Transduction is a mode of genetic transfer between bacteria mediated by bacteriophages. During viral replication, fragments of bacterial DNA can become packaged within viral particles. These particles may then infect other bacteria and insert the donor DNA into the recipient genome. There are two types of transduction - generalized, where any bacterial DNA fragment can be transferred, and specialized, where only DNA near the site of viral integration is transferred. Cotransduction frequencies can also be used to map the relative locations of bacterial genes, as genes closer together are more likely to be cotransferred within the same viral particle. Transduction is useful for genetic engineering and mapping bacterial chromosomes.

1. TRANSDUCTION
Dr. M. Sonia Angeline
Assistant Professor
Kristu Jayanti College

2. INTRODUCTION
 Transduction is a mode of genetic transfer from one bacterium to another through a
virus.
 There is no direct contact between the bacterial cells.
 Transduction is a method of gene transfer in bacteria from donor to recipient using
bacteriophage.
 In this process, bacteriophages, which infect bacteria, use host cells to multiply and while
assembling they sometimes pack the bacterial DNA with them.
 Later, when these viruses/ bacteriophages infect new bacterial cells, the bacterial genome
that they carry may get inserted into the host genome.
 Transduction is commonly used in genetic engineering for inserting foreign DNA into
the host cell.
 Transduction was discovered by Zinder and Lederberg in Salmonella. Hershey and
Chase used transduction as a tool to confirm that DNA is the genetic material.

3. STEPS INVOLVED IN TRANSDUCTION:
 Infection of the bacterial cell by bacteriophage.
 The virus uses the host machinery to make multiple copies either directly by the
lytic cycle or first gets incorporated into the bacterial genome by the lysogenic
cycle and followed by the lytic stage.
 During assembly of bacteriophages, the bacterial genome also gets packed by
mistake in the viral head alongside the viral genome.
 In the lysogenic cycle, during excision of prophage, some parts of the bacterial
genome that flank the prophage are also excised and go inside the assembled viral
head together with the viral genome.

4. STEPS INVOLVED IN TRANSDUCTION:
 When these viruses infect another bacterial cell, they inject the viral DNA as
well as donor DNA into the host cell.
 The bacterial DNA either forms plasmids or gets inserted into the recipient
DNA if it is homologous to the recipient genome.
 Most of the time it remains as an extrachromosomal DNA.
 It can also get inserted with the prophage if it is a temperate phage.
 So the fate depends on the portion of bacterial DNA and also on the nature
of bacteriophages.

5. GENERALIZED TRANSDUCTION
 If all the fragments of donor DNA from any region of chromosome have a chance to enter into
transducing bacteriophage then it is known as generalized transduction.
 In this type of transduction, at first bacteriophage infects donor cell and begins lytic cycle.
 When virus enter into bacterial cell, virus hijack host cell and synthesize virus components such as
genome, enzymes, capsid, head tail and tail fibers. Then viral enzyme hydrolyses host cell DNA
into small fragments.
 During assembly of virus component to form progeny viruses, sometime any of the fragments of
donor DNA get incorporated into the virus capsid (bacteriophage head).
 Such abnormal bacteriophage when infects a new cell, it can transfer this donor DNA into new
bacteria.
 Since this donor DNA is not viral DNA, it does not replicates inside recipient bacteria but
undergoes homologous recombination with recipient cell’s chromosomal DNA forming
recombinant cell.

7. SPECIALIZED TRANSDUCTION
 In specialized transduction, bacteriophage transfer only a few restricted gene
(DNA fragments) from donor bacteria to recipient bacteria.
 Specialized transduction is carried only by temperate bacteriophage which
undergoes lysogenic cycle in donor cell.
 At first temperate bacteriophage enter into donor bacteria and then its genome
gets integrated with host cell’s DNA at certain location and remains dormant and
pass generation to generation into daughter cell during cell division.
 The bacteriophage which follows lysogenic cycle is known as temperate phage.
 When such lysogenic cell is exposed to certain stimulus such as some chemicals
or UV lights, it causes induction of virus genome from host cell genome and
begins lytic cycle.

8. SPECIALIZED TRANSDUCTION
 On induction from donor DNA, this phage genome sometimes carries a
part of bacterial DNA with it.
 The bacterial DNA lies on sides of integrated phage DNA are only carried
during induction.
 When such bacteriophage carries a part of donor bacterial DNA infects a
new bacteria, it can transfer that donor DNA fragments into new recipient
cell.
 So, in this specialized transduction only those restricted gene are situated
on the side of integrated viral genome have a chance to enter into
recipient cell.

11. CO TRANSDUCTION
 Transduction can also be used to map the relative locations of different bacterial
genes on the chromosome.
 The principle is simply that the amount of DNA carried by any one transducing
particle represents a very small portion of the total DNA of the bacterial cell.
 Transduction can also be used to map the relative locations of different bacterial
genes on the chromosome.
 The principle is simply that the amount of DNA carried by any one transducing
particle represents a very small portion of the total DNA of the bacterial cell.
 Any two genes that can be contained within a single transducing particle
(meaning they can be "cotransduced") must, therefore, be relatively close to
each other on the chromosome.

12. CO TRANSDUCTION
 By measuring the cotransduction frequency of two or more genes, one can
estimate the relative proximity of those genes.
 The simplest analysis is to select for the transfer of one gene (for example, a
gene encoding antibiotic resistance), then to screen the population of
transductant cells for the inheritance of a second gene.
 The cotransduction frequency is measured as the percentage of cells that have
inherited both genes, rather than the selected first gene by itself.
 The higher their cotransduction frequency, the closer two genes must be to
each other.

13. Genetic mapping chromosome transfer by Hfr
strains
Due to the F factor's inherent tendency to transfer itself
during conjugation, the rest of the bacterial genome is
dragged along with it.
Therefore, unlike a normal F+ cell, Hfr strains will attempt to
transfer their entire DNA through the mating bridge, in a
fashion similar to the normal conjugation.

14. Hfr strains
 A strain of bacterial that possesses the F factor integrated into the
bacterial genome, hence, when it conjugates with another bacterium, it
attempts to transfer a copy of the F factor as well as a portion of or the
entire chromosome to the recipient bacterium.
 Therefore, we can immediately determine the order of the genetic
markers simply by looking at the percentage of recombinants for any
marker among the leu+ recombinants.
 Because the inheritance of thr+ is the highest, this must be the first
marker to enter after leu. The complete order is leu, thr, ile, mal, trp.

15. Co Transduction for Mapping
 Cotransduction can be used to determine
a genetic map of the relative distances
between genetic markers and the order of
the markers.
 The use of cotransduction to determine
gene order is based on the fact that the
closer genes are to each other, the greater
the probability they will be
cotransduced.
 That is, the frequency of cotransduction
is inversely proportional to the distance
between two genes.

16. Genetic Mapping in bacteria by transduction
Transductants - once the donor genetic material has been
introduced into the recipient. It may undergo genetic
recombination with a homologous region of the recipient
chromosome.
The recombinant recipient is called transductants.
Phage Lysate- the suspension of released progeny phages is
called as a phage lysate.

17. Genetic Mapping in bacteria by transduction
 Generalized transduction can be used to make linkage map and also to find gene
order and map distance between co-transduced genes.
 Selected markers - a marker gene which help in the selection of transduced cells
among the population of transduced and non transduced cells.
 Unselected markers- rest other markers are considered as unselected markers.

19. E. coli genetic map
 Two principal techniques of genetic mapping will be mentioned in the following discussion. The first
method is mapping by "time of entry," in which genetic markers are positioned on the chromosome by
determining the time when each marker first enters a recipient cell during the course of chromosome
transfer from an Hfr donor strain.
 The second, and generally more accurate, mapping technique relies on transduction crosses mediated
by bacteriophage P1 of E. coli.
 Two-factor transduction crosses provide estimates of the distance between pairs of markers separated
by not more than 1.5 to 2 min by making use of the empirical "minutes-to-per cent frequency of
cotransduction".
 Multifactor transduction crosses involving three or more cotransducible markers permit precise
determinations of the sequence of markers with respect to each other and to the linkage map as a
whole.
 Groups of closely linked markers for which the gene sequence relative to the whole map is known are
referred to in this review as having a known orientation.