Page 20 - 84 human physiolofy part-1

Chapter 2

organelles. Eukaryotes include all protozoans, fungi, plants, and animals (including humans), and these
cells are characterized by a nucleus (which houses the chromosomes) as well as a variety of other
organelles. Human cells vary considerably (consider the differences between a bone cell, a blood cell,
and a nerve cell), but most cells have the features described below.

• Nucleus

Controls the cell;
houses the genetic material
(DNA). The nucleus is the
largest of the cells
organelles. Cells can have
more than one nucleus or
lack a nucleus all together.
Skeletal muscle cells
contain more than one
nucleus whereas red blood
cells do not contain a A comparison and Eukaryote and Prokaryote cells.
nucleus at all. The nucleus
is bounded by the nuclear envelope, a phospholipid bilayer similar to the plasma membrane. The space
between these two layers is the nucleolemma Cisterna.

The nucleus contains the DNA,
as mentioned above, the hereditary
information in the cell. Normally
the DNA is spread out within the
nucleus as a threadlike matrix
called chromatin. When the cell
begins to divide, the chromatin
condenses into rod-shaped bodies
called chromosomes, each of
which, before dividing, is made up
of two long DNA molecules and
various histone molecules. The
histones serve to organize the
lengthy DNA, coiling it into
bundles called nucleosomes. Also
visible within the nucleus are one
or more nucleoli, each consisting of
DNA in the process of
manufacturing the components of
ribosomes. Ribosomes are shipped A cross-sectional diagram of a cell.
to the cytoplasm where they
assemble amino acids into proteins. The nucleus also serves as the site for the separation of the
chromosomes during cell division.

• Chromosomes

20 | Human Physiology