2.1. Cell physiology[[Student version, December 8, 2002]] 39
Figure 2.10: (Schematic; electron micrograph.) One of the 46 chromosomes of a somatic (ordinary, non-germ)
human cell. Just prior to mitosis, every chromosome consists of two identical “chromatids,” each consisting of tightly
folded “chromatin fibers.” Each chromatin fiber consists of a long DNA molecule wrapped around histone proteins
to form a series of “nucleosome particles.”[After WM Becker and DW Deamer The world of the cell, 2d edition, fig
13-20 (Benjamin–Cummings, 1991).] [Copyrighted figure; permission pending.]
Figure 2.11: (Scanning electron micrograph.) Cell crawling. At the front of this fibroblast cell (left), filopo-
dia, lamellipodia, and ruffles project from the plasma membrane. The arrow indicates the direction of movement.
[Copyrighted figure; permission pending.]
nerve cell, orneuron,has a central cell body (thesoma)with a branching array of projections
(orprocesses). The processes on a neuron are subdivided into many “input lines,” thedendrites,
and one “output line,” theaxon.The entire branched structure has a single interior compartment
filled with cytoplasm. Each axon terminates with one or moreaxon terminals(or “boutons”),
containing synaptic vesicles. A narrow gap, orsynapse,separates the axon terminal from one of
the next neuron’s dendrites. Chapter 12 will discuss the transmission of information along the axon,
and from one neuron to the next.
Still other elements of the external anatomy of a cell are transient. For example, consider the cell
shown in Figure 2.11. This cell is a “fibroblast”; its job is to crawl between other cells, laying down