MICROBIAL MOLECULAR BIOLOGY & GENETICS
1. The two kinds of nucleic acid, deoxyribonucleic acid (DNA) and
ribonucleic acid (RNA), differ from one another in chemical composition
and structure. In procaryotic and eucaryotic cells, DNA serves as the
repository for genetic information.
2. DNA is associated with basic proteins in the cell. In eucaryotes these are
special histone proteins, whereas in procaryotes nonhistone proteins are
complexed with DNA.
3. The flow of genetic information usually proceeds from DNA through RNA
to protein. A protein’s amino acid sequence reflects the nucleotide
sequence of its mRNA. This messenger is a complementary copy of a
portion of the DNA genome.
4. DNA replication is a very complex process involving a variety of proteins
and a number of steps. It is designed to operate rapidly while minimizing
errors and correcting those that arise when the DNA sequence is copied.
5. Genetic information is contained in the nucleotide sequence of DNA (and
sometimes RNA). When a structural gene directs the synthesis of a
polypeptide, each amino acid is specified by a triplet codon.
6. A gene is a nucleotide sequence that codes for a polypeptide, tRNA, or rRNA.
7. Most bacterial genes have at least four major parts, each with different
functions: promoters, leaders, coding regions, and trailers.
8. Mutations are stable, heritable alterations in the gene sequence and usually,
but not always, produce phenotypic changes. Nucleic acids are altered in
several different ways, and these mutations may be either spontaneous or
induced by chemical mutagens or radiation.
9. It is extremely important to keep the nucleotide sequence constant, and
microorganisms have several repair mechanisms designed to detect
alterations in the genetic material and restore it to its original state. Often
more than one repair system can correct a particular type of mutation.
Despite these efforts some alterations remain uncorrected and provide
material and opportunity for evolutionary change.
DNA as Genetic Material
The early work of Fred Griffith in 1928 on the transfer of virulence
in the pathogen Streptococcus pneumoniae set
the stage for the research that first showed that DNA was the genetic
material. Griffith found that if he boiled virulent bacteria
and injected them into mice, the mice were not affected and no
pneumococci could be recovered from the animals. When he injected
a combination of killed virulent bacteria and a living nonvirulent
strain, the mice died; moreover, he could recover living
virulent bacteria from the dead mice. Griffith called this change
of non virulent bacteria into virulent pathogens transformation.
Oswald T. Avery and his colleagues then set out to discover
which constituent in the heat-killed virulent pneumococci was responsible
for Griffith’s transformation. These investigators selectively
destroyed constituents in purified extracts of virulent pneumococci,
using enzymes that would hydrolyze DNA, RNA, or
protein. They then exposed nonvirulent pneumococcal strains to
the treated extracts. Transformation of the nonvirulent bacteria
was blocked only if the DNA was destroyed, suggesting that
DNA was carrying the information required for transformation.
The publication of these studies by O. T. Avery,
C. M. MacLeod, and M. J. McCarty in 1944 provided the first evidence
that Griffith’s transforming principle was DNA and therefore
that DNA carries genetic information.
Fig: Griffith’s Transformation Experiments
Some years later (1952), Alfred D. Hershey and Martha
Chase performed several experiments that indicated that DNA
was the genetic material in the T2 bacteriophage. Some luck
was involved in their discovery, for the genetic material of many
viruses is RNA and the researchers happened to select a DNA
virus for their studies. Imagine the confusion if T2 had been an
RNA virus! The controversy surrounding the nature of genetic
information might have lasted considerably longer than it did.
Hershey and Chase made the virus DNA radioactive with 32P or
labeled the viral protein coat with 35S. They mixed radioactive
bacteriophage with E. coli and incubated the mixture for a few
minutes. The suspension was then agitated violently in a Waring
blender to shear off any adsorbed bacteriophage particles.
Subsequent studies on the genetics of viruses and bacteria
were largely responsible for the rapid development of molecular
genetics. Furthermore, much of the new recombinant DNA technology
has arisen from recent progress in bacterial
and viral genetics. Research in microbial genetics has had a
profound impact on biology as a science and on the technology that affects everyday life.
Central Dogma Of Molecular Biology:
Biologists have long recognized a relationship between DNA,
RNA, and protein , and this recognition has guided a
vast amount of research over the past decades. DNA is precisely
copied during its synthesis or replication. The expression of the information
encoded in the base sequence of DNA begins with the
synthesis of an RNA copy of the DNA sequence making up a gene.
A gene is a DNA segment or sequence that codes for a polypeptide,
an rRNA, or a tRNA. Although DNA has two complementary
strands, only the template strand is copied at any particular point on
DNA. If both strands of DNA were transcribed, two different
mRNAs would result and cause genetic confusion. Thus the sequence
corresponding to a gene is located only on one of the two
complementary DNA strands. Different genes may be encoded on
opposite strands. This process of DNA-directed RNA synthesis is
called transcription because the DNA base sequence is being written
into an RNA base sequence. The RNA that carries information
from DNA and directs protein synthesis is messenger RNA
(mRNA). The last phase of gene expression is translation or protein
synthesis. The genetic information in the form of an mRNA nucleotide
sequence is translated and governs the synthesis of protein.
Thus the amino acid sequence of a protein is a direct reflection of
the base sequence in mRNA. In turn the mRNA nucleotide sequence
is a complementary copy of a portion of the DNA genome.
Fig: Central Dogma of Molecular Biology
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