Notes For Biology Chapter 9

“The Cell Cycle”__________________Biology

9th Chapter

A. The Cell Cycle Events that occur in the life of a cell Includes 3 major stages • Interphase • Karyokinesis • Cytokinesis

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1. Interphase G1 Phase – carries out basic functions duration variable cell “decides” to o divide o enter a quiescent phase o die G0 Phase – the cell maintains specialized characteristics, but doesn’t divide S Phase – cells replicate chromosomes and synthesize proteins G2 Phase – cell synthesizes additional properties and assembles / stores membrane material 2. Karyokinesis (mitosis) Equal distribution of replicated chromosomes Each replicated chromosome now consists of two identical copies (sister Chromatids) * Centromere – attachement point of chromatids

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Prophase

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- replicated chromosomes condense - mitotic spindle forms - nucleolis disappears Prometophase o Nuclear membrane breaks down o Spindle fibers attach to centromeres of chromosomes Metaphase o Chromosomes are lined up single file along equator of mitotic spindle

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Anaphase o Centromeres part, sister chromotids (now called chromosomes) separate o Chromosomes move toward opposite poles Telophase o Mitotic spindle breaks down

o Chromosomes decondense o Nuclear membranes reform around two nuclei o Nucleoli reappear

3. Cytokinesis Distribution of cytoplasm to daughter cells •

begins during anaphase or telophase depending on cell type

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

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Cytokinesis in plant cells vesicles fuse, forming a cell plate across midline of cell cell plate gives rise to two primary cell walls

Does Cytokinesis always accompany karykinesis? No. Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleal 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

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

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* Apoptosis (apo = from and ptosis = falling) Programmed Cell Death - absorption of tadpole tails during metomorphosis Cell Death is highly regulated

Cells are phagocytized which prevents potentially harmful debris

Necrosis *Necrosis (Nexpoc = Dead) Cell Death Stimulated by: Injury, Infection, Cancer, etc. Cell Ruptures and releases potentially harmful substances into the intracellular matrix Results in inflammation Debris cleaned by phagocytes

Steps of Apoptosis: • • • • •

The cell loses shape DNA starts degrading Nuclear envelope becomes discontinuous and the DNA fragments Plasma membrane blebs The cell is phagocytosed (eaten by lysosomes or white blood cells)

Causes of Apoptosis: -

Infection Damage Development