Cell Cycle and Cell Division

Cell Cycle Overview

• The cell cycle is conserved in all eukaryotes.
• It is a sequence of events where cell contents are duplicated and divided into two.
• Observing animal cell division in culture reveals cells do not divide at the same time, but when they do it follows the same stages in mitosis.
• Video 18.5 and Chapter 18 Animations provide further details, available in Essential Cell Biology, 6th Edition.
• A link to more information is located at https://digital.wwnorton.com/ecb6

Cell Cycle Phases

• The M phase involves nuclear and cytoplasmic division.
• Mitosis is nuclear division.
• Cytokinesis is cytoplasmic division.
• Interphase is the period between cell divisions.
• Metabolic activity, cell growth, and repair happen during interphase.
• G1 phase, S phase (DNA synthesis), and G2 phase comprise Interphase.
• DNA synthesis happens way before the M phase.

Cell Cycles in Multicellular Organisms

• Mature cells often do not divide to make new cells
• Terminally differentiated cells include nerve, muscle, and red blood cells; they lose the ability to divide after differentiation.
• Some cells divide only when stimulated appropriately.
• An example: liver cells start dividing to repair damage.
• Some cells, such as hematopoietic and epithelial stem cells, normally divide on an ongoing basis.
• Cells that do not divide are in G0.
• Cells in G0 are still metabolically active and carry out cell functions.
• Cells can exit the cell cycle into G0.
• In G0, cells are resting and not undergoing proliferation.

Cell-Cycle Control System

• The cell-cycle control system delays later events until earlier ones are complete.
• Start transition (G1→S) assesses if the environment is favorable.
• G2/M transition (G2→M) checks if DNA is replicated and any damage is repaired.
• Metaphase-to-anaphase transition assesses if all chromosomes are properly attached to the mitotic spindle assembly.
• Problems in checkpoints can cause chromosome segregation defects.
• Cell cycle progression is controlled by molecular switches.
• Entry into the next phase of the cell cycle is triggered by cyclin-dependent protein kinases (Cdks).
• Cyclin-Cdk complex activates for entry, then inactivates.
• Entry into M phase involves M-Cdk (Cdk activated by M cyclin).
• M-Cdk phosphorylates other regulatory proteins.
• Entry into the next phase can be paused by other regulators.

Mitosis

• Mitosis is observable under a microscope.
• It consists of prophase, prometaphase, metaphase, anaphase, and telophase.
• Cytokinesis often starts before mitosis finishes.
• Mitosis can be observed using Video 18.8 and Video 18.9.
• Additional information can be found in Chapter 18 Animations; Essential Cell Biology, 6th Edition at https://digital.wwnorton.com/ecb6

Prophase

• G1 phase initiates centrosome duplication.
• Chromosomes are replicated during the S-phase
• Replicated chromosomes condense.
• Mitotic spindle assembly starts and requires duplicated centrosomes during prophase.
• Cohesins hold sister chromatids together, deposited during S phase.
• In G2, replicated chromosomes are dispersed and tangled, requiring reorganization for mitosis.
• Chromosome condensation (chromatid compaction) and sister-chromatid resolution (separable units) occur.
• Cohesins are removed from chromosome arms but remain at centromeres.
• Condensins condense DNA in each sister chromatid.
• Sister chromatids are resolved but remain associated at the centromere by cohesins.

Dynamic Microtubules

• Dynamic microtubules are required for mitosis.
• Non-dividing cells have microtubules arranged in a radial pattern.
• (+) ends radiate out; (-) ends stabilize at the MTOC (centrosome).
• Mitotic spindle assembly starts.
• It requires disassembly and reassembly of microtubules and duplicated centrosomes.
• Structure comprises a pair of centrioles organized at right angles, composed of nine fibrils of three microtubules each.
• The centrosome matrix surrounds the pair of centrioles.
• It contains γ-tubulin ring complexes (γ-TuRCs) and nucleating sites that assemble new microtubules.
• Centrosomes duplicate once per cell cycle during interphase.
• Duplication is initiated in G1 phase and completed by G2.
• Each centriole serves as a site for assembly of a new centriole.
• Duplicated centrosomes form poles of the mitotic spindle.

Mitotic Spindle Assembly

• Mitotic spindle assembly starts in prophase (M phase).
• It requires microtubule dynamics (disassembly and assembly).
• Duplicated centrosomes separate.
• Radial array of microtubules extends out from each to position centrosomes.
• The next event is nuclear envelope breakdown.

Nuclear Envelope Breakdown

• Occurs at the boundary between prophase and prometaphase.
• Nuclear lamina consists of a meshwork of interconnected nuclear lamin proteins.
• The 2D lattice formed is on the inner nuclear membrane.
• Phosphorylation of lamins and nuclear pore proteins triggers disassembly of the nuclear envelope, forming small membrane vesicles.
• It requires microtubule dynamics (disassembly and assembly) and microtubule motor protein activity.
• Kinetochore microtubules attach to duplicated chromosomes.
• Chromosome movement commences.

Mitotic Spindle Assembly Function

• Astral microtubules position the mitotic spindle with cytoplasmic dynein.
• Non-kinetochore microtubules are cross-linked and stabilize throughout the mitotic spindle.
• Kinesin-5 is present, as are other microtubule-associated proteins.
• Kinetochore microtubules attach duplicated chromosomes to the spindle poles.

Kinetochore Microtubules

• Kinetochores locate at the centromeres of chromosomes.
• One kinetochore is present for each sister chromatid in the duplicated chromosome.
• Microtubules from both spindle poles must attach to sister chromatid kinetochores.
• Tension is generated to line up chromosomes at the spindle equator.
• Connecting protein complexes bind to the microtubule near the plus end.
• Microtubule is exposed, allowing growing or shrinking for chromosome movement.
• Video 18.6 provides a 3D reconstruction of the mitotic spindle.

Metaphase Alignment

• Chromosomes align on the metaphase plate (equator of the spindle).
• Microtubule dynamics maintain the metaphase spindle.

Anaphase

• Also called Spindle Assembly Checkpoint.
• Anaphase starts when all chromosomes align on the metaphase plate.

Metaphase-Anaphase Transition

• Tubulin flux is continuous during metaphase.
• Tubulin subunits are added at the plus end and removed at the minus end.
• Kinetochore microtubule length remains constant.
• The cell separates sister chromatids.
• Cleaved cohesin allows chromatid separation.
• Kinetochore microtubules shorten and pull sister chromatids to opposite poles.
• The APC/C triggers separation by promoting destruction of cohesins.
• Spindle poles also move outward, powered by kinesin and cytoplasmic dynein.

Sister Chromatid Separation

• Anaphase A involves chromosomes pulled poleward
• Anaphase B involves poles pushed and pulled apart.
• Sliding force between overlapping, non-kinetochore microtubules pushes the poles apart.
• A pulling force at the cortex drags the poles apart.
• Microtubule growth at plus ends of non-kinetochore microtubules further pushes poles apart.

Telophase

• Chromosomes separate into two groups.
• Each set sits at a spindle pole.
• The nuclear envelope reassembles.
• It requires the removal of phosphate groups using phosphatases.
• The mitotic spindle disassembles, and chromosomes decondense into chromatin.
• Mitosis ends, and cytokinesis starts in anaphase.
• Chromosomes now separated into two groups, one at each spindle pole, and also reassembly of nuclear envelope.
• Marks the end of mitosis, and the cell starts assembling the contractile ring for cytokinesis (starts in Anaphase)

Cytokinesis

• The cytoplasm divides in two via a contractile ring of actin and myosin filaments.
• The contractile ring contracts at the cleavage furrow, eventually disassembling.
• The process results in two daughter cells, each with its own nucleus, centrosome, and interphase microtubules.
• The contractile ring of actin and myosin filaments create cleavage furrow.
• The contractile force is caused by the same contractile ring
• It brings the cell membrane in as contractile ring becomes smaller until remaining non-kinetochore microtubules contract from their central spot.

Dynamic Cytoskeletal Structures

• Cytoskeletal dynamics are important phase in animal cells.
• Interphase microtubules, actin, and myosin arrays disassemble before mitosis.
• Mitotic spindle and contractile ring assemble for mitosis and cytokinesis.
• Mitotic structure assembles rapidly after the task is complete.
• Cytokinesis in animal cells starts with the assembly of the contractile ring, dividing the cells in 2
• It differs from plants
• It ends with the cleavage furrow, midway in between spindle
• The contractile force is caused by the contraction of the ring, bringing the cell membrane closer

Cell Division in Plant Cells

• It is similar to animal cells
• But there is no cell wall and has another method/system to divide the Mitotic spindle.
• Plant cell cytokinesis is very different, caused by the formation of cell wall
• After cell division, there is telophase involving chromosome segregation
• Then the phragmoplast comes into action to form into a cell plate, the specific structure involves microtubules, actin and vesicle from Golgi
• Cell plate forms with transmembrane which is a component from protein

Meiosis and Mitosis

• Meiosis generates four nonidentical haploid nuclei, whereas mitosis produces two identical diploid nuclei.
• A comparison between Meiosis and Mitosis:
• During the metaphase and what happens in the meiosis, spindle fibre attaches to the chromosome during metaphase
• Meiosis homologous chromosomes are paired on the metaphase, while chromosomes are not paired during mitosis
• Meiosis and mitosis both need to generate cells, but the types of cells they generate differ.

Stem Cells & Cell Division

• Stem Cells, for example in humans:
• Pluripotent, embryonic
• Adult, somatic
• In both, the cells need to self renew and stem cells are used to reprogramme cells for treatment of diseases.
• For example University of Toronto have identified 2 researchers: James Till and Ernest McCulloch

Learning Outcomes

• The successful user will be able to;
• Describe all phases of the Cell Cycle
• Understand how cell cycle moves and system regulates
• Understand the difference of terms, M Phase Vs Mitosis and Cytokineses
• Describe the different phases of mitosis
• Understand the role of microtubules (dynamics and motor proteins) in spindle form
• Outline how microfilaments and actin filaments are involved in cell division
• Compare cytokinesis in plants Vs. animals