UPSC Prelims 2023 Q70: Microsatellite DNA and Its Uses — Explained
'Microsatellite DNA' is used in the case of which one of the following?
- Studying the evolutionary relationships among various species of fauna
- Stimulating 'stem cells' to transform into diverse functional tissues
- Promoting clonal propagation of horticultural plants
- Assessing the efficacy of drugs by conducting series of drug trials in a population
Answer: (a) Studying the evolutionary relationships among various species of fauna
Statement-wise explanation
- a: correct — Microsatellites are stretches of DNA in which a very short motif of 1–6 base pairs repeats over and over (for example, CACACA...). The number of repeats at a given site changes quickly between generations, so two individuals, populations or related species seldom carry the same set of repeat lengths. Comparing these lengths across many sites shows how closely groups are related, which makes microsatellites a standard tool for studying phylogeny and population genetics in animals.
- b: incorrect — Stem cells are turned into specialised tissues by growth factors, signalling molecules and transcription factors. Adult cells are reprogrammed into induced pluripotent stem cells by four 'Yamanaka factors' (Oct4, Sox2, Klf4, c-Myc). Microsatellites are non-coding repeat sequences that serve as markers to read. They are not used as signals to change what a cell becomes.
- c: incorrect — Horticultural plants are cloned by vegetative methods such as cuttings, grafting and layering, and by plant tissue culture (micropropagation). Totipotency, nutrient media and plant hormones like auxins and cytokinins make these methods work. Microsatellite markers may later confirm that the clones are genetically true to type, but they do not propagate plants.
- d: incorrect — A drug's efficacy is measured in clinical trials. These are phased studies on human volunteers (Phase I to IV), and in India they are governed by the New Drugs and Clinical Trials Rules, 2019. The trials compare clinical outcomes between treated and control groups. Microsatellite analysis is not how a drug's efficacy is judged.
How to eliminate
Think of a microsatellite as a DNA marker, a stretch of DNA you read and compare. It is not a tool that makes something happen. Of the four options, only one involves comparing DNA between organisms. Stem cell differentiation is driven by growth factors and transcription factors. Clonal propagation is a tissue-culture and vegetative-propagation technique. Drug efficacy is measured with clinical outcomes. The word 'DNA' in the question points to genetics, and the only option about genetic comparison is evolutionary relationships. If you remember NCERT Class XII's DNA fingerprinting section, where repetitive or satellite DNA varies between individuals, you can settle it quickly. Fact to remember: microsatellites, also called SSRs or STRs, are highly variable repeat markers used in DNA fingerprinting, population genetics and phylogeny.
Know more
Satellite DNA is DNA made of short sequences repeated many times in a row (tandem repeats). It does not code for proteins. NCERT Class XII (Molecular Basis of Inheritance) uses it to explain DNA fingerprinting. Satellite DNA is grouped by the size of the repeat unit:
| Type | Repeat unit | Other names | Main uses |
|---|---|---|---|
| Microsatellite | 1–6 bp | SSR (simple sequence repeat), STR (short tandem repeat) | Forensics, paternity tests, population genetics, phylogeny, crop breeding |
| Minisatellite | ~10–60 bp | VNTR (variable number of tandem repeats) | Early DNA fingerprinting (Alec Jeffreys, 1984–85) |
| Satellite (classical) | longer, up to hundreds of bp | — | Centromere and heterochromatin structure |
Why microsatellites are useful markers
- Highly polymorphic: the number of repeats mutates quickly, mostly through DNA-polymerase slippage during replication. Individuals and populations therefore differ.
- Codominant: both alleles (the copies from each parent) can be read, which helps parentage and population studies.
- PCR-friendly: a tiny or degraded sample, such as hair, scat or old tissue, is often enough.
Applications to remember
- Forensics: human DNA profiling uses a standard panel of STR sites (for example, the CODIS panel in the US).
- Wildlife conservation: estimating tiger and elephant numbers from scat, tracing animal parts seized from poachers, and measuring inbreeding in isolated populations.
- Evolution and taxonomy: measuring genetic distance between closely related species or populations.
- Agriculture: SSR markers for marker-assisted breeding and for registering crop varieties.
In India, the Centre for DNA Fingerprinting and Diagnostics (CDFD, Hyderabad) and CSIR-CCMB's LaCONES lab apply these markers to forensics and to conserving endangered species.