Cell Division

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  • Created by: Emily
  • Created on: 01-05-13 19:08

Preperation for cell division.


  • Before cells divide they need an exact copy of their DNA by using a supply of organic bases, pentose sugar molecules and phosphates.
  • The method by which DNA is copied is Semi-Conservative Replication.
  • The DNA unwinds by the influence of the enzyme DNA Helicase.
  • Hydrogen Bonds between the two chains break and the two DNA strands seperate.
  • Each complementary strand the acts as a template to build its opposite strand from free nucleotides.
  • The enzyme DNA Polymerase joins the nucleotides toegther.
  • This results in the production of two identical copies of double stranded DNA.
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Evidence for semi-conservative replication.


  • Meselson and Stahl experiment.
  • Bacteria were cultured with a heavy isotope of nitrogen located in the organic bases of their DNA.
  • The bacteria were then supplied with bases containing the normal light nitrogen.
  • They replicated their DNA using these bases.
  • Their population increased.
  • Each molecule of DNA of the next generation had one strand containing heavy nitrogen and one strand containing light nitrogen.
  • The mass of DNA was therefore midway between the original heavy and the normal light DNA.
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Types of cell division.


  • Cells divide for growth, repair and reproduction.
  • When the chromosomes are in pairs the cell is said to be Diploid.
  • The pairs of chromosomes are Homologous pairs.
  • Mitosis produces genetically identical copies of cells with the same number of chromosomes for growth and repair.
  • Meiosis produces cells that have half the number of chromosomes, one from each pair.
  • The cells are Haploid and are used as gametes.
  • Meiosis introduces variation because the pairs can be split up in many different ways; Independent Assortment.
  • Genetic material can also be exchanged between homologous chromosomes by crossing over.
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The cell cycle.


  • The length of time between a cell being formed and it dividing is called the cell cycle.
  • G1 phase- the cell grows making new proteins and more organelles.
  • S phase- the DNA of the chromosome is replicated by semi-conservative replication.
  • G2 phase- more organelles are made and a spindle forms.
  • M phase- this is mitosis involving the seperating of the genetic material into 2 nuclei.
  • C phase- cytokinesis, where the cell divides in 2.
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  • Interphase is the period of time between cell divisions.
  • It is made up of G1, S and G2 phases.

1. Prophase: Each chromosome forms 2 chromatids joined by a centromere. 2 centrioles begin to move forming a spindle.

2. Metaphase: The chromatids, still joined by a centromere move to the middle of the cell. Each of the 2 chromatids has identical DNA to the other.

3. Anaphase: The spindle fibres join to the centromeres. The spindle fibres shorten and the centromeres split. The seperated chromatids are now chromosomes.

4. Telophase: Identical chromosomes move to each pole. The nuclear membrane re-forms, the cell membrane narrows at the middle and two daughter cells are formed.

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