The Cell Division Cycle. • Almost 90% of the cycle is taken up with Interphase during which DNA in the nucleus is replicated. • Mitosis and cytokinesis only take . CELL CYCLE. Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. The cell cycle, mitosis and meiosis. Learning objective. This learning material is about the life cycle of a cell and the series of stages by which genetic materials.
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Describe the mechanisms for bacterial cell division and eukaryotic mitosis and meiosis and suggest how failures in the two later processes might lead to. Oegema, K. and Hyman, A. A. Cell division (January 19, ), WormBook, ed. The C. elegans Research Community, WormBook, doi//. | Lecture - Cell Division ANAT Lecture PDF Cell division uses up a lot of energy, so cells ensure they have enough.
In the following, we focus on bacterial divisome elements, for which we summarize relevant work regarding their in vitro reconstitution. We then discuss recent progress towards the de novo design of membrane-transforming and divisome-positioning elements. Although we focus on cell division driven by specific membrane-transforming elements at the division site, it is important to note that cytokinesis can also occur without such machineries.
A prominent example is presented by L-forms, bacterial variants lacking a cell wall that can be generated for both Gram-positive and Gram-negative bacteria [ 38 ].
Even in the absence of the highly conserved protein FtsZ, L-form bacteria have been shown to divide by biophysical mechanisms involving excess membrane synthesis coupled to cell shape changes [ 38 , 39 ].
Moreover, certain bacteria, including Mycoplasma genitalium, divide via motility of the nascent daughter cells on solid surfaces, when FtsZ is deleted [ 40 ].
The Szostak lab has worked extensively on protocell model systems using vesicles that self-assemble from fatty acid micelles [ 41 ]. They could show that in a solution where solute permeation across the membranes is slow, modest shear forces introduced by blowing puffs of air onto the sample from a distance were then sufficient to cause the vesicles to divide into multiple daughter vesicles without content loss [ 41 , 42 ].
It is plausible that processes resulting in similar fluid shear stresses might have occurred on the early Earth, pointing to a potential avenue for simple, physical division mechanisms employed by primitive cells. Moreover, their further study may provide principles that can be employed to realize similar mechanisms in the context of synthetic cells and their division. Bacterial cell division Bacterial cytokinesis is a complex dynamic process that involves the synthesis of new cell envelope material, membrane constriction and fission as well as remodelling and separation of the peptidoglycan layer [ 43 ].
Cell division in the vast majority of bacteria involves the GTPase protein and tubulin homologue FtsZ [ 43 ]. FtsZ Figs. Importantly, the FtsZ ring is not a uniform, cohesive structure, but comprised of smaller, overlapping filaments [ 48 ].
These filaments are highly dynamic and exhibit treadmilling behaviour [ 33 , 49 , 50 ]. Interestingly, FtsZ treadmilling is coupled to circumferential movement of the cell wall synthesis machinery in the periplasm [ 49 , 50 ], although the molecular mechanism of this coupled motion remains unclear [ 51 ]. Moreover, in Escherichia coli, cell wall synthesis and not FtsZ limits the rate of constriction [ 52 ].
Thus, it has been suggested that FtsZ has mostly an organizing function and that it is the cell wall synthesis machinery which generates constrictive force via the pushing of newly inserted peptidoglycan against the inner membrane from the periplasm [ 43 ]. However, in vitro reconstitution experiments have suggested that FtsZ actively generates forces capable of membrane remodelling [ 34 ]. Thus, the individual contributions of FtsZ and cell wall synthesis are interesting open questions and motivate further research in this area [ 51 ].
Mechanisms for the localization of FtsZ to the division site differ between bacteria, and both positive and negative regulatory mechanisms have been reported [ 29 ]. The first involves the protein SlmA and inhibits Z-ring assembly across the chromosome [ 53 ]. The MinCDE system inhibits assembly near the poles via a self-organized gradient of the FtsZ inhibitor MinC, which has the highest concentration at the poles and lowest at the mid-cell [ 54 , 55 ].
Importantly, pole-to-pole oscillations, and consequently correct gradient formation, arise from a sensitive interplay of geometric boundary conditions and other parameters, such as interaction rates [ 58 , 59 ]. Out of those two positioning systems, the E.
Synthetic cell division via reconstitution of E. The corresponding machinery in E.
However, due to the sensitivity of the Min system to the geometry and dimensions of the surrounding membrane system [ 59 — 62 ], the vesicle will likely need to be shaped in a way to enable robust gradient formation, and FtsZ localization, by the Min system.
Towards reconstituting vesicle division based on E. In the following, we briefly summarize the outcomes of reconstitution experiments with these components, which are reviewed in more detail elsewhere [ 63 — 66 ]. When reconstituted inside multilamellar liposomes, FtsZ-YFP-MTS was capable of membrane deformation [ 34 ], although it is unclear whether this force would suffice for constriction in vivo [ 43 ].
Furthermore, FtsZ-YFP-MTS was found to display an intrinsic curvature in its polymeric state, facilitating its self-assembly along membranes of negative curvature [ 67 ]. On supported lipid bilayers, FtsZ self-organizes into dynamic ring structures, in which individual FtsZ filaments undergo treadmilling to drive chiral rotations of the rings [ 33 ] Fig.
Initially, it has been suggested that formation of these dynamic rings requires the simultaneous presence of non-MTS-fused FtsZ and the anchor protein FtsA, which exerts a negative feedback on membrane-bound FtsZ filaments [ 33 ]. However, a subsequent study from our lab demonstrated that, under certain biochemical conditions, FtsZ-YFP-MTS alone also self-organizes into dynamic ring patterns [ 36 ].
This result has important implications for Z-ring formation within the context of synthetic cell division, as complexity can now be reduced to a single chimeric protein and because the required conditions for correct assembly are better defined.
Very recently, it has been found by in vitro reconstitution that the essential divisome proteins FtsN and FtsQ co-migrate with treadmilling FtsZ filaments via a diffusion-capture mechanism [ 68 ]. FtsZ variants have also been reconstituted inside lipid droplets [ 69 ], coacervates [ 70 ], crowding-induced phase-separated condensates [ 71 ] and lipid vesicles [ 35 , 72 , 73 ]. Besides the already mentioned deformations observed for FtsZ-YFP-MTS in multilamellar vesicles [ 34 ], the simultaneous presence of FtsZ and different ZipA or FtsA variants has been reported to give rise to membrane deformations when reconstituted or expressed inside giant unilamellar vesicles [ 35 , 72 , 73 ].
In some cases, these deformations have been suggested to be responsible for observed constriction and division of vesicles [ 35 ]. Among different positioning systems, the Min oscillator is a promising option for localizing an FtsZ-based divisome in the middle of a vesicle in vitro as it contains only a few, relatively well-understood components and the influence of biochemical and geometrical factors has been comprehensively analyzed.
When MinD and MinE are reconstituted on a flat supported membrane, topped by a uniform buffer, these proteins self-organize into traveling waves via an ATP-driven reaction-diffusion mechanism [ 74 ]. Such simplified flat membrane systems have been used extensively by us and others to investigate the effects of lipid and buffer composition, as well as mutations in MinD and MinE, on the formation and properties of Min patterns [ 75 — 79 ], and to achieve external photo- control over self-organization [ 80 ].
Although the experiments above were performed in the presence of a two-dimensional, non-enclosed membrane system, the simplicity of the setup allowed the efficient establishment of suitable conditions for the functionality and compatibility of different components as well as the potential to modulate the spatiotemporal properties of Min patterns in a predictable fashion.
We and others have also reconstituted Min protein patterns in more cell-like settings, such as in PDMS microcompartments [ 60 , 61 , 84 ], in lipid droplets [ 85 ], on the outside of lipid vesicles [ 86 ], and—most recently and relevant for this review—inside lipid vesicles [ 87 ] Fig. These studies established which types of patterns form under different geometric constraints and, with regard to the reconstitution in droplets and vesicles, confirmed that Min oscillations can occur inside lipid-mono- or -bilayer-enclosed compartments.
Potential roles of these mechanical effects in cell division could be explored in future studies. Notably, the Min system has also been combined with additional division-related proteins in some of the above-mentioned cell-like systems.
Moreover, oscillations of Min proteins in PDMS microcompartments resulted in a time-averaged concentration gradient of MinC with maxima at the poles and minimum in the middle [ 61 ]. Very recently, our lab has shown that—even in the absence of MinC—MinD and MinE can support the anticorrelated movement and oscillation of model membrane proteins, including mCherry fused to various membrane targeting sequences, lipid-anchored streptavidin and FtsZ-YFP-MTS [ 88 ].
Moreover, if the proteins are permanently anchored to the membrane, MinDE oscillations can localize them to the middle of a microcompartment [ 88 ]. This implies that MinD and MinE are sufficient to generate a generic cue for the localization of membrane proteins, which may also be relevant for simplified divisome localization machineries. Challenges for the in vitro reconstitution of divisome elements Despite the progress in reconstituting bacterial divisome elements in vitro, several challenges remain to be addressed.
First, it has still not been experimentally demonstrated that FtsZ can reproducibly exert sufficient forces to constrict and divide a lipid vesicle from the inside. Quantitative measurements of potential forces generated by FtsZ could resolve its sufficiency or contribution for vesicle division. Second, while FtsZ forms dynamic rings and the Min system is capable of gradient-forming pole-to-pole oscillations in vitro, the integration of both phenomena is not as trivial as may seem. Although several factors, like lipid composition, crowding agents and the concentration and functional features of Min proteins have been identified that modulate the length scale of Min patterns [ 60 , 75 , 76 , 78 ], Min oscillations have not yet been realized in a cell-sized compartment, but rather in compartments scaled to the dimensions of in vitro Min patterns [ 60 , 84 ], which are around an order of magnitude larger than the in vivo patterns [ 74 ].
Pachytene : Crossing over occurs between non-sister chromatids of ho- mologous chromosomes. Diplotene : Dissolution of synaptonemal complex occurs and the recom- bined chromosomes separate from each other except at the sites of crossing over. These X-shaped structures are called chaismata.
Anaphase I : Homologous chromosomes separate while chromatids re- main associated at their centromeres. Interkinesis : Stage between two meiotic divisions. Significance of Meiosis 1.
Formation of gametes : In sexually reproducing organisms. Genetic variability 3. Maintenance of chromosomal number : By reducing the chromosome number in gametes.
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