DNA - Meselson and Stahl Flashcards
What does it mean if DNA is semi-conservative?
That each strand acts as a template…
…and that a new double helix (daughter DNA) is made from 1 old strand and 1 new strand of DNA
Describe the Meselson and Stahl experiment
https://www.youtube.com/watch?v=O-P_NJwa-h4
Bacteria (with 14N) are grown in a broth containing the heavy (15N) isotope
A sample of DNA from the 15N culture of bacteria was extracted and spun in a centrifuge
The bacteria containing only 15N DNA was then taken out of the 15N broth and added to a broth containing only the lighter 14N nitrogen. The bacteria were left for enough time for one round of DNA replication to occur before their DNA was extracted and spun in a centrifuge
The DNA molecules would now contain both the heavy 15N and light 14N nitrogen and would therefore settle in the middle of the tube (one strand of each DNA molecule would be from the original DNA containing the heavier nitrogen and the other (new) strand would be made using only the lighter nitrogen)
Process of centrifuging DNA [3 marks] MS
1) A centrifuge separates substances with different densities.
2) DNA replication is SEMI CONSERVATIVE
3) This means that each strand acts as a template.
4) And that the new double helix (daughter DNA) is made from one old strand and one new strand of DNA.
What were the two isotopes in the experiment?
N14 and N15
Which is denser, N15 or N14?
N15 goes to bottom, bc it is heavier/denser
What are the types of DNA replication models?
Semi-conservative
Conservative
What did the conservative model suggest?
A DNA double helix is formed containing 2 new strands
(This molecule contains none of the original DNA)
Another double helix is formed with 2 original strands (from before)
= STEP 1 =
> Took a sample of bacteria, containing N-14 in the DNA.
Extracted the DNA, placed it in a solution, and spun at high speeds in a centrifuge
The DNA formed a HIGH band near the top of tube
= GENERATION 0 =
> Cultured the bacteria in a growth medium containing ONLY N-15 over a FEW generations
Extracted & centrifuged - formed a LOWER band.
= GENERATION 1 =
> Took a sample of bacteria growing in N-15, and transferred it to N-14 solution
Allowed them to replicate DNA only ONCE
Extracted & centrifuged - formed an INTERMEDIATE band
(Shows that DNA contained 1 strand with N-14 and 1 stand with N-15
Therefore DNA replicated semi-conservatively)
= GENERATION 2 =
> Allowed bacteria with N-14 & N-15 strand to replicate again on N-14 solution
Extracted & centrifuged - formed an INTERMEIATE and a HIGHER band
What did the Meselson-Stahl experiment prove?
That DNA replication is semi-conservative
What happened during Step 1 of the experiment?
1~ Took a sample of bacteria, containing N-14 in the DNA.
2~ Extracted the DNA, placed it in a solution, and spun at high speeds in a centrifuge
3~ The DNA formed a HIGH band near the top of tube
What happened during the Generation 0 stage of the experiment?
1~ Cultured the bacteria in a growth medium containing ONLY N-15 over a FEW generations
2~ Extracted & centrifuged - formed a LOWER band.
What happened during the Generation 1 stage of the experiment?
1~ Took a sample of bacteria growing in N-15, and transferred it to N-14 solution
Allowed them to replicate DNA only ONCE
2~ Extracted & centrifuged - formed an INTERMEDIATE band
What happened during the Generation 2 stage of the experiment?
1~ Allowed bacteria with N-14 & N-15 strand to replicate again on N-14 solution
2~ Extracted & centrifuged - formed an INTERMEIATE and a HIGHER band
What does an intermediate band forming in the tube suggest?
That the DNA contained 1 strand with N-14 and 1 stand with N-15
Therefore DNA replicated semi-conservatively
Suggest what the results would show if DNA replicated conservatively [4 marks]
= CONSERVATIVE MODEL =
1 molecule of DNA would contain 2 original strands
1 molecule of DNA would contain 2 new strands
= Experiment =
1)
If 15N bacteria (after gen 0) cultured in 14N solution…
would produce 1 molecule of DNA containing ONLY 15N, and 1 molecule with ONLY 14N
2)
…if replicated again in 14N…
would produce 1 molecule of ONLY 15N, and 3 molecules of ONLY 14N
…therefore it would produce a THICK higher band (for 14N) and a LOWER band (dense 15N)
