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.
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 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.