with colorectal and endometrial cancer, not photosensitivity. Answer: (b).
Q2. A patient ingests death cap mushrooms and develops hepatic failure over several days. Which enzyme is inhibited, and why is the onset delayed?
Pick an option to check your answer.
Show explanation
Solution. Alpha-amanitin selectively inhibits RNA polymerase II, which transcribes messenger RNA.
Existing messenger RNA and protein continue to function until they turn over naturally, so cellular failure develops over days rather than immediately.
That delayed course is characteristic and often means the patient presents after apparent recovery from the initial gastrointestinal illness. Answer: (b).
Q3. A neonate with cardiac defects, hypocalcaemia and immunodeficiency has a normal karyotype. Which investigation should follow, and why?
Pick an option to check your answer.
Show explanation
Solution. This is DiGeorge syndrome, caused by a microdeletion at 22q11.2.
A standard karyotype resolves only large chromosomal abnormalities, and a deletion of this size falls below its detection threshold, so a normal result does not exclude it.
FISH uses a labelled probe targeting the specific region and will detect the deletion. Answer: (b).
7. Common exam traps
- Assuming DNA polymerase can start a chain. It cannot, which is why primase and RNA primers exist.
- Confusing proofreading with mismatch repair. Proofreading is the polymerase's own 3' to 5' exonuclease acting immediately; mismatch repair corrects errors that escape it.
- Pairing the wrong repair pathway with a cancer syndrome. BRCA is homologous recombination, Lynch is mismatch repair, xeroderma pigmentosum is nucleotide excision repair.
- Forgetting that peptidyl transferase is a ribozyme. The catalytic activity is in ribosomal RNA, not protein.
- Mixing up 30S and 50S antibiotic targets. Aminoglycosides and tetracyclines act on 30S; chloramphenicol, macrolides, clindamycin and linezolid on 50S.
- Assuming a nonsense mutation is always worse than a frameshift. A frameshift corrupts every downstream codon and is usually more damaging.
- Treating a normal karyotype as excluding a genetic cause. Microdeletions require FISH or microarray.
- Reversing the epigenetic directions. Methylation silences; acetylation activates.
Summary
- Replication is semiconservative, and polymerase's inability to start a chain or to work in reverse explains primers, Okazaki fragments and ligase.
- Topoisomerase is a drug target twice over, inhibited by fluoroquinolones in bacteria and by etoposide and irinotecan in human cells.
- Telomerase is a reverse transcriptase active in germ and stem cells and reactivated in most cancers, conferring replicative immortality.
- Nucleotide excision repair handles ultraviolet pyrimidine dimers and fails in xeroderma pigmentosum.
- Mismatch repair failure causes Lynch syndrome with microsatellite instability, since repeated sequences accumulate errors first.
- Homologous recombination is accurate but restricted to S and G2 phase and fails in BRCA-associated cancers; non-homologous end joining is error-prone but always available and fails in ataxia telangiectasia.
- RNA polymerase I makes rRNA, II makes mRNA and is inhibited by alpha-amanitin, and III makes tRNA; prokaryotes have one polymerase, inhibited by rifampicin.
- Transcripts are capped, polyadenylated and spliced, and the snRNPs performing splicing are the autoantigens targeted by anti-Smith antibodies in lupus.
- Many beta-thalassaemia mutations are splice-site rather than coding mutations.
- The genetic code is degenerate, unambiguous, non-overlapping and near-universal, with degeneracy concentrated at the wobble third base.
- Frameshift mutations are usually the most damaging, because every downstream codon is misread, unless the change is a multiple of three.
- Peptidyl transferase is a ribozyme, and the 70S versus 80S ribosome difference underlies all antibiotic selectivity.
- Aminoglycosides and tetracyclines target 30S; chloramphenicol, macrolides, clindamycin and linezolid target 50S.
- Diphtheria and Pseudomonas exotoxin A inactivate elongation factor 2, while Shiga toxin inactivates the 60S subunit.
- Southern blotting detects DNA, Northern RNA and Western protein, and PCR depends on a heat-stable polymerase to permit thermal cycling.
- FISH detects microdeletions that a karyotype cannot resolve, and epigenetically, methylation silences while acetylation activates.
- Sanger sequencing terminates chains with dideoxynucleotides and interrogates one gene accurately, while next-generation sequencing screens many genes in parallel.
- CRISPR-Cas9 cuts at a guide RNA-specified site, and whether the gene is knocked out or repaired depends on whether the cell uses end joining or homologous recombination.
