Cell Cycle: Replication, Segregation and Division

• Cell cycle is an ordered series of events where cells divide to make new cells

Introduction

• The cell cycle consists of cell growth and chromosome replication first
• Then, chromosome segregation during mitosis
• Cytokinesis is the third step involving cell division

Phases

• The cell cycle has two major events: S-phase and M-phase
• S-phase involves DNA Synthesis
• M-phase has; chromosome segregation / nuclear division / karyokinesis (mitosis), and cytoplasmic division (cytokinesis)
• The cell cycle can be divided into two major phases by visibility of chromosomes
• "Visible chromosomes" includes mitosis
• "Non-visible chromosomes" includes interphase
• Mitosis comes from the Greek word for "thread" because chromosomes look like pieces of thread when pulled apart

Interphase

• During interphase chromosomes are not visible
• Interphase lasts a long time (23 hours)
• The phases of interphase are G1 -> S -> G2
• G: GAP phases that involve; cell growth, monitoring of environmental signals
• S: DNA Synthesis

Mitosis

• Chromosomes are visible during mitosis
• Mitosis is short (1 hour)
• The main events of Mitosis are; chromosome condensation, nuclear envelope breakdown, mitotic spindle formation, and chromosome segregation
• If you see condensed chromosomes in a cell, that cell is in Mitosis
• Chromosomes condense to enable mitosis
• Each chromosome has a centromere and chromosomal DNA molecules
• Chromosome duplication creates sister chromatids that separate

Time-Lapse Microscopy

• DIC Time lapse microscopy of cell division shows the cell division process
• Cytoskeletal rearrangements occur from interphase to mitosis
• During mitosis, microtubules form the mitotic spindle
• Microtubules are hollow tubes, 25 nm with 15-nm lumen
• Tubulin consists of α-tubulin and β-tubulin
• Microtubules maintain cell shape, cell motility, chromosome movements in cell division, and organize organelle movements
• MTOC (microtubule organizing center) of animal cells includes the centrosome
• The metaphase plate involves a tug of war

Mitosis Stages

• The key stages of mitosis are: interphase, prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis
• In prophase the intact nuclear envelope contains condensing replicated chromosomes consisting of 2 sister chromatids connected in length. The centrosome will form the mitotic spindle
• In prometaphase, the nuclear envelope fragement, and the chromosomes move in active motion. The kinetochore microtubule is also visible
• In metaphase, the chromosomes align at the equator of the spindle, midway between the spindle poles.
• In anaphase, sister chromatids separate to create two daughter chromosomes
• Each chromatid is pulled slowly inward to the spindle pole
• Kinetochore microtubules shorten, and spindle poles also move apart, enabling chromosome segregation
• During telophase, daughter chromosomes arrive at the pole of the spindle and decondense
• A new nuclear envelope reassembles to complete the formation of two nuclei
• Cytokinesis begins with the start of a contractile ring
• During cytokinesis, the cytoplasm divides via a contractile ring with actin and myosin filaments
• This pinches the cell to create two daughters, with one nucleus each

Additional points

• During interphase the nuclear envelope and chromatin are present and centrosomes duplicate
• Mitosis involves duplicated chromosomes condensing, nuclear envelope breaking down, spindle fibers attaching to kinetochores, all chromosomes arranging at the metaphase plate, chromosomes segregating and spindle breaking down
• Daughter cells cytoplasm separate during cytokinesis
• Organelles are re-organized and re-distributed from interphase to mitosis
• The ER re-organizes and the Golgi disassembles during mitosis
• Mitochondrial dynamics occur through cell division
• Cytoskeleton reararrangement occurs during mitosis
• Cell shape changes during mitosis for animal cells
• Chromosomes have been separated and segregated to opposite poles which enables Cytokinesis
• Cytokinesis is possible via an actomyosin ring (A.K.A contractile ring) composed of actin (microfilements) and Myosin

Summary

• The stages of the cell cycle are G1 -> S -> G2 -> M
• The sub-phases of mitosis are prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis
• Chromosome segregation occurs via the mitotic spindle
• Cytokinesis is enabled by the actomyosin ring
• Not only chromosomes but also other organelles are reorganized

Regulation

• Surveillance mechanisms and checkpoints regulate the cell cycle
• Cell cycle events are dependent on each other
• Eukaryotes have similar cell division principles
• These all have common principles and unique features
• Basic principles of the cell cycle; defined order of events, many decision-making steps, accurately completed processes, robust processes, irreversible events
• It also keeps the memory that events occur once per cell cycle, and is highly adaptable
• Plant vs animal cytokinesis have clear differences
• Plant cytokinesis occurs via cell plate formation and vesicles forming the cell plate, also daughter cells form
• Animal cells separate via a cleavage furrow and contractile ring formed of microfilaments to form daughter cells
• Centrosomes are absent in plants
• Some cells do not undergo nuclear envelope breakdown
• The two versions of mitosis are "closed" and "open"
• Cells that undergo "closed" mitosis have their MTOC embedded in the nuclear envelope
• The components of the cell-cycle control system trigger cell cycle events
• The environment must be appropriate to start the transition
• It is important that all DNA is replicated
• Cyclin-dependent protein kinases (Cdks) control the cycle
• Cdks use a cyclin
• A kinase phosphorylates proteins
• Kinase and phosphates work together to turn proteins on or off
• Cyclin Dependent Kinases Regulate the Cell Cycle by phosphorylating several proteins
• The location of serines and threonines control how each kinase is controlled
• Positive and negative feedback creates switches and control the order of cell cycle phases
• Previously: The Cell cycle is a clock with gates and dominos with barriers called checkpoints
• Also, Cyclin dependent kinases (CDKs) in association with cyclins drive the cell cycle

Model organisms

• Breakthroughs in cell cycle discoveries came from frog egg, sea urchin embryo, and yeast models
• Model Organisms in Cell Cycle Analysis comprise of; S. cerevisiae, Arbacia punctulata, Caenorhabditis elegans, Drosophila melanogaster, S. pombe, Xenopus laevis
• All eukaryotic cells employ similar machinery duplicate and divide themselves, have a similar timing, and coordination of cycle events
• Synchrony, is one property that helps in cell-cycle studies
• Divisions of Xenopus laevis is easy to study and use in experiments
• Xenopus (frog) oocytes are arrested in the G2 phase of the cell cycle until hormonal stimulation triggers entry into the M phase of meiosis
• Egg morphology shows a dark pigmented hemisphere where the nucleus, or germinal vesicle, is found
• Xenopus egg extracts are a cell free system that enable mitosis studies
• Using these extracts you can check; DNA & centrosome, proteins, inhibitors, activators, antibodies, and siRNAs
• Cell-free mitotic cycle lasts 40-60 mins
• MPF (Maturation Promoting Factor) can be discovered from eggs