DNA Replication I - Lecture 4

Question 1

What is semiconservative replication?

Answer 1

End up with 2 identical strands same as parent DNA, each daughter strand has half the information from the parent

Question 2

What were the other possible types of replication?

Answer 2

○ Conservative replication: one original DNA molecule and one new DNA molecule
○ Dispersive replication: old and new DNA are interspersed within each daughter molecule

Question 3

Why was the semiconservative replication thought to be impossible?

Answer 3

○ DNA is plectonemic: strands can’t be separated without unwinding the double helix structure
○ Many DNA molecules are circular

Question 4

What was the Meselson-Stahl experiment?

Answer 4

○ A culture of bacteria were grown in a medium containing heavy nitrogen (15NH4Cl)
○ Transfer to 14NH4Cl medium
○ Makes DNA with different weights
○ Extract DNA and use density gradient centrifugation

Question 5

What were the expected bands for each type of replication and the actual number of bands for the 1st cell division?

Answer 5

○ Conservative = 2 bands
○ Dispersive = 1 band
○ Semiconservative = 1 band
○ Actual band = 1 band

Question 6

What were the expected bands for each type of replication and the actual number of bands for the 2nd cell division?

Answer 6

○ Dispersive = 1 band
○ Semiconservative = 2 bands
○ Actual = 2 bands

Question 7

What does type I DNA topoisomerase do?

Answer 7

○ Cuts a DNA strand once
○ Enzyme holds on to the end and fuses the two ends together when they have been unwinded

Question 8

What does type II DNA topoisomerase do?

Answer 8

○ Major enzyme used
○ Makes a double stranded cut
○ Pass second segment of the DNA molecule through the gap
○ Rejoins the two cut strands