[Unit 2.2] | DNA Replication

Question 1

In what stage of the cell cycle does DNA replication occur?

Answer 1

S-Phase

Question 2

Describe how DNA is replicated.

Answer 2

• DNA Helicase breaks the hydrogen bonds between the base pairs & allows the 2 strands to unwind.

• The exposed polynucleotide strands act as templates for free, complementary nucleotides to bind to.

• DNA polymerase joins the free nucleotides in a condensation reaction & hydrogen bonds form between the base pairs.

• Both new DNA molecules contain one original strand and one new strand. Half the original DNA is conserved in each molecule.
• (Semi-Conservate Replication.)

Question 3

What are the 3 models that were proposed for DNA replication?

Answer 3

• Conservative: Produces one completely new molecule and one original molecule.

• Semi-Conservative: Produces two molecules where one strand in each is new and the other is original. (Correct Model)

• Dispersive: New DNA molecules consist of original & new parts dispersed throughout the molecule.

Question 4

Who confirmed Semi-Conservative replication in 1958?

Answer 4

Meselson & Stahl.

Question 5

How did Meselson and Stahl prove semi-conservative replication is correct?

Answer 5

• Grew E. Coli in Nitrogen-15 so the heavy isotope could be found in all of the DNA’s bases.

• E. Coli switched to a new medium containing the lighter Nitrogen-14. (So that new DNA contained it.)

• Left in the medium to replicate 1 or more times.

• DNA was then extracted and centrifuged in caesium chloride.

• DNA produced bands based on its density.
• G0: 100% of DNA is N-15 + N-15
G1: 100% of DNA is N-15 + N-14
G2: 50% of DNA is N-15 + N-14, 50% of DNA is N-14 + N-14
G3: 75% of DNA is N-14 + N-14, 25% of DNA is N-15+N-14