the retirement of "junk DNA"
When scientists first realized that most of the genome does not code for protein, the leftover DNA picked up a dismissive nickname: 'junk DNA'. The image was of an attic full of clutter the cell carries around but never uses. Decades of work have made that label look hasty — not because all of it turned out to be precious, but because much of it turned out to do real work.
The term was coined in 1972 to describe DNA that seemed to have no protein-coding function, especially the genome's abundant repeats and pseudogenes. As tools improved, large fractions of this 'junk' were shown to be functional or at least active: regulatory sequences (enhancers, promoters) that control genes; DNA transcribed into functional non-coding RNAs; structural elements like centromeres and telomeres; and origins of replication. The ENCODE project reported biochemical activity across most of the genome, which intensified an honest scientific debate: biochemical activity (such as being transcribed at low levels) is not the same as being functional in the sense of being selected for and mattering to the organism.
So the careful position today is to retire the blanket word 'junk' without overcorrecting. Some non-coding DNA is clearly functional and conserved; some is best described as parasitic or self-propagating (transposons that copy themselves); and some may indeed be largely inert filler that the genome tolerates. The lesson is one of scientific humility: 'we don't yet know what it does' is not the same as 'it does nothing', and naming our ignorance 'junk' once slowed the search for the regulatory genome.
Long stretches once dismissed as junk near the gene for hemoglobin turned out to hold enhancers — switches that boost the gene in red-blood-cell precursors. Mutations in these 'junk' regions can cause inherited blood disorders even though they touch no protein-coding letter.
Sequence once labelled junk often holds the switches that control our genes.
Beware of two opposite errors: writing off all non-coding DNA as useless, or insisting every base must be functional. The honest answer is mixed — some is functional, some is selfishly self-copying, and some may be tolerated filler.