Mitosis and Mitosis and CytokinesisCytokinesis

Section 6-3Section 6-3

In Mitosis, Chromatids In Mitosis, Chromatids are pulled by are pulled by MicrotubulesMicrotubules During mitosis the nucleus divides to During mitosis the nucleus divides to

form two nuclei, each containing a form two nuclei, each containing a complete set of chromosomes and the complete set of chromosomes and the chromatids on each chromosome are chromatids on each chromosome are physically moved to opposite sides of physically moved to opposite sides of the dividing cell with the help of the the dividing cell with the help of the spindle.spindle.

During cytokinesis the cytoplasm is During cytokinesis the cytoplasm is divided between the two resulting divided between the two resulting cells.cells.

SpindlesSpindles – cell structures made up of – cell structures made up of both centrioles and individual both centrioles and individual microtubule fibers that are involved in microtubule fibers that are involved in moving chromosomes during cell moving chromosomes during cell division.division.

Forming the Spindles in Animal CellsForming the Spindles in Animal Cells

1. One pair of centrioles, at right angles 1. One pair of centrioles, at right angles to one anotherto one another

2. During G2. During G22 phase – the centriole pair phase – the centriole pair is replicated so that the cell has two is replicated so that the cell has two pairs of centrioles as it enters the pairs of centrioles as it enters the mitotic phase.mitotic phase.

3. When the cell enters mitotic phase, 3. When the cell enters mitotic phase, centriole pairs begin to separate, centriole pairs begin to separate, moving towards the opposite poles of moving towards the opposite poles of the cell.the cell.

4. As centrioles separate, spindles 4. As centrioles separate, spindles begin to form.begin to form.

5. Centrioles and spindle fibers – both 5. Centrioles and spindle fibers – both made of hollow tubes of protein made of hollow tubes of protein called called microtubulesmicrotubules. .

6.6. Each spindle fiber made of an Each spindle fiber made of an individual microtubule. Each individual microtubule. Each centriole, however, is made of nine centriole, however, is made of nine triplets of microtubules arranged in triplets of microtubules arranged in a circle.a circle.

Forming Spindle Fibers in Plant CellsForming Spindle Fibers in Plant Cells Plant cells do not have centrioles, Plant cells do not have centrioles,

but they form a spindle almost but they form a spindle almost identical to that of an animal cell.identical to that of an animal cell.

The Mitotic SpindleThe Mitotic Spindle

Separating Chromatids Separating Chromatids by Attaching Spindle by Attaching Spindle FibersFibers Some microtubules in spindle Some microtubules in spindle

interact with each other, others interact with each other, others attach to a protein structure attach to a protein structure found on each side of the found on each side of the centromere.centromere.

The two sets of microtubules The two sets of microtubules extend out toward opposite extend out toward opposite poles of the cell.poles of the cell.

Once microtubules attach to Once microtubules attach to centromeres and poles, chromatids can centromeres and poles, chromatids can be separated.be separated.

Chromatids moved to each pole of cell Chromatids moved to each pole of cell in a manner similar to bringing in a fish in a manner similar to bringing in a fish with a fishing pole. When microtubule with a fishing pole. When microtubule is “reeled in”, the chromatids are is “reeled in”, the chromatids are dragged to opposite poles.dragged to opposite poles.

Reeling occurs because ends of spindle Reeling occurs because ends of spindle fibers are broken down bit by bit at fibers are broken down bit by bit at each of the poles. Fibers become each of the poles. Fibers become shorter – chromatids move closer.shorter – chromatids move closer.

As soon as chromatids separate As soon as chromatids separate from each other – called from each other – called chromosomes.chromosomes.

When finally arrive at the poles, When finally arrive at the poles, one complete set is present.one complete set is present.

Mitosis and Cytokinesis Mitosis and Cytokinesis Divide CellsDivide Cells

Mitosis - Continuous process, but Mitosis - Continuous process, but divided into four stages divided into four stages

PP – – ProphaseProphase

MM – – MetaphaseMetaphase

AA – – AnaphaseAnaphase

TT – – TelophaseTelophase

Stage One – ProphaseStage One – Prophase

Chromosomes coil up and Chromosomes coil up and become visiblebecome visible

Nuclear envelope dissolvesNuclear envelope dissolves Spindle formsSpindle forms

Stage Two – MetaphaseStage Two – Metaphase

chromosomes move to center chromosomes move to center of cell and line up along the of cell and line up along the equatorequator

spindle fibers link chromatids spindle fibers link chromatids to opposite polesto opposite poles

Stage Three – AnaphaseStage Three – Anaphase

centromeres dividecentromeres divide two chromatids (now called two chromatids (now called

chromosomes) move toward chromosomes) move toward opposite poles of cell as opposite poles of cell as spindle fibers shortenspindle fibers shorten

Stage Four – TelophaseStage Four – Telophase

nuclear envelope forms around nuclear envelope forms around chromosomes at opposite pole chromosomes at opposite pole of cellof cell

chromosomes uncoilchromosomes uncoil spindle dissolves spindle dissolves

After four stages are complete, so After four stages are complete, so is mitosis.is mitosis.

http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html

http://www.johnkyrk.com/mitosis.html

http://www.accessexcellence.org/RC/VL/GG/mitosis.html

http://www.cellsalive.com/mitosis.htm

CytokinesisCytokinesis

Begins as mitosis endsBegins as mitosis ends cytoplasm of cell is divided in halfcytoplasm of cell is divided in half cell membrane grows to enclose cell membrane grows to enclose

each cell, which forms two cellseach cell, which forms two cells end result of mitosis and end result of mitosis and

cytokinesis – two identical cellscytokinesis – two identical cells

Animal cells and other cells without Animal cells and other cells without cell walls – cell is pinched in half by cell walls – cell is pinched in half by a belt of protein threadsa belt of protein threads

Plant cells and other cells with cell Plant cells and other cells with cell walls have a different method of walls have a different method of dividing cytoplasm.dividing cytoplasm.

vesicles formed by the Golgi apparatus vesicles formed by the Golgi apparatus fuse at midline of dividing cell and fuse at midline of dividing cell and form a cell plateform a cell plate

Cell PlateCell Plate – membrane bound cell wall – membrane bound cell wall that forms across the middle of a plant that forms across the middle of a plant cell in cytokinesiscell in cytokinesis

New cell wall forms on both sides New cell wall forms on both sides of the cell plate.of the cell plate.

When complete – cell plate When complete – cell plate separates plant cell into two new separates plant cell into two new plant cells.plant cells.

In plant and animal cells, In plant and animal cells, offspring cells are about the offspring cells are about the same size. Each has an same size. Each has an identical copy of the identical copy of the original cell’s chromosomes original cell’s chromosomes and about ½ of the and about ½ of the cytoplasm and organelles.cytoplasm and organelles.

SummarySummary Before mitosis begins, the cell will Before mitosis begins, the cell will

form the mitotic spindle during the G2 form the mitotic spindle during the G2 phase of the cell cycle.phase of the cell cycle.

Mitosis occurs in four stages – PMATMitosis occurs in four stages – PMAT Cytokinesis occurs after mitosis – Cytokinesis occurs after mitosis –

division of the cell’s cytoplasmdivision of the cell’s cytoplasm http://school.discoveryeducation.com/http://school.discoveryeducation.com/

quizzes6/muskopf/mitosis1.htmlquizzes6/muskopf/mitosis1.html

HOMEWORKHOMEWORK

Section 6-3 Review QuestionsSection 6-3 Review Questions Math Lab p. 129 Math Lab p. 129