Mitosis

• What is it likely that the first cell was?

  The first cells were likely some sort of membrane surrounding self-replicating molecules such as RNA.
• What is the cell cycle?

  Two main phases: Interphase (the growth period) and Cell division (Mitosis or meiosis- division of the nucleus) and (cytokinesis - division of the cytoplasm and organelles)
• What is Interphase?


  Interphase has three stages:

  G1 (synthesis of cell components

  S phase (DNA replication)

  G2 (preparation for meiosis/mitosis)

  DNA replication, to form two identical DNA strands, occurs in the S phase of interphase. After replication, the two genetically identical strands of DNA are known as sister chromatids. Sister chromatids are held together at the centromere, which adheres sister chromatids to each other vias cohesin (a protein complex) and is the site of kinetochore and microtubule attachment for movement of chromosomes. The cohesin forms a ring-like structure that wraps around strands of DNA, connecting the sister chromatids until anaphase. During interphase, nuclei are rounded or oval, DNA is granular and in chromatin form. Chromosomes are not present. Microtubules are distributed around the nucleus.
• What is the centromere?

  The section where sister chromatids are held together, via cohesin. It is also where kinetochores allow microtubule attachment
• What is cohesin?


  A protein complex. The cohesin forms a ring-like structure that wraps around strands of DNA, connecting the sister chromatids until anaphase
• What are kinetochores?


  Protein structures located at the centromere and they allow microtubule attachment
• How is DNA packaged?

  DNA is a double helix and is made of two antiparallel strands of nucleotides with two strands linked by hydrogen bonding between complimentary base pairs. Eukaryotic DNA is wrapped around histone proteins, forming nucleosomes. These nucleosomes coil and stack together to form chromatin fibres, and it is the chromatin fibres that form chromosomes.
• Why is DNA in chromatin form during interphase?

  Because DNA needs to be accessible to enzymes, during interphase most of the DNA is in chromatin form and not tightly coiled.
• What are the four stages of mitosis?

  Mitosis is the second stage of the cell cycle, and is divided into four stages:

  Prophase

  Metaphase

  Anaphase

  Telophase

  (PMAT)
• What occurs during prophase?

  DNA condenses- During prophase, replicated DNA in chromatin form condenses to become a chromosome.

  Chromosomes are held together by cohesin- Each replicated chromosome is a pair of sister chromatids that are joined at the centromere, held together with cohesin.

  Kinetochore attaches to the centromere- The kinetochore attaches to the centromere, which is ready to attach to the spindle fibres, which will then allow the chromosomes to move around.

  Microtubules grow and shrink to find the chromosomes- This "search-and-capture" process ensures that the spindle fibres attach correctly to the kinetochores

  Mitotic spindle is created- The microtubules grow out from the centrosomes, which move to opposite ends of the cell to prepare to pull the chromosomes apart, organising themselves into long, strong fibres stretching across the cell known as spindle fibres. The spindle fibres will later attach to the chromosomes to help pull them apart, and are known as the mitotic spindle.

  Nuclear envelope breaks down- The nuclear envelope starts breaking down so that the chromosomes can move freely.
• What occurs during metaphase?

  Chromatids of the replicated chromosomes are still attactched to one another via cohesin in the centromere

  The mitotic spindle attaches to the kinetochores on each chromosome

  Chromosomes are moved to be equidistant from the two poles of the cell, lined up in a region known as the metaphase plate.

  It is double checked that every chromosome is correctly attached to a spindle fibre via a kinetochore
• What occurs during anaphase?

  The cohesin holding the sister chromatids together is cut via the enzyme separase, which is kept inactive by a protein known as securin before anaphase.

  Motor proteins of the microtubules pull the sister chromatids along the spindle fibres which begin to shorten and pull the sister chromatids towards the opposite poles of the cell. Each sister chromatid is now considered a full chromosome, known as daughter chromosomes, and as the spindle fibres pull them apart, the chromosomes move toward opposite sides of the cell.
• What occurs during telophase?

  Occurs at the same time as cytokinesis

  Chromosomes are pulled into tight groups at each pole of the cell

  The nuclear membrane forms around each set of daughter chromosomes

  The chromosomes decondense/ uncoil to form chromatin

  The mitotic spindle breaks down and disappears
• What regulates mitosis progression and may result in cancer when is deregulated?

  The phases of mitosis and their progression is regulated by proteins known as cyclins.
• What is cytokinesis?

  Cytokinesis is the division of the cytoplasm and the organelles into two daughter cells.
• How does cytokinesis occur in plant cells?

  The Golgi buds off vesicles that move towards the cell equator

  The vesicles fuse to create a disc-shaped structure: the cell plate

  The cell plate extends and fuses with the sides of the parent cell, completely separating the two new daughter cells.

  The vesicles bring in pectin and other materials via exocytosis between the two membranes

  the lamella that will link the new cell walls is formed

  The daughter cells bring in cellulose via exocytosis to the equator

  The cellulose builds the cell wall of each daughter cell
• How does cytokinesis occur in animal cells?

  Contractile proteins known as actin and myosin form rings at the cell equator

  Contractile proteins contract and pull the cell membrane inwards to form a cleavage furrow

  The cell membrane cleavage furrow pinches in until the cells split into two daughter cells
• What is budding (unequal cytokinesis)?

  Budding is an asymmetric division mechanism used by yeast to reproduce asexually. During cytokinesis, the daughter cell receives only a small portion of the cytoplasm. Initially, the daughter cell is attactched to the parent cell, and eventually the cells are separated and the daughter cell matures into a new yeast cell.
• What is oogenesis (unequal cytokinesis)?

  Oogenesis is the production of an egg cell (ovum). Before a girl is born, she contains oogonia, which are not egg cells yet. When the girl is born, these become primary oocytes, which stay frozen in the ovaries until puberty when hormones are distributed which stimulate one primary oocyte to develop further per month. During Meiosis I, the primary oocyte splits into two cells, one secondary oocyte (which will eventually become an egg) and a polar body which has no use. During Meiosis II, if the secondary oocyte is fertilised, it splits into an ovum and another polar body, which is not needed. The cytoplasm is divided unequally during cytokinesis, as the polar bodies are smaller than the secondary oocyte/ovum. They are divided unequally because he large volume of cytoplasm in the ovum/secondary oocyte is used to provide all organelles energy to sustain the developing embryo.
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