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What is a Nucleolus? - Definition & Function

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  • 0:01 What is a Nucleolus?
  • 0:32 Functions of the Nucleolus
  • 1:15 Cells Without Nuclei
  • 1:38 Lesson Summary
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Lesson Transcript
Instructor: Nicholas Gauthier
The nucleolus is that mysterious round structure we are all taught to draw inside the nucleus of a cell. We know that it is difficult to spell, but more importantly, what does it do? Find out in this lesson!

What is a Nucleolus?

The nucleolus is a round body located inside the nucleus of a eukaryotic cell. It is not surrounded by a membrane but sits in the nucleus. The nucleolus makes ribosomal subunits from proteins and ribosomal RNA, also known as rRNA. It then sends the subunits out to the rest of the cell where they combine into complete ribosomes. Ribosomes make proteins; therefore, the nucleolus plays a vital role in making proteins in the cell.

Function of the Nucleolus

Ribosomes are made of two subunits, a large and a small. Each subunit is manufactured separately in the nucleolus. The proteins needed to make them enter the nucleus through pores in the nuclear membrane. The rRNA is made in the nucleolus itself. Once made, the subunits leave through the nuclear pores and combine elsewhere in the cell.

Ribosomes can be found in various places in the cell, including the cytosol, the endoplasmic reticulum and the nuclear membrane. They are responsible for creating all of the proteins that the cell needs. The more proteins a cell needs to make, the more ribosomes it needs, and therefore, the larger the nucleolus.

Cells Without Nuclei

Prokaryotic cells, such as bacteria, do not have a nucleus or a nucleolus. In such cells, ribosomes are made in the cytoplasm. The DNA for making these ribosomes is in the nucleoid. While prokaryotic cells may have some fairly complex biochemical processes, they do not possess the specialized structure of the mitochondrion.

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