The Cell Cycle


Nuclear Division is the process by which the nucleus divides. Two types: Mitosis and Meiosis. Cell Division follows nuclear division and is the process by which the whole cell divides. Before a nucleus divides it must be replicated (copied). Ensures all daughter cells have genetic information to produce the enzymes and other proteins they require.Very precise. As nearly all cells are more or less identical to the original.For semi-conservative replication to take place there are four requirements:The four types of nucleotide, each with their bases of A, T, C, G must be present.Both strands of DNA must act as a template for the attachment of these nucleotides. The enzyme DNA Polymerase is needed to catalyse the reaction.A source of chemical energy is required. The Process of Semi-Conservative replication: The enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of DNA. As a result the double helix separates into two strands and unwinds. Each exposed polynucleotide strand then acts as a template to which complementary nucleotides attracted. Energy is needed to activate these nucleotides. The activated nucleotides are joined together by the enzyme DNA polymerase to form the missing polynucleotide on each of the two original polynucleotide strands of DNA. Each of the new DNA molecules contains one of the original strands and one new strand. This is called semi0conservative replication. Evidence for Semi-conservative Replication: All the bases in DNA contain Nitrogen. Nitrogen has two forms: lighter N14 and the heavier isotope N15. Bacteria will incorporate nitrogen from their growing medium into any new DNA that they make. To separate the different DNA types, they centrifuged the extracted DNA in a special solution. The lighter the DNA, the nearer the top it was and the heavier it was the nearer the bottom.They also analysed the DNA after two and three generations. In generation 0 there will be one piece of DNA, which will have two heavy strands. In generation 1 there will be two pieces of DNA, which will both be hybrid, with one heavy and one


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