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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.
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 rDNA gene cluster of eukaryotes consists of the genes for the 18S, 5.8S and 28S rRNA, separated by the two ITS-1 and ITS-2 spacers. The active genome of eukaryotes contains several hundred copies of the rDNA transcriptional unit as tandem repeats, they are organized in nucleolus organizer regions (NORs), [4] which in turn can be present at ...
The structure of a eukaryotic protein-coding gene. Regulatory sequence controls when and where expression occurs for the protein coding region (red). Promoter and enhancer regions (yellow) regulate the transcription of the gene into a pre-mRNA which is modified to remove introns (light grey) and add a 5' cap and poly-A tail (dark grey).