• Nuclear division is the process by which the nucleus divides. There are two types of nuclear division, mitosis and meiosis
  • Cytokinesis follows the nuclear division and is the process where the rest of the cell divides
  • Before the nucleus can divide the DNA must be replicated to ensure that the resulting daughter cells have the same genetic code for to produce the correct enzymes and other proteins

Conservative Replication

  • The conservative replication model suggests that the original DNA molecule remains intact and that a separate daughter DNA copy was built from new molecules of deoxyribose, phosphate and organic bases. Of the two molecules produced, one would be mode of entirely new materials whilst the other would be entirely original material.

Semi Conservative Replication

  • The semi conservative model suggests that the original DNA molecule splits into two separate strands that new nucleotides fill in the opposite chain. The result is one strand of new polynucleotide chains and one strand of the original polynucleotide chain.
  • The semi conservative replication to occur it needs 4 requirements
  1. The 4 types of nucleotides (adenine, guanine, thymine and cytosine)
  2. Both strands of the DNA molecule act as a template for the attachment of the nucleotide
  3. The enzyme polymerase
  4. A source of chemical energy which is required to fuel the process

Process of Semi Conservative Replication

  1. The enzyme DNA Helicase breaks the hydrogen bonds linking the base pairs of DNA. The DNA molecule therefore unwinds into two separate strands
  2. Each separate strand has the exposed polynucleotide that acts like a template to which complementary free nucleotides that are abundant around in the nucleus bind by specific base pairings
  3. The nucleotides are joined through a condensation reaction by the enzyme DNA polymerase to form the new polynucleotide strand on each of the two original polynucleotide strands of DNA
  4. The result is that there are now 2 DNA molecules, with each have 1 strand of the old DNA and 1 strand of the new DNA

Proving the Semi Conservative Model

  • Meselsohn and Stahl devised an experiment to discover which model was correct
  • They based their experiment on 3 key areas
  • All the bases in DNA contain nitrogen
  • Nitrogen has 2 forms, the lighter nitrogen (14N) and the heavier isotope (15N)
  • Bacteria will incorporate nitrogen from their growing medium into any new DNA that they make
  • Bacteria was grown in 2 mediums, one in the 14N and the other in 15N
  • The grew the 15N bacteria for several generations before transferring them to 14N for one generation to replicate.
  • The mass of this molecule of DNA was measured using a centrifuge, with the heavier molecules being at the bottom and the lighter at the top.
  • The 14N was at the top
  • The 15N was at the bottom
  • The 14N and 15N was in the middle proving that half of the DNA comes from the old DNA and the other half made of new DNA; thus the semi conservative model was correct.