Bacteria are prokaryotes. They have a circular genome, usually present as a single DNA plasmid. Bacteria are not specifically sexual or asexual organisms. Binary fission, a form of asexual reproduction, occurs among many bacteria, but some also reproduce sexually via conjugation. Research is still delving into the intricacies of bacterial reproduction, partly to understand how bacteria share genes in the context of antibiotic resistance.
Asexual reproduction of bacteria
The most basic form of bacterial reproduction, binary fission is a form of asexual reproduction in which the bacterial cell grows twice its size, duplicates its genome and splits into two bacterial cells, similar to mitosis in animal cells. Essentially the bacterial cell clones itself. As this diagram from Cornell University shows, the FtsZ ring is an important part of the splitting process, which is called cytokinesis in eukaryotes. Probably the most important part of binary fission is replication and segregation of the genetic material. The replicated and original genomes attach to the cell membrane and opposite ends of the cell to anchor them into the two new cells as the halves of the cell separate.
Some cells lack the central FtsZ ring and replicate two offspring cells within its own cytoplasm using FtsZ rings at the poles. This process is called intracellular offspring production. In this method, the mother cell dies and releases two offspring (as opposed to resulting in one mother and one offspring).
A similar method of asexual reproduction is baeocyte formation. A baeocyte is a small cell that enters a replicative phase. It then undergoes multiple rounds of fission to produce a number of bacterial cells. Budding also occurs among some bacterial species. Rather than undergo doubling and fission, the bacterial replicates its DNA and then encases it at the cell membrane and wall for release as a new cell.
Sexual reproduction in bacteria
Sexual reproduction is the creation of an organism that contains the genetic material from two donor parents. Via protein tubes called pili (singular, pilus), bacteria can share genetic material, swapping genes in a process called conjugation, which is considered one type of horizontal gene transfer. When bacteria accept genetic material from a donor it is incorporated into the genome when replication occurs, creating an offspring with different genetics than the parent.
Donor bacteria (termed “male”) carry a plasmid called F factor (F+). This is copied and transmitted to F- bacteria (termed “female”). This process is shown in an animation from HHMI in the context of drug resistance. The genetic recombination results in a bacterial cell with a new genome with contributions from both parents.
Some bacterial species, primarily Gram-positive bacteria, can create a clone and close it off as an endospore when experiencing an environmental assault or stress. An endospore is essentially a specially coated cyst with a cortex of the membranes and cell wall and a core containing the genetic information for the bacterium. The spore can withstand environmental stressors and hazards, allowing the bacterium’s genetics to survive.
Over time bacterial populations have developed various ways to reproduce and pass on their genes. The ability to vary the process allows bacteria to survive in various conditions and contexts. Some of which science has yet to understand.