Chapter 10

 What are some reasons/examples where you can think of smaller being better?

 As living organisms get older, they naturally tend to grow in size  The growth in size is due to the accumulation of more cells, not the growth of each individual cell  On average, the cells of adult organisms are the same size as young organisms

 There are 2 main reasons why cells divide rather than grow indefinitely 1. Larger cells place more demands on cells DNA 2. Larger cells have trouble exchanging nutrients and waste across the membrane

DNA Overload  The DNA controls all of the cells functions  ie – what the cell does and what type of cell it is  Information stored in DNA can meet the needs of small cells, but wouldn’t be enough to meet those of a larger cell  No extra DNA copies are produced after a cell is made (library in a small town example)  If there are any DNA “issues” there would be more trouble for the organism

Exchanging & Using Materials  The rate of exchange depends on a cells Surface Area  The rate of usage depends on a cells Volume  As a cell grows in size, its volume increases much faster than its surface area  As a result, the cell would be using materials at a faster rate than what it can get rid of waste and take in new nutrients  (small town and 2 lane road example)

 Before a cell has the chance to become too large, it will divide into 2 “daughter” cells  Cell Division – the process by which a cell divides into 2 daughter cells  Cell division solves any problems associated with cell growth  Each daughter cell gets a copy of the DNA  Each daughter cell will have an increase in surface area to volume

 A cell can not just simply divide without any type of preparation  The process of cell division includes 1. Preparation interphase 2. Nucleus division 3. Cytoplasm division M Phase

 The genetic information that is passed from one generation to the next is carried by chromosomes  Chromosomes are made of DNA (includes genetic info and proteins)  Chromosomes are mostly only visible during cell division  Prior to cell division each chromosome is copied into 2 sister chromatids  The chromatid pair is attached at a centromere located in the center  During cell replication, one chromatid is sent to each daughter cell

 The cell cycle is the series of events cells go through as they grow and divide  Cell cycle consists of 4 phases 1. G-1 phase 2. S phase 3. G-2 phase 4. M Phase

Interphase  the “in-between” period in the cell cycle when the cell is preparing for division  Is a period of intense cell growth and activity  Consists of 3 phases 1) G-1 Phase  Phase where the cell does most of its growing  Proteins and organelles can also be produced

2) S Phase  Chromosomes are replicated and DNA is synthesized  Remember ~ S for SYNTHESIS 3) G-2 Phase  Last and shortest phase in interphase  Many of the organelles and molecules required for cell division are produced  When G-2 is complete the cell is ready to enter M-phase

M Phase  The process of cell division  Comprised of 2 steps 1. Mitosis – division of the cell nucleus 2. Cytokinesis – division of the cell cytoplasm  Remember ~ M for MITOSIS!

 There are 4 main phases of mitosis 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase  Depending on the organism, each phase can last anywhere from a few minutes to several days!

 The 1 st and longest phase of mitosis  Takes 50% - 60% of mitosis’ total time  The chromatin condenses into visible chromosomes  The nuclear envelope breaks down The cell prior to Prophase (interphase)

 The centrioles move to opposite sides of the nucleus  Tiny structures in the cytoplasm which produce spindle fibers  Spindle fibers form from the centrioles  Fanlike microtubule structure that acts to separate the chromosomes The end of prophase moving into metaphase

 The 2 nd phase of mitosis  Often lasts only a few minutes  The chromosomes line up across the middle of the cell  The spindle fibers attach to the centromere  Remember ~ M for MIDDLE

 The 3 rd phase of mitosis  The spindle fibers pull at the centromere, separating the sister chromatids  Each sister chromatid moves away/apart toward separate poles of the cell  Anaphase is complete when the individual chromosomes (chromatids) stop moving  Remember ~ A for AWAY

 The 4 th and final phase of mitosis  The condensed chromosomes begin to reform back to chromatin  The nuclear envelope begins to reform around the chromosomes

 At the end of telophase, mitosis is complete and we are left with 2 new nuclei  Cytokinesis is the division of the cell’s cytoplasm  Is the completion of the M phase  Cytokinesis usually takes place at the same time as telophase  As a result we have 2 new “daughter” cells which are identical to the “parent” cell

 Cell growth and cell division must be carefully controlled  Different cells of the body divide at different rates  Nervous and muscles cells rarely divide once formed  Cells of skin, digestive tract and marrow divide often  Newly divided cells make it possible to replace old cells that are worn out or break down

 Within the laboratory we can look at cell growth  Cells tend to stop growing when they come into contact with other cells  When cells are removed, growth will start again

 In our bodies we can see the same event  When there is an injury such as a cut, cells at the edge are stimulated to divide  New cells are produced and healing begins  By the end of the healing process, cells stop diving

 Scientists wanted to know what tells cells to divide  Performed experiments on dividing cells and noticed level of protein cyclin rose and fell with progression of cell cycle  Cyclin – regulates the cell cycle Caused spindle formation when placed in non- dividing cells  Cyclins – family of proteins that regulate the timing of the cell cycle in eukaryotic cells

 Internal Regulators – proteins that respond to events inside the cell  Allow the cell cycle to proceed only when certain processes have happened inside the cell  Ex) makes sure the cell doesn’t enter mitosis until all chromosomes have been replicated

 External Regulators – proteins that respond to events outside the cell  Direct the cell to speed up or slow down the cell cycle  Ex) Growth Factors – tell the cell when to grow and/or divide Especially important in embryonic development and wound healing  Ex) Molecules on Neighboring Cells – cause cells to slow down growth or stop their cycle Prevents excessive cell growth or tissues interrupting others

 The consequences of uncontrolled cell growth can be severe on an organism  Cancer – disorder where cells do not respond to the bodies signal to regulate cell growth  As a result, cells divide uncontrollably and form masses called tumors  Cancer cells can break loose from tumors and spread throughout the body

 Control has broken down for some reason…  Cells no longer respond to internal or external regulators  Exposure  Tobacco  Radiation  Viral infections  Genetic Defect  P53 gene – supposed to halt cell cycle no longer functions