1B DNA Replication Flashcards
What is the process by which DNA replicate known as?
Semi-conservative replication
What is the first stage of DNA replication?
DNA helicase breaks the hydrogen bonds between the bases on the two strands that make up a DNA double helix. This allows the helix to unwind to form two single polynucleotide strands.
Why is the semi-conservative replication of DNA important to an organism?
The semi-conservative replication of DNA is important as it ensures genetic continuity between generations of cells (i.e. it means the cells produced by cell division have exactly the same DNA as their parent cells).
The diagram on the right shows DNA replication taking place.
Name the enzyme labelled A and describe its function.
The enzyme labelled A is DNA polymerase. The function of this enzyme is to catalyse the condensation reactions that join the adjacent nucleotides of the newly forming DNA strands together.
How many template strands are shown in the diagram?
How do these template strands allow the creation of new strands?
There are two template strands in the diagram (shown in green). Free-floating nucleotides are able to pair up with the exposed bases on each template strand through complementary base pairing, before being joined together by DNA polymerase. Each template strand therefore allows the creation of one new DNA strand.
A scientist examined semi-conservative DNA replication in bacteria. He grew one sample of bacteria in a medium containing a heavy nitrogen (15N) isotope and one in a medium containing a light nitrogen (14N) isotope.
He grew a third sample of bacteria for several generations in a 15N medium, then for a short time in a The results are shown in tubes A to C on the right. 14N medium. As the bacteria grew, they incorporated the nitrogen from each medium into their DNA. DNA samples were taken from all the bacteria and spun in a centrifuge. Explain how these results provide support for the idea that DNA replicates semi-conservatively.
What do you notice about the new strands created by each template strand?
The sequence of bases of the first template strand, is the same as the the new strand created by the seond template strand.
The sequence of bases of the second template strand is the same as the new strand created by the first template strand.
What do you need to remember about the DNA strands in a DNA double helix?
Each strand in a DNA double helix has a ‘three prime end, 3’, and a five prime end, 5’.
The strands run in opposite directions - they’re antiparallel.
Why does the polymerase enzyme on one strand of a DNA double helix, work in the opposite direction to the polymerase working on the other strand?
The active site of DNA polymerase is only complementary to the 3’ end of of the newly forming DNA strand. The enzyme can only add nucleotides to this end.
This means that DNA ploymerase moves down the template strand in the 3’ to 5’ direction and that the new strand is made in the 5’ to 3’ direction.
Because the strands in a double helix are antiparallel, the DNA polymerase working on one of the template strands moves in the opposite direction to the DNA polymerase working on the other template strand.
How is onservative replication different to semi-conservative replication?
If the method was conservative, the original DNA strands would stay together and the new DNA molecules would contain two new strands.
What two types of nitrogen are used in semi-conservative replication?
heavy nitrogen (15N) and light nitrogen (14N).
What is the first step of semi-conservative relication?
1) Two samples of bacteria were grown - one in a nutrient broth containing light nitrogen, and one in a broth with heavy nitrogen. As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA.
What is the second step of semi-conservative relication?
spun in a centrifuge - the DNA from the heavy nitrogen bacteria settled lower because it’s heavier.
What is the third step of semi-conservative relication?
Then the bacteria grown in the heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was taken out and spun in the centrifuge.
What is the fourth step of semi-conservative relication?
If replication was conservative, the original heavy DNA, which would still be together, would settle at the bottom and the new light DNA would settle at the top.
Explain the results of semi conservative replication.
If replication was semi-conservative, the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen.
So the DNA would settle out between where the light nitrogen DNA settled out and where the heavy nitrogen DNA settled out.
DNA settled out in the middle, showing that the DNA molecules contained a mixture of heavy and light nitrogen. The bacterial DNA had replicated semi-conservatively in the light nitrogen.
