Cell Division 2

• Prophase: The nucleolus disappears, the nuclear envelope starts to fragment, and the DNA and associated proteins found within a cell, known as chromatin, start to coil up and condense. Mitotic spindle starts to form.
• Chromatin condensation: Condensation of DNA into compact chromosomes facilitates the accurate transmission of genetic information during M phase.
• Centrosomes and microtubules: Microtubule fibres in the cytoskeleton are organised by the microtubule organising centre (MTOC)- this is the centrosome in animal cells.
• Centrosome duplication occurs during S phase.
• During mitosis, the cytoplasmic microtubule complex is resorbed, and each of the replicated centrosomes assembles a radial 'astral' array of microtubules that separate to define the mitotic spindle and its poles.
• Meiosis generates haploid gametes.
• Mitosis is asexual reproduction and meiosis is sexual reproduction.
• Aneuploidy: Errors in mitosis and/or meiosis that can lead to major chromosomal abnormalities.
• M phase: Mitosis + Cytokinesis.
• Microtubule poisons interfere with the function of microtubules, they bring about a prolonged cell cycle arrest during M phase that eventually leads to cell death.
• Prometaphase:

  ~In cells other than fungal cells, the nuclear membrane breaks down fully.

  ~Microtubules reorganise into the mitotic spindle and attach to chromosomal centromeres, at regions known as kinetochores.
• Metaphase:

  ~Middle stage of mitosis.

  ~The replicated chromosomes line up along the equator of the dividing cell (the metaphase plate).
• Spindle assembly checkpoint (SAC):

  ~Cells arrest in Metaphase until all chromosomes are attached to the mitotic spindle correctly (in a stable bi-polar fashion that creates equal tension on opposite spindle poles).
• Anaphase:

  ~The sister chromatids synchronously separate to form two daughter chromosomes; each is pulled towards opposing spindle poles (driven by microtubule dynamics).
• Telophase:

  ~Set of daughter chromosomes at spindle pole.

  ~Mitotic spindle disappears and the nuclear envelope reassembles around the chromosomes.

  ~Contractile ring starting to contract.
• Cytokinesis:

  ~Completed nuclear envelope surrounds decondensing chromosomes.

  ~Parent cell is split at the equator (contractile ring) creating cleavage furrow to create two daughter cells.

  ~Re-formation of interphase array of microtubules nucleated by the centrosome.
• During cytokinesis in plant cells instead of a cleavage furrow a cell plate splits the parent into two daughter cells. The cell plate is made from vesicles containing cell wall material.
• Some eukaryotic cells divide asymmetrically by budding: (Saccharomyces cerevisiae, the yeast species used in baking), hydra.

  
  

  ~Asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site.

  ~The small build coming out the parent cell is called a bud.

  ~The bud remains attached as it grows, separating when it is mature, leaving behind scar tissue.
• ~Mitosis: Asexual reproduction (clones)

  ~Meiosis: Sexual reproduction (unique)
• Meiosis 1 is a reduction division. Halves the number of chromosomes within the resulting daughter cells.

  Recombination between non-homologous sister chromatids (crossing over) occurs which increases the genetic variation.
• Prophase 1: Nuclear envelope breakdown; formation of the microtubule spindle and chromatin condensation occurs.

  ~Each homologous pair (bivalent) of duplicated chromosomes (maternal and paternal) align closely together; this tight pairing is called a synapsis. Linked together their entire length by a protein scaffold called the synaptonemal complex.

  ~Sections of non-sister chromatids may exchange with each other through a process known as crossing over which results in increased genetic variation.
• Metaphase 1:

  ~The homologous chromosome pairs are aligned at the central plane of the cell (the metaphase plate) by the spindle microtubules.

  ~Kinetochores assemble on one side (the external side) of each chromatid in meiosis 1.

  ~Independent assortment of maternal and paternal chromosomes occurs, again increasing genetic diversity in daughter cells.
• Anaphase 1: The homologous are pulled apart and move apart to opposite ends of the cell. The sister chromatids of each chromosome, however, remain attached to one another and don't come apart.
• Telophase 1: The chromosomes arrive at opposite poles of the cell. In some organisms, the nuclear membrane re-forms and the chromosomes de-condense, although in others, it is skipped- since cells with soon go through meiosis II.
• Cytokinesis usually occurs at the same time as telophase I, forming two haploid daughter cells.
• Meiosis II is very similar to mitosis. In meiosis II sister chromatids separate to form haploid cells and in mitosis sister chromatids separate to form diploid cells.
• Meiosis causes genetic diversity in gametes. Each gamete has a unique makeup due to:

  ~Crossing over events in Meiosis I

  ~Random (independent) sorting of homologous chromosomes in meiosis.

  ~Random (independent) sorting of sister chromatids in meiosis II.
• Not all forms of meiosis are the same.

  ~Human males: prophase I lasts about a week, and the entire meiotic cycle takes about a month. 4 cells are produces from each primary spermatocyte.

  ~Human females: prophase I begins before birth but then arrests; continuing years/ decades later, during a monthly ovarian cycle post-puberty. Only one ovum is produced from each primary oocyte.
• Errors in chromosomal segregation during mitosis and/ or meiosis.

  ~Failure to correctly separate the pairs of chromosomes or sister chromatids to different daughter cells is referred to as non-disjunction.

  ~This can be a result of numerical aberrations (too many or too few chromosomes- that is, aneuploidy.