Information Technology Reference
In-Depth Information
m
2
cross-sectional area) with a 4.6
million basepair chromosome has the equivalent of a 9.2-megabit memory. This
memory codes for as many as 4300 different polypeptides under the inducible
control of several hundred different promoters. These polypeptides perform
metabolic and regulatory functions that process the energy and information,
respectively, made available to the cell. This complexity of functionality allows
the cell to interact with, influence, and, to some degree, control its environment.
Compare this to the silicon semiconductor situation as described in the Inter-
national Technology Roadmap for Semiconductors (ITRS) [54]. ITRS predicts
that by the year 2014, memory density will reach 24.5 Gbits/cm
2
, and logic
transistor density will reach 664 M/cm
2
. Assuming four transistors per logic
function, 2
bacterial cell such as
Escherichia coli
(2
µ
m
2
of silicon could contain a 490-bit memory or approximately
three simple logic gates. Even this level of functionality depends on an unsure
path of technology development that will require breakthroughs in lithography,
materials, processing, defect detection, and many other challenging areas. We
can confidently say that silicon technology will not approach bacterial-scale
integration within the foreseeable future.
Aside from this functional density, microorganisms have other attributes de-
sirable for engineered synthetic systems. Cells are relatively rugged components
that subsist in extreme environments such as deep-sea thermal vents, subzero
arctic seawaters, hypersaline solutions, water saturated with organic solvents,
contaminated soils, and industrial wastes. Prokaryotic cells are relatively easy
to manipulate genetically, and they have a diverse set of gene regulation sys-
tems that allow their inclusion in the types of hybrid systems considered here.
Furthermore, these cells can easily be incorporated into a 3D structure (i.e.,
3D integration) instead of the 2D structure of integrated circuits (ICs). And
cells self-replicate and self-assemble into groups (e.g., biofilms), making them
easy to manufacture with no requirement of lithography, mask alignment, or
other technologically challenging processing steps to produce highly functional
components.
µ
COMPLEXITY OF FUNCTIONALITY: WILD-TYPE ORGANISMS
Before proceeding down the path of engineered cellular components, it is in-
structive to consider the complex functionality exhibited in nature, both by
individual cells and by the more complex behavior of interconnected groups of
cells. Here we present three examples.
Magnetotaxis
Individual cells are capable of performing extremely complex tasks as illus-
trated by magnetotaxis, a type of directed motility. Many microorganisms in
aqueous environments are motile in an active search for nutrients. Some bacteria
