Basic Neurosciences · Paper A
Molecular biology basics
The 'central dogma' describes how genetic information flows one way: from DNA, to RNA, to protein. DNA acts like a master recipe book kept safely in a library, the cell nucleus. The cell never removes this original book: it only copies out the recipe it needs, as RNA, and carries that copy to the kitchen, the cytoplasm, where the protein is actually built. A fault at any single step in this flow produces a genetic disease with its own distinct pattern.
DNA replication is SEMICONSERVATIVEeach new double strand contains one original (parental) strand and one newly-made strand, built by DNA polymerase working in one direction (5' to 3'), with a helper enzyme called ligase joining the short fragments made on the lagging strand. A fault in this ligase step leaves the lagging-strand fragments unjoined, so the new DNA strand becomes unstable. RNA polymerase transcribes (copies) a DNA template into RNA; in human cells this precursor RNA is then capped, given a tail (polyadenylated), and spliced to cut out non-coding sections called introns before it becomes mature messenger RNA. Ribosomes only ever read this finished, spliced mRNA in the cytoplasm, using transfer RNAs to build the protein one amino acid at a time. E.g., a single faulty splice site can leave an intron in the mature mRNA, producing a nonfunctional protein despite a normal DNA sequence.
A vignette locates a fault at one step of the DNA to RNA to protein flow: replication, transcription, splicing or translation; and asks which step is involved. Watch for a retrovirus running the flow backwards with reverse transcriptase, the step its drugs specifically block.
Almost every molecular genetics question is secretly asking the same thing: which single step of DNA-to-RNA-to-protein has broken, and what does breaking that specific step look like? Learning the CENTRAL DOGMA (the rule that genetic information flows from DNA, to RNA, to protein, and not normally in reverse) as a sequence of named, ordered steps turns a wall of enzyme names into a short checklist you can walk through under exam pressure.
DNA (deoxyribonucleic acid) is the cell's long-term genetic archive, stored safely inside the nucleus. RNA (ribonucleic acid) is a short-lived working copy of one gene, made when that gene is needed. TRANSCRIPTION is the process of copying a DNA gene into RNA, and TRANSLATION is the process of reading that RNA to build a protein. The whole system exists so the master archive, DNA, never has to leave the nucleus or be used up doing the actual work of building proteins.
Candidates often assume genetic information can flow in any direction they like, or that RNA is simply a smaller version of DNA doing the same job. Neither is quite right. Information normally flows only one way, DNA to RNA to protein, and RNA and DNA differ in real chemistry, not just length: RNA is single-stranded, uses the base uracil instead of thymine, and is chemically far less stable, which is exactly why it is disposable and DNA is not.
DNA stores information as a sequence of four bases: adenine, thymine, cytosine and guanine. When a cell divides, it must copy this whole archive faithfully, a process called REPLICATION, and this copying is SEMICONSERVATIVE: each new double strand keeps one original parental strand and pairs it with one freshly built strand. The enzyme DNA polymerase builds the new strand in one direction only, and because DNA is double-stranded and antiparallel, one new strand, the LAGGING STRAND, is built in short fragments that a separate enzyme, ligase, must join together afterwards. When a specific gene is needed rather than the whole archive copied, the cell instead performs transcription: RNA polymerase reads one DNA strand and builds a matching RNA copy of just that gene, base by base.
Replication: happens before cell division, copies the entire genome, is semiconservative, and uses DNA polymerase plus ligase. For example, consider a fault at the ligase step: the lagging strand's fragments are left unjoined, producing an unstable new DNA molecule.
Transcription: happens in the nucleus, copies one gene from DNA into a matching strand of RNA, and is carried out by RNA polymerase. In human cells the resulting precursor RNA is processed before it can be used: a protective cap is added at one end, a tail of repeated bases (polyadenylation) is added at the other, and non-coding stretches called INTRONS (sections of a gene that do not code for protein and are cut out before translation) are removed by a process called splicing, leaving mature messenger RNA (mRNA).
Translation: happens in the cytoplasm, where ribosomes read the mature mRNA three bases (one CODON) at a time. Transfer RNA (tRNA) molecules each carry one specific amino acid and match it to the correct codon, so the ribosome can link amino acids together in the exact order the mRNA specifies, building the finished protein.
The genetic code itself is TRIPLET, meaning three bases specify one amino acid, and DEGENERATE, meaning several different codons can specify the same amino acid, which is exactly why some single-base changes cause no change at all in the final protein.
Replication versus transcription is settled by the product and the timing: replication copies the whole genome as DNA, before cell division, while transcription copies one gene as RNA, whenever that gene's product is needed. Transcription versus translation is settled by location and product: transcription happens in the nucleus and makes RNA, while translation happens in the cytoplasm and makes protein. A retrovirus such as HIV is the deliberate exception worth knowing: it carries an enzyme, reverse transcriptase, that runs the usual direction backwards, building DNA from an RNA template, which is precisely the step that reverse transcriptase inhibitor drugs are designed to block.
A suspected fault anywhere in this pathway, from a single point mutation to a whole missing exon, is confirmed at the DNA level by sequencing, most often next-generation sequencing that reads out the actual base sequence of the gene in question. Where the downstream RNA or protein product itself needs checking, laboratories can use RNA sequencing or targeted protein assays, though DNA sequencing remains the standard first investigation once a specific gene is suspected from clinical or family history.
Molecular biology basics themselves are a foundation topic rather than a directly treatable condition, but confirming exactly where in this pathway a specific inherited disease's fault sits shapes real management: it determines whether GENETIC COUNSELLING, presymptomatic testing of relatives, or a specific molecular therapy is appropriate. In HIV, understanding that reverse transcriptase reverses the normal DNA-to-RNA direction directly explains why reverse transcriptase inhibitors, one pillar of WHO-recommended antiretroviral therapy, work specifically at that one reversed step.
When a molecular biology question describes a broken process, first place it on the DNA-to-RNA-to-protein line, then ask whether it is a copying problem, a nucleus-to-message problem, or a message-to-protein problem: that single placement usually points straight to the responsible enzyme.
The central dogma is a one-way pipeline, DNA to RNA to protein, and nearly every molecular genetics question is just asking which single pipe segment has broken.