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Ribosomal DNA

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Title: Ribosomal DNA  
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Ribosomal DNA

The gene segment of eukaryotic rDNA contains 18S, 5.8S, and 28S tracts and forms a tandem repetitive cluster; the 5S rDNA is coded separately. NTS, nontranscribed spacer, ETS, external transcribed spacer, ITS, internal transcribed spacers 1 and 2, numbered from 5' end.

Ribosomal DNA (rDNA) is a DNA sequence that codes for acrocentric chromosomes 13,14,15,21 and 22.

Sequence homogeneity of the repeat unit

In the large rDNA array, polymorphisms between rDNA repeat units are very low, indicating that rDNA tandem arrays are evolving through

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Notes

The rDNA transcription tracts have low rate of polymorphism among species, which allows interspecific comparison to elucidate phylogenetic relationship using only a few specimens. Coding regions of rDNA are highly conserved among species but ITS regions are variable due to insertions, deletions, and point mutations. Between remote species as human and frog comparison of sequences at ITS tracts is not appropriate.[5] Conserved sequences at coding regions of rDNA allow comparisons of remote species, even between yeast and human. Human 5.8S rRNA has 75% identity with yeast 5.8S rRNA.[6] In cases for sibling species, comparison of the rDNA segment including ITS tracts among species and phylogenetic analysis are made satisfactorily.[7][8] The different coding regions of the rDNA repeats usually show distinct evolutionary rates. As a result, this DNA can provide phylogenetic information of species belonging to wide systematic levels.[9]

Sequence divergence to clarify phylogeny

5S tandem repeat sequences in several Drosophila were compared with each other; the result revealed that insertions and deletions occurred frequently between species and often flanked by conserved sequences.[4] They could occur by slippage of the newly synthesized strand during DNA replication or by gene conversion.[4]

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