Chapter 8 THE CELL CYCLE

A. The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: F Interphase F Karyokinesis (mitosis) F Cytokinesis

1. Interphase Cell is not dividing. ] G 1 Phase - carries out basic functions & performs specialized activities. F duration is extremely variable F contains restriction checkpoint ~ cell “decides” to: - divide - enter a quiescent phase (G 0 ) - die

] G 0 Phase - cell maintains specialized characteristics, but does not divide. Ex. neurons & muscle cells ] S Phase - cell replicates chromosomes & synthesizes proteins. [animal cells replicate centrioles as well]

] G 2 Phase - cell synthesizes additional proteins (ex. tubulin) & assembles/stores membrane material.

2. Karyokinesis (mitosis; M phase) Equal distribution of replicated genetic material.

] Prophase F replicated chromosomes condense* F centrosomes separate & migrate toward opposite sides of cell F mitotic spindle forms (microtubules grow out from centrosomes) F nucleolus disappears

] Prometaphase F nuclear membrane breaks down F spindle fibers attach to centromeres of chromosomes

] Metaphase F chromosomes are lined up single-file along equator of mitotic spindle.

] Anaphase F centromers part, sister chromatids (now called chromosomes) separate F chromosomes move toward opposite poles

] Telophase F mitotic spindle breaks down F chromosomes decondense F nuclear membranes reform around two nuclei F nucleoli reappear

3. Cytokinesis Distribution of cytoplasm to daughter cells. F begins during anaphase or telophase F differs in animal & plant cells

Cytokinesis in animal cells: ] cleavage furrow (slight indentation) forms around equator of cell. ] actin & myosin microfilaments act like a drawstring to pinch the cell in two. ] usually an equal division.

Cytokinesis in animal cells

Cytokinesis in plant cells: F phragmoplast (microtubule structure) forms in cytoplasm & traps vesicles containing cell wall material. F vesicles fuse, forming a cell plate across midline of cell. F 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  Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence.

 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.

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

B. Apoptosis Programmed cell death; part of normal development.

Steps of Apoptosis:

C. Cancer (loss of cell cycle control) Condition resulting from excess cell division or deficient apoptosis. Characteristics of Cancer Cells: ] can divide uncontrollably & eternally ] are heritable & transplantable ] lack contact inhibition ] readily metastasize ] exhibit angiogenesis ] exhibit genetic mutability

Causes of Cancer: ] Over-expression of oncogenes Oncogenes are genes that trigger limited cell division. ] Inactivation of tumor suppressor genes Tumor suppressor genes prevent a cell from dividing or promote apoptosis.

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