Eukaryotic cells contain multiple copes of rrna genes(or called rDNA) to support the production of a large numbers of ribosomes. For example, the human genome contains about 200 copies of the gene encoding the single transcription unit for 5.8S, 18S and 28S rRNAS, and approximately 2000 copies of the gene encoding 5S rRNA. The genes for 18S, 5.8S and 28S rRNAS are distributed into 10 clusters in tandem array near the tip of one of the two copies of five different chromosomes(13, 14, 15, 21, and 22); the 5S rRNa genes locate in a single tandem array on chromosome 1. Another typical example is Xenopus oocytes, rRNA genes of mature oocytes (in diameter lmm) are amplified about 2000-fold resulting in -million copies of rRNA gene and nearly 1012 ribosomes per cell. The cells need take 500 years to produce such huge numbers of ribosome if the oocytes were without the selective amplification of rrna genes
Eukaryotic cells contain multiple copes of rRNA genes (or called rDNA) to support the production of a large numbers of ribosomes. For example, the human genome contains about 200 copies of the gene encoding the single transcription unit for 5.8S, 18S and 28S rRNAs, and approximately 2000 copies of the gene encoding 5S rRNA. The genes for 18S, 5.8S and 28S rRNAs are distributed into 10 clusters in tandem array near the tip of one of the two copies of five different chromosomes (13, 14, 15, 21, and 22); the 5S rRNA genes locate in a single tandem array on chromosome 1. Another typical example is Xenopus oocytes, rRNA genes of mature oocytes (in diameter 1mm) are amplified about 2000-fold, resulting in ~1million copies of rRNA gene and nearly 1012 ribosomes per cell. The cells need take ~500 years to produce such huge numbers of ribosome, if the oocytes were without t h e s e l e c t i v e a m p l i f i c a t i o n o f r R N A g e n e s
3′45S 41s 32S ④ 28S 目5.8s
lkb 18S5.85 28S Frog Nontranscribed spacer Gene Terminators promoter Mouse 25kb 5.8S 28S RNA transcription unit. The top drawing whose corresponding RNAs are degraded d fo nortion ofthe dna from a nucleo- are shown in vellow The nontranscribed sp
These multiple even amplified rrna genes are distributed to different numbers of the nucleolus, from one to thousands. Namely, following each cell division, nucleoli form around the chromosomal regions that contain the 5.8S 18S and 28S rRNA genes, these regions are therefore called nucleolar organizing regions(NOR). The variety of numbers of nucleolus of cells is not only due to the differentiation of species but depending on the development stages. Many species have two nucleolus numbers. But some have only one, like human; some have more than two nucleoli, such as a mature Xenopus oocyte three years old have thousands of the nucleoli The transcription and processing ofrRNAs Each nuclear organizing region contains a cluster of tandemly repeated large(5.8S,18S and 28S)rRNA genes
These multiple even amplified rRNA genes are distributed to different numbers of the nucleolus, from one to thousands. Namely, following each cell division, nucleoli form around the chromosomal regions that contain the 5.8S, 18S and 28S rRNA genes, these regions are therefore called nucleolar organizing regions(NOR). The variety of numbers of nucleolus of cells is not only due to the differentiation of species but depending on the development stages. Many species have two nucleolus numbers. But some have only one, like human; some have more than two nucleoli, such as a mature Xenopus oocyte three years old have thousands of the nucleoli. The transcription and processing of rRNAs Each nuclear organizing region contains a cluster of tandemly repeated large (5.8S, 18S and 28S ) rRNA genes
that are separated from each other by nontranscribed spacer DNA. These genes are very actively transcribed by rna polymerase I. In addition, most small nuclear RNAs, SnRNA (and mRNAs) are synthesized by rna polymerase Il, 5s RNA(and tRNA)are synthesized by rna polymerase III As mRNA, rRNAs are transcribed in the initial precursor RNA form, called primary transcript or pre-RNA, which derived from corresponding transcription unit segment of DNA. The primary transcript of large (5.8S, 18S and 28S) FRNa genes is the large 45S pre-rRNa with two external transcribed spaces at both 5 and 3 ends and two internal transcribed spaces(at between 18S/5.8S and 5.8S/28S) These transcribed spaces then are removed by a series of ordered cleavage steps during the early stages of and after the transcription, finally resulting in formation of mature rRNA (5.8S, 18S and 28S). Each of the tandemly arrayed
that are separated from each other by nontranscribed spacer DNA. These genes are very actively transcribed by RNA polymerase I. In addition, most small nuclear RNAs, snRNA (and mRNAs) are synthesized by RNA polymerase II, 5S RNA (and tRNA) are synthesized by RNA polymerase III. As mRNA, rRNAs are transcribed in the initial precursor RNA form, called primary transcript or pre-RNA, which derived from corresponding transcription unit segment of rDNA. The primary transcript of large (5.8S, 18S and 28S ) rRNA genes is the large 45S pre-rRNA with two external transcribed spaces at both 5’ and 3’ ends and two internal transcribed spaces (at between 18S/5.8S and 5.8S/28S). These transcribed spaces then are removed by a series of ordered cleavage steps during the early stages of and after the transcription, finally resulting in formation of mature rRNA (5.8S, 18S and 28S ). Each of the tandemly arrayed