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Other functional regions of the non-coding DNA fraction include regulatory sequences that control gene expression; scaffold attachment regions; origins of DNA replication; centromeres; and telomeres. Some non-coding regions appear to be mostly nonfunctional, such as introns , pseudogenes , intergenic DNA , and fragments of transposons and viruses .
Junk DNA (non-functional DNA) is a DNA sequence that has no known biological function. [ 1 ] [ 2 ] Most organisms have some junk DNA in their genomes —mostly, pseudogenes and fragments of transposons and viruses—but it is possible that some organisms have substantial amounts of junk DNA.
Pseudogenes are nonfunctional segments of DNA that resemble functional genes.Pseudogenes can be formed from both protein-coding genes and non-coding genes. In the case of protein-coding genes, most pseudogenes arise as superfluous copies of functional genes, either directly by gene duplication or indirectly by reverse transcription of an mRNA transcript.
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]
The non-intron sequences that become joined by this RNA processing to form the mature RNA are called exons. [ 3 ] Introns are found in the genes of most eukaryotes and many eukaryotic viruses and they can be located in both protein-coding genes and genes that function as RNA ( noncoding genes ).
The A form occurs under non-physiological conditions in partly dehydrated samples of DNA, while in the cell it may be produced in hybrid pairings of DNA and RNA strands, and in enzyme-DNA complexes. [54] [55] Segments of DNA where the bases have been chemically modified by methylation may undergo a larger change in conformation and adopt the Z ...
Many repeat sequences are likely to be non-functional, decaying remnants of Transposable elements, these have been labelled "junk" or "selfish" DNA. [7] [8] [9] Nevertheless, occasionally some repeats may be exapted for other functions. [10]
A conserved non-coding sequence (CNS) is a DNA sequence of noncoding DNA that is evolutionarily conserved. These sequences are of interest for their potential to regulate gene production. [1] CNSs in plants [2] and animals [1] are highly associated with transcription factor binding sites and other cis-acting regulatory elements.