DNA Replication
Review
• The structure of DNA is a ____________________
• The sides of the ladder is composed of
_______________ and ________________
• The middle rungs of the ladder is composed of
________________
• Purines are ______________ and pyrimidines are
_______________
• A always binds to T and C always binds to G
because __________________________________
DNA Replication
• When cells divide, all of the DNA in one cell need to
be replicated
• Each strand of DNA acts as a template for a new
strand
• We needed to know how
• Scientists Meselson and Stahl came up with a
clever experiment to show this
3 Different Possibilities
• Three models were suggested for the
mechanism of replication:
• 1) conservative replication
• Each of the two strands is replicated
without strand separation
• One DNA helix contains all old strands,
other helix contains all new strands
3 Different Possibilities
• 2) semiconservative replication
• Every newly created helix contains one old
strand and one new strand
3 Different Possibilities
• 3) dispersive replication
• After replication, every helix contains parts
of the new and old strands randomly
dispersed throughout the DNA molecule
The Experiment
• Step 1: bacteria is grown for many generations in
an environment containing 15N (this is a heavy
isotope of the normal 14N nitrogen)
• Over many generations, most of the DNA is made up of
15N
• DNA containing 15N is denser
The Experiment
• Step 2: the bacteria grown in 15N were transferred
to an environment with 14N for one round of
replication
• What results would you expect if the replication
was:
• Semi-conservative?
• Conservative?
• Dispersive?
The Experiment
• The actual results
after one generation:
• Therefore, replication
is either
semiconservative or
dispersive
The Experiment
• Step 3: The bacteria were allowed to replicate once
more
• What results would you expect if the replication
was:
• Semi-conservative?
• Dispersive?
The Experiment
• The actual results:
• Therefore, replication is semiconservative
Practice questions
• Messelson and Stahl’s experiment indicated that
DNA replicates semiconservatively. What
percentage of DNA double helixes would contain
one of the original parent strands after three
generations of replication?
• The four nitrogenous bases are paired to make up
the rungs of the DNA ladder. Differentiate between
the purine and pyrimidine structures of each of
these nitrogenous bases, and explain the
significance of the pattern of their arrangement