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The nonfunctional DNA in bacterial genomes is mostly located in the intergenic fraction of non-coding DNA but in eukaryotic genomes it may also be found within introns. There are many examples of functional DNA elements in non-coding DNA, and it is erroneous to equate non-coding DNA with junk DNA.
The common theme is that the original proponents of junk DNA thought that all non-coding DNA was junk. [2] [6] This claim has been attributed to a paper by David Comings in 1972 [28] where he is reported to have said that junk DNA refers to all non-coding DNA. [19] But Comings never said that.
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DNA sequences that carry the instructions to make proteins are referred to as coding sequences. The proportion of the genome occupied by coding sequences varies widely. A larger genome does not necessarily contain more genes, and the proportion of non-repetitive DNA decreases along with increasing genome size in complex eukaryotes. [38]
The mtDNA control region is an area of the mitochondrial genome which is non-coding DNA. This region controls RNA and DNA synthesis. [1] It is the most polymorphic region of the human mtDNA genome, [2] with polymorphism concentrated in hypervariable regions. The average nucleotide diversity in these regions is 1.7%. [3]
Non-functional DNA elements such as pseudogenes and repetitive DNA, both of which are types of junk DNA, can also be found in intergenic regions—although they may also be located within genes in introns. [2] It is possible that these regions contain as of yet unidentified functional elements, such as non-coding genes or regulatory sequences. [3]
Cis-regulatory elements (CREs) or cis-regulatory modules (CRMs) are regions of non-coding DNA which regulate the transcription of neighboring genes.CREs are vital components of genetic regulatory networks, which in turn control morphogenesis, the development of anatomy, and other aspects of embryonic development, studied in evolutionary developmental biology.
Eventually pseudogenes may be deleted from their genomes by chance of DNA replication or DNA repair errors, or they may accumulate so many mutational changes that they are no longer recognizable as former genes. Analysis of these degeneration events helps clarify the effects of non-selective processes in genomes.