Assembly of ribosomes
•Ribosomal RNA is transcribed as a 45S precursor RNA, synthesized in the nucleolus by polI from thousands of copies of the gene.
•The 45S precursor
(13,000 nt)is processed into 3 smaller RNAs
28S (5000 nt), 18S (2000 nt) and 5.8S (160
nt)
•The 5S subunit is synthesized by polIII from a cluster of 2000 genes located separately from the other ribosomal genes
•Some 80 proteins associate with the rRNAs to make up complete ribosome.
•Small ribosomal
subunit (40S) contains 18S rRNA while the
large 60S subunit contains the remaining rRNAs
•