45S pre-ribosomal RNAs
45s pre-ribosomal rnas
The sequences coding for ribosomal RNAs are present as rDNA repeating units, designated RNR1 through RNR5, in the p12 region of chromosomes 13, 14, 15, 21 and 22. A 45S rRNA which serves as the precursor for the 18S, 5.8S and 28S rRNA, is transcribed from each rDNA unit by RNA polymerase I. The number of rDNA repeating units varies between individuals and from chromosome to chromosome, although usually 30 to 40 repeats are found on each chromosome. [Note that the clusters are extremely difficult to sequence and are therefore not represented on the reference genome assembly. Each gene symbol below is representative of one 45S pre-ribosomal RNA repeat unit per repeat cluster]