Final answer:
Conjugation in bacteria involves an F+ cell with an F plasmid forming an F pilus that contacts an F- cell, leading to the transfer of genetic material via a cytoplasmic bridge. The F pilus retracts to bring the cells together, allowing the transfer of DNA, which is incorporated into the recipient to replicate, turning it into an F+ cell. This process is crucial for genetic diversity and can spread traits like antibiotic resistance.
Step-by-step explanation:
Conjugation and DNA Transfer
Conjugation is a process of genetic exchange where DNA is directly transferred from one bacterium (donor) to another (recipient). In Escherichia coli, donor F+ cells contain a fertility factor known as the F plasmid, which endows the cells with the ability to initiate conjugation. The F plasmid encodes for the formation of a structure called the F pilus, also referred to as the sex pilus. During conjugation, the F pilus extends from the donor cell and makes contact with the recipient F- cell. After the initial recognition, retraction of the F pilus draws the two cells together.
The cell membranes of both bacteria merge, creating a conjugation tube or a cytoplasmic bridge, which allows the transfer of DNA to occur. Once in contact, the F plasmid DNA is nicked and a single strand begins to transfer through the bridge by rolling circle replication. The recipient cell duplicates this incoming DNA strand to form a double-stranded plasmid, effectively converting the F- cell into an F+ cell with the ability to form its own conjugation pili. This entire process is instrumental in spreading genetic material, such as antibiotic resistance genes, between bacteria, thereby facilitating bacterial adaptation and evolution.
In some cases, the F plasmid may integrate into the bacterial chromosome and form an Hfr (High-frequency recombination) cell. The Hfr cell is capable of transferring not only the F plasmid but also portions of chromosomal DNA to recipient cells during conjugation. An incorrect excision of this integrated plasmid can create an F' plasmid carrying chromosomal genes, which are also transferable through conjugation, providing another route for genetic variation.