MICB 201- Learning Objectives


Chapter 4

• Describe the role of FtsZ in cell division.

FtsZ is a protein that is involved in the cell division of MOST prokaryotes. It is a protein that acts as a subunit and it assembles as a ring when the dividing cell is expanding and its genetic material is copied. The ring is assembled between the duplicated nucleoids – the nucleoids are on opposite poles and they are basically identical.

Anchored to the inner surface of the CM, the FtsZ ring contracts by shedding subunits into the cytoplasm; pulling the membrane inward eventually pinching them off.

So basically FtsZ is assembled while/after the genetic material has been copied and starts contracting when the nucleoids are in the emerging cells.

• Describe the different ways the synthesis of rigid cell wall polysaccharides is coordinated with

FtsZ ring constriction.

For organisms without a rigid cell wall, they just pinch off their membrane – easy.

For organisms with a rigid cell wall, the situation is more variable for cell division. The FtsZ ring not only pinches off the membranes but it serves also as a scaffold for the synthesis of new cell wall material at the division site.

In some prokaryotes, the joint b/t the new cell wall material is degraded at the same time the membrane is constricted by FtsZ. Then the cells just pinch off.

In the other prokaryotes, a cross wall (septum) is completed in the final stages of FtsZ constriction. Later, enzymes digest part of the cross wall/septum.

• Explain the difference between FtsZ-based symmetric cell division, asymmetric cell division, budding and bipolar cell division with respect FtsZ ring localization, synthesis of new cell wall relative to the FtsZ ring and the nature of the daughter cells relative to the parental cell.

Symmetric cell division is the most common in prokaryotes, exhibited by cocci, rods, spirilla, etc.

In a cell, there is a FtsZ ring in the middle with new wall material synthesized on either side equally. The daughter cells are identical. This means each cell is half new, half old. So symmetric cell division = binary fission.

In non-spherical cells, symmetric cell division first involves the synthesis of new cell wall material at other sites in addition to the division site.

Another method is fragmentation done by Bdellovibrio. Fragmentation is simply a variation on symmetric cell division but instead of 2 daughter cells there are lots more. A long chain or filament of incompletely separated cells is initially formed. The filament then undergoes FtsZ dependent cell division resulting in simultaneous formation of unicells. Bdellovibrio reproduces in the periplasm of other cells.

Incomplete cell division – In some prokaryotes, a significant fraction of cells never completely separate after symmetric cell division. Therefore, chains/filaments or clumps of cells result depending on the orientations of the division planes. If they only divide on one plane you get a chain. If they divide on 3 planes then you get a clump.

Asymmetric cell division – Caulobacter has a stalk that sticks to surfaces (adhesive polysaccharide glue) and flagella (flagella is not part of stalk). FtsZ localizes to the midpoint of the stalked cell. As cell expansion proceeds, differential gene expression in the pre-daughter cells causes them to grow differently and have different characteristics. One pre-daughter cell retains a stalked morphology while the other synthesizes a flagellum prior to cell separation.

Budding – less common among prokaryotes. A bud is formed on the cell surface. It enlarges and eventually separates from mother cell. Some budding organisms have FtsZ. Some do not.

Budding clearly creates asymmetric products. However, the term asymmetric cell division does not include budding by convention of terminology. Asymmetric cell division is just binary fission where the two daughter cells are different. In budding, the division plane is not roughly at the midpoint of the expanded mother cell. Also, budding involves the synthesis of new cell material on only one side of the FtsZ ring while in asymmetric cell division it’s both.

Recall Epulopiscium fishelsoni which lives in the surgeon fish. This single celled organism does not undergo binary fission, but it still uses FtsZ. This is called bipolar cell division – it ‘fissions’ at both poles using FtsZ. Each of these polar fissions generates an intracellular daughter cell. Both cells expand to nearly the full length of the mother cell and they emerge from the mother cell as it disintegrates.

• Describe the role of MreB in cell division.

The MreB protein serves a function in nonspherical cells during cell division. This is because nonspherical cells need to elongate prior to cell separation. The MreB determines how they elongate, which is important to maintain the ‘correct’ shape of the cells. It does this by forming helical shaped bands around the inside of nonspherical cells, just underneath the CM like FtsZ.

Acts as a scaffold around which to synthesize wall material (like FtsZ earlier). CreS is also used.

• Describe the distribution of FtsZ rings, Cdv rings and MreB in prokaryotes and state those groups that lack these proteins.

Most Bacteria and Archaea use FtsZ but some Archaea use Cdv which stands for cell division protein.

Bacteria that lack FtsZ also lack a rigid PG – Mycoplasmas, Planctomyces. Planctomyces use the stalk budding method but they do budding, not asymmetric cell division. Some budding does not have FtsZ, like the planctomyces. Some Archaea use snapping division which does not involve FtsZ nor Cdv. FtsZ is not only important for pinching off the CM but also for providing a scaffold for synthesis of cell wall.

• Describe what is known about elongation in those rod-shaped organisms and hyphal organisms that lack MreB.

Not all nonspherical prokaryotes possess MreB cytoskeleton. Usually MreB used to synthesize cell wall, used as a scaffold to control the shape. But there are some MreB-less Archaea like the square Archaea. Also, Mycoplasmas. They must have other proteins that serve as cytoskeleton.

Some Bacteria that don’t have MreB can also use snapping division. These Bacteria have a FtsZ but snapping Archaea do not.