What is the Nucleolus?

Greer Hed

The nucleolus, also called a nucleole, is a cellular structure found within the nucleus of eukaryotic cells. There can be many nucleoli within a single cell nucleus, although normal human cells all have only one nucleolus. It is composed of nucleic acids and proteins and is responsible for the transcription and assembly of ribosomal ribonucleic acid (rRNA). rRNA is a major component of the cellular organelles known as ribosomes, which manufacture proteins for use by the rest of the cell.

The cell's nucleus in the control center and contains genetic information.
The cell's nucleus in the control center and contains genetic information.

A cell's nucleus is often described as its "control center," as it contains much of the cell's important genetic information. The nucleus also contains a number of structures referred to as subnuclear bodies, of which the nucleolus is one of the most well-known. Nucleoli are found in the nucleus around chromosomal regions called nucleolar organizing regions.

The nucleus of each human somatic cell contains 46 chromosomes.
The nucleus of each human somatic cell contains 46 chromosomes.

The transcription of rRNA takes place within this structure. Transcription is a process in which rRNA is synthesized using the existing genetic sequences found in deoxyribonucleic acid (DNA) as a template. There are three main stages of this process: initiation, chain elongation, and termination.

In the initiation stage of rRNA transcription, enzymes that promote the action of other enzymes — called RNA polymerases — bind to genes in a strand of DNA. Next, the RNA polymerase enzymes analyze and copy the DNA strand, which is one of two strands that form the DNA double helix. The other, complementary strand of DNA is what the enzymes recreate. Termination is the end of the process that occurs for different reasons in eukaryotic cells and prokaryotic cells like bacteria.

Once rRNA has been transcribed in the nucleolus, it is combined with a variety of protein molecules. Then the rRNA and proteins are assembled into two subunits, one large and one small, that will eventually combine to create a single ribosome. These subunits leave the cell's nucleus via pores found in the nuclear membrane. They enter the cytoplasm of the cell, where they come together to form a functioning ribosome. Since ribosomes' major function in a human body is to synthesize proteins from amino acids, cells that need more protein in order to function tend to have larger nucleoli.

Most ribosomes that are actively engaged in protein synthesis within a eukaryotic cell are found on the rough endoplasmic reticulum. Messenger ribonucleic acid (mRNA) provides the ribosomes with information that they translate into a specific amino acid sequence. A third type of RNA, called transfer ribonucleic acid (tRNA), then transfers amino acids to the interior of the ribosome, where they are assembled into protein chains.

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Discussion Comments


I would like to ask do rough endoplasmic reticulum have more amino acids than a nucleus?


@croydon - I think that argument is a bit flimsy though. I don't know enough evolutionary science to know all the details, but the way DNA works has been observed and it has been shown that it could have evolved by itself and how that works. I believe the RNA transcription process in the nucleolus structure is one of those things that can go "wrong" resulting in a new set of DNA and possibly, eventually, a new kind of animal (or plant, or whatever).

I actually think it's more of a miracle that this complex and beautiful system happened on its own. The idea that a god put it together like a kid playing with Lego doesn't seem nearly as wonderful and miraculous, it just seems kind of patronizing.


@pleonasm - To me, this isn't necessarily evidence for evolution. It could just as easily be evidence for intelligent design. I mean, a designer who gets a system right the first time doesn't need variations, so why put them in? So, the cell nucleolus and all the other bits inside a cell could be the product of a creator who was simply very clever and able to put together something that would work for any kind of animal or plant.


It always amazes me when I find some description of how amazingly complex life really is. I mean, this process of the different types of RNA to copy the DNA happens in almost every kind of cell, even bacteria cells, which is one of the strongest evidences of evolution, as it shows we all have a common ancestry.

And it's so complicated, but it managed to arise all by itself, and it is one of the processes responsible for all the many varied forms of life on the planet.

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