The Cell Cycle Ch. 12

Cell Cycle – life of a cell from its origin in the division of a parent cell until its own division into two. Cell division allows sexually reproducing cells to develop from a single cell (zygote) Cell division functions in growth, repair, and reproduction There are two types of cell division: Mitosis – produces two genetically identical “daughter cells” Meiosis – occurs with sexually reproducing organisms and results in cells with half the number of chromosomes of the parent cell

Chromosomes Coiled and condensed strand of DNA and proteins The fiber made of DNA and proteins is called chromatin A replicated chromosome has two “sister” chromatids One is an exact copy of the other The centromere holds the two chromatids together

The kinetochore is a disc-shaped protein on the centromere that attaches the chromatid to the mitotic spindle during cell division Somatic cells – all body cells except reproductive cells Gametes – reproductive cells (sperm and egg) Human somatic cells have 46 chromosomes Human gamete cells have 23 chromosomes

Mitotic Cell Cycle Cycle gone through when a somatic cell is dividing Two things limit cell size and promote cell division Ratio of volume of cell to surface area Capacity of Nucleus Interphase Cell grows and copies its chromosomes 3 subphases G1 Phase (first gap) S Phase (DNA synthesis) G2 Phase (second gap)

More than 90% of the life of a cell is spent in interphase When a cell is in interphase and not dividing, the chromatin is not condensed, it is threadlike Centrosomes – consisting of two centrioles are in the cytoplasm of an animal cell MTOC’s (microtubules organization centers) are found in plant cells instead of centrosomes Mitosis Dividing of the nucleus Four parts: prophase, metaphase, anaphase, telophase Cytokinesis Division of the cytoplasm

Steps of Mitosis Prophase Nuclear membrane begins to disintegrate Chromosomes condense Nucleolus disappears Mitotic spindle begins to form Metaphase Chromosomes line up at the equator Centrosomes are at opposite poles of the cell

Anaphase Centromeres of each chromosome separate Spindle fibers pull apart sister chromatids Telophase Chromosomes cluster at opposite sides of cell Nuclear membrane reforms Chromosomes begin to unravel

Cytokinesis Cytoplasm divides In animal cells a cleavage furrow forms down the middle of the cell as actin and myosin microfilaments pinch in the cytoplasm Plant cells have a cell plate that forms during telophase from golgi vesicles Daughter plant cells do not separate from each other, a sticky middle lamella cements the cells together

If cells lose their dependence and inhibition, then they begin to divide uncontrollably These cells are considered to be “cancerous”