Fundamentals of Anatomy and Physiology

Cell Structure

Nuclear Membrane
A unique feature of the nuclear membrane or envelope is
that it is composed of two membranes (Figure 3-6). The in-
ner membrane surrounds and contains the nucleoplasm

Nucleus

Nuclear pores

Two membranes (^) Nucleus
of nuclear (^)
envelope (^) Pore
Chromatin (^)
Nucleolus (^)
(A) Cisterna
of rough ER
Nucleus
Inner membrane
Outer membrane
Pore
complex
(^) ®
Learning
(^) (B) Cengage ©
Figure 3- 6 The structure of the nuclear membrane or
envelope-. (A) Diagrammatic view of the internal
anatomy of the nucleus and the connection of the outer
nuclear membrane with the rough endoplasmic
reticulum. (B) Diagrammatic view of the pore complex.

45

and its materials. The outer membrane is continuous with

the endoplasmic reticulum (ER), an organelle discussed

later. The electron microscope has revealed the presence of

pores or openings in the double nuclear membrane. These

pores have a very fine partition to hinder the free transport

or leakage of materials of the nucleoplasm but which allow

the passage of materials from the nucleo-plasm, which

must gain access to the cytoplasm-. For example,- when

protein synthesis must take place, the code to make the

protein is on the DNA in the nucleus but the protein is

made at a ribosomal site in the cytoplasm. The code is

copied from the DNA by a special molecule called

messenger RNA (mRNA), which leaves the nucleus

through a pore to go to the ribosome. This process is dis-

cussed in more detail later in the chapter-. The structure of

the nuclear membrane is the same fluid mosaic model as

The Cell Membrane.

Nucleoplasm

The fluid medium of the nucleus is called the nucleo-plasm.

It consists of a colloidal suspension of proteins; the nucleic

acids DNA, deoxyribonucleic acid (dee-ock-see-rye-

boh-noo-KLEE-ik ASS-id), and RNA, ribonucleic acid

(rye-boh-noo-KLEE-ik ASS-id); enzymes; and other

chemicals of the nucleus. Many chemical reactions occur-

in the nucleoplasm and are essential to cellular function and

Laboratory Exercise: Cellular Reproduction.

Chromatin

When the cell is stained, fine dark threads appear in the

nucleus. This material is called chromatin - (KROH-mah-

tin) and is the genetic material of the cell. The cells of the

human body contain 46 chromosomes (22 pairs of

autosomes and one pair of sex chromo-somes: one member

of each pair comes from the father and one member from

the mother). The egg cell and the sperm cell contain one-

half that number, or 23 chromo-somes. Chromosomes are

made of DNA molecules and proteins. When the DNA

molecules duplicate during cell division, they shorten and

thicken and become visible. We now call the DNA

chromosomes. When the cell is not dividing, the DNA

molecules are long and thin and vis-ible only as chromatin.

All of the above terms are used to describe the different

levels of chromosomal organi-zation -(Figure 3-7). This is

discussed in greater detail in -Chapter 4. DNA controls

many of the functions of the cell.

Nucle olus

The nucleolus (noo-KLEE-oh-lus) is a spherical particle

within the nucleoplasm that does not have a covering

membrane around it. It is composed of primarily DNA,