Biology: Chapter 5: Mitotic Cell cycle: Mitosis

Biology: Chapter 5: Mitotic Cell cycle: Mitosis

• Part of a precisely controlled process called the cell cycle.
• Nuclear division: Splits the cell's nucleus into two daughter nuclei that are genetically identical and contain the same number of chromosomes as the parent cell.

Cell cycle

• Regular sequence of events between one cell division to the next.
• Three phases: Interphase, nuclear division (mitosis) and cell division (cytokinesis)

Interphase:

1. G1 phase (Gap 1): Cell grows to normal size after cell division. Carries out normal function eg. Protein synthesis. At the end of G1, the cell receives a signal telling it whether or not it will divide. If a cell receives the go signal to divide, it will continue into the S phase.
2. S phase (synthesis): DNA in the nucleus replicates - two sets of each chromatid that can form chromosomes.
3. G2 phase (Gap 2): Cell continues to grow and the new DNA is checked for any errors and are repaired. Preparations made for division eg. Increased production of Tubulin, which is used to make spindle.

Nuclear division (mitosis):

• Nucleus divides into two genetically identical daughter nuclei
• Growth temporarily stops.

Cell division (cytokinesis):

• After the two new nuclei move to the opposite poles of the cell, the cell splits into two by constricting the cytoplasm between the two nuclei. 
• Cleavage furrow: Constriction of the cytoplasm between two nuclei.
• Creates two new identical cells.
• In plant cells, vesicles carry building materials (eg. cellulose) from the golgi body and fuse together to form a cell plate between the two nuclei.

Cell cycle diagram

Mitosis

• Four phases: Prophase, Metaphase, Anaphase and Telophase (PMAT)

Prophase:

• Chromatin coil up and chromosomes start to appear. 
• Nuclear envelope, nucleolus and organelles 'disappear' - break up into smaller vesicles
• Centrosomes replicate and start moving to opposite poles where they form the poles of the spindle fibre.

Metaphase:

• Centrosomes at opposite poles help organize production of spindle microtubules.
• Chromosomes line up across the middle of the spindle, attached by their centromeres.

Anaphase:

• Spindle fibre attached to the centromeres of the chromosomes pull the two chromatids to separate poles, centromere first.

Telophase:

• Chromatids reach the pole of their spindle and uncoil (each one contains 1 DNA molecule, which replicates itself during interphase for the next cell division)
• Nuclear envelope and nucleolus reform around the chromatin
• Remaining spindle fibre breaks down
• Cytokinesis starts 

Significance of mitosis

• Growth: Two daughter cells are identical and have the same number of chromosomes - allow for multicellular growth from a unicellular zygote. Can occur over the entire body or in specific regions.
• Repair and replacement of cells: Dead or faulty cells can be replaced by identical cells.
• Asexual reproduction: Production of new genetically identical individuals by a single parent organism. Can occur in unicellular organisms eg. Amoeba or in some plants (budding)
• Immune response