Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY DNA (deoxyribonucleic acid)

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The relationship between time and temperature deter-

mines the speed of sterilization. The higher the temperature

the more quickly a sample can be sterilized. Typical combina-

tions of temperature and pressure are 115° C [239° F]–10

pounds per square inch (psi), 121° C [249.8° F]–15 psi, and

132° C [269.6° F]–27psi. Which combination is used depends

on the material being sterilized. For example, a large and

bulky load, or a large volume of cultureshould be kept in

longer. Shorter sterilizations times are sufficient for contami-

nated objects such as surgical dressing, instruments, and

empty glassware.

The third method of treatment of microorganisms and

material contaminated with microorganisms is incineration.

On a small scale incineration is practiced routinely in a

microbiology laboratory to sterilize the metal loops used to

transfer microorganisms from one place to another. Exposing

the metal loop to a gas flame will burn up and vaporize any

living microbes that are on the loop, ensuring that infectious

organisms are not inadvertently transferred elsewhere. The

method of incineration is also well suited to the treatment of

large volumes of contaminated fluids or solids. Incineration is

carried out in specially designed furnaces that achieve high

temperatures and are constructed to be airtight. The use of a

flame source such as a fireplace is unsuitable. The incinera-

tion needs to occur very quickly and should not leave any

residual material. The process needs to be smoke-free, other-

wise microbes that are still living could be wafted away in the

rising smoke and hot air to cause infection elsewhere.

Another factor in proper incineration is the rate at which sam-

ple is added to the flame. Too much sample can result in an

incomplete burn.

Disposal of microorganisms also requires scrupulous

record keeping. The ability to back track and trace the disposal

of a sample is very important. Often institutions will have

rules in place that dictate how samples should be treated, the

packaging used for disposal, the labeling of the waste, and the

records that must be maintained.

See alsoLaboratory techniques in microbiology; Steam pres-

sure sterilizer

DNA (DNA (deoxyribonucleic acid) DEOXYRIBONUCLEIC ACID)

DNA, or deoxyribonucleic acid, is the genetic material that

codes for the components that make life possible. Both

prokaryotic and eukaryotic organisms contain DNA. An

exception is a few virusesthat contain ribonucleic acid,

although even these viruses have the means for producing

DNA.

The DNA of bacteriais much different from the DNA

of eukaryotic cells such as human cells. Bacterial DNA is dis-

persed throughout the cell, while in eukaryotic cells the DNA

is segregated in the nucleus, a membrane-bound region. In

eukaryotics, structures called mitochondria also contain DNA.

The dispersed bacterial DNA is much shorter than eukaryotic

DNA. Hence the information is packaged more tightly in bac-

terial DNA. Indeed, in DNA of microorganismssuch as

viruses, several genes can overlap with each other, providing

information for several proteins in the same stretch of nucleic

acid. Eukaryotic DNA contains large intervening regions

between genes.

The DNA of both prokaryotes and eukaryotesis the

basis for the transfer of genetic traits from one generation to

the next. Also, alterations in the genetic material (mutations)

can produce changes in structure, biochemistry, or behavior

that might also be passed on to subsequent generations.

Genetics is the science of heredity that involves the

study of the structure and function of genes and the methods

by which genetic information contained in genes is passed

from one generation to the next. The modern science of genet-

ics can be traced to the research of Gregor Mendel

(1823–1884), who was able to develop a series of laws that

described mathematically the way hereditary characteristics

pass from parents to offspring. These laws assume that hered-

itary characteristics are contained in discrete units of genetic

material now known as genes.

The story of genetics during the twentieth century is, in

one sense, an effort to discover the geneitself. An important

breakthrough came in the early 1900s with the work of the

American geneticist, Thomas Hunt Morgan (1866–1945).

Working with fruit flies, Morgan was able to show that genes

are somehow associated with the chromosomesthat occur in

the nuclei of cells. By 1912, Hunt’s colleague, American

geneticist A. H. Sturtevant (1891–1970) was able to construct

the first chromosome map showing the relative positions of

different genes on a chromosome. The gene then had a con-

crete, physical referent; it was a portion of a chromosome.

During the 1920s and 1930s, a small group of scientists

looked for a more specific description of the gene by focusing

their research on the gene’s molecular composition. Most

researchers of the day assumed that genes were some kind of

protein molecule. Protein molecules are large and complex.

They can occur in an almost infinite variety of structures. This

quality is expected for a class of molecules that must be able

to carry the enormous variety of genetic traits.

A smaller group of researchers looked to a second fam-

ily of compounds as potential candidates as the molecules of

heredity. These were the nucleic acids. The nucleic acids were

first discovered in 1869 by the Swiss physician Johann

Miescher (1844–1895). Miescher originally called these com-

pounds “nuclein” because they were first obtained from the

nuclei of cells. One of Miescher’s students, Richard Altmann,

later suggested a new name for the compounds, a name that

better reflected their chemical nature: nucleic acids.

Nucleic acids seemed unlikely candidates as molecules

of heredity in the 1930s. What was then known about their

structure suggested that they were too simple to carry the vast

array of complex information needed in a molecule of hered-

ity. Each nucleic acid molecule consists of a long chain of

alternating sugar and phosphate fragments to which are

attached some sequence of four of five different nitrogen

bases: adenine, cytosine, guanine, uracil and thymine (the

exact bases found in a molecule depend slightly on the type of

nucleic acid).

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