Lecture 17: Dividing the Cell I: Cell Cycle and Mitosis Bio Sci 93

Why cells divide

Essential for all organisms: reproduction (e.g., single-celled amoeba), tissue renewal (replacement and repair of cells), and growth and development (one cell becomes about 200 trillion cells).

Stem cells

Cells that can divide and develop into specialized cells. Used to study human tissue development and disease, test drugs, and make specialized cells for cell-based therapies.

Key concept of cell division

Identical genetic material must be passed from parent cell to offspring cells. This happens through cell cycle regulation and mitosis.

Cell cycle

Describes the growth and division of all types of cells. Two parts: interphase and the mitotic phase. Its purpose is precise replication of chromosomes and proper distribution of chromosomes to daughter cells.

Interphase

Period of the cell cycle between divisions. Has three parts: G1, S, and G2.

G1 phase

Metabolic activity and growth: synthesis of macromolecules and cytoplasmic organelles. Chromosomes are unduplicated.

S phase

DNA is replicated (and there is metabolic activity, growth, and DNA synthesis). Chromosomes become duplicated.

G2 phase

Additional growth and preparation for cell division. Chromosomes are duplicated but uncondensed.

Mitotic phase

Includes mitosis (with five subphases) and cytokinesis.

Chromosome

A single DNA molecule. It undergoes replication in S phase.

Sister chromatids

The two identical copies of a duplicated chromosome, held together by cohesin. They separate during mitosis, one to each daughter cell.

Cohesin

Protein complex that holds sister chromatids together until anaphase.

Centromere

Region where sister chromatids are joined. The kinetochore forms at the centromere.

Kinetochore

Protein structure at the centromere of each chromatid where spindle microtubules attach.

Mitosis

Equal distribution of genetic material to two daughter cells: the sister chromatids separate, one to each daughter cell.

Cytokinesis

The process of division of the cytoplasm.

Mitotic spindle

Apparatus of microtubules that moves chromosomes during mitosis.

Centrosome

Microtubule-organizing center that duplicates during the cell cycle and moves to opposite poles of the cell. Contains a pair of centrioles; microtubules radiate from it, forming the spindle.

Aster

Array of short microtubules radiating from a centrosome.

Kinetochore microtubules

Spindle microtubules that attach to the kinetochores of chromosomes.

Nonkinetochore microtubules

Spindle microtubules that do not attach to chromosomes. They overlap at the spindle midzone.

What to remember for each phase

For each subphase of mitosis, know: what is happening to the chromosomes, what is happening to the spindle apparatus, and what is happening to the nuclear envelope.

Prophase

Chromosomes condense into two sister chromatids. The early mitotic spindle starts to form and the centrosomes begin moving to opposite poles. The nucleolus disappears.

Prometaphase

The nuclear envelope breaks into fragments. Kinetochores form at the centromeres, and kinetochore microtubules attach to them. Nonkinetochore microtubules extend from each pole.

Metaphase

Chromosomes line up on the metaphase plate (an imaginary plane in the middle of the cell). A centrosome sits at each spindle pole and the spindle is complete.

Anaphase

Cohesin is cleaved. Sister chromatids separate and move as daughter chromosomes toward opposite poles of the spindle. The cleavage furrow begins forming.

Telophase and cytokinesis

Two nuclei form: the nuclear envelope and nucleolus re-form around each set of chromosomes, and the chromosomes decondense. Cytokinesis divides the cytoplasm.

Cytokinesis in animal cells

A cleavage furrow forms and pinches the cell in two.

Cytokinesis in plant cells

A cell plate forms and develops into a new cell wall between the daughter cells.

How chromosomes move

Experiment: spindle microtubules were labeled yellow and a mark was bleached (not cut) between the pole and the chromosome. The microtubule length on the pole side did not change, while the chromosome side shortened. Conclusion: chromosomes "walk" along the microtubule track using motor proteins at the kinetochore, while tubulin subunits are lost.

Spindle and centrosome mutations

Mutations in genes whose protein products take part in the mitotic spindle and centrosome cause many human disorders. Spindle defects are associated with brain disorders and cancer.

Microcephaly

Small brain that is structurally normal but has a reduced number of cortical neurons. Caused by mutations in 9 genes whose protein products all localize to centrosomes.

Lissencephaly

Small brain with a smooth surface. Caused by mutations in the Lis1 gene, whose protein product stabilizes the microtubule/dynein complex.

Hypothesis

A testable explanation for a set of observations, based on the available data and guided by inductive reasoning. It is a statement you believe to be true. Example: the inhibitor "93stop" will stop cell proliferation at a specific stage of the cell cycle.

Control group vs. test group

Control group is not treated with the variable being tested (here, cells grown without the inhibitor). Test group receives the treatment (cells grown with the inhibitor "93stop").

Steps of the scientific method

Hypothesis, experiment to test the hypothesis (experimental design with control and test groups), data analysis (organizing and presenting the data), interpretation of results (what do the data mean?), and conclusion (how do the results fit in the big picture?).

Flow cytometry

Technique that measures the fluorescence of individual cells. Here, a fluorescent chemical that binds DNA gives a snapshot of which cell cycle phase each cell is in.

Histogram

A type of bar graph used to organize and present data. Here, the x-axis is the amount of fluorescence per cell and the y-axis is the number of cells.

What the fluorescence axis shows

Fluorescence per cell indirectly shows the amount of DNA in each cell, which tells you the phase of the cell cycle.

Reading a cell cycle histogram

Peak A (lowest fluorescence) is cells in G1 with unduplicated DNA. Peak B (in between) is cells in S phase replicating DNA. Peak C (highest fluorescence, about twice the DNA content of A) is cells in G2 or M. A treated sample with a taller peak at one phase suggests the drug stops cells at that phase.