(89)
6. Angive at rRNA findes som repeterede sekvenser i det humane genom
Devlin, s.218 – fig.5.7
Issinger, 1.12-13

Ribosomal RNA genes are arranged in many copies one after the other. Each copy is transcribed separately and each transcript is processed into three separate RNA species. Promoter and enhancer genes are located in the nontranscribed regions on the tandemly repeated sequences. 

Each transcriptional unit contains sequences for the 28S, 5.8 S and 18S rRNA, in that order. Several hundred copies (ca. 280) for each transcriptional unit occur tandemly (one after another) on five different chromosomes. 

rRNA primary product of each transcription unit is a long rRNA, termed 45S RNA, containing only one copy of 28S, 5.8S and 18S  sequences, insuring equal amounts of each of these unit. The primary transcript is then processed and three mature rRNA spices are created.  

RNA polymerase I is used as a catalysing enzyme.  

All rRNA genes, except the 5S rRNA, are located in the nucleolus, on a specific chromosomal region called the “nucleolar organiser“. The 5S rRNA is transcribed by RNA polymerase III in the nucleoplasm and has 2000 copies in the human genome.

The reason for this many repeats of the rRNA genes is that rRNA is a structural protein. Each transcript of the gene produces only one copy of each rRNA molecule.

In comparison to other structural proteins that can be produced many times, if the mRNA that codes for them is translated by many ribosomes before it is destroyed by the exonucleases in the cytoplasm. 

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