Topic 2c: Mitosis

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The process of cell division in most eukaryotic cells

exception: gametes; mature nerve and muscle cells in animals.

1 diploid cell divides into 2 diploid daughter cells

chromosomes: when the cell is preparing to divide, each chromosome is replicated to be made up of 2 sister chromatids. When the cell is not diving, the chromatids are the chromosomes. Sister chromatids are attached at the centromere

diploid number in humans = 46 (this is 2N)

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Because mitosis is used for growth of organisms, it requires regulation.

regulated by cell cycle

Interphase: the first three phases of the cell cycle (G1, S, and G2); preparation for cell division, cells spend most of their time in this phase

G1 phase: the first gap phase of the cell cycle. The parent cell is growing larger, adding additional cytoplasm, and replicating organelles

S phase: synthesis; chromosomes are replicated. At the end, each chromosome consists of 2 sister chromatids

G2 phase: the second gap phase. The cell continues to grow in size and make final preparations for cell division (continuing to synthesize proteins and gather energy)

M phase: mitosis and cytokinesis occur to form the daughter cells

G0 phase: quiescent (inactive) phase; cells that are unable to divide are in this phase, this can be permanent or temporary; ex: mature muscle and nerve cells

possible triggers to enter this phase: lack of food, contact inhibition (crowding), or mutation/cell damage

Cell cycle control:

checkpoints: points in the cycle where progression is paused until conditions are favorable

G1 checkpoint: monitors for DNA damage, adequate growth, energy stores, etc.

G2 checkpoint: ensures all chromosomes are replicated and are undamaged

M checkpoint: all chromosomes must be attached to spindle microtubules for cycle to proceed

regulatory molecules:

cyclins must be bound to cyclin-dependent kinases to form the cyclin-CDK complex

cyclin-CDK complex is active only when it is phosphorylated

the complex acts as a positive regulator as it phosphorylates other molecules to advance the cycle forward

proto-oncogenes are positive regulators of the cell cycle

negative regulators

p53: halts the cell cycle and recruits DNA repair enzymes; if DNA is not repaired, triggers apoptosis

p21: production is triggered by secretion of p53; prevents progression of the cycle by binding to the cyclin-CDK complex

Rb: a protein that halts the cell cycle when dephosphorylated by binding E2F

failure of regulatory mechanisms:

when telomeres shorten to their threshold point, the cell is programmed to go into apoptosis.

If p53 is nonfunctional, this allows the cell to pass the G1 checkpoint despite being damaged, resulting in the formation of daughter cells with mutated genes, resulting in eventual tumor formation

proto-oncogenes become oncogenes when they are mutated; oncogenes contribute to cancer formation

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M Phase in detail:

prophase:

chromosomes are in the form of chromatin but begin to condense

nuclear membrane dissolves to release the chromosomes into the cytoplasm

centrioles replicate and move to opposite ends of the cell; mitotic spindle forms

metaphase:

spindle fibers attach to the chromosomes at the kinetochore

the chromosomes line up on the metaphase plate

anaphase:

sister chromatids are pulled apart to opposite ends of the cell; centromere splits so each chromosome has its own centromere

telophase:

chromosomes begin to decondense

nuclear membranes reform, secluding the piles of genetic material

mitotic spindle dissolves

cytokinesis:

division of cytoplasm and plasma membrane into 2 cells via pinching by the cleavage furrow (in animals)

in plants, cell plate forms the new cell wall

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Mitotic spindle apparatus: made up of microtubules; assist in movement of chromosomes (change in microtubule size causes movement of chromosomes)

centrioles: centers that help form the spindle fibers

asters: star shaped radial arrays that form around the centrioles that help guide the chromosomes to ensure that both daughter cells have the right amount/complements; also have a role in cleavage furrow formation and maintenance of the apparatus

spindle: fibers that attach to and move chromosomes around

Centromeres and kinetochores:

centromere: highly specialized region of chromatin that serves as the site of sister chromatid attachment as well as kinetochore assembly

without a centromere, there will be no kinetochore and the sister chromatids will not be separated