In what are non-coding regions of DNA? post we have briefly explained about amount of DNA and coding and non-coding regions of DNA.
The vertebrates with the greatest amount of DNA per cell are amphibians, which are surely less complex than humans in their structure and behaviour. The unicellular protozoa species Amoeba dubia has 200 times more DNA per cell than humans.
Many plant species also have considerably more DNA per cell than humans have. For example, tulips have 10 times as much DNA per cell as humans. The DNA content per cell also varies considerably between closely related species.
All insects or all amphibians would appear to be similarly complex, but the amount of haploid DNA in species within each of these phylogenetic classes varies by a factor of 100. The genomes of higher eukaryotes contain large amounts of noncoding DNA. For example, only a small portion of the β-globin gene cluster of humans, about 80 kb long, encodes protein.
Non-coding Regions of DNA
Moreover, compared with other regions of vertebrate DNA, the β-globin gene cluster is unusually rich in protein-coding sequences, and the introns in globin genes are considerably shorter than those in many human genes.
In contrast, a typical 80-kb stretch of DNA from the yeast S. cerevisiae, a single-celled eukaryote contains many closely spaced protein-coding sequences without introns and relatively much less noncoding DNA.
Of the 94 percent of human genomic DNA that has been sequenced, only ≈1.5 percent corresponds to protein-coding sequences (exons). Most human exons contain 50–200 base pairs, although the 3’ exon in many transcription units is much longer.
Human introns vary in length considerably. Although many are ≈90 bp long, some are much longer; their median length is 3.3 kb. Approximately one-third of human genomic DNA is thought to be transcribed into pre-mRNA precursors, but some 95 percent of these sequences are in introns, which are removed by RNA splicing.
Different selective pressures during evolution may account, at least in part, for the remarkable difference in the amount of non-functional DNA in unicellular and multicellular organisms.
For example, microorganisms must compete for limited amounts of nutrients in their environment, and metabolic economy thus is a critical characteristic.
Since synthesis of non-functional (i.e., noncoding) DNA requires time and energy, presumably there was selective pressure to lose non-functional DNA during the evolution of microorganisms.
On the other hand, natural selection in vertebrates depends largely on their behaviour. The energy invested in DNA synthesis is trivial compared with the metabolic energy required for the movement of muscles; thus there was little selective pressure to eliminate non-functional DNA in vertebrates.