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.

• 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.

• 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.

• 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.

• 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.