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|Chapter Three: Cell Structures|
The nucleus is usually the easiest organelle to see under a microscope. It contains all of the genetic material (DNA) of a cell. If you'd like a more information on DNA, Chapter One provides an overview and Chapter Six discusses it in detail. Through a long process which will be discussed in Chapter Six, the DNA within the nucleus controls the production of proteins in the cell which in turn affects many of the cell's organelles and processes. That's why the nucleus is sometimes called the cell's "brain."
In eukaryotic cells, the nucleus is surrounded by a nuclear membrane (sometimes called the nuclear envelope). The membrane contains tiny pores which allow RNA (ribonucleic acid, a molecule similar to DNA) to pass through. RNA is used outside of the nucleus in the production of proteins.
Within the nucleus is a structure (although there can be as many as four of them) called the nucleolus where organelles called ribosomes are created. Under a microscope, the nucleolus looks like a large dark spot within the nucleus. When a cell reproduces (see Chapter Five), the nucleolus disappears.
As mentioned in other sections, DNA is ultimately involved in the production of proteins. These proteins are synthesized (produced) at special sites called ribosomes. Ribosomes consist of ribosomal RNA (sometimes written as rRNA) and protein, grouped into one large subunit and one small subunit.
The rRNA which is a main element of ribosomes is created in the nucleolus, and the protein portion of the ribosome is synthesized in the cell's cytoplasm. These proteins enter the nucleus through the pores in the nuclear membrane and combine with the rRNA to form the small and large subunits. The completed subunits leave the nucleus and can be found in the cytoplasm or in an organelle known as the endoplasmic reticulum.