Microbiology and Immunology

Asilomar conference WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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species the eggs may or may not be fertilized; fertilized eggs

produce females, while unfertilized eggs produce males.

There are a number of crop plants that are propagated

asexually. The advantage of asexual propagation to farmers is

that the crops will be more uniform than those produced from

seed. Some plants are difficult to cultivate from seed and asex-

ual reproduction in these plants makes it possible to produce

crops that would otherwise not be available for commercial

marketing. The process of producing plants asexually is called

vegetative propagation and is used for such crops as potatoes,

bananas, raspberries, pineapples, and some flowering plants

used as ornamentals. Farmers plant the so-called “eyes” of

potatoes to produce duplicates of the parent. With banana

plants, the suckers that grow from the root of the plant are sep-

arated and then planted as new ones. With raspberry bushes,

branches are bent and covered with soil. They then grow into

a separate plant with their own root system and can eventually

be detached from the parent plant.

See alsoCell cycle and cell division

AAsilomar conferenceSILOMAR CONFERENCE

Soon after American microbiologist Hamilton Smith’s 1970

discovery of the first restriction enzyme, it became possible to

combine DNAfrom different sources into one molecule, pro-

ducing recombinant DNA. Concern by scientists and lay peo-

ple that some of this recombinant-technology DNA might be

harmful to humans prompted the research to stop until scien-

tists could evaluate its risks.

In February 1975 over 100 internationally respected

molecular biologists met at the Asilomar conference center in

California. There they decided upon a set of guidelines to be

followed by all scientists doing recombinant DNA research.

They considered every class of experiment and assigned each

a level of risk: minimal, low, moderate, or high. Each level of

risk required a corresponding set of containment procedures

designed to minimize the chance of vectors (carriers) contain-

ing recombinant DNA molecules from escaping into the envi-

ronment, where they could potentially harm humans or other

parts of the ecosystem. Because these projected experiments

had never been done, assignment to a risk category was, of

course, somewhat speculative and subjective. Accordingly, the

potential risks were arrived at by estimate.

At all risk levels, the guidelines called for the use of bio-

logical barriers. Bacterial host cells should be from strains

unable to survive in natural environments (outside the test

tube). Vectors carrying recombinant DNA, including plas-

mids, bacteriophages, and other viruses, were to be nontrans-

missible and unable to survive in natural environments.

For experiments having minimal risk, the guidelines

recommended that scientists follow general microbiology

safety procedures. These included not eating, drinking, or

smoking in the lab; wearing laboratory coats in the work area;

and promptly disinfecting contaminated materials.

Low-risk procedures required a bit more caution. For

example, procedures producing aerosols, such as using a

blender, were to be performed under an enclosed ventilation

hood to eliminate the risk of the recombinant DNA being lib-

erated into the air.

Moderate-risk experiments required the use of a laminar

flow hood, the wearing of gloves, and the maintenance of neg-

ative air pressure in the laboratory. This would ensure that air

currents did not carry recombinant DNA out of the laboratory.

Finally, in high-risk experiments, maximum precautions

were specified. These included isolation of the laboratory

from other areas by air locks, having researchers shower and

change their clothing upon leaving the work area, and the

incineration of exhaust air from the hoods.

Certain types of experiments were not to be done at all.

These most potentially dangerous experiments included the

cloningof recombinant DNA from highly pathogenic organ-

isms or DNA containing toxin genes. Also forbidden were

experiments involving the production of more than 10 liters of

cultureusing recombinant DNA molecules that might render

the products potentially harmful to humans, animals, or plants.

The scientists at the Asilomar conference also resolved

to meet annually to re-evaluate the guidelines. As new proce-

dures were developed and safer vectors and bacterial cells

became available, it became possible to re-evaluate and relax

some of the initially stringent and restrictive safety standards.

See alsoRecombination; Viral genetics; Viral vectors in gene

therapy

AAtomic force microscopeTOMIC FORCE MICROSCOPE

In 1985, the development of the atomic force microscope

(AFM) allowed scientists to visualize the surface of cellular

structures during some physiological processes. Along with

the use of field ion microscopes and powerful scanning tun-

neling microscopes (STM), these advances in microscopy rep-

resent the most fundamental greatest advances since the

development of the electron microscope.

Budding yeast cells undergoing asexual reproduction; yeast are

single-celled fungi.

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