APR January/February 2022 - 34

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DRUG DEVELOPMENT
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Transduction Process
* Transduction is the process by which foreign DNA is introduced
into a cell by a virus or viral vector.1
of DNA from one bacterium to another and hence an example
of horizontal gene transfer.2
Transduction does not require
physical contact between the cell donating the DNA and the cell
receiving the DNA (which occurs in conjugation), and it is Dnase
resistant (transformation is susceptible to Dnase). Transduction is
a common tool used by molecular biologists to stably introduce
a foreign gene into a host cell's genome (both bacterial and
mammalian cells), see schematic depicted by Figure 6.
* Note: A deoxyribonuclease (Dnase, for short) is an enzyme that
catalyzes the hydrolytic cleavage of phosphodiester linkages in
the DNA backbone, thus degrading DNA. Deoxyribonucleases
are one type of nuclease, a generic term for enzymes capable of
hydrolyzing phosphodiester bonds that link nucleotides.
Type of Transduction
Transduction has three forms:
a. Generalized transduction: It occurs in the lytic cycle
of phage virus. DNA of phages virus enter into E.coli
bacteria. This DNA replicates and develops many
new DNA and capsids. The DNA of bacteria is broken.
Some pieces of DNA also enter into the capsid of virus.
Bacteria burst and release new phage viruses. Now
this phage enters into recipient bacteria and transfer
the DNA of donor bacteria into the DNA of recipient
bacteria. Bacterial endonucleases enzymes destroy
the phage virus. Now these bacteria incorporate
genes of donor bacteria and replicates.
b.
Specialized transduction: It occurs in the Lysogenic
cycle of phage virus. In this cycle viral DNA incorporate
into bacterial DNA as prophage. It remains peacefully
there. But sometimes, it becomes lytic. It comes out
of bacterial DNA. Some part of bacterial DNA remain
attached to it. Viral DNA with a piece of bacterial DNA
replicates and lops new capsids. Bacteria burst. Virus
infects other bacteria and transfer the genes of the
donor bacteria to recipient bacteria.
c. Restricted transduction: Certain phages carry out
a more restricted kind of transduction. They carry
only a specific section of bacterial genetic material.
They transduce only a few genes. Retroviruses carry
out specific or restricted transduction. These viruses
can cause the formation of tumors (oncogenesis) in
animals. It is now known that these viruses exchange
a small portion of their genome for a mutant cellular
gene that has a role in gene regulation or replication.
These viruses carrying mutant genes infect cells. They
transform these cells into tumor cells.
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| January/February 2022
An example is the viral transfer
Bacterial Transformation
What is Bacterial Transformation? Bacterial transformation is a process
of horizontal gene transfer by which some bacteria take up foreign
genetic material (naked DNA) from the environment. It was first
reported in Streptococcus pneumoniae by Griffith in 1928.5
DNA as the
transforming principle was demonstrated by Avery et al in 1944.6
The process of gene transfer by transformation does not require a
living donor cell but only requires the presence of persistent DNA in the
environment. The prerequisite for bacteria to undergo transformation
is its ability to take up free, extracellular genetic material. Such bacteria
are termed as competent cells.
The factors that regulate natural competence vary between various
genera. Once the transforming factor (DNA) enters the cytoplasm, it
may be degraded by nucleases if it is different from the bacterial DNA.
If the exogenous genetic material is similar to bacterial DNA, it may
integrate into the chromosome. Sometimes the exogenous genetic
material may co-exist as a plasmid with chromosomal DNA.
Reasons for Transformation
The phenomenon of natural transformation has enabled bacterial
populations to overcome great fluctuations in population dynamics
and overcome the challenge of maintaining the population numbers
during harsh and extreme environmental changes. During such
conditions some bacterial genera spontaneously release DNA from
the cells into the environment free to be taken up by the competent
cells. The competent cells also respond to the changes in the
environment and control the level of gene acquisition through the
natural transformation process.
Competence of Bacteria
Not all bacteria are capable of taking up exogenous DNA from their
environment. The practical approach to acquire competent cells is
to make the bacterial cells artificially competent using chemicals or
electrical pulses.
* Chemical induction of competence involves the
following steps:
»
chilling the cells in the presence of calcium phosphate
to make them permeable
»
»
incubation with DNA
heat shock treatment at 42°C for 60-120 seconds that
causes the DNA to enter the cells. To endure the heat
shock treatment, it is important the cells used are in the
log phase of growth

APR January/February 2022

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