MitosisChapter 9

Cell Division

• process by which new identical cells are produced from existing cells– stomach and intestine cells every 2-3 days– skin cells every 20 days– liver cells every 200 days– brain cells reproduce every 30-50 years

Chromosomes• short, stringy structures of DNA• carriers of genetic material• usually chromatin: long strands of DNA

which are wrapped around histone proteins• just before cell division, chromatin compacts

and reorganizes into chromosomes– each chromosome internally duplicated– consists of two identical DNA chains (sister

chromatids) attached together at a single point (centromere)

Chromosome Structure

Mitosis in Animal Cells• outside nucleus – centrosome

(microtubule organizing center)– organizes mitotic spindle

• contains many fibers each composed of a bundle of microtubules

– in animals, contains two centrioles• oriented at right angles to each other

• each with 9 triplets of microtubules arranged in a cylinder

• Centrosome was also replicated in S-phase, so now two centrosomes

Mitosis Phase One: Prophase• longest phase of mitosis• chromatin has condensed (visible in

microscope)• nuclear envelope and nucleolus disintegrate• centrioles move to ends of cell (in animals)

– form microtubules in star-like arrays called asters

• spindle forms– football-shaped, cagelike structure make up

of microtubules– in plants, spindle does not need centrioles

Pictures of Prophase

Prometaphase• centromeres develop

two kinetochores– specialized protein

complex– 1 over each sister

chromatid• physically hook sister

chromatids up with specialized microtubules (kinetochore fibers)

• connect sisters to opposite poles of mother cell

Mitosis Phase Two: Metaphase• spindle fibers attach to chromosomes at

kinetochores• chromosomes are lined up across

equatorial plane– metaphase plate (looks like glass) shows

where mother cell will be divided

Mitosis Phase Three: Anaphase• centromeres dissolve• daughter chromosomes separate and

are pulled to opposite ends of cell by the shortening of the kinetochore fibers

Mitosis Phase Four: Telophase• begins as daughter chromosomes reach

opposite ends• chromosomes unwind to become chromatin• spindle breaks down• nucleolus reappears; nuclear envelope forms

Cytokinesis• cytoplasm divides to form two separate,

identical cells

• animal cytokinesis:

– A cleavage furrow appears between daughter nuclei

– Formed by a contractile ring of actin filaments

– Like pulling on a draw string

– Eventually pinches mother cell in two

Cytokinesis in Animals

Cytokinesis in Plants• cell walls do not permit furrowing• begins with formation of a cell plate

– many small membrane-bounded vesicles– eventually fuse into one thin vesicle

extending across the mother cell– membranes of the cell plate become the

plasma membrane between the daughter cells•contents of vesicles become the middle

lamella between the two daughter cells•daughter cells later secrete primary cell

walls on opposite sides of middle lamella

Cytokinesis in Plants

Animation of Stages of Mitosis

Mitosis takes 30 minutes to 1 ½ hours depending on the type of cell

Remember: I P MAT C

Results of Mitosis• guarantees genetic continuity with two

identical cells• unicellular organisms simply multiply

into two organisms; they do not become multicellular organisms

• multicellular organisms form tissues, organs, and organ systems as a result of mitosis

Identify which of the following examples in the video clip are actually mitosis

The End