TRANSFORMATION
Transformation is a method of genetic recombination in which a naked DNA from a donor bacteria is
transferred to a competent recipient bacteria and incorporated into chromosome of the latter, e.g.
.in Bacillus, Haemophilus, Neisseria, Pneumococcus
.Transformation occurs in nature
.It is widely used in recombinant DNAtechnology
In Gram+ve bacteria the DNA is taken up as a single stranded molecule and the complementary
.strand is synthesized in the recipient
.In Gram-ve bacteria double stranded DNAis transformed
Mechanism of Transformation:
1. Donor bacterium dies, releasing dsDNA into the environment.
2. Nucleases enzymes cut DNA into 20 genes long fragments.
3. Fragments bind to DNA-binding proteins on competent recipient’s surface.
4. DNA enters cytoplasm.
5. RecA protein facilitates genetic exchange between donor and recipient DNA.
Transformation – step I
A donor bacterium dies and is degraded.
Transformation – step II
A fragment of DNA from the dead donor bacterium binds to DNA binding proteins on cell wall of a
competent live recipient bacterium .
Transformation – step III
The Rec A protein promotes genetic exchange between a fragment of the donor's DNA and the
recipient's DNA.
Transformation – step IV
Exchange is complete.
Factors affecting transformation–
DNA size and state : Sensitive to nucleases (at least 5 X 105 daltons )
Competence of the recipient (Bacillus, Haemophilus, Neisseria, Streptococcus)
•The ability to take up DNA from the environment is known as competence
• only DNA from closely related bacteria (competent cells) would be successfully transformed
• Competence factor (a specific protein produced at a particular time in the growth cycle of
competent bacteria and enable it to take up DNA naturally)
• Induced competence, e.g. by CaCl2.
Transduction
Definition: Gene transfer from a donor to a recipient bacteria through a bacteriophage
• Bacteriophage (phage): A virus that infects bacteria.
• Types of transduction: Generalized – Specialized
Lytic Cycle
A bacteriophage life cycle in which the phage replicates inside the host, causes lysis (bursting) of the
bacterial cell, and releases new phage particles.
Lysogenic Cycle
A bacteriophage life cycle in which the phage DNA integrates into the host genome as a prophage
and replicates passively with the host cell without causing immediate lysis
Generalized Transduction
Generalized transduction can transfer any gene of donor bacteria to recipient bacteria
During the replication of a lytic phage the capsid sometimes enclose a small fragment of
lysed bacterial DNA, instead of phage DNA, by a "head-full" mechanism. This is a
defective phage.
Such a phage cannot lyse another bacterium because the DNA in the phage head does
not have the genetic information to produce pha ge genome and proteins.
On infection of another bacterium defective phage injects the fragment of donor
bacterial DNA into the recipient bacteria, where it can be exchanged for a piece of the
recipient's DNA, if their sequences are homologous.
Steps in Generalized Transduction
• Release of phage
• Phage replication and degradation of host DNA
• Assembly of phages particles and encapsidation of host DNA
• Infection of recipient
• Homologous recombination
• Infection of Donor
Specialized transduction
A transduction in which only certain donor genes can be transferred to the recipient
Occur during the lysogenic life cycle of a temperate phage
During spontaneous induction of lysogeny, a small piece of bacterial DNA may
sometimes be exchanged for a piece of phage genome.
This piece of bacterial DNA replicates as a part of the phage genome and is
incorporated into capsid of each phage progeny
On infection of a recipient bacteria, the phage DNA containing donor bacterium
genes are injected into the recipient bacterium where donor DNA fragment can be
exchanged for a piece of the recipient's DNA, if their sequences are homologous
Different phages may transfer different genes but an individual phage can only
transfer certain genes
Lysogenic (phage) conversion occurs in nature and is the source of virulent strains of
bacteria, e.g. toxin production in Cl. botulinum, C. diptheriae, STEC, etc
Steps in Specialized Transduction
• Excision of the prophage
• Replication and release of phage
• Infection of the recipient
• Lysogenization of the recipient
Conjugation – “Bacterial Mating”
Conjugation is a process of direct transfer of DNA from one bacterium (donor) to another
(recipient) through cell-to-cell contact, typically via a sex pilus (F pilus).
Key Features:
Requires physical contact between two bacterial cells.
Involves a donor cell (F⁺) and a recipient cell (F⁻).
DNA transferred is usually a plasmid (like the F plasmid), but sometimes
chromosomal genes can also be transferred.
Plays a key role in horizontal gene transfer and genetic recombination in bacteria.
⚙️Mechanism of Conjugation:
1. F Plasmid Basics:\\\
F plasmid (fertility factor) carries genes needed to make the sex pilus.
A bacterium with the F plasmid is called F⁺ (donor).
A bacterium without it is F⁻ (recipient).
2. F⁺ × F⁻ Mating:
F⁺ cell forms a pilus that attaches to an F⁻ cell.
A copy of the F plasmid DNA is transferred to the F⁻ cell.
After transfer, the recipient becomes F⁺.
3. Hfr Strain (High frequency recombination):
Sometimes, the F plasmid integrates into the bacterial chromosome → forms an Hfr
cell.
During conjugation, part of the bacterial chromosome (starting from F plasmid site)
is transferred.
Usually, only a portion is transferred, and the recipient remains F⁻ but may gain new
genes through recombination.
4. F′ (F-prime) Conjugation:
If an F plasmid excises from the chromosome but carries some bacterial genes with
it, it becomes an F′ plasmid.
F′ plasmid can be transferred to F⁻ cells → results in partial diploidy (two copies of
some genes).
Role in DNA Recombination:
DNA transferred via conjugation can recombine with the recipient’s genome if
sequences are homologous.
Leads to new genetic traits in bacteria (e.g., antibiotic resistance, metabolic
abilities).
