Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Monod, Jacques Lucien

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made of sugar and phosphate molecules are connected by

rungs of nitrogen-containing chemicals called bases (nitroge-

nous bases). Each strand is a linear arrangement of repeating

similar units called nucleotides, which are each composed of

one sugar, one phosphate, and a nitrogenous base. Four differ-

ent bases are present in DNA adenine (A), thymine (T), cyto-

sine (C), and guanine (G). The particular order of the bases

arranged along the sugar-phosphate backbone is called the

DNA sequence; the sequence specifies the exact genetic

instructions required to create a particular organism with its

own unique traits.

Each time a cell divides into two daughter cells, its full

genome is duplicated; for humans and other complex organ-

isms, this duplication occurs in the nucleus. During cell divi-

sion the DNA molecule unwinds and the weak bonds between

the base pairs break, allowing the strands to separate. Each

strand directs the synthesis of a complementary new strand,

with free nucleotides matching up with their complementary

bases on each of the separated strands. Nucleotides match up

according to strict base-pairing rules. Adenine will pair only

with thymine (an A-T pair) and cytosine with guanine (a C-G

pair). Each daughter cell receives one old and one new DNA

strand. The cell’s adherence to these base-pairing rules ensures

that the new strand is an exact copy of the old one. This min-

imizes the incidence of errors (mutations) that may greatly

affect the resulting organism or its offspring.

Each DNA molecule contains many genes, the basic

physical and functional units of heredity. A gene is a specific

sequence of nucleotide bases, whose sequences carry the

information required for constructing proteins, which provide

the structural components of cells and as well as enzymesfor

essential biochemical reactions.

The chromosomes of prokaryotic microorganismsdiffer

from eukaryotic microorganisms, in terms of shape and organ-

ization of genes. Prokaryotic genes are more closely packed

and are usually is arranged along one circular chromosome.

The central dogma of molecular biology states that

DNA is copied to make mRNA (messenger RNA), and mRNA

is used as the template to make proteins. Formation of RNA is

called transcriptionand formation of protein is called transla-

tion. Transcription and translation processes are regulated at

various stages and the regulation steps are unique to prokary-

otes and eukaryotes. DNA regulation determines what type

and amount of mRNA should be transcribed, and this subse-

quently determines the type and amount of protein. This

process is the fundamental control mechanism for growth and

development (morphogenesis).

All living organisms are composed largely of proteins,

the end product of genes. Proteins are large, complex mole-

cules made up of long chains of subunits called amino acids.

The protein-coding instructions from the genes are transmitted

indirectly through messenger ribonucleic acid (mRNA), a

transient intermediary molecule similar to a single strand of

DNA. For the information within a gene to be expressed, a

complementary RNA strand is produced (a process called

transcription) from the DNA template. In eukaryotes, messen-

ger RNA (mRNA) moves from the nucleus to the cellular cyto-

plasm, but in both eukaryotes and prokaryotes mRNA serves

as the template for protein synthesis.

Twenty different kinds of amino acids are usually found

in proteins. Within the gene, sequences of three DNA bases

serve as the template for the construction of mRNA with

sequence complimentary codons that serve as the language to

direct the cell’s protein-synthesizing machinery. Cordons

specify the insertion of specific amino acids during the syn-

thesis of protein. For example, the base sequence ATG codes

for the amino acid methionine. Because more than one codon

sequence can specify the same amino acid, the genetic codeis

termed a degenerate code (i.e., there is not a unique codon

sequence for every amino acid).

Areas of intense study by molecular biology include the

processes of DNA replication, repair, and mutation (alterations

in base sequence of DNA). Other areas of study include the

identification of agents that cause mutations (e.g., ultra-violet

rays, chemicals) and the mechanisms of rearrangement and

exchange of genetic materials (e.g. the function and control of

small segments of DNA such as plasmids, transposable ele-

ments, insertion sequences, and transposons to obtain

recombinant DNA).

Recombinant DNA technologies and genetic engineer-

ing are an increasingly important part of molecular biology.

Advances in biotechnologyand molecular medicine also carry

profound clinical and social significance. Advances in molec-

ular biology have led to significant discoveries concerning the

mechanisms of the embryonic development, disease, immuno-

logic response, and evolution.

See alsoImmunogenetics; Microbial genetics

MONOCLONAL ANTIBODY•seeANTIBODY, MON-

OCLONAL

MMonod, Jacques LucienONOD, JACQUESLUCIEN(1910-1976)

French biologist

French biologist Jacques Lucien Monod and his colleagues

demonstrated the process by which messenger ribonucleic acid

(mRNA) carries instructions for protein synthesisfrom

deoxyribonucleic acid(DNA) in the cell nucleusout to the ribo-

somesin the cytoplasm, where the instructions are carried out.

Jacques Monod was born in Paris. In 1928, Monod

began his study of the natural sciences at the University of

Paris, Sorbonne where he went on to receive a B.S. from the

Faculte des Sciencesin 1931. Although he stayed on at the

university for further studies, Monod developed further scien-

tific grounding during excursions to the nearby Roscoff

marine biology station.

While working at the Roscoff station, Monod met André

Lwoff, who introduced him to the potentials of microbiology

and microbial nutrition that became the focus of Monod’s

early research. Two other scientists working at Roscoff sta-

tion, Boris Ephrussi and Louis Rapkine, taught Monod the

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