Cell Division

• Cell division is the process by which a parent cell divides into two (or more) daughter cells.
• All cell divisions, regardless of organism are preceded by a single round of DNA replication.
• Depending on the type of cell division, daughter cels can either be genetic (and phenotypic) clones of each other (and the parent cell) (asexual reproduction) or they can be genetically distinct (sexual reproduction).
• In unicellular organisms (prokaryotes), cell division used solely for (asexual) reproduction.
• In multicellular organisms, cell division is the means of tissue growth, tissue homeostasis (maintenance), repair and (sexual) reproduction.
• Prokaryotes divide by asexual reproduction strategies: binary fission, budding, fragmentation, sporulation.
• In eukaryotic cells (unicellular or multicellular organisms), cell division occurs as part of a larger, complex programme:

  ~Asexual reproduction: Mitosis

  ~Sexual reproduction: Meiosis
• Regulation of cell division is crucial for maintaining the balance between growth and tissue homeostasis.
• Errors or disruptions in these regulatory processes can lead to uncontrolled cell division, which can cause cancer.

  Control of cell division is mediated by both external and internal 'signals'.
• Favourable environment for cell division:

  ~ Nutrients are available

  ~ No DNA damage

  ~ Cell size (mass) has increased sufficiently.
• Causes of DNA replication to be incomplete:

  ~ Lack of nutrients

  ~ Cell is too small

  ~ DNA is damaged
• Unicellular organisms divide as fast as they can and their rate of proliferation depends on the availability of nutrients in the environment.

  
  

  Nutrients - Cell growth - Cell Division
• Cells of multicellular organisms divide only when more cells are needed by the organism. For an animal cell to proliferate, nutrients are not enough:

  ~ Growth Factor + Mitogen + nutrients.

  Growth factors = cell growth

  Mitogen = cell division

  
  

  ~Extracellular factor + nutrients = Cell growth + Cell division
• Growth factors (and mitogens) stimulate cell growth (and cell division) without crossing the cytoplasmic membrane.
• Platelet-derived growth factor (PDGF) has been proposed to be one of the growth factors that drive proliferation during normal embryonic development in mammals.
• Binary fission is a continuous process. Cell growth, DNA replication and cell division occur simultaneously. It is a form of asexual reproduction/ cell division is also used by some organelles within eukaryotic organisms.

  In prokaryotes.
• Steps of binary fission:

  ~Prokaryotic parent cell initiates replication.

  ~A copy of the cell's DNA is created.

  ~Cell elongates and cross wall forms.

  ~Cross wall forms completely and daughter cells separate.
• FtsZ proteins play a key role in regulating binary fission in bacteria.

  ~Bacteria cell division is orchestrated by the divisome, a protein complex centred on the tubulin homolog, FtsZ.

  ~FtsZ (can form microtubules) polymerises into a dynamic ring (the Z-ring) that defines the division site, recruited downstream proteins, and directs peptidoglycan synthesis to drive constriction and cell division.
• Most prokaryotes rely on binary fission for propagation, some species use alternative mechanisms to reproduce:

  ~Budding (commonly used by eukaryotic yeast cells).

  ~A small bud forms at one end of the mother cell or on filaments called prosthecae.

  ~As growth proceeds, the size of the mother cell remains constant, but the bud enlarges. New nucleotide moves into it.

  ~When the bud is about the same size as the mother cell, it separates (daughter cell produced).
• Some species of prokaryotes use fragmentation to reproduce.
• Fragmentation involves breaking up of the parent cell into smaller pieces which then develop into new individuals.
• To reproduce some prokaryotes use sporulation. Where the mother cell produces a spore with a hard coat that can survive harsh environments.

  ~Happens more in stress environments.
• Forms of asexual reproduction in prokaryotes:

  ~Binary fission

  ~Budding

  ~Fragmentation

  ~Sporulation

  
  

  Daughter cells produced are genetic and phenotypic clones of each other (and the original parent cell).
• Ways genetic variation happens within a prokaryote through horizontal gene transfer.

  ~Conjugation: DNA (Plasmid) is transferred from a donor to a recipient bacterium by direct contact.

  ~Transformation:  The uptake of DNA from a dead bacterial cell into a live bacterial cell through the cell membrane becoming a part of the cell genome.

  ~Transduction: A virus transferring genetic material from one bacterium into another.
• Transposons are a class of genetic elements that can 'jump' to different locations within a genome.
• Transposons are maintained in an integrated site in the genome, eventually they become inactive and no longer move.
• The simplest kind of transposons contain a copy of the transposes gene (used to cut/paste the transposon within the genome) with no additional genes.
• Transposons behave as parasitic elements and usually have no known associated function that is advantageous to the host.
• Transposable elements have additional genes associated with them, such as antibiotic resistance factors.
• These additional genes typically occur when an infecting bacterium acquires a plasmid that carries a gene encoding resistance to one or more antibiotics via horizontal gene transfer.
• In eukaryotes DNA replication, cell growth and cell division are segregated.

  ~M phase + Interphase (G1 + S + G2)
• Interphase:

  • Most eukaryotic cells are held in G1 (or G0) until they are instructed to divide.
  • Mitogens trigger G1/G0- arrested cells to commit to cell division.
• M phase:

  • Nuclear + cell division.
  • Different processes for Mitosis vs Meiosis.
• Calculation for the number of cells.

  
• G1 is when cellular contents, excluding the chromosomes are duplicated.
• S phase: Each of the 46 chromosomes is duplicated by the cell.
• G2: The cell 'double checks' the duplicated chromosomes for error, making any needed repairs.
• Mitosis: M phase or Spindle assembly checkpoint (SAC).
• Restriction checkpoint, R (or start in yeast) is at the end of the G1 checkpoint.
• Eukaryotic cell cycle checkpoints are controlled by key cell cycle regulators: CDKs and cyclins. There is different kinds of cyclins and CDKs.