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