2.7 – DNA replication,
transcription, and translation
DNA Replication
DNA Replication animation
– Focus only on the role of helicase and DNA
polymerase…the other enzymes are HL
knowledge.
– Make an annotated version of this on paper!
How do we know that this is how DNA
replicates itself?
The Meselson and Stahl experiment
In order for cell division to occur, DNA must be
duplicated to ensure that progeny cells have the
same genetic information as parent cells.
The process of duplicating DNA is termed
replication. The Meselson-Stahl experiment
sought to understand the mechanism of
replication.
The Meselson and Stahl experiment
Did replication occur in a conservative fashion, a
semi-conservative fashion, or in a dispersive
fashion?
The Meselson and Stahl experiment
They grew E. coli in a medium containing “heavy”
nitrogen, 15N, for a number of generation.
They then transferred the bacteria to a 14N medium.
Samples of the bacteria were taken over a period of
time and separated by density gradient
centrifugation, a method in which heavier
molecules settle further down in a centrifuge tube
than lighter ones.
The Meselson and Stahl experiment
A single band of DNA at the start (0 generations)
had a density of 1.742 g/cm3. The main band of
DNA after four generations had a density of
1.710 g/cm3.
Explain how DNA with a lower density had been
produced by the bacteria.
The Meselson and Stahl experiment
Estimate the density of DNA after one
generation.
The Meselson and Stahl experiment
Explain whether the density of DNA after one
generation falsifies any of the three possible
mechanisms for DNA replication.
The Meselson and Stahl experiment
Describe the results after two generations,
including the density of DNA.
The Meselson and Stahl experiment
Explain whether the results after two
generations falsify any of the three possible
mechanisms for DNA replication.
The Meselson and Stahl experiment
Explain the results after three and four
generations.