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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 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 ...
Non-B DNA can have significant implications for DNA biology and human health. For example, Z-DNA has been implicated in immunity and autoimmune diseases, such as lupus and arthritis. [6] H-DNA has been implicated in genomic instability and cancer, and G-quadruplexes have been linked to telomere maintenance, [7] oncogene activation, and cancer. [8]
Non-functional DNA is rare in bacterial genomes which typically have an extremely high gene density, with only a few percent being not protein-coding. [60] However, in most animal or plant genomes, a large fraction of DNA is non-functional, given that there is no obvious selective pressure on these sequences.
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]
Different TLRs in human detect different PAMPS: lipopolysaccharides by TLR4, viral dsRNA by TLR3, viral ssRNA by TLR7/TLR8, viral or bacterial unmethylated DNA by TLR9. TLR9 has evolved to detect CpG DNA commonly found in bacteria and viruses and to initiate the production of IFN (type I interferons ) and other cytokines. [44]
A study that included, but was not limited to, 478 bacterial genomes, concluded that as genome size increases, the number of genes increases at a disproportionately slower rate in eukaryotes than in non-eukaryotes. Thus, the proportion of non-coding DNA goes up with genome size more quickly in non-bacteria than in bacteria.
Spacer DNA is a region of non-coding DNA between genes. [1] [2] The terms intergenic spacer (IGS) or non-transcribed spacer (NTS) are used particularly for the spacer DNA between the many tandemly repeated copies of the ribosomal RNA genes. [3] In bacteria, spacer DNA sequences are only a few nucleotides long.