Human Physiology, 14th edition (2016)

62 Chapter 3

3.3 Cell Nucleus and Gene Expression

The nucleus is the organelle that contains the DNA of a cell. A

gene is a length of DNA that codes for the production of a spe-

cific polypeptide chain. In order for genes to be expressed,

they must first direct the production of complementary RNA

molecules. That process is called genetic transcription.

The reverse of exocytosis is endocytosis, as previously
described; the membranous vesicle formed by that process is an
endosome. Some cellular proteins that were released by exocy-
tosis are recycled by a pathway that is essentially the reverse of
the one depicted in figure  3.12. This reverse pathway is called
retrograde transport, because proteins within the extracellular
fluid are brought into the cell and then taken to the Golgi appa-
ratus and the endoplasmic reticulum. Some toxins, such as the
cholera toxin, and proteins from viruses (including components
of HIV) rely on retrograde transport for their ability to infect cells.
LEARNING OUTCOMES

After studying this section, you should be able to:

5. Describe the structure of the nucleus and of
   chromatin, and distinguish between different types
   of RNA.


6. Explain how DNA directs the synthesis of RNA in
   genetic transcription.

Inner and outer

nuclear membranes

Nucleus

Chromatin

Pore Nucleolus

Nucleus

Inner

membrane

Outer

membrane

Pore

complex

Ribosome

Figure 3.13 The nuclear membranes and pores.

A diagram showing the inner and outer membranes and the

nuclear pore complexes. The nucleolus within the nucleus is

also shown.

| CHECKPOINTS

3a. Explain why microtubules and microfilaments can be

thought of as the skeleton and musculature of a cell.

3b. Describe the functions of lysosomes and

peroxisomes.

3c. Describe the structure and functions of mitochondria.

3d. Explain how mitochondria can provide a genetic

inheritance derived only from the mother.

3e. Describe the structure and function of ribosomes.

4. Distinguish the two types of endoplasmic
   reticulum and explain the relationship between the
   endoplasmic reticulum and the Golgi complex.

Most cells in the body have a single nucleus ( fig. 3.13 ). Excep-

tions include skeletal muscle cells, which have many nuclei,

and mature red blood cells, which have none. The nucleus is

enclosed by two membranes—an inner membrane and an outer

membrane—that together are called the nuclear envelope. The

outer membrane is continuous with the endoplasmic reticu-

lum in the cytoplasm. At various points, the inner and outer