UPSC Prelims 2022 Q99: DNA Barcoding and Its Uses — Explained

Consider the following statements:

DNA Barcoding can be a tool to:

1. assess the age of a plant or animal.

2. distinguish among species that look alike.

3. identify undesirable animal or plant materials in processed foods.

Which of the statements given above is/are correct?

  1. 1 only
  2. 3 only
  3. 1 and 2
  4. 2 and 3

Answer: (d) 2 and 3

Statement-wise explanation

  • 1: incorrect — A DNA barcode is a short, standard stretch of DNA, such as the mitochondrial COI gene in animals. Its sequence is the same in every cell, and it does not change over the organism's life. A barcode taken from a seedling is the same as one taken from the old tree, so it can say which species a sample is but not how old the individual is. Age is estimated with other tools, such as growth rings in trees or radiocarbon dating for dead material.
  • 2: correct — Barcoding was built for this purpose. Species that look almost the same, called cryptic species, usually still differ clearly in their barcode sequence. Matching an unknown sample against a reference library such as BOLD (Barcode of Life Data System) can tell them apart. It also links eggs, larvae or fragments to the right species when there is no adult body to examine.
  • 3: correct — Processing removes the features you could identify by eye, such as shape, colour and scales, but DNA fragments often survive. Barcoding processed fish, meat or herbal products has repeatedly shown cheaper or undeclared species being substituted. One Brazilian study found about 26–30% of seafood samples from fisheries and restaurants were mislabelled. This makes barcoding a standard tool for detecting food fraud and adulteration.

How to eliminate

Ask what a barcode really is. It is a fixed label for a species, much like a product barcode, and it does not change over time. That rules out statement 1, because something that stays the same throughout life cannot measure age. With statement 1 gone, only the options without it are left: '3 only' and '2 and 3'. Statement 2 is the textbook definition of barcoding (identifying species from a short DNA sequence). If you know that, the answer follows. Fact to remember: barcoding identifies WHAT species a sample is, from any tissue or life stage. It never tells WHEN the organism was born.

Know more

DNA barcoding identifies a species from a short, standardised gene region (about 400–800 base pairs). The sample's sequence is compared with a reference library. Paul Hebert (University of Guelph, Canada) proposed the method in 2003. A good barcode region varies little within a species and clearly between species.

Group Standard barcode region Location
Animals COI (cytochrome c oxidase subunit I), ~650 bp Mitochondrial DNA
Land plants rbcL + matK Chloroplast DNA
Fungi ITS (internal transcribed spacer) Nuclear ribosomal DNA

COI works poorly in plants because plant mitochondrial genes change too slowly. That is why plants use chloroplast genes instead.

Uses

  • Telling apart cryptic (look-alike) species and finding new ones
  • Identifying eggs, larvae, seeds or fragments that have no diagnostic features
  • Detecting food fraud: fish substitution, undeclared meat, adulterated herbal products
  • Wildlife forensics: identifying poached animal parts, ivory, or traded timber and fish
  • Biosecurity: spotting invasive pests and disease vectors at borders

Limits

  • It cannot give age, sex or health. The sequence is the same across the whole lifespan.
  • It is only as good as the reference library. A species missing from the library cannot be named.
  • Recently diverged species or hybrids may share barcodes.
  • Heavily processed or heated samples may have DNA that is too broken to read.

Related terms

  • Metabarcoding: reads many species at once from a mixed sample.
  • eDNA: DNA collected from water or soil. Metabarcoding of eDNA lets scientists survey biodiversity without catching animals.

India: the Zoological Survey of India (ZSI) runs a Centre for DNA Taxonomy. India joined the International Barcode of Life (iBOL) consortium in 2020, with ZSI as the member body.

In the news

In 2020 India joined the International Barcode of Life (iBOL) consortium through the Zoological Survey of India. The Union Cabinet approved the ZSI–iBOL MoU in October 2020, which kept DNA barcoding in the news before the 2022 paper. The question itself tests static science.