meselson-stahl experiment Flashcards
(8 cards)
what were they testing
were trying to determine how dna replicates
conservative replication or semi conservative
the experiment
they used e.coli bacteria which reproduce quickly and are easy to grow
they grew bacteria in a medium containing the heavy isotope of nitrogen N-15. this caused all the bacterial DNA to become labelled with heavy nitrogen, making it denser. they then transferred the bacteria to a medium containing the lighter isotope, N-14, and allowed the bacteria to replicate once.
after each round of replication, they extracted the DNA and used density gradient centrifugation to separate the DNA based on its density. this method causes heavier DNA to form bands lower down in the centrifuge tube and lighter DNA to stay higher up.
after one round of replication in N14, the DNA formed a single intermediate band between the heavy and light positions. this ruled out conservative model, which would have produced one heavy and one light band
after two rounds of replication, there were two bands: one at intermediate position and one at the light position. this provided strong evidence for the semi-conservative model of replication. in this model, each new DNA molecule consists of one original strand and one newly synthesised strand
this pattern of results is not consistent with the disperse model, which would produce a single band that gets gradually lighter with each generation. therefore, the experiment showed that DNA replication is semi conservative
after one round of replication in N14 medium, only one band of DNA is seen at an intermediate position. what does this tell us about the DNA strands in each molecule
each DNA molecule contains one original N15 strand and one newly synthesised N14 strand- evidence for semi conservative replication
why would the conservative model of replication produce two bands after one round in N14 medium
because one DNA molecule would remain entirely heavy N15 and the other would be entirely light N14, forming separate bands
in a variation of the experiment, bacteria were grown in N14 medium for three generations after being switched from N15. predict the relative intensity of the bands
there would be a strong light band and a weaker intermediate band
why is it important that bacteria were first grown in only N15 before being moved to N14
to ensure all original DNA strands were fully labelled with heavy nitrogen, allowing clear tracking of old vs new strands
how did the use of density gradient centrifugation allow meselson and stahl to distinguish between DNA molecules
it separated DNA based on its density, heavier DNA with N15 sank further than lighter DNA with N14, revealing different band positions
a student claims their DNA sample showed both heavy and light bands after one replication. what could have gone wrong
contamination or incomplete mixing of N15/N14
centrifugation may not have run long enough to resolve the bands properly
