Evidence for Semi-Conservative Replication Flashcards
Who discovered the structure of DNA and when?
Watson and Crick in 1953.
What helped Watson and Crick discover the structure of DNA?
Rosalind Franklin’s research on x-ray diffraction.
What did Watson and Crick hypothesise?
DNA must replicate either conservatively or semi-conservatively.
What is conservative replication?
The original DNA remains intact following DNA replication and the newly synthesised and original DNA join together.
Who proved that DNA replication must be semi-conservative?
Meselson and Stahl.
What are the two isotopes of nitrogen?
14N and 15N
Which is lighter 14N or 15N?
14N
What is an isotope?
Different number of neutrons same number of electrons.
What do bacteria do with the nitrogen isotopes?
They take in the nitrogen isotopes to make new DNA nucleotides.
What will happen if bacteria is grown in a medium containing 14N?
The DNA will only contain isotope 14N and it will be lighter.
What will happen if bacteria is grown in a medium containing 15N?
The DNA will only contain isotope 15N and it will be heavier.
How can you confirm the densities of the DNA to figure out which isotope of nitrogen it has?
Spin the DNA samples in a centrifuge. 15N DNA will settle lower as it is more dense and 14N DNA will settle higher as it is less dense.
What does a generation mean in semi-conservative replication?
How many rounds of DNA replication has occurred.
What happens at generation 0?
No replication has occurred it is the starting point.
Step 1 of the Meselson and Stahl experiment?
Grew e.coli in a growth medium containing only 15N. All of the DNA contained 15N because the centrifuge had all the DNA settled at the bottom meaning it was dense meaning it was 15N.
Step 2 of the Meselson and Stahl experiment?
Took all the e.coli out of the growth medium and put it in a medium only containing 14N. Allowed bacteria to divide once. The DNA contained 1 strand of 14N and 1 strand fo 15N. The position in the centrifuge was a bit a higher.
Step 3 of the Meselson and Stahl experiment.
Still in the 14N bacterium. Replicated again. Out of the 4 new double helixes 2 were 15N and 14N however 2 were only 14N and 14N.
Step 4 of the Meselson and Stahl experiment.
Still in the 14N bacterium. 25% of the DNA was 14N and 15N. 75% was 14N and 14N.
