DNA Replication Flashcards
Why is the process of DNA synthesis described as semi-conservative?
Each daughter duplex has one parental strand and one new strand (2 strands separate and each acts as a template)
Who did an experiment to prove DNA synthesis is semi-conservative?
Meselson Stahl
-used radioactive nitrogen (heavy nitrgen) and normal nitrogen (light nitrogen)
LOOK AT SLIDESHOW
What enzyme catalyses the polymerisation of deoxyribonucleotides into the DNA strand?
DNA polymerase
What is DNA polymerase described as and why?
Described as template dependent
-as it reads the sequence of bases on the template strands and inserts the appropriate complementary base into the new strand
Where does the DNA polymerase add nucleotides?
Can only add nucleotides to the free 3’ OH of the new strand
This extends new strand from starting point in a 5’ to 3’ direction
What is at the 5’ end, the 3’ end and what bonds nucleotides together?
-5’ free phosphate
-3’ free OH
-phosphodiester bond between nucleotides
Why must the double helix unwind and what enzyme catalyses this?
The double helix must unwind for DNA replication to proceed
This proces is catalysed by helicase enzymes that break the hydrogen bonds between the base pairs of the parent DNA molecule.
What does the action of helicase create?
A replication fork exposing 2 strands of parental DNA
-has a leading and lagging strand
What did Okazaki deduce?
-one strand being synthesised is made continuously in the 5’ to 3’ directions as the fork opens - leading strand
-the other is made discontinuely as short segments of DNA, in the 5’ to 3’ direction, that would be joined together to make the completed new strand - lagging strand
What are Okazaki fragments?
Short segments that are joined together to make the completed new strand in lagging strand
What is needed for initiation of DNA syntheis?
A primer - provides the free 3’OH onto which nucleotides are added at 3’ end
-Primers are short RNA pieces laid down by a primase enzyme
