Question

If you mix together an equal number of \(\mathrm{F}^{+}\)and \(\mathrm{F}^{-}\)cells, how would you expect the proportions to change over time? In other words, do you expect an increase in the relative proportion of \(\mathrm{F}^{+}\) or of \(\mathrm{F}^{-}\)cells? Explain your answer.

Short answer

Over time, in an equal mixture of F+ and F- cells, you would expect an increase in the relative proportion of F+ cells. This is because F- cells receive the F plasmid from F+ cells during conjugation, thus transforming into F+ cells. Consequently, the proportion of F- cells decreases.

Step-by-step solution

Understand Bacterial Conjugation

Bacterial conjugation is a way by which a bacterium transfers genetic material to another bacterium. In the case of cells with F factor, F+ bacteria have a plasmid (F factor) which F- bacteria lack. F+ bacteria can transfer their F plasmid to F- bacteria, turning them into F+ bacteria.

Expected change in relative proportions

In a mixed culture of equal number of F+ and F- bacteria, over time as bacterial conjugation proceeds, the F- bacteria (which initially lack the F plasmid) start receiving the F plasmid from F+ bacteria. Therefore, previously F- bacteria start becoming F+.

Final Expectation

This means that over time the proportion of F- bacteria in the culture would decrease while the proportion of F+ bacteria would increase. This is because F- bacteria are transforming into F+ bacteria due to the transfer of F plasmid during bacterial conjugation.

Key concepts

F Factor

Understanding the 'F factor,' or fertility factor, is crucial in bacterial genetics. It is a specific type of plasmid, which is a DNA molecule separate from the chromosomal DNA and capable of autonomous replication. The F factor is noteworthy because it enables a bacterial cell (designated as F+) to produce a structure called a pilus and engage in a unique form of DNA transfer known as bacterial conjugation.

In a population of bacteria, possessing the F factor essentially means that a cell has the genetic 'equipment' required to donate genetic material to a recipient cell, the F- cell, which lacks the F factor. After contact is made via the pilus, the F factor can be replicated and a copy transferred to the F- cell, converting it into a donor cell. This exchange can rapidly alter the genetic landscape of a bacterial colony, leading to an increase in F+ cells over time.

Genetic Material Transfer

The transfer of genetic material between bacterial cells, primarily through processes like bacterial conjugation, plays a pivotal role in bacterial adaptation and evolution. Unlike sexual reproduction in eukaryotes, which involves the fusion of gametes, bacterial conjugation is a unidirectional transfer of DNA.

During this process, an F+ cell will extend its pilus to make contact with an F- cell. Once contact is established, a bridge-like connection forms, allowing the transfer of the F factor. This can include not only the plasmid DNA itself but also any additional genes carried on the plasmid. Therefore, conjugation allows not only for the spread of the ability to conjugate (F+ status) but also for the potential dissemination of other advantageous traits, such as antibiotic resistance. As a result of this transfer, the recipient F- cells are converted to F+ status, leading to a dynamic shift in the composition of the bacterial population over time.

Plasmid

Plasmids are circular, double-stranded DNA molecules that exist independently of the chromosomal DNA within bacterial cells. They are often referred to as 'replicons' because of their ability to replicate autonomously. Plasmids come in various sizes and can carry a range of genes that may confer advantageous traits to their bacterial hosts, such as antibiotic resistance or the ability to use unusual nutrients.

Importantly, plasmids like the F factor play an essential role in horizontal gene transfer among bacteria, including conjugation. Given that plasmids are not typically required for a bacterium's basic functions, their loss or acquisition can happen with little to no immediate detrimental effect on the cell. However, they can dramatically affect the cell's abilities and its fitness in certain environments. As such, understanding plasmids is vital to comprehending how bacteria can rapidly adapt to new challenges and spread important traits throughout a population.