3.2 From One to Many: the Cell Cycle (SNAB Revision)

snab biology revision pages 114 -124 

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Intense and organized activity (preperation for division); the cell synthesizes new material (DNA and cell organelles). 
3 sections to interphase, G1, S, and G2.. the length of the G1 phase determines how often the cell will divide.
Individual chromosomes are unravelled exposing the genetic material and new proteins are synthesized and cell contains enough contents for 2 new cells.. 


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Chromosomes condense (shorter and thicker); chromatids of each chromosome visible. Two strands = identical copies (DNA replication). Two chromosomes joined @ centromere. 


Spindle formed --> centrioles organize spindle fibres at opposite sides of cell. 
Nuclear envelope breaks down = end of prophase.  

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Nuclear envelope breaks down = beginning of metaphase. 
Centromeres attach to spindle fibres @ the equator.


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The centromeres split and the spindle fibres shorten. The two halves of each centromere are pulled in opposite directions and one chromatid of each chromosome is pulled to each of the poles. Chromatids reach poles and spindle breaks down = end of anaphase. (http://micro.magnet.fsu.edu/micro/gallery/mitosis/earlyanaphase.jpg)

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Reverse of prophase (effectively) 
Chromosomes unravel, nuclear envelope reforms ---> two sets of genetic information enclosed in two seperate nuclei.


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Mitosis is Important because..

Growth and Repair 

  • mitosis ensures genetic consistency which is important for growth and repair 
  • mitosis ensures that all the cells in the body have the same genetic information 
  • allows old and damaged cells to be replaced with new copies 

Asexual Reproduction 

  • alot of organisms reproduce by growing copies of themselves through mitosis (offspring genetically identical). 
  • bacteria under go binary fission (cell grows and then divides into two new cells)
  • Hydra and the single celled fungus reproduce by budding (outgrowth detaches to form new individual) 
  • Some organisms (e.g. mosses, liver worts and all plants) will reproduce asexually at one stage in their life cycle and sexually at another. 
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Stem Cells and How they Work

There are three different stages of stem cells, before they specialize they are totipotent, then they become pluripotent, then multipotent. Only embryonic stem cells are totipotent, and adult stem cells are multipotent.

totipotent: all powerful; can form any type of cell - you get these from embryos 

pluripotent: can form almost any type of cell, but not all cells (not cells of the amniotic sac) 

multipotent: they can form the type of cells that are a part of that tissue; e.g. they've specialized to being blood stem cells but not what type of stem cells within the blood. 

 *Most plant cells are all totipotent - (with the exception of bark) 

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Ethics of Stem Cell Research

Argument One: Are Embryos People?
o the biggest controversial issue with discarding an embryo is that it is potentially destroying a human life
o Catholic Church sees the use of stem cell research, as abortion
o when is it safe to say a life is a life? and who decides this?

Argument Two: People With Life Threatening Diseases That Can be Cured by Stem Cell Research (e.g. Parkinson's Disease) 
o stem cell research offers the potential to cure incurable diseases
o if it helps patients it should be justified to use stem cell research
o its healthy people, that dont understand the patients point of view, making the choice on what they can benefit from; this isn't right
o ending a human beings life is always wrong

Argument Three: IVF leftovers 
o loads of discarded embryos are kept in freezers, never to be used or looked at again
o if these embryos will otherwise be discarded, cant they be used as stem cells to cure other patients?

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