Summarize the following text to main points and sub-points and make detailed explain for them:

ferred by plasmids and transposons. 
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GENE TRANSFER IN BACTERIA 
Recombination between two genetically 
different DNA molecules take place only 
when the bacterial cell (recipient) acquires a 
foreign genetic element derived from a 
different cell (donor). Acquisition of foreign 
genetic elements in procaryotes may occur 
by three main mechanisms: 
1) transformation; 2) transduction; and 3) 
conjugation. These means of transfer of 
genes will be discussed briefly. 
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GENETIC TRANSFORMATION 
Bacterial cell can take small naked DNA 
fragments from its surroundings, which brings 
about a genetic change in the recipient cell. 
This process is called genetic transformation. The 
source of transforming DNA may be the released 
DNA from bacterial cells upon their lysis. 
The released DNA molecule ultimately fragments 
into small segments of DNA each carrying few 
genes. Other live bacterial cells in the vicinity of 
the fragmented DNA, are sometimes able to take 
up (absorb) one or few of these fragments. 
Bacterial cells that are able to absorb free DNA 
fragments are called competent bacteria. 
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In some types of bacteria, the state of 
competency occurs only within a brief period 
during the logarithmic phase of growth. 
Some bacteria, such as E. coli, do not show 
natural competency. However, low efficiency 
competency can be induced in these bacteria by 
treatment with high concentration of calcium 
chloride, aided by overnight incubation at cold 
temperature, or by heat shock. Uptake of double-
strand DNA molecules is usually accompanied by 
the degradation of one strand and the integration 
of the second strand into the recipient DNA by a 
recombination process. 

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Transformation in bacteria was first demonstrated in the 
encapsulated bacterium Streptococcus pneumoniae by the 
English scientist Fred Griffith in 1928. S. pneumoniae 
colonies are smooth glistening when the organism possesses 
normal capsule. 
These are described as "S" colonies and are characterized by 
high virulence to experiments mice. 
Upon prolonged cultivation of the bacterium on artificial 
medium they spontaneously lose their capsules permanently, 
"S" colonies turn into rough variant "R" colonies that in the 
same time, lose their virulence to mice. This is known as 
S→R variation. 
Griffith made the important observation that upon injection 
of dead "S" cells along with live "R" cells, the R variants 
regains its virulence completely and kills the injected 
animals. In addition, "S" variants could be recovered from 
killed animals. 
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It was clear that something in the dead R cells 
caused a permanent change in the live R cells to S 
cells . 
Transformation events in some Gram positive 
bacteria: 
1- Competency of the recipient cell must develop 
naturally during exponential growth, or be 
induced by calcium/cold treatment. 
2- Double strand DNA fragments only bind to the 
surface of the recipient cell. 
Binding is believed to occur at receptor-like 
structures on cell membrane. 
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TRANSDUCTION 
Some bacteriophages act as vehicles for transfer 
of genes from one bacterial cell to another. 
Bacterial viruses are involved in gene transfer by 
two mechanisms: 
1) Generalized transduction, and 
2) Specialized transduction. 
A transducing phage is that phage capable of 
mobilizing bacterial genetic material from one 
cell to another. 
Not all bacteriophages are transducing, and not 
all bacteria are transducible as well. 
A transducing phage is not able to cause lysis of 
the infected cell because it is always defective. 
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GENERALIZED TRANSDUCTION : 
Generalized transduction is characterized 
by the transfer of a generalized group of 
genes, i.e. any gene on the bacterial 
chromosome have the same chance of 
being transferred by the viral carrier to 
another bacterial cell. 
Generalized transduction usually follows a 
lytic attack of a bacteriophage on bacterial 
cell. 
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At the end of the lytic cycle, bacterial 
genome undergoes fragmentation, often 
to a virussized fragments of DNA, which 
can be incorporated inside viral capsids, 
in place of the original virus genome. 
After release from the lysed cell, the 
formed pseudo-virus can attack and 
injects its bacterial DNA into new cells. 

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The injected bacterial DNA 
segment recombines at 
homologous segment on the main 
chromosome of the newly infected 
cell. 
It can be mediated by virulent 
phages, or by temeprate Phages 
that revert to lytic cycle at later 
stage.
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SPECIALIZED TRANSDUCTION : 
It is the process by which a specific set of genes is transferred 
by temperate phages from one bacterial cell into another. 
Temperate phages are those capable of integrating their 
genetic material into the bacterial chromosome (Iysogony). 
These phages recombine with the bacterial DNA at precise 
sites on the bacterial chromosome. 
A classical example of this type of transduction is that of 
lambda (A) phage which infects E.coli and integrates itself in 
a specific site on the chromosome. Lambda phage inserts its 
DNA in a region immediately adjacent to the galactose 
metabolism genes cluster on the genetic map of E. coli.
Upon reversion of the virus into its virulent form, it excises 
(cuts) itself out of the chromosome to resume its lytic cycle. 
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In rare occasion, excision of the integrated phage from the 
bacterial chromosome is incorrect. The excised virus leaves a 
part of it DNA in the chromosome, while some of the 
adjacent galactose genes remain attached to the excised virus 
DNA. 
In other words, the excised DNA is a hybrid of viral and 
bacterial DNA. In addition, the excised virus is defective 
because it misses the viral genes left on the bacterial 
chromosome. 
The defective phage cannot replicate itself properly because 
of its missing genes. A helper lambda virus (nondefective) 
must be present in the same cell (mixed infection), to 
complement the missing segments of the defective virus so 
that it can be replicated and becomes a transducing phage. 
Upon infecting a new cell, the transducing phage recombines 
at the same site on the chromosome to transduce galactose 
genes to the new bacterial cell. 
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CONJUGATION 
Plasmids mobility from one bacterial cell into another, can be 
achieved by cell-to-cell contact process known as 
conjugation. 
A portion of the bacterial chromosome can also be mobilized 
from one cell to another by plasmids during conjugation. 
A conjugative plasmid (first recognized in E. coli), which 
codes for sex pili, is known as F factor (F for fertility) or some 
times sex factor. 
Cells ,which carry F factor are usually designated F+ ( 
tentatively described as male cell),while cells which lack F 
factor and act as recipient of that factor are known as 
(female) . 
Sometimes, the F factor integrates itself in the main bacterial 
chromosome. The plasmid recombines with the bacterial 
genome by a site-specific mechanism. Both the genome and 
the plasmid have several homologous insertion sequences. 
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When stable integration is achieved, plasmid can initiate the 
mobilization of the bacterial chromosome to an F- cell. 
A copy of large segment of the bacterial chromosome (and 
even the whole chromosome) can be transferred to the 
recipient cell (single strand), with portion of the plasmid 
leading the transfer. 
The transferred DNA does not circularize and extensive 
recombination takes place. A copy of the whole 
chromosome may be transferred in about 100 minutes. 
Bacterial strains which carry a chromosome-integrated P 
factor, and show high genetic recombination are known as 
high frequency of recombination strains or Hfr. 
Because both plasmid and Hfr chromosome replicate during 
conjugation by the rolling circle method, the F+ remains as 
F+ and Hfr stays as Hfr after conjugation
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F- recipient of F factor from F+ dononr, 
becomes F+; while F- cell which received a 
part of chromosome from Hfr cell does not 
convert to Hfr, unless the whole 
chromosome carrying the remaining of the 
plasmid is completely transferred.
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GENE CLONING RECOMBINANT DNA 
TECHNOLOGY/GENETIC ENGINEERING 
GENE CLONING is a special gene manipulation 
procedure intended for preparing large amounts 
of specific pure genes. 
These cloned genes are used in further 
characterization and manipulation of genetic 
material as well as in expression of valuable gene 
products (proteins) that has applications in many 
fields. 
Gene cloning is brought about by procedures of 
genetic recombination collectively known as 
recombinant DNA technology or genetic 
engineering. 
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USEFUL TERMINOLOGY: 
CLONE: A group of identical genes or organisms produced 
asexually from a common progenitor. 
CLONING: The process of propagating identical genes or 
organisms in a clone. 
SHOTGUN METHOD: Cloning of random fragments of a 
whole genome for selecting a specific DNA fragment. 
RESTRICTION ENDONUCLEASE:

Summary :