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16. Angive fordelingen af rRNA i ribosomerne
Stryer, s.824-6, fig. 29.17; 29.16
Devlin, s.83, s.243-tab.6.7

rRNA constitutes for 2/3 of the entire mass of the ribosome (1/3 made of protein). rRNAs are folded into defined structures with many short duplex regions. The key sites of ribosomes are made of RNA, while the proteins serve as structural scaffolding.  

Each ribosome is made up of a big and a small subunit, containing different rRNAs. The rRNA molecules in the ribosomes are named after their sedimentation constant.

There is a difference in the rRNA content in prokaryotes and eukaryotes:

• Eukaryotes – one ribosome (80S) is made up of 4 rRNAs and 82 proteins. The small subunit (40S) is made up of 18S RNA and 33 proteins. The large subunit (60S) is made up of 28S, 5.8 and 5S rRNA and 49 proteins.
   

• Prokaryotes - one ribosome (70S) is made up of 3 rRNAs and 55 proteins. The small subunit (30S) contains 16S rRNA and 21 proteins, while the large subunit (50S) has 34 proteins and 5S and 23S rRNA.

In eukaryotes, all rRNAs are transcribed by RNA polymerase I in the nucleolus, except 5S rRNA which is synthesized by RNA polymerase III in the nucleoplasm.

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