Nucleus

Most of the functions of a eukaryotic cell are controlled by the nucleus, shown in Figure 4-12. The nucleus is filled with a jellylike liquid called the nucleoplasm, which holds the contents of the nucleus and is similar in function to a cell's cytoplasm.

The nucleus houses and protects the cell's genetic information. The hereditary information that contains the instructions for the structure and function of the organism is coded in the organism's DNA, which is contained in the nucleus. When a cell is not dividing, the DNA is in the form of a threadlike material called chromatin. When a cell is about to divide, the chromatin condenses to form chromosomes. Chromosomes are structures in the nucleus made of DNA and protein.

The nucleus is the site where DNA is transcribed into ribonucleic acid (RNA). RNA moves through nuclear pores to the cytoplasm, where, depending on the type of RNA, it carries out its function.

Nuclear Envelope

The nucleus is surrounded by a double membrane called the nuclear envelope. The nuclear envelope is made up of two phos-pholipid bilayers. Covering the surface of the nuclear envelope are tiny, protein-lined holes, which are called nuclear pores. The nuclear pores provide passageways for RNA and other materials to enter and leave the nucleus.

Nucleolus

Most nuclei contain at least one denser area, called the nucleolus (noo-KLEE-uh-luhs). The nucleolus (plural, nucleoli) is the site where DNA is concentrated when it is in the process of making ribosomal RNA. Ribosomes (RIE-buh-SOHMZ) are organelles made of protein and RNA that direct protein synthesis in the cytoplasm.

Nuclear envelope

Nuclear pores figure 4-12

The nucleus of a cell is surrounded by a double membrane called the nuclear envelope. The nucleus stores the cell's DNA.

Nuclear pores

Nuclear envelope

Outer membrane figure 4-13

Mitochondria convert organic molecules into energy for the cell. Mitochondria have an inner membrane and an outer membrane. The folds of the inner membrane, called cristae, are the site of energy conversion.

Outer membrane figure 4-13

Mitochondria convert organic molecules into energy for the cell. Mitochondria have an inner membrane and an outer membrane. The folds of the inner membrane, called cristae, are the site of energy conversion.

Large subunit

Small subunit figure 4-14

Large subunit

Small subunit

Ribosomes are the organelles responsible for building protein. Ribosomes have a large and small subunit, each made of protein and ribosomal RNA. Some ribosomes are free in the cell. Others are attached to the rough endoplasmic reticulum.
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