Introduction 
genes it uses (and thus the kinds of proteins it 
produces) or the amounts of each protein it 
makes (Figure 5)? In the next section we describe 
the considerable progress that has been made 
recently in deciphering the DNA sequence ele- 
ments that determine whether a gene is tran- 
scribed. Proteins, commonly known as tran- 
scriptional factors, bind to these DNA 
sequence elements and determine when, and 
how frequently, a gene is transcribed. Since this 
often influences how much of the encoded pro- 
tein is produced by a cell, such information pro- 
vides 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 with distinctive 
pores that separates it from the cytoplasm, 
which comprises the rest of the cell. Nearly all 
cellular functions are compartmentalized in 
other cellular structures collectively referred to 
as organelles. Among the more prominent or- 
ganelles in animal cells are the mitochondria, 
which are the cells' principal energy source; ly- 
sosomes, which are concerned largely with the 
degradation of foreign materials and of cellular 
proteins whose functions have been fulfilled; the 
rough endoplasmic reticulum, a complex, ri- 
bosome-studded system of membranes responsi- 
ble for the synthesis of proteins secreted by the 
cells; and the Golgi apparatus, which both mod- 
ifies proteins (by adding other chemical groups 
such as sugars) and also packages them for trans- 
port to their appropriate locations, such as the 
cell membrane. Most of these organelles are 
themselves surrounded by membranes that sepa- 
rate their functions from those of the rest of the 
cell. It is easy to see how such a compartmental- 
ized structure allows the cell to organize its dif- 
ferent processes efficiently, but it poses an organi- 
zational problem that is of considerable current 
interest in cell biology: How are particular pro- 
teins routed to the correct organelles? As we be- 
gan to learn about the structures of individual 
proteins, it was discovered that there are specific 
"signals" built into proteins that target them to 
particular organelles or particular locations 
within the cell and that there are distinctive cel- 
lular machineries that "detect" these signals and 
steer the proteins in particular directions (Figure 
6). Thus certain proteins are directed to the nu- 
cleus, 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 cellular 
structures known collectively as the cytoskele- 
ton. The cytoskeleton consists of several types of 
elongated threads or filaments (microfilaments, 
microtubules, intermediate filaments), each 
made of specific proteins that are so designed as 
to assemble spontaneously into filaments. 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 mo- 
tors that can move proteins and organelles that 
attach to them along particular cytoskeletal fila- 
ments to various locations within the cell. We 
also know that the appropriate organization of 
cytoskeletal filaments and motor proteins can, in 
some cases, contribute to cell motility, that is, 
the movement of the entire cell from one loca- 
tion to another. Such cellular movements are es- 
pecially important in development but continue 
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, divi- 
sion of the cell into two daughter cells, the re- 
