AP BIOLOGYMitosis

Mitosis vs. Meiosis• Mitosis: Cell Division that produces two diploid cells that

are identical to each other and to the parent cell.• This produces body cells (somatic cells) so the organism

can grow or replace dead/damaged cells.

• Meiosis: Cell division that produces four haploid cells that are not identical genetically.

• This produces sex cells (gametes) that are used in sexual reproduction.

“C” Words:

• Chromatin-Strands of DNA in a cell that is not dividing—unwound and indistinct.

• Chromosomes-Strands of DNA in a cell that is dividing—wound up and distinct.

Chromatin is unwound and indistinct in the nucleus of the cell.

More “C” Words:

• Chromatids—one of 2 identical strands of DNA in a chromosome (called sister chromatids)

• Centromere—the structure that holds together the 2 sister chromatids. Histones-protein “beads” that keep

the strand of DNA from tangling up.

More “C” Words

• Centrosome/Centriole-Structure found in animal cells that anchors the spindle fibers; made of bundles of microtubules.

• Cytokinesis—the division of the cytoplasm following mitosis (the dividing of the nucleus)

Chromosome Structure

• When cells begin to divide, the first thing that happens is that the chromatin in the nucleus begins to wind up, separating the strands from each other.

• This forms individual chromosomes, each with two identical chromatids.

Each chromosome consists of 2 identical strands of DNA (called sister chromatids) held together with a centromere.

Cell Cycle

Interphase

Interphase

A cell in Interphase is not actively dividing. It is working very hard, however—growth, replicating DNA and preparing to divide.

DNA is in the form of chromatin here:

Prophase

Centrioles migrate to the poles. Nuclear Membrane disappears.

Chromatin winds up to form chromosomes.

Spindle forms.

Prophase

Metaphase

Metaphase

Metaphase

Chromosomes move to the equator of the cell.

Anaphase

Chromosomes split in half at centromere.

Each chromatid moves to opposite poles.

Chromosome Structure At Anaphase

Centromere splits and chromatids go to opposite poles of cell

Chromosomes prior to Anaphase

Kinetochores

• Kinetochores are structures that are part of the spindle fiber system.

• They attach to chromosomes at the centromere.

• During Anaphase, they shorten and move toward the poles, pulling apart the chromatids.

Telophase

TelophaseChromosomes unwind to form chromatin again.

New nuclear membranes form.

Spindle disappears

Cytokinesis

Cytokinesis in an Animal Cell: A cleavage furrow forms, eventually pinching in the cell membrane to form 2 cells.

Cytokinesis in a Plant Cell: A cell plate forms between the 2 new nuclei—eventually dividing the cell into 2 cells. The cell plate becomes the new cell wall.

Cytokinesis in Animal Cells (on left) and Plant Cells (on right)

Regulation of Mitosis• How often cells divide depends on many factors. For

example, the age of the organism and type of cell greatly affects how often it divides.

• Example: skin cells divide frequently; mature nerve cells and muscles cells never divide.

• What determines when a cell divides? Signals!

Signal Transduction• Transduction = the relaying of a message from one

molecule to another• When a cell has a receptor for a particular molecule on its

surface, it will send a “signal” into the cell when that molecule arrives.

• This “signal” is usually relayed

to other molecules and

activates a certain response

inside the cell.

The Cell Cycle Control System

• The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock

• The cell cycle control system is regulated by both internal and external controls

• The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

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Fig. 12-14

SG1

M checkpoint

G2M

Controlsystem

G1 checkpoint

G2 checkpoint

• For many cells, the G1 checkpoint seems to be the most important one

• If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide

• If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the G0 phase

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Fig. 12-15

G1

G0

G1 checkpoint

(a) Cell receives a go-ahead signal

G1

(b) Cell does not receive a go-ahead signal

The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases

• Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks)

• The activity of cyclins and Cdks fluctuates during the cell cycle

• MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase

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Fig. 12-17a

Time(a) Fluctuation of MPF activity and cyclin concentration during the cell cycle

Cyclinconcentration

MPF activity

M M MSSG1 G1 G1G2 G2

Fig. 12-17b

Cyclin isdegraded

Cdk

MPF

Cdk

MS

G 1G2

checkpoint

Degradedcyclin

Cyclin

(b) Molecular mechanisms that help regulate the cell cycle

G2

Cyclin

accum

ulatio

n

Stop and Go Signs: Internal and External Signals at the Checkpoints

• An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase

• Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide

• For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture

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Fig. 12-18

Petriplate

Scalpels

Cultured fibroblasts

Without PDGFcells fail to divide

With PDGFcells prolifer-ate

10 µm

• Another example of external signals is density-dependent inhibition, in which crowded cells stop dividing

• Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide

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Fig. 12-19

Anchorage dependence

Density-dependent inhibition

Density-dependent inhibition

(a) Normal mammalian cells (b) Cancer cells25 µm25 µm

Cancer cells exhibit neither anchorage dependence nor density-dependent inhibition.

Loss of Cell Cycle Controls in Cancer Cells

• Cancer cells do not respond normally to the body’s control mechanisms

• Cancer cells may not need growth factors to grow and divide:• They may make their own growth factor• They may convey a growth factor’s signal without the presence of the

growth factor• They may have an abnormal cell cycle control system

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• A normal cell is converted to a cancerous cell by a process called transformation

• Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue

• If abnormal cells remain at the original site, the lump is called a benign tumor

• Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors

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Fig. 12-20

Tumor

A tumor growsfrom a singlecancer cell.

Glandulartissue

Lymphvessel

Bloodvessel

Metastatictumor

Cancercell

Cancer cellsinvade neigh-boring tissue.

Cancer cells spreadto other parts ofthe body.

Cancer cells maysurvive andestablish a newtumor in anotherpart of the body.

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Mitosis Animations• http://www.cellsalive.com/mitosis.htm

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__mitosis_and_cytokinesis.html

• http://www.youtube.com/watch?v=VlN7K1-9QB0