Question

List, and briefly discuss, (1-2 sentences max for each) the THREE different ways bacterial cells can exchange genetic information.

Answer 1

There are three ways for bacteria to transfer their DNA horizontally :-

1. Conjugation.

2. Transformation.

3.Transduction.

Conjugation:-
Conjugation is DNA is transferred from one bacterium to another. After the donor cell pulls itself close to the recipient using a structure called a pilus, DNA is transferred between cells. In most cases, this DNA is in the form of a plasmid.
1. An F+ donor cell contains its chromosomal DNA and an F plasmid. It has a rodlike pilus. A recipient F- cell has only a chromosome and no F plasmid.

2. The donor cell uses its pilus to attach to the recipient cell, and the two cells are pulled together.

3. A channel forms between the cytoplasms of the two cells, and a single strand of the F plasmid is fed through.

4. Both of the cells now have an F plasmid and are F+. The former recipient cell is now a new donor and can form a pilus.
An F+ donor cell contains its chromosomal DNA and an F plasmid. It has a rodlike pilus. A recipient F- cell has only a chromosome and no F plasmid.
The donor cell uses its pilus to attach to the recipient cell, and the two cells are pulled together.
A channel forms between the cytoplasms of the two cells, and a single strand of the F plasmid is fed through.
Both of the cells now have an F plasmid and are F+. The former recipient cell is now a new donor and can form a pilus.

Steps of bacterial conjugation :-

Step I: Pilus formation :-
Donor cell (F+ cell) produces the sex pilus, which is a structure that projects out of the cell and begins contact with an F– (recipient) cell.
Step II: physical contact between donor cell and recipient cell:-
The pilus enables direct contact between the donor and the recipient cells forming conjugation tube
Step III: transfer of F- plasmid:-
F-factor opens at replication origin (Ori T site).
one strand of F-factor is cut down at origin and then 5’end of this strand enters into recipient cell.
Step IV: complementary strand synthesis:-
In the last step, the donor cell and the recipient cell, both containing single-stranded DNA of F-plasmid
A complementary strand is then synthesized in both donor and recipient cell,
Now the recipient cell also contain a copy of F-plasmid and become a donor cell.

Bacterium Mobile Plasmid Pilus Chromosome 2. Donor Recipient Relaxosome Transferosome 4. New Donor New Donor

When conjugation is initiated, via a mating signal, a complex of proteins called the relaxosome creates a nick in one plasmid DNA strand at the origin of transfer, The transferred, , is unwound from the duplex and transferred into the recipient bacterium in a 5'-terminus to 3'-terminus direction. The remaining strand is replicated, either independent of conjugative action (vegetative replication) or in concert with conjugation (conjugative replication similar to the rolling circle replication of lambda phage.

Transformation:-

Unlike humans, bacteria are capable of taking up DNA directly from their enviroment and incorporating it into their genomes.This process is known as natural transformation. This DNA usually comes from dead bacteria lysing (splitting open) and releasing their genetic contents into the surrounding area.

Recipient cell DNA fragments from donor cells Chromosomal DNA Recipient cell takes up donor DNA Degraded unrecombined DNA 2 Recombination occurs between donor DNA and recipient DNA Genetically transformed cell

Transduction:-

lytic cycle: The normal process of viral reproduction involving penetration of the cell membrane, nucleic acid synthesis, and lysis of the host cell.
lysogenic cycle: A form of viral reproduction involving the fusion of the nucleic acid of a bacteriophage with that of a host, followed by proliferation of the resulting prophage.
transduction: Transduction is the process by which DNA is transferred from one bacterium to another by a virus.It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector. 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 DNAase resistant (transformation is susceptible to DNAase). Transduction is a common tool used by molecular biologists to stably introduce a foreign gene into a host cell’s genome.

When bacteriophages (viruses that infect bacteria) infect a bacterial cell, their normal mode of reproduction is to harness the replicational, transcriptional, and translation machinery of the host bacterial cell to make numerous virions, or complete viral particles, including the viral DNA or RNA and the protein coat.

Transduction is especially important because it explains one mechanism by which antibiotic drugs become ineffective due to the transfer of antibiotic-resistance genes between bacteria. In addition, hopes to create medical methods of genetic modification of diseases such as Duchenne/Becker Muscular Dystrophy are based on these methodologies.

The Lytic Cycle and the Lysogenic Cycle
Transduction happens through either the lytic cycle or the lysogenic cycle. If the lysogenic cycle is adopted, the phage chromosome is integrated (by covalent bonds) into the bacterial chromosome, where it can remain dormant for thousands of generations. If the lysogen is induced (by UV light for example), the phage genome is excised from the bacterial chromosome and initiates the lytic cycle, which culminates in lysis of the cell and the release of phage particles. The lytic cycle leads to the production of new phage particles which are released by lysis of the host.

Transduction is a method for transferring genetic material. The packaging of bacteriophage DNA has low fidelity and small pieces of bacterial DNA, together with the bacteriophage genome, may become packaged into the bacteriophage genome. At the same time, some phage genes are left behind in the bacterial chromosome.

There are generally three types of recombination events that can lead to this incorporation of bacterial DNA into the viral DNA, leading to two modes of recombination.

Generalized transduction is the process by which any bacterial gene may be transferred to another bacterium via a bacteriophage, and typically carries only bacterial DNA and no viral DNA. In essence, this is the packaging of bacterial DNA into a viral envelope. This may occur in two main ways, recombination and headful packaging.

If bacteriophages undertake the lytic cycle of infection upon entering a bacterium, the virus will take control of the cell’s machinery for use in replicating its own viral DNA. If by chance bacterial chromosomal DNA is inserted into the viral capsid which is usually used to encapsulate the viral DNA, the mistake will lead to generalized transduction.

If the virus replicates using “headful packaging,” it attempts to fill the nucleocapsid with genetic material. If the viral genome results in spare capacity, viral packaging mechanisms may incorporate bacterial genetic material into the new virion.

The new virus capsule, now loaded with part bacterial DNA, continues to infect another bacterial cell. This bacterial material may become recombined into another bacterium upon infection.

TRANSDUCTION PROCESS Generalized Transduction 1) Bacterial host cell (Donor cell) infected by phage. Specialized Transduction 1) Prophage DNA integrated into the bacterial DNA. Prophage DNA Donor DNA Bacterial host DNA Mrus Phage DNA 2) Host cell DNA is broken down into smaller pieces. Proteins and phage EDNA is also synthesized. 2) Prophage DNA cuts incorrectly and exchanges its DNA to that of bacterial host cell. Beginning of excision After complete excision 3) Bacterial host DNA is packaged in one of the viral capsids that are released through lysis of the bacterial cell. Phage capsid + donor cell DNA 3) Phage capsids contain bacterial host DNA, which is is transferred to a new bacterial recipient cell. Phage capsid+ Donor DNA and Prophage DNA 4) Transducing phage with host DNA infest new recipient cell. Recombination also occurs here. Recipient DNA 4) Transduced phage is transferred to a new cell and recombination occurs. Recipient DNA 5) The recombinant cell DNA has a different genotype, which is different from the donor or recipient cell genotype. 5) Recombinant new cell has a mixture of the donor DNA 1 and one of its own DNA. Its genotype is different. New recombinant cell Prophage DNA Host DNA Recipient DNA