Chapter 10

1. Too many demands on DNA  DNA is the instruction set for proteins  The larger the cell is, the more proteins need to be made to operate the cell  Eventually the DNA cannot send out instructions fast enough to keep up with the demands  Think of what happens when too many friends try to chat with you online at once…. 2. The cell cannot transport material fast enough across the membrane  As the cell grows, it’s nutrient needs increase as well as the waste production  Cell division prevents cell from becoming so large, they cannot function

 DNA is wound tightly around proteins  DNA + Proteins = Chromatin  Long and stringy, like spaghetti  When chromatin is condensed and coiled, it is called chromatid  Chromatin and chromatid are words that describe the appearance of DNA in the nucleus

 Long before the cell divides, the chromatin copies itself  These two strings of chromatin are attached at a single location called a centromere and the whole structure is called a Chromosome  When the chromatin condenses and becomes tightly wound chromatids, each Chromosome is made of a pair of sister chromatids  These make the chromosome look like an “x”  There are 46 chromosomes in every single cell of the Human body!!!!!

 The entire process of cell division is referred to as the Cell Cycle  When a cell is made, it grows larger and then divides into two daughter cells  These daughter cells can then grow larger and divide again into two more daughter cells each  So on and so forth  Every cell division, doubles the cell population  The Cell Cycle is divided into 4 Phases OR 6 Stages

 G1 Phase: the cell grows larger  G for Growth  S Phases: the cell duplicates its DNA during DNA Replication  G2 Phase: the cell prepares for Mitosis  M Phase: the cell goes through two events 1. Mitosis - NUCLEUS DIVISION (4 steps involved) 2. Cytokinesis – CYTOPLASM DIVISION

 This is the old-school way of thinking 1. Interphase – combines G1, S and G2 phases into one stage  Scientists originally though nothing happened here and therefore lumped the 3 phases together 2. Prophase 3. Metaphase 4. Anaphase 5. Telophase 6. Cytokinesis MITOSIS Cytoplasm Division

 Longest Stage  Includes:  G1 – Growth  S – DNA Replication  G2 – Prep. For Mitosis  During S Phase:  DNA is in chromatin form  Each of the 46 chromatin strings of DNA is replicated  The original string and the copied string are attached at the centromere  Centrioles are also duplicated

 This is the beginning of  mitosis  Longest stage in mitosis  Lots of things happen 1. Nuclear membrane (envelope) breaks apart 2. Chromosomes condense into a visible “X” shape  each X = a pair of sister chromatids 3. Chromosomes migrate towards middle of cell 4. Spindle fibers grow from centrioles 5. Centrioles move to opposites sides of cell

 “m” in Metaphase means middle-phase 1. Chromosomes line up in the middle of the cell 2. Spindle fibers attach to each chromatid in a chromosome from opposite sides of the cell 3. Astral Rays form from centrioles  These anchor the centrioles in place

 3 rd step of Mitosis, ana = 3 letters  The division of the nucleus officially begins 1. Spindle fibers retract towards centrioles, ripping the chromatids apart in each chromosome chromatids move to each side of the cell  92 chromatids in total are moving. Remember however, there are 46 identical pairs of chromatids. This allows each side of the cell to get ONE complete set of 46 chromatids

 Nucleus division finishes: 1. Chromatids gather at opposite ends of the cell and decondense (Become chromatin again) 2. Two new nuclear envelopes (membranes) form around each set of chromatin 3. Cleavage occurs  The membrane pinches inward at the middle of the cell

 After mitosis finishes, the cytoplasm pinches in half  There are now two daughter cells  Each cell has it’s own set of 46 chromosomes (in the form of single copies of chromatin)  Each new cell can now start the cell cycle all over  G1, S, G2 and M Phases

 Proteins regulate the cell cycle  This group of proteins is called cyclins  Some cyclins work internally (inside) of the cell  Some make sure the cell doesn’t divide before the DNA is replicated  Others make sure the chromosomes are attached to the spindle fibers  Some cyclins work externally (outside) of the cell  Some prevent the cells from dividing too fast which can harm the surrounding tissue  Other may speed up the cycle  During embryo development  During the healing of a wound  Uncontrolled Mitosis and Cell division is called Cancer  A tumor is a mass of cells that have been produced by uncontrolled division