In most prokaryotes such as bacteria, what is the usual method of asexual reproduction and cell division?

Introduction / Context:
Bacteria are single celled prokaryotic organisms that reproduce rapidly under favorable conditions. Understanding how they divide is fundamental in microbiology, medicine, and biotechnology. Unlike many eukaryotes, bacteria do not undergo mitosis, but they still need a reliable mechanism to copy their genetic material and split into daughter cells. This question asks about the primary method by which bacteria carry out asexual reproduction and cell division.

Given Data / Assumptions:

• The organisms in question are bacteria, not eukaryotic cells or viruses.
• The question focuses on the usual method of reproduction under normal conditions.
• Options include binary fission, a reference to spirilla, RNA, and fusion.
• We assume no special conditions such as conjugation or spore formation are being considered.

Concept / Approach:
The primary method of bacterial reproduction is binary fission. In this process, the bacterial chromosome is replicated and each copy attaches to different parts of the plasma membrane. The cell grows, and the membrane and cell wall pinch inward, eventually splitting the cell into two genetically identical daughter cells. Spirilla refers to spiral shaped bacteria, not a reproductive method. RNA replication is a feature of many viruses, especially RNA viruses, but bacteria are cellular organisms that replicate DNA. Fusion of two adult bacterial cells is not the standard description of bacterial reproduction, although horizontal gene transfer can occur by conjugation, which is different from fusion followed by division.

Step-by-Step Solution:
Step 1: Recall that bacteria usually have a single circular DNA molecule located in the nucleoid region. Step 2: During binary fission, this DNA is copied so that the cell has two identical chromosomes. Step 3: The two DNA molecules attach to different sites on the inner surface of the plasma membrane. Step 4: The cell elongates, and new cell wall and membrane material are added between the two DNA attachment sites, pulling them apart. Step 5: A septum forms, and the cell membrane and wall pinch inward until the original cell divides into two daughter cells. Step 6: Recognize that this entire process is called binary fission and is the standard method of bacterial cell division.

Verification / Alternative check:
Microbiology textbooks and diagrams consistently describe binary fission as the characteristic method of prokaryotic reproduction. Laboratory growth curves of bacterial cultures, showing exponential growth, are often explained by repeated rounds of binary fission. While bacteria can exchange genetic material by conjugation, transformation, or transduction, these processes do not replace binary fission as the main means of producing more cells. This confirms that binary fission is the correct answer.

Why Other Options Are Wrong:
Option B: Spirilla describes bacteria with a spiral shape. Shape terms such as cocci, bacilli, and spirilla indicate morphology, not division mechanisms.

Option C: RNA replication is central to the life cycle of RNA viruses, which must copy their RNA genomes. Bacteria have DNA genomes and use DNA replication before dividing by binary fission.

Option D: Fusion of two adult bacterial cells into one is not a common reproductive strategy. Bacteria may transfer plasmids by conjugation, but each cell still divides by binary fission.

Common Pitfalls:
Students sometimes confuse the various processes bacteria use to exchange genes with the actual process of cell multiplication. Another pitfall is assuming that mitosis applies to all forms of cell division, even in prokaryotes. Remember that binary fission is simpler than mitosis and does not involve a mitotic spindle or multiple chromosomes, which is appropriate for bacterial cells.

Final Answer:
Most bacteria reproduce asexually by binary fission, in which one cell divides into two genetically similar daughter cells.