The Mechanics of Cell Division M Phase: takes about an hour in most animal cells of the 12- 24 hours for the total cell-cycle. The Main Goal

The Theme Polymerization

The Cast The chromosome The kinetochore

The Cast (cont’d) Microtubule-activating Proteins (MAPs) Attached to the kinetochore Dynein Kinesins Purpose: to help transport along the spindle’s microtubules. Centrosomes

Human mitotic chromosomes Cohesins: protein complexes that “glue” the sister chromatids together. (A) Red stain indicates a linear central axis around which the sister chromatids are organized. (B) Two sister chromatids stopped in metaphase (C) This scaffolding around which the DNA is formed is made of a protein called condensin.

The bipolar mitotic spindle is responsible for the replicated chromosomes to be distributed to each of the daughter cells. Microtubule-dependent motor proteins help the microtubules of the spindles to accomplish this goal. Animal Cells: contractile ring is responsible for cytokinesis. Plant Cells: phragmoplast is responsible for cytokinesis.

Two Mechanisms Prevent Cytokinesis Without Mitosis Cyclin-dependent kinases (CdKs) phosphorylate proteins required for mitosis and some proteins required for cytokinesis are phosphorylated and become inactivated. This way M-Cdk (Mitosis-dependent Cdk) must be inactivated before the cell can move into cytokinesis. Once the chromosomes are separated to different poles of the cell, the vacated central region of the spindle is required to form the contractile ring.

DNA must be replicated and centrosomes must be duplicated prior to M phase. Centrosomes are the principal microtubule organizing center in animal cells.

Centriole pairs are held together by fibers Centriole pairs are held together by fibers. The mother centriole is larger than the daughter centriole. Each centriole pair becomes part of the microtubule organizing center (the aster) from which microtubules radiate.

The Centrosome Cycle The centrioles are associated with the centrosome matrix that nucleates microtubule outgrowth.

Green Stain: microtubules Blue Stain: chromatin Green Stain: microtubules Nuclear Envelope breaks down and this allows the spindle fibers to interact with condensed chromosomes.

Mitosis in a plant cell (A) Prophase (B and C) Prometaphase: n. envelope breaks down and chromosomes interact with microtubules. (D) Metaphase

Summary: Cell division occurs during the ___ phase, which consists of nuclear division or mitosis followed by ________________. The DNA is replicated in the preceding S phase; the two copies of each replicated chromosome, called ___________________ ___________________ remain glued together by ___________________. At the start of M phase, ________________ bind to the replicated chromosomes and condense them. A microtubule-based mitotic spindle is responsible for chromosome segregation in all eucaryotic cells. The ____________ _____________ in animal cells develops from the microtubule ________(plural) that form around each of the two centrosomes produced when the centrosome duplicates, beginning the S phase; at the onset of M phase, the duplicated ______________________ separate and move to opposite sides of the nucleus to initiate the formation of the two poles of the spindle. An actin and ____________- based contractile ________ is responsible for cytoplasmic division in animal cells and in many unicellular eukaryotes, but not in __________ cells.

Spindle begins to form outside the nucleus during prophase. When nuclear envelope breaks down during prometaphase, then the microtubules can capture the chromosomes. Anaphase, the spindles actually elongate which increases the separation of the two poles of the cell. This continues into telophase.

Microtubule-dependent Motor Proteins Kinesin Proteins: move towards the (+) end of the microtubules. Dyneins move toward the minus end. Both motor proteins operate at or near the ends of the MTs. It is here where the MTs are assembled and disassembled. The motor proteins are also thought to be involved in generated force for the movement that separates the poles of the cell.

Types of Microtubules Astral MTs Kinetochore MTs Overlap MTs or non-kinetochore MTs

Microtubules and the M Phase Interphase Dynamic Instability Catastrophe Rescue

Microtubules and the M Phase (cont’d) Prophase Changes in the MTs, M-Cdk, microtubule motor proteins, microtubule-associated proteins (MAPs) and catastrophins.

Antiparallel microtubules: the associated motor protein can move MTs in opposite directions. This would separate the centrosomes. This is the force generated by the kinesin motors that moves the poles of the cell apart.

Less overlap between the MTs as the kinesins push the centrosomes apart.

A Balance Between Motor Proteins Constructs Proper Spindle Length Yeast Cells with green fluorescent anti-tubulin antibodies. C: overexpression of the (-) end-directed motor protein producing short spindles D: overexpression of the (+) end-directed motor protein producing long spindles

Microtubules and the M Phase (cont’d) Prometaphase Breakdown of the nuclear envelope is triggered by MCdk phosphorylating the inner lining of the nuclear membrane called the nuclear lamina. This allows for the MTs to contact the condensed chromosomes. “Search and capture” The kinetochores attach the chromosomes to the mitotic spindle.

Microtubules and the M Phase (cont’d) Prometaphase (cont’d) The microtubules that attach to the kinetochore become stabilized. Bipolar Attachment Oscillation at the metaphase plate waiting a signal to separate.

The Spindle Just Doesn’t Stick To The Kinetochore

At the Metaphase Plate Kinetochores are red. Continuous oscillations at the metaphase plate. Mitotic cells spend about half of the M phase in metaphase, oscillating and waiting for the signal to enter anaphase.

(A) Astral Ejection Force: when the sister chromatids are cut the part without the kinetochore ends up away from the cell’s pole.

Microtubules and the M Phase (cont’d) Anaphase All the chromosomes must be positioned at the metaphase plate for anaphase to begin. Spindle-attachment checkpoint

Sudden separation at anaphase. Cohesin must release the hold it has on the sister chromatids. Anaphase promoting complex (APC) will inactivate some phosphorylating actions and activates another enzyme (separase) that unglues the cohesin.

Where does this shortening occur?

Pushing in the central spindle and a pulling by the astral microtubules at the poles.

How motor proteins and microtubule depolymerization at the kinetochore combine to move chromosomes to the poles is a mystery but two ideas are:

Microtubules and the M Phase (cont’d) Telophase Nuclear envelope reforms from nuclear membrane fragments. Nuclear pores are formed within the envelope. Nuclear envelope becomes continuous with the ER. Pores then pump in nuclear proteins Chromosomes decondense. Transcription can resume.

Mitosis begins with _______ which is triggered by MCdk Mitosis begins with _______ which is triggered by MCdk. In animal cells, a dynamic microtubule array forms around each of the duplicated centrosomes, which separate to initiate the formation of the two spindle ______. Prometaphase _______ with the breakdown of the ________ ____________. This allows for the __________ on the condensed ______________ to contact the ____________________ from each of the ___________ poles. The kinetochore microtubules from opposite spindle poles pull in opposite directions on each duplicated chromosome. There exists also a polar ejection or force pushing the chromosomes to the spindle equator to form the _______________ _______. Anaphase begins with the cleavage of the linkage by ________ to the sister chromatids. At this time then the chromosomes are ____________ to opposite poles and also at the same the spindle poles move apart. In telophase, the _____________ __________ reforms.

Cytokinesis The furrowing of the cell membrane created by the dividing frog egg is caused by the contractile ring underneath it.

Cytokinesis in a plant cell The new cell plate begins in late anaphase and guided by a the phragmoplast. The phragmoplast is filled with small vesicles from the Golgi apparatus that are filled with polysaccharides and glycoproteins required for the new cell wall. The cell plate is formed inside out.