Cell Division

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Introduction

Cartoon of mitosis and meiosis
Mitosis and meiosis

Mitosis is the form of cell division in all cells, except germ cells, occurs by 2 mechanical processes that initially divide the nucleus then the cell cytoplasm. Each mitosis of a diploid cell produces 2 diploid cells. Historically, the cells produced by this division are called "daughter" cells.

  • Mitosis segregation of chromosomes and formation of 2 nuclei
  • Cytokinesis splitting of the cell as a whole into 2 daughter cells

See main page Mitosis

Meiosis is a special type of "reductive" cell division occurring only in the generation of germ cells. Reductive because it produces from the original double genome copy (diploid) a single copy (haploid) of the cell nuclear genome. The mechanism is the same in male and female, though it differs substantially as to when it occurs and how many germ cells are produced from each original diploid cell.

  • female - (oocyte, egg) formed before birth, only a single haploid cell is produced, meiosis is only completed in the uterus at fertilization.
  • male - (spermatozoa, sperm) formed after puberty, four haploid cells are produced, meiosis is completed in the testis.

See main page Meiosis


Cell Division Links: meiosis | mitosis | Lecture - Cell Division and Fertilization | spermatozoa | oocyte | fertilization | zygote | Genetics

Some Recent Findings

  • The nucleoporin ELYS/Mel28 regulates nuclear envelope subdomain formation in HeLa cells[1] "In open mitosis, the nuclear envelope (NE) reassembles at the end of each mitosis. This process involves the reformation of the nuclear pore complex (NPC), the inner and outer nuclear membranes, and the nuclear lamina. In human cells, cell cycle-dependent NE subdomains exist, characterized as A-type lamin-rich/NPC-free or B-type lamin-rich/NPC-rich, which are initially formed as core or noncore regions on mitotic chromosomes, respectively. Although postmitotic NE formation has been extensively studied, little is known about the coordination of NPC and NE assembly. ...Our data show, that ELYS/Mel28 plays a role in NE subdomain formation in late mitosis."
More recent papers
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Movies

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Mitosis 01 icon.jpg
Movie shows a cell dividing by mitosis with a fluorescently labelled protein that is located at the kinetochores and along the axes of the chromosome arms.

The labelling allows you to see the chromosomes and the linking region (kinetochore) between chromosome pairs and the mitotic spindle microtubules.

Final part of movie shows the two cells in bright field illumination.

Links: MCB Movie - The stages of mitosis and cytokinesis in an animal cell

Cell Changes

  • Nucleus
    • Chromosome condensation
    • Nuclear envelope breakdown
  • Cytoplasm
    • Cytoskeleton reorganization
    • Spindle formation (MT) Contractile ring (MF)
    • Organelle redistribution
  • Mitosis Energy
    • Cell division uses up a lot of energy, so cells ensure they have enough resources to complete the job before committing to it.

Mitosis Phases

  • Based on light microscopy of living cells light and electron microscopy of fixed and stained cells
  • 5 Phases - prophase, prometaphase, metaphase, anaphase, and telophase
    • Cytokinesis 6th stage overlaps the end of mitosis

MBC The stages of mitosis and cytokinesis in an animal cell


Interphase

  • not a mitotic phase (discussed in cell cycle)
  • Chromosomes dispersed in nucleus
  • Gene expression
  • Cytoskeleton and cell organelles - Distributed and functioning
  • Mitochondria undergo independent proliferation/division

Chromosome Changes

Mitosis fl.jpg

Prophase

Mammalian cell - prophase[2]
  • Chromosome DNA has been earlier duplicated (S Phase)
  • Chromosomes begin condensing
  • Chromosome pairs (chromatids) held together at centromere
  • Microtubules disassemble
  • Mitotic spindle begins to form

Spindle Apparatus

  • 3 sets of microtubules - (+) ends point away from centrosome at each pole.
  1. astral microtubules - anchor the pole end in position
  2. kinetochore microtubules - connected to chromosomes
  3. polar microtubules - form the structure of the spindle apparatus

Spindle Apparatus EM | Spindle Apparatus | MBC Movie- Microtubule dynamics during mitosis

At end of prophase nuclear envelope breaks down

Prometaphase

  • Microtubules now enter nuclear region
  • Nuclear envelope forms vesicles around mitotic spindle
  • Kinetochores form on centromere attach to some MTs of spindle

Dynamic instability and the capture of chromosomes

Centromeric attachment of microtubules

At end of prometaphase chromosomes move to metaphase plate

Metaphase

Mitosis - Metaphase
  • Kinetochore MTs align chromosomes in one midpoint plane.
    • Astrin is a spindle-associated protein required for chromosome alignment at the metaphase plate.[3]

Proposed alternative mechanisms for chromosome congression

Metaphase ends when sister kinetochores separate

Anaphase

Chromosome motility anaphase
  • Separation of sister Kinetochores
  • shortening of Kinetochore microtubules pulls chromosome to spindle pole.
  • Katanin is a microtubule-severing complex involved with this stage of microtubule dynamics.[4]

Experiment - during anaphase A chromosomes move poleward along stationary kinetochore microtubules, which coordinately disassemble from their kinetochore ends

Anaphase ends as nuclear envelope (membrane) begins to reform.

Telophase

Mitosis - Telophase
  • Chromosomes arrive at spindle poles
  • Kinetochore MTs lost
  • Condensed chromosomes begin expanding
    • Continues through cytokinesis

Links: Figure 19-41 Microtubule dynamics during mitosis | Figure 19-34. The stages of mitosis and cytokinesis in an animal cell | Cytokinetic abscission: cellular dynamics at the midbody

Cytokinesis

  • Division of cytoplasmic contents
  • Contractile ring forms at midpoint under membrane
  • Microfilament ring - contracts forming cleavage furrow
    • myosin II is the motor
  • Eventually fully divides cytoplasm

Links: Cytokinesis | Cytokinesis in Plants

Cell Organelles

Mitochondria

  • Divide independently of cell mitosis
  • distributed into daughter cells

Peroxisomes

  • localise at spindle poles

Endoplasmic Reticulum

Golgi

  • 2 processes - disassembly and reassembly[5]
  • Golgi stack undergoes a continuous fragmentation process
  • fragments are distributed into daughter cells
  • are reassembled into new Golgi stacks

Disassembly

  • Unstacking - mediated by two mitotic kinases (cdc2 and plk)
  • Vesiculation - mediated by COPI budding machinery ARF1 and the coatomer complex

Reassembly

  • Fusion - formation of single cisternae by membrane fusion
  • Restacking - requires dephosphorylation of Golgi stacking proteins by protein phosphatase PP2A

References

  1. <pubmed>22555603</pubmed>
  2. Russan NM. Let's Build a Spindle. ASCB Image & Video Library. 2008;CYT-190. Available at: http://cellimages.ascb.org/u?/p4041coll12,521
  3. <pubmed>21402792</pubmed>
  4. <pubmed>17452528</pubmed>
  5. <pubmed>18156178</pubmed>

Reviews

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Cite this page: Hill, M.A. (2019, December 11) Embryology Cell Division. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Cell_Division

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© Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G