Introduction 
transcribed. Proteins, commonly known as tran- 
scriptional factors, bind to these DNA se- 
quence elements and determine when, and how 
frequently, a gene is transcribed. Since this often 
influences how much of the encoded protein is 
produced by a cell, such information provides 
one level of insight about how cells differ from 
one another. 
Another large issue concerns the way in which 
proteins are deployed in the cell. A cell is not just 
a bag of randomly distributed molecules; it has a 
highly organized internal structure. The DNA that 
makes up the genes and the machinery for gene 
transcription are packaged in the nucleus, which 
is surrounded by a membrane that separates it 
from the cytoplasm, which comprises the rest of 
the cell. Nearly all cellular functions are com- 
partmentalized in other cellular structures collec- 
tively referred to as organelles. Among the more 
prominent organelles in animal cells are the mi- 
tochondria, which are the cells' principal en- 
ergy source; lysosomes, which are concerned 
largely with the degradation of foreign materials 
and of cellular proteins whose functions have 
been fulfilled; the rough endoplasmic reticu- 
lum, a complex, ribosome-studded system of 
membranes responsible for the synthesis of pro- 
teins secreted by the cells; and the Golgi appara- 
tus, which both modifies proteins (by adding 
other chemical groups such as sugars) and pack- 
ages them for transport to their appropriate loca- 
tions, such as the cell membrane. Most of these 
organelles are themselves surrounded by mem- 
branes that separate their functions from those of 
the rest of the cell. It is easy to see how such a 
compartmentalized structure allows the cell to 
organize its different processes efficiently, but it 
poses an organizational problem that is of consid- 
erable current interest in cell biology: How are 
particular proteins routed to the correct organ- 
elles? As we began to learn about the structures of 
individual proteins, it was discovered that there 
are specific "signals" built into proteins that tar- 
get them to particular organelles or particular lo- 
cations within the cell and that there are distinc- 
tive cellular machineries that "detect" these 
signals and steer the proteins in particular direc- 
tions (Figure 6). Thus certain proteins are di- 
rected to the nucleus, while others are targeted 
for insertion into the surface membrane of the 
cell, and yet others are destined for export out of 
the cell as secretory products. 
At another level of organization, it has become 
evident that the organelles within a cell are also 
not distributed randomly. In many cells one can 
identify a distinct "top" and "bottom." Other 
cells, while not polarized in this manner, have 
asymmetric structures arranged in such a way that 
given organelles are distributed in different, but 
quite reproducible, patterns. Still other cells, es- 
pecially nerve cells, have unusual extensions or 
processes that may be many hundred times as 
long as the body of the cell. In each case organ- 
elles have to be transported to particular loca- 
tions and maintained there; they do not drift 
about haphazardly inside the cell. 
The asymmetric shapes of cells and the loca- 
tions of their organelles both rely on cellu- 
lar structures known collectively as the cyto- 
skeleton. The cytoskeleton consists of several 
types of elongated threads or filaments (micro- 
filaments, microtubules, intermediate fila- 
ments) , each made of specific proteins that are 
so designed as to assemble spontaneously into fil- 
aments. These cytoskeletal elements serve as a 
form of internal scaffolding to maintain the shape 
of the cell, and as a system of tracks along which 
organelles can be transported. Recent research 
has disclosed a variety of proteins that function as 
molecular motors that can move proteins and or- 
ganelles that attach to them along particular cyto- 
skeletal filaments to various locations within the 
cell. We also know that the appropriate organiza- 
tion of cytoskeletal filaments and motor proteins 
can, in some cases, contribute to cell motility, 
that is, the movement of the entire cell from one 
location to another. Such cellular movements are 
especially important in development but con- 
tinue to play an integral role in the life of certain 
cells even in adult life. 
All these processes — gene transcription, pro- 
tein targeting, organelle movement, and cell mo- 
tility — must be carefully regulated so that cells 
respond appropriately to different situations. The 
same is true of many other cellular processes. For 
example, the proliferation of cells that takes 
place by cell division involves copying or repli- 
cating the genes, the breakdown of the nuclear 
membrane, the separation of the duplicated 
chromosomes into two equivalent sets, division 
of the cell into two daughter cells, the re-forma- 
tion of a nucleus in each of the daughter cells, 
and finally the resumption of normal function in 
both cells. This whole process, which is known as 
the cell cycle, takes place whenever cells divide 
and remains an important part of the life of all but 
a few cell types. Many types of cells — for exam- 
ple, the cells in the blood and skin — are contin- 
