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