Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Donnan equilibrium

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ganisms; Laboratory techniques in immunology; Laboratory

techniques in microbiology; Molecular biology and molecular

genetics

DNA HDNA Hybridization YBRIDIZATION

Evolutiondeals with heritable changes in populations over

time. Because DNAis the molecule of heredity, evolutionary

changes will be reflected in changes in the base pairs in DNA.

Two species that have evolved from a common ancestor will

have DNA that has very similar base pair sequences. The

degree of relatedness of two species can be estimated by

examining how similar their base pair sequences are. One

method of assessing relatedness uses hybridization of DNA.

In the molecular genetic technique of hybridization of

DNA, single strands of DNA from two different species are

allowed to join together to form hybrid double helices. These

hybrid segments of DNA can be used to determine the evolu-

tionary relatedness of organisms by examining how similar or

dissimilar the DNA base pair sequences are.

The technique of DNA hybridization is based on two

principles: the first, that double strands of DNA are held

together by hydrogen bonds between complementary base

pairs, and the second is that the more closely related two

species are, the greater will be the number of complementary

base pairs in the hybrid DNA. In other words, the degree of

hybridization is proportional to the degree of similarity

between the molecules of DNA from the two species.

Hybridization of DNA is accomplished by heating

strands of DNA from two different species to 86° C

[186.8° F]. This breaks the hydrogen bonds between all

complementary base pairs. The result is many single-

stranded segments of DNA. The single-stranded DNA from

both species is mixed together and allowed to slowly cool.

Similar strands of DNA from both species will begin to

chemically join together or re-anneal at complementary base

pairs by reforming hydrogen bonds.

The resulting hybrid DNA is then reheated and the tem-

perature at which the DNA once again becomes single-

stranded is noted. Because one cannot observe DNA

separating, another technique must be used simultaneously

with heating to show when separation has occurred. This tech-

nique employs the absorption of UV light by DNA. Single

strands of DNA absorb UV light more effectively than do dou-

ble strands. Therefore, the separation of the DNA strands is

measured by UV light absorption; as more single strands are

liberated, more UV light is absorbed.

The temperature at which hybrid DNA separation occurs

is related to the number of hydrogen bonds formed between

complementary base pairs. Therefore, if the two species are

closely related, most base pairs will be complementary and the

temperature of separation will be very close to 86° C [186.8° F].

If the two species are not closely related, they will not share

many common DNA sequences and fewer complementary base

pairs will form. The temperature of separation will be less than

86° C [186.8° F] because less energy is required to break fewer

hydrogen bonds. Using this type of information, a tree of evo-

lutionary relationships based on the separation temperature of

the hybrid helices can be generated.

See alsoEvolution and evolutionary mechanisms; Evolu-

tionary origin of bacteria and viruses

DONNAN, FREDERICKGEORGE

(1870-1956)Donnan, Frederick George

British chemist

Frederick George Donnan was a British chemist whose work

in the second decade of the twentieth century established the

existence of an electrochemical potential between a semiper-

meable membrane. The membrane allows an unequal distribu-

tion of ionic species to become established on either side of

the membrane. In bacteria, this Donnan equilibriumhas been

demonstrated to exist across the outer membrane of Gram-

negative bacteria, which separates the external environment

from the periplasm. The energy derived from this ionic

inequity is vital for the operation of the bacteria.

Donnan was born in Colombo, Ceylon (now known as

Sri Lanka). He was educated at Queen’s College in Belfast,

Northern Ireland, at the University of Leipzig in Berlin, and at

the University College, London. He taught at Liverpool

University from 1904 until 1913, when he rejoined the faculty

of University College as a Professor of Inorganic and Physical

Chemistry. He remained there until his retirement in 1937.

In 1911, Donnan began his studies of the equilibrium

between solutions separated by a semipermeable membrane

that led to the establishment of the Donnan equilibrium. He

also was involved in important studies in physical chemistry,

which included the study of colloids and soap solutions,

behavior of various gases, oxygen solubility, and the manu-

facture of nitric acid.

Of all his research achievements, Donnan’s major

accomplish was the theory of membrane equilibrium. In his

productive research career, Donnan authored more than one

hundred research papers.

See alsoBacterial membranes and cell wall

DDonnan equilibriumONNAN EQUILIBRIUM

Donnan equilibrium (which can also be referred to as the

Gibbs-Donnan equilibrium) describes the equilibrium that

exists between two solutions that are separated by a mem-

brane. The membrane is constructed such that it allows the

passage of certain charged components (ions) of the solutions.

The membrane, however, does not allow the passage of all the

ions present in the solutions and is thus a selectively perme-

able membrane.

Donnan equilibrium is named after Frederick George

Donnan, who proved its existence in biological cells. J.

Willard Gibbs had predicted the effect some 30 years before.

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