Chapter 9

THE CELL CYCLE

Jill before teaching go to this hyperlink (on these words)

Continuous balance between cell division and cell death Mitosis – A process that forms two genetically

identical cells from one Product of mitosis: daughter cells

Apoptosis – natural cell death – precise – geneticallly programmed(?)

The Cell Cycle (hyperlink)Checkpoints – 1. when certain proteins

interact in a way that ensures the proper sequence of events is unfolding2. allows a pause so newly formed DNA can be checked and repaired before dictating orders

Cell cycle, con’tEvents that occur in the life of a cell.Includes 3 major stages:• Interphase • Karyokinesis (mitosis) • Cytokinesis

InterphaseCell is not dividing, but there is GREAT activity

• G1 Phase - carries out basic functions & performs specialized activities.• duration is extremely variable• Synthesizes proteins, lipids and carb in case

of cell division

G1 Phase, con’t• contains restriction checkpoint ~ cell “decides”

to:- Divide- Stops to repair DNA damage- enter a quiescent phase (G0)

- die

• G0 Phase – a cell can exit the cycle at G1 to enter this phase• The cell maintains specialized characteristics,

but does not divide.• No replication of DNA• Must be at this stage in an egg for cloning to

work Ex. neurons & muscle cells

Interphase, con’t

Interphase, con’t•S Phase –

•Great Synthetic activity – replicating DNA

• cell replicates chromosomes & synthesizes associated proteins. (also those that coordinate events of nucleus and cytoplasm

(animal cells replicate centrioles as well)

• G2 Phase – • Makes more proteins – especially

tubulin for microtubules• Membrane materials stored in

vesicles under the cell membrane• DNA winds tightly around proteins

to start mitosis• Interphase ends

Karyokinesis ( aka mitosis hyperlink); M phase)Equal distribution

of replicated genetic material (chromosomes).

Nucleus actively dividing

(hyperlink)See mitotic spindle

(diagram pg 141)

Participants in Mitosis Centromere – link sister

chromatids

Participants, cont 2 identical copies of

chromosomes (sister chromatids)

Participants, cont Spindle grows from the centrosome/centrioles Proteins around the centriole initiate spindle

growth

• Prophase (hyperlink)• DNA coils tightly around

proteins• replicated chromosomes

condense• centrosomes separate &

migrate toward opposite sides of cell

• mitotic spindle forms (microtubules grow out from centrosomes)

• nucleolus disappears

Mitosis Phases - hyperlink

• Prometaphase• nuclear membrane

breaks down- into small pieces and lay parallel to the cell membrane

• spindle fibers attach to centromeres of chromosomes

• Metaphase• chromosomes are lined up

single-file along equator of mitotic spindle.

• Chromosomes seem motionless because they are pulled equally by both sides of the cell

• Anaphase• Centromeres, one

per chromatid, move apart separating the chromatids to opposite

• Microtubules in the spindle shorten and some lengthen in a way that moves the poles farther apart

• Telophase• Cell begins to look like a

dumbell (cytokinesis has begun)

• mitotic spindle breaks down

• chromosomes decondense

• nuclear membranes reform around two nuclei

• nucleoli reappear• End of Mitosis

3. CytokinesisDistribution of cytoplasm and all other

contents to daughter cells.• begins during anaphase or telophase

depending on the cell type• differs in animal & plant cells

Cytokinesis in animal cells:• cleavage furrow (hyperlink)(slight indentation)

forms around equator of cell.• Contractile ring of actin & myosin

microfilaments act like a drawstring to pinch the cell in two.

• Asters determine the number of cleavage furrows

• usually an equal division.

Cytokinesis in animal cells

Cytokinesis in plant cells hyperlink:New cell wall must be built

• phragmoplast (microtubule structure) forms in cytoplasm & traps vesicles containing cell wall material. (between daughter cells)

• vesicles fuse, forming a cell plate across midline of cell.

• cell plate gives rise to two primary cell walls.

Does cytokinesis always accompany karyokinesis?

Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleated cells).

Control of the Cell Cycle hyperlink( what turns mitosis on or off)

Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence.

Survivins override signals that tell the cell to die, keeping it in mitosis, not apoptosis

Telomeres

At tip of chromosomes 100’s-1000’s of

repeating sequences on end of chromosome

Each time mitosis occurs, DNA looses 50-100 sequences

After 50+ divisions DNA signals cell division to cease

Telomere, con’t A few cells, DNA does

NOT shrink (bone marrow, small intestine, blood cells, germ cells for sperm)

If DNA shrinks, no telomerase made Telomerase add

DNA to tips of chromosome

Plant cells produce telomerase and divide more than 50 times

Shortening of telomeres - loss of telomere DNA signals cell to stop dividing.

Some cells produce telomerase (enzyme that continually adds telomere DNA).

Contact Inhibition - healthy cells stop dividing when they come in contact with other cells.

Signals to Divide: Signals from outside the cell effect cell cycles

Hormones - stimulate cell division.Ex. Estrogen stimulates uterine cell division

Growth factors - proteins that stimulate local cell division.Ex. Epidermal growth factor (EGF) stimulates

epithelial cell division filling in new skin underneath a scab

Ex. Produced in salivary glands of animals – aids in wound healing

Interaction of kinases & cyclins - activate genes that stimulate cell division.

Signals to Divide, con’t

Stem Cells (hyperlink)• Cells used to replenish tissues

Control at the Tissue level – stem cells and cell populations

Stem cell’s con’t• When a stem cell divides,

one daughter cell will specialize and the other daughter cell will remain a stem cell

Ex: basal layer of skin, bone marrow, & small intestine, heart and ventricles of brain

Cell populations – up to 3% are dividing. (expanding population) or if all are dividing it is called a renewel population

Static populations- cells are no longer dividing in the tissue- nerves and muscles (these enlarge-not divide)

B. Apoptosis (hyperlink)Programmed cell

death; part of normal development.

Eliminates excess cells and cells that could grow uncontrollably.

Tadpole tail, webbing between fingers)

Cell death is part of life

Steps of Apoptosis:

Apoptosis rapidly and neatly dismantle cell into

membrane bound pieces that phagocyte will mop up

(as opposed to necrosis due to injury death receoptor receives signal caspases (enzymes that snip cell components)

are activated within caspases destroy proteins and other

components. caspases destroy adhesion molecules so cell

can’t cling to another cell cell undulates, forming bulges called blebs nucleus bursts releasing chromatin cell shatters loose membrane surrounds pieces phagocytes mop up Similar in plant cells, but parts are digested by

enzymes

Why cells die Brain cell example pg. 158 To distinguish self from non-self protective function-to detect and

weed out cells that could grow uncontrollably

C. Cancer (loss of cell cycle control) Condition resulting from excess cell

division or deficient apoptosis.Characteristics of Cancer Cells:• can divide uncontrollably & eternally• dedifferentiation• are invasive• are heritable & transplantable• lack contact inhibition• readily metastasize• exhibit angiogenesis• exhibit genetic mutability

Cancer (hyperlink)- con’t given nutrients and space, cancer cells reproduce

uncontrollably growth rate depends on the type of cell fast growing must be 1 centimeter in diameter- may

produce 1 million new cells/hour loss of cell cycle control is inherited by descendents injectable lack contact adhesion often undergo mutations stimulate angiogenesis (blood vessel growth)

Causes of Cancer: mistimed or misplaced mitosis Absence of normal apotosis (hyperlink) Over-expression of oncogenes Oncogenes are genes that trigger limited cell division. Inactivation of tumor suppressor gene Tumor suppressor genes prevent a cell from dividing or

promote apoptosis.

Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors:• carcinogens• radiation• viruses• diet• exercise habits