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DNA Replication
The Model
- 2 possible models to explain how this happens:
- DNA must replicate (duplicate itself)
- Free-floating nucleotides in nucleus are assembled into a copy of parental DNA
- Before mitosis cell doubles its genetic information so each daughter cell has identical genetic information to parent cellF
DNA Replication
Model #1: Conservative Replication
Model #2: Semi-conservative Replication
Free floating nucleotides
Parental DNA
Daughter DNA
Parental DNA is conserved
- Parental DNA molecule remains unchanged (conserved)
- One daughter DNA molecule is constructed entirely from free-floating nucleotides
Model #1: Conservative Replication
- Each daughter DNA molecule has one strand of parental nucleotides and one strand made from free-floating nucleotides- Half the parental material is conserved in each daughter DNA molecule (semi-conserved)
Model #2: Semi-conservative Replication
Free floating nucleotides
Parental DNA
Daughter DNA
Parental DNA is partially conserved
*Recall that nitrogen is essential component of DNA
- Used cultures of Escherichia coli (E. coli) bacteria that were given nutrients containing either "heavy" nitrogen (15N) or a more common isotope of nitrogen (14N)
- Matthew Meselson and Franklin Stahl performed an experiment
DNA Replication
Matthew Meselson
FranklinStahl
- Bacteria that contain DNA made of 15N have DNA “heavier” than DNA containing 14N- When DNA is extracted from bacteria and spun in a centrifuge, samples with 15N will sink lower in test tube than samples with 14N
The Meselson-Stahl Experiment
Step 1:
Step 2:
Step 3:
Grow E. coli bacteria in presence of 15N only (parental DNA)
Take parental bacteria and allow them to reproduce in presence of 14N only (generation 1)
Take generation 1 bacteria and again allow them to reproduce in presence of 14N only (generation 2)
The Meselson-Stahl ExperimentResults
Parental Generation 1 Generation 2
Layer of 15N Intermediate
Layer
Layer of 14N
Analysis of Results
Step 1: Step 2: Step 3:
Step 1: Step 2: Step 3:
Parental DNADaughter DNA Daughter DNA
m=15
m=14.5 m=14m=14.5
The Meselson-Stahl Experiment
RESULTS SUPPORT SEMI-
CONSERVATIVE MODEL
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