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In eukaryotes, genes encoding ribosomal RNA and spacers occur in tandem repeats that are thousands of copies long, each separated by regions of non-transcribed DNA termed intergenic spacer (IGS) or non-transcribed spacer (NTS). Each eukaryotic ribosomal cluster contains the 5' external transcribed spacer (5' ETS), the 18S rRNA gene, the ITS1 ...
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.
In humans, intergenic regions comprise about 50% of the genome, whereas this number is much less in bacteria (15%) and yeast (30%). [4] As with most other non-coding DNA, the GC-content of intergenic regions vary considerably among species. For example in Plasmodium falciparum, many intergenic regions have an AT content of 90%. [5]
Eukaryotic transcription occurs within the nucleus where DNA is packaged into nucleosomes and higher order chromatin structures. The complexity of the eukaryotic genome necessitates a great variety and complexity of gene expression control. Eukaryotic transcription proceeds in three sequential stages: initiation, elongation, and termination. [1]
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.
RISA involves PCR amplification of a region of the rRNA gene operon between the small and large subunits called the intergenic spacer region ISR. [2] By using oligonucleotide primers targeted to conserved regions in the 16S and 23S genes, RISA fragments can be generated from most of the dominant bacteria in an environmental sample.
The term "repeated sequence" was first used by Roy John Britten and D. E. Kohne in 1968; they found out that more than half of the eukaryotic genomes were repetitive DNA through their experiments on reassociation of DNA. [5] Although the repetitive DNA sequences were conserved and ubiquitous, their biological role was yet unknown.
The H/ACA box and C/D box snoRNAs are ncRNAs found in archaea and eukaryotes. RNase MRP is restricted to eukaryotes. Both groups of ncRNA are involved in the maturation of rRNA. The snoRNAs guide covalent modifications of rRNA, tRNA and snRNAs; RNase MRP cleaves the internal transcribed spacer 1 between 18S and 5.8S rRNAs.