Composition and structure of chromosomal and amplified ribosomal DNA's of Xenopus laevis

摘要: Abstract Ribosomal DNA of Xenopus laevis is a homogenous component high buoyant density which contains genes for ribosomal RNA. This comprises about 0.2% the chromosomal somatic cells (chromosomal rDNA † ). In oocytes X. , amplified 1000-fold and additional copies (amplified rDNA) are located extrachromosomally. Both rDNA's have been isolated several their properties compared. The single unequivocal difference between these DNA's in content 5-methyl deoxycytidylic acid. Chromosomal 4.5% its residues as 5-MeC, whereas no detectable 5-MeC. 5-MeC on both strands rDNA, including gene regions transcribed vivo . base composition otherwise very similar; contain 67% deoxyguanylic acids (with 13% latter methylated rDNA). exhibit biphasic melting curve reflecting two with markedly different compositions. Differences thermal stability most likely due to contents were compared by ability hybridize rRNA complementary RNA, had from either Escherichia coli RNA polymerase. such hybrids nucleotide cRNA's indistinguishable. therefore considered be identical except content. A model presented includes data laboratories. consists repeating unit 9 × 10 6 daltons recurs 450 times at each nucleolar organizer region. sequence 40 s precursor molecule alternates equal length not into (“spacer region”). presents our estimates lengths region arrangement rDNA.