Domains of Life; Prokaryotic vs. Eukaryotic Cells

ID #1491

I heard that the reason why nucleolus in the nucleus looks darkest (dense) is that there are many ribosomes. I think there are more ribosomes on rough ER. Am I wrong? Also, I think nucleolus looks dense because of DNA. Am I wrong?

The reason the nucleolus looks so dense under transmission electron microscopy has to do with ribosomes, but it's not functional ribosomes themselves that make it dense. It's because the nucleolus is where the parts are assembled (and some of them synthesized) that will go into ribosomes. Since there are so many ribosomes needed in the cell, this little "factory" is very busy, and there are a great number of molecules in a relatively small region. This makes it look very dense, because it is! But these ribosomes that are being assembled won't have the finishing touches put on them until they are exported out of the cytoplasm, and it is in the cytoplasm where ribosomes are facilitating protein synthesis.

Regarding your question about the nucleolus looking dark because of DNA, that is not the complete concept. Yes, there is a great deal of DNA found in that region, because it is being transcribed into pieces of RNA that will become part of ribosomes. But there's also a whole lot of DNA in the rest of the nucleus, and most of it doesn't have anything to do with ribosomes. So it won't be in the nucleolus, and won't contribute to the density of that functional domain. We'll be talking a lot more about this in a few weeks.

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