Non-coding DNA
DNA sequences that do not encode proteins, including functional and junk regions.
Non-coding DNA (ncDNA) refers to DNA sequences that do not encode proteins. While some non-coding DNA is transcribed into functional non-coding RNA molecules (such as transfer RNA, microRNA, and ribosomal RNA) or contains regulatory sequences, other regions appear to be mostly nonfunctional and are termed junk DNA. The fraction of non-coding DNA varies greatly between organisms, from about 12% in bacteria to 98–99% in the human genome.
- type
- genomic component
- key_finding
- 98–99% of human genome is non-coding DNA
- bacterial_noncoding_fraction
- 12%
- human_coding_fraction
- 1–2%
- pufferfish_noncoding_fraction
- 90%
- bladderwort_noncoding_fraction
- 70%
- human_noncoding_gene_fraction
- at least 6%
Lore & Background
Non-coding DNA includes a variety of functional elements such as non-coding RNA genes (e.g., transfer RNA, ribosomal RNA, microRNA, piRNA), regulatory sequences that control gene expression, scaffold attachment regions, origins of DNA replication, centromeres, and telomeres. Other regions, like introns, pseudogenes, intergenic DNA, and fragments of transposons and viruses, appear to be mostly nonfunctional. The concept of junk DNA emerged from observations that much repetitive DNA in eukaryotes lacks known function.
Reader's Guide
The study of non-coding DNA has reshaped understanding of genome evolution and function. The C-value paradox—the observation that genome size varies widely even among closely related species—was resolved by recognizing that most differences come from expansion and contraction of repetitive DNA, much of which may be junk. The G-value paradox highlights that the number of genes does not correlate with perceived complexity; for example, the unicellular Polychaos dubium has over 200 times the DNA of humans, while the pufferfish Takifugu rubripes has a genome one-eighth the size of the human genome yet a comparable number of genes. The bladderwort plant Utricularia gibba demonstrates that a multicellular organism can function with a greatly reduced genome by expunging junk DNA, retaining only necessary sequences. Disputes remain over the number of functional noncoding genes in humans, with estimates ranging from about 5,000 to over 100,000, largely due to debate over long noncoding RNA genes.
Did You Know?
- In bacteria, coding regions typically take up 88% of the genome, leaving 12% non-coding.
- The human genome contains somewhere between 1–2% coding DNA, meaning 98–99% is non-coding.
- The pufferfish Takifugu rubripes genome is about one-eighth the size of the human genome but has a comparable number of genes.
- The bladderwort plant Utricularia gibba has a genome of 100.7 Mb, likely evolved from a 1,500 Mb ancestral genome, and has roughly the same number of genes as other plants.
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