Download 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.