Inquiry Question
How does reproduction ensure the continuity of a species?
Content description
- Explain the mechanisms of reproduction that ensure the continuity of a species, by analysing sexual and asexual methods of reproduction in a variety of organisms, including bacteria: binary fission
The prokaryotes
The organisms known as prokaryotes are subdivided into two groups: the eubacteria (or called bacteria) and the archaebacteria. Prokaryotes lack membrane-bound organelles. Both bacterial and archaebacterial cells have a nucleoid region where the chromosomal DNA is located. They divide by binary fission.
At the start of the binary fission process, the chromosomal DNA of the bacterial cell replicates, as shown in the following figure. After replication, the DNA molecules are transported away from each other to the cell’s opposite ends.
The bacterial cell begins to elongate (becomes stretched along the longitudinal axis). At this time, a specific protein in the cell’s cytoplasm accumulates in the middle of the elongating cell. This protein, called FtsZ, links with each other to form a ring in the middle of the cell. The ring is attached to the cell membrane. After separating the replicated chromosomes, the FtsZ ring contracts, pulling the cell membrane to the interior (forming a cleavage furrow). Eventually, the cell membrane fuses in the middle, segregating the cytoplasm into two. This structure is called a septum. Then, bacterial cell wall materials are laid at the septum, resulting in a new cell wall separating the two daughter cells. This completes binary fission.
Other components of the bacterial cell cytoplasm are also divided between the two daughter cells during binary fission. Plasmid DNA is replicated and apportioned between the daughter cells. Protein complexes, such as ribosomes, are also distributed between the daughter cells. After cell division, other cellular components are rapidly manufactured in the daughter cells.
The two daughter cells produced through binary fission are genetically identical (other than base changes introduced through mutations during DNA replication). Therefore, as a reproductive strategy, binary fission maintains the genetic structure of bacterial populations through generations. Bacteria use other mechanisms (such as conjugation, transduction and transfection) to introduce genetic change in their gene pools.
Bacteria can reproduce rapidly through binary fission under laboratory conditions (i.e. ideal growth conditions).
| Bacteria | Generation time (minutes) | Notes |
|---|---|---|
| Escherichia coli | 17 | Intestinal bacterium |
| Staphylococcus aureus | 30 | A human pathogen that causes skin, joint, blood infections, and food poisoning. |
| Lactobacillus acidophilus | 60-80 | An intestinal bacterium (part of the gut microflora) |
| Mycobacterium tuberculosis | 800-900 | A human pathogen that causes tuberculosis |
For example, consider the case of Escherichia coli. Let us assume that this bacterium divides every 20 minutes (i.e. doubles every 20 minutes). If we were to start with one bacterial cell in culture, after 24 hours, we would have:
2 x 1072 = 4,722,366,482,869,645,213,696 cells
That is 4.7 x 1021 cells. To put this in context, it is estimated that there have been 1 x 1011 humans on Earth since we first appeared. Of course, the bacterial growth numbers are idealised calculations, and many factors, such as nutrient availability and competition for space, will limit their growth. However, the comparison illustrates one advantage of binary fission as a reproductive strategy: under good growth conditions, asexually-reproducing organisms can grow rapidly and occupy an ecological niche. Furthermore, gene mutations that confer advantageous phenotypes can spread rapidly in such gene pools.