DNA replication - L5 Flashcards
What was Meselson and Stahl’s experiment on semi-conservative replication of DNA?
What were their results?
- They grew E.coli in many generations in 15N
- Then transferred to 14N medium
- Continued growth in 14N medium
- Then isolated DNA from cells and mixed with CsCl solution and placed in a centrifuge.
- Light DNA, which is less dense at the top and heavier DNA, that is more dense at the bottom of the test tube.
After one generation – They saw a band further to the left than the band they originally started with, so discounts conservative model - where one set of strands is old and one is new.
After two generations – They saw two bands (cant be dispersive as would show one band if it was - where chunks of old and new DNA)
After several generations, matched results for semi conservative model – where original strand has its hydrogen bonds broken apart, and then at the replication fork, new strand is added and new hydrogen bonds are made with new DNA added.
How does lead and lagging strand synthesis occur?
Leading strand – primer is added, then DNA polymerase can act from 5’-3’ ends towards the replication fork. Can proceed continuously.
Lagging strand – As the template is antiparallel goes from 5’-3’ end away from the replication fork, where it will stop once it finds a primer. Then it starts again as it discontinuous so must be primed numerous times.
Okazaki fragments = the pieces of DNA that are stuck together to make up the lagging strand of replication.
In DNA replication what is the role of the enzyme…
helicase?
Topoisomerase?
exonuclease?
DNA ligase?
Telomerase?
Helicase – breaks hydrogen bonds between the two DNA strands, separating them. Unzipps DNA helix.
Topoisomerase - relieves pressure from over winding around the replication bubble by making and resealing breaks in the DNA
Exonuclease - an enzyme which removes successive nucleotides from the end of a polynucleotide molecule.
DNA ligase - join stretches of new DNA together.
Telomerase - maintains telomere length
