DNA replication Flashcards
When does DNA replication occur
DNA replication occurs before mitosis or meiosis
why is DNA replication semi-conservative
Because one strand is from parent cell/original strand and one is newly synthesised
What is the process of DNA replication (semi-conservative)
- DNA helicase breaks the hydrogen bonds between the complementary base pairs between the double helix strand causing it to unwind
- Each original DNA will now become a template so free floating activated nucleotides are attracted to their complementary base pair and align opposite the parental DNA strands meaning the hydrogen bonds are holding them together
- DNA polymerase catalyses the joining of adjacent activated nucleotides via a condensation reaction forming phosphodiester bonds by removing 2 phosphate groups
- Now each new DNA contains one strand of original and new synthesised strand
what are activated nucleotides
what does DNA polymerase do to activated nucleotides
They are free nucleotides that contain 3 phosphate groups
DNA polymerase cleaves (breaks off) the two extra phosphates and uses the energy released to create the phosphodiester bonds (between adjacent nucleotides) via a condensation reaction
Who suggested the model of semi-conservative replication and who proved it
Watson and crick suggested the model
Meselson and Stahl proved it
How was the control samples produced in Meselson and Stahl’s experiment
- E.coli bacteria was grown on known forms of nitrogen
N14 and N15 - DNA was extracted from the bacteria then was centrifuged
- Different bonding patterns seen as N15 is denser than N14
Strand for N15 is lower in the test tube once centrifuged
Look up example of the test tubes if need be to visualise
Why was Nitrogen used and why do we need control tests
Nitrogen - to produce DNA as nitrogen is needed in nitrogenous bases
Control tests - To compare with
What was Meselson and Stahl’s investigation
- A culture of heavy nitrogen (N15) was placed in a growth medium with light nitrogen (N14)
- When replicating the heavy nitrogen will contain heavy nucleotides but to make new strands only light nucleotides will be available
- Once centrifuged we will be able to see the first generation will contain both heavy and light nucleotides as the strand is shown between the light and heavy control strands
- However when replicated again in the 2nd generation the light strand is shown also due to the light strands also having to become a template to replicate
What is the action of DNA polymerase
DNA polymerases active site is only complementary to the 3’ end of the newly forming DNA strand so it can only add nucleotides in the 3’-5’ direction
this means the new strand is made in the 5’- 3’ direction and DNA polymerase moves down the template strand at the 3’ - 5’ direction
