Questions and Answers about BIOLOGY

by Ken Miller and Joe Levine


QUESTION: I have a question that my students asked me while we were studying cells. I was puzzled. If the nucleolus of a eukaryotic cell is responsible for ribosome production, how do prokaryotic cells have ribosomes? (Melissa, a teacher from Sugarland, Texas)

That's a great question, and it points up one of the big differences between eukaryotic and prokaryotic cells. Most prokaryotic cells have only a handful of genes for the ribosomal RNA molecules that make up ribosomes. They transcribe those genes and also synthesize the 70 or 80 proteins that make up ribosomes, and the ribosomes are assembled at dispersed sites throughout the cell that are too small to see in the light microscope.

Eukaryotic cells, however, are generally much bigger. Therefore, they need to produce many more ribosomes, and they have multiple copies of the ribosomal RNA genes.... sometimes several hundreds. The transcription of so many rRNA molecules in the same region draws in a host of ribosomal components, and this aggregation of protein and RNA crowds out other molecules, producing the dense spot in the nucleus that we call the nucleolus.

Interestingly, when some organisms (like frogs) form the huge cells that are their eggs (a frog egg may be several millimeters in diameter) they also produce thousands of extra copies of some of their ribosomal RNA genes.... these extra clusters of rRNA do the same thing, causing ribosome components to aggregate at several places in the cells large nuclei, and this forms what cell biologists call "accessory nucleoli."

So, it's all a matter of size. The sheer number of rRNA genes in eukaryotes is what produces the visible nucleolus.

I hope this helps to answer your student's excellent question!

Ken Miller (10/12/03)

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(A web site developed by Ken Miller and Joe Levine to provide scientific and educational support for teachers and students using our textbooks)