1. tRNAs are transcribed
in the nucleoplasm by
RNA polymerase III.
The primary transcript is processed to the mature
tRNA and transported to the cytoplasm. In the
cytoplasm the tRNAs
are coupled to their
2. Pre-mRNAs, containing
the sequences coding for the ribosomal proteins, are
RNA polymerase II
in the nucleoplasm. The pre-mRNAs are processed (see
splicing) and transported (see transport) as
mRNP-particles to the
they are picked up by the ribosomes
3. The ribosomal RNAs (except 5S rRNA) are transcribed in the nucleolus as one giant precursor RNA by RNA polymerase I. The precursor is processed (see rRNA processing) to 18S,5.8S and 28S rRNA. 5S rRNA is transcribed in the nucleoplasm by RNA polymerase III and transported to the nucleolus. The rRNAs are folded and associate with ribosomal proteins to form the 40S and 60S ribosomal subunits. The subunits are then transported from the nucleolus to the cytoplasm.
An example of the Rate of Ribosome Synthesis:
*HeLa cells (a type of human
tumour cells) divide each 24 hours.