Topics 18-19: DNA Replication

Question 1

Explain the differences among semi-conservative, conservative, and dispersive models of DNA replication.

Answer 1

DNA Replication

Semi-conservative: Hypothesis where both the parent and new DNA interwind with one another.

Conservative: Hypothesis where the parent DNA served as a templet to from a new and complete DNA strand exact copy of the parent DNA

Dispersive: Hypothesis where the parent and new strand of DNA is in random places of the strand.

Remember: DNA Replication is actually semi-conservative.

Question 2

Describe the Meselson-Stahl experiment and predict the results after one and two replications given different possible models of DNA replication.

Answer 2

The Messelson-Stahl experiment involved E.coli bacteria to determine if DNA is semi-conservative, conservative, to dispersive.
In their experiment they grew E.coli in a heavy 15Nitrogen solution. The DNA of the E.coli contained only the heavy 15Nitrogen in it.
Then they transferred it to a light 14Nitrogen solution where new DNA replication began to form.
Then they centrifuged it into an CsCl solution. During DNA replication the DNA weighed the same in their 2nd generation. This meant that it would not be conservative.
They balanced each other perfectly which clearly meant that is was semi-conservative.

Question 3

What is the direction of DNA replication? Explain why there is a leading and lagging strand.

Answer 3

The direction of DNA replication is from 5’ to 3’. The 3’ prime end is the growing end where the ribose sugar is located and phosphate can bond.

Question 4

Draw out a replication bubble with two replication forks. For each replication fork, which should be the leading strand and which should be the lagging? Don’t forget the direction for both the new and old DNA.

Answer 4

The Leading strand is continuous. Growing from the 5’prime end to the 3’prime end.
The lagging strand is discontinuous, growing from the 5’prime end to the 3’prime but on opposite directions of leading strand.
Parent/Old DNA acts as the template for the new DNA.
Parent DNA
5’ to 3’
3’ t 5’

Question 5

Describe all the enzymes and other proteins involved in DNA replication.

Answer 5

Enzymes involved:
Helicase
Topoisomerase
DNA polymerase III
DNA polymerase I
Primase
DNA Ligase

Proteins:
Sliding Clamp
Single stranded binding proteins

Question 6

Why is it impossible to replicate the ends of eukaryotic chromosomes? How is this problem fixed in eukaryotes?

Answer 6

Its impossible to replicate the ends of eukaryotic chromosomes because the ends are RNA primers and when removed it can’t replicate.

This problem is fixed thanks to enzymes Telomerase. Telomerase attach to these ends and use the RNA attached into it to replicate DNA. making the same copy over and over again called telomeres.

Enzyme telomerase are an example of reverse transcriptase which is when RNA is used as a template instead of DNA.