What is semi-conservative replication?
- DNA copies itself before cell division so that the new cell has the full amount of DNA
- half of the strands in each new DNA molecule are from the DNA molecule
- there’s genetic continuity between generations of cells
Semi-conservative replication (stage 1)
- the enzyme DNA helicase breaks the hydrogen bonds between bases on the two polynucleotide DNA strands
- the helicase unwinds to form two single strands
Semi-conservative replication (stage 2)
- each original strand acts as a template for a new strand
- complementary base pairings means free floating DNA nucleotides are attracted to their exposed complementary base pairing
Semi-conservative replication (stage 3)
- condensation reactions join the nucleotides of the new strands together
- the reaction is catalysed by the enzyme DNA polymerase
- hydrogen bonds form between the bases on the new and original strands
What is the structure of a DNA strand?
- each end of the DNA strand is different to its structure
- one end is called the 3’ and the other is called the 5’
- in a DNA helix the strands run in opposite directions (anti-parallel)
Who are Meselson and Stahl?
- they created an experiment using two isotopes of nitrogen (heavy and light) that showed DNA replication was semi-conservative not conservative
- their experiment lead other scientists carry out experiments showing semiconservative replication is the universal method for DNA replication in all living things
semi-conservative replication experiment (stage 1)
- two samples of bacteria were grown
- one in a nutrient broth containing light nitrogen and one in heavy nitrogen
- as the bacteria reproduces it takes in the nitrogen help make new nucleotides for its DNA
- the nitrogen gradually becomes part of the bacterias DNA
Semi-conservative replication experiment (stage 2)
- a sample of DNA was taken from each batch of bacteria and spun in a centrifuge
- the DNA from the heavy nitrogen settled lower down the centrifuge tube than the light nitrogen because its heavier
Semi-conservative replication experiment (stage 3)
- the bacteria containing heavy nitrogen is removed and put into a broth of light nitrogen
- the bacteria was left for one round of DNA replication
- a sample of DNA was taken and spun in a centrifuge
Semi-conservative replication experiment(stage 4)
-in conservative replication the heavy DNA would still be together and settle at the bottom with the new light nitrogen at the top
Semi-conservative replication experiment (stage 5)
- the experiment showed that the DNA settled in the middle meaning the DNA molecule contained a mixture of heavy and light nitrogen
- this means the bacterial DNA had replicated semi-conservatively in the light nitrogen
Semi-conservative replication (stage 4)
Each new DNA molecule contains one new strand and one original strand
who first came up with the theory of semi-conservative DNA replication?
Watson and crick
what is an isotope?
two different forms of the same element
what does 3’ mean?
three prime
How does DNA polymerase work?
- the active site of DNA polymerase is only complementary to 3’ end of the DNA strand so this is where nucleotides are added
- a new strand is made in a 5’ to 3’ direction
- DNA polymerase moves down the template strand in a 3’ to 5’ direction since the strands are anti-parallel
- DNA polymerase working on two DNA template strands moves in the opposite directions
