DNA replication Flashcards
Step 1 of DNA replication
Enzyme DNA helicase catalysed the separation of a polynucleotide strands in a molecule by breaking hydrogen bonds between complementary base pairs
Step 2 of DNA replication
Free activated nucleotides are attracted to its complementary pair on template strands and forms hydrogen bonds to it
Step 3 of DNA replication
Enzyme DNA polymerase catalyses the formation of phosphodiester bonds in activated nucleotides in a condensation reaction which releases water
Step 4 of DNA replication
Now have 2 identical strands of DNA consisting of the original template strand and a newly synthesises strand
Which direction does DNA polymerase catalyse formation of phosphodiester bonds?
only at the carbon 3
Why is it called semi conservative replication?
Because 1 molecule of DNA is replicated into 2 identical molecules but each molecule has 1 original strand and 1 newly synthesised strand
What would hypothetical conservative replication look like?
1 DNA molecule never separates into strands but replicates another molecule which contains both newly synthesises strands
What would hypothetical dispersion replication look like?
Sections of nucleotides are copied so each new strand has newly synthesised sections
How did scientists work out which replication method was correct
Using Meslson and Stahls method
What is the Meslson and Stahl study?
Showing which method of DNA replication is correct
Using the different isotopes of nitrogen to differentiate the different strands of DNA
Background of Meselson and Stahl
2 isotopes of N: N-14 (light) and N-15 (heavy)
In nitrogenous bases, the N14 is present in all DNA of a bacteria
Step 1 of Meselson and Stahl
Extract DNA from bacteria with N-14 in bases
Step 2 or Meslson and Stahl
Centrifuge N-14 DNA sample at very high speeds:
It will produce a band at the top of the test tube because only light dna is present and moves to the top
Step 3 of Meselson and Stahl
Bacteria DNA is grown for several generations in N-15 so all nitrogenous bases contain heavy nitrogen
Step 4 of Meselson and Stahl
Centrifuge new N-15 grown sample which produces band at bottom because heavy N moves to bottom and only that’s present
100% of DNA is at heavy band
Step 5 of Meselson and stahl
Grown heavy nitrogen sample in N-14 growth medium and allow to only go through one generation of DNA replication
So, half of DNA is heavy template N-15 and half is synthesised N-14
Step 6 of Meselson and Stahl
Centrifuge half and half sample in a centrifuge
Produces band in MIDDLE because DNA molecules have 1 light strand and 1 heavy. So falls midway
100% of DNA is at the middle density
What rules out conservative replication in Meselson and Stahl?
When 1 molecule with half N-14 and N-15 is centrifuged, the band is produced in middle Because it’s middle heavy so falls to middle
However, in cons replication the molecules wouldn’t have mixture of light and heavy so would be at top and bottom
density would be spread out: 50% at top 50% at bottom
Step 7 of Meselson and Stahl
Allow half and half DNA molecule to divide once more in light growth medium
Step 8 of Meselson and Stahl
Centrifuge to produce a band at intermediate because half and half molecule present but also band at N14 because a fully light nitrogen base detected
How is dispersion method disproved?
After undergoing 2 replications, the centrifuge produces band at intermediate because all molecules contain both isotopes but band at N14 shows how semi conservative occurred
Description of half and half molecule dividing
The heavy chain from half and half molecule used as template to synthesise light chain = 2 half and half
Then light chain from half and half used to synthesises another light chain = fully N14
Why in the formation of phosphodiester bonds between monomers in DNA replication are the nucleotides only attached to carbon 3 of sugar?
Because DNA polymerase which catalyses the formation of phosphodiester bonds is an enzyme with a tertiary structure of active site specific to the carbon 3 end (complementary)
How does DNA having hydrogen bonds between complementary pairs mean it is adapt for replication
Because the H bonds can be broken easily so can be separated easily
