55 WJEC Biology AS Level - Study And Revision Guide - Neil Roberts (1.5 Nucleic Acids And Their Functions) Flashcards
What does the process of semi-conservative DNA replication require?
The process requires
- ATP
- Free nucleotides
- Enzymes.
What are the steps in semi-conservative DNA replication?
DNA helicase breaks the hydrogen bonds between the bases
This causes the double helix to unwind and separate into two strands.
The exposed bases bind to free floating nucleotides in the nucleoplasm.
DNA polymerase binds the complementary nucleotides (forming the phosphodiester bond).
One strand acts as the template for the new molecule.
Therefore newly synthesised DNA contains one parent strand and a complementary newly synthesised strand.
What is the Meselson-Stahl experiment?
This was an important experiment carried out to determine the exact mechanism for DNA replication.
What is the first step in the Meselson-Stahl experiment?
- Growing bacteria on a (15)N medium.
- (15)N is a heavy isotope of nitrogen so all DNA produced would be of a heavier weight than normal.
- When DNA was extracted by centrifuging in caesium chloride, the DNA band appeared low down in the tube.
What is the second step in the Meselson-Stahl experiment?
What does this rule out?
Bacteria were then grown on a (14)N medium (normal weight nitrogen)
After one generation the DNA extracted formed an intermediate band half way up the tube.
This is because the DNA molecule contained one strand from the heavy parent DNA and one newly synthesised light DNA strand.
(Because one band only was produced this rules out conservative replication.)
What is the third step in the Meselson-Stahl experiment?
What does this rule out?
The bacteria were grown for a further generation using (14)N medium.
The DNA extracted formed an intermediate band half way up the tube, and a lighter band towards the top of the tube.
Because half of the DNA was intermediate weight and half light, this rules out dispersive replication.
What is the final conclusion in the Meselson-Stahl experiment?
DNA replicates semi-conservatively.
What would happen if bacteria were grown for further generations using 14(N) medium?
If grown for further generations using (14)N medium, whilst intermediate weight DNA would remain, the proportion of light DNA produced would increase.
Draw a diagram showing the Meselson-Stahl experiment results.
Does DNA polymerase form hydrogen bonds?
DNA polymerase does NOT form hydrogen bonds.
What bonds does DNA polymerase form?
It binds the complementary nucleotides (forming the phosphodiester bond).
How can DNA be separated according to size?
DNA can be separated according to size by centrifuging samples at very high speeds (over 100,000 rpm) in a dense solution of caesium chloride.
DNA will settle out where the density of the DNA equals that of the caesium chloride, and a visible band will form.
