Question

Briefly describe two ways in which chromosomal DNA from a donor cell may be transferred to a recipient cell during the process of conjugation.

Short answer

Chromosomal DNA may be transferred via a plasmid that contains chromosomal DNA or through direct transfer of a portion of the donor's chromosome into the recipient cell.

Step-by-step solution

Understanding Conjugation

Conjugation is a process of horizontal gene transfer where a donor cell transfers genetic material to a recipient cell through direct contact. This contact is often mediated by a sex pilus, which connects the two cells.

Plasmid Transfer

One way chromosomal DNA may be transferred is through a plasmid. A plasmid containing a portion of the donor's chromosomal DNA can be replicated and transferred to the recipient cell through the pilus.

Chromosome Mobilization

Another method involves the mobilization of the donor cell's chromosome itself. A segment of the donor's chromosomal DNA is transferred directly into the recipient cell, usually starting at the origin of transfer, which becomes integrated into the recipient's chromosome.

Key concepts

Horizontal Gene Transfer

Horizontal gene transfer (HGT) is a fundamental process in bacterial evolution, allowing for the sharing of genetic information between organisms that are not parent and offspring. Unlike vertical gene transfer, which occurs when genes are transmitted from parent to progeny, HGT involves the movement of genes across different species or strains of bacteria, enriching the genetic pool.

There are several mechanisms of HGT, with conjugation being one of the most well-studied. In conjugation, DNA is deliberately transferred between bacteria through direct cell-to-cell contact. This process enables bacteria to rapidly adapt and acquire new traits such as antibiotic resistance or virulence factors. This is also an example of how bacteria can evolve without the slow process of mutation over generations, showcasing their ability to quickly respond to environmental pressures.

Sex Pilus

The sex pilus is a thin, thread-like structure that emerges from the surface of a donor bacterial cell. Think of it as a microscopic grappling hook that establishes a physical bridge between two bacterial cells. This structure is crucial for the process of conjugation as it allows for the direct transfer of genetic material.

The sex pilus is not a passive structure; it is composed of protein subunits called pilins. The pilus is able to extend and retract, which enables the donor cell to reach out to potential recipient cells and draw them close, setting the stage for DNA transfer. Once retracted, the two cells almost 'kiss', creating a narrow channel for genetic exchange. This close contact is essential for the successful transfer of genetic information during conjugation.

Plasmid Transfer

Plasmids are small, circular pieces of DNA that are distinct from the chromosomal DNA of the bacterium. These DNA molecules can replicate independently and often carry genes that can be beneficial to the bacterium, such as those conferring antibiotic resistance. In the context of conjugation, plasmid transfer is a significant method through which bacteria can share these advantageous genes.

During plasmid transfer, a copy of the plasmid is made in the donor cell and is then passed to the recipient cell via the conjugation bridge established by the sex pilus. The process usually involves rolling circle replication, a mechanism by which the plasmid is replicated one strand at a time, allowing a single-stranded copy to enter the recipient cell. Once inside, this strand is converted into a double-stranded plasmid, ready to be expressed or further replicated in the new host.

Chromosome Mobilization

Chromosome mobilization is a more complex form of gene transfer during bacterial conjugation because it involves segments of the actual chromosomal DNA rather than plasmids. Unlike plasmids, chromosomal DNA houses the essential genetic instructions for the bacterium's survival and reproduction.

In chromosome mobilization, a specific part of the chromosome called the origin of transfer is where the process begins. Enzymes cut the DNA at this site, and the mobilized segment enters the recipient cell through the conjugation bridge. Once inside, the DNA either remains as an extra-chromosomal element or integrates into the recipient's chromosome through recombination, bestowing new properties to the host. This method of transfer is often less frequent than plasmid transfer due to the size and complexity of chromosomal DNA.