MESELSON-STAHL EXPERIMENT
The Meselson-Stahl experiment, conducted in 1958 by Matthew Meselson
and Franklin Stahl, provided crucial evidence supporting the semiconservative
model of DNA replication. This experiment helped to confirm that DNA
replication involves the formation of two new DNA molecules, each consisting of
one original (parental) strand and one newly synthesized (daughter) strand.
Here's an overview of the Meselson-Stahl experiment:
Objective: To investigate the mode of DNA replication by determining
whether it is conservative, semiconservative, or dispersive.
Experimental Design:
- Isotope Labelling:
- E. coli
bacteria were grown in a medium containing a heavy isotope of nitrogen, 15N,
which is a non-radioactive, heavier form of nitrogen. This resulted in
the incorporation of 15N into the bacterial DNA.
- After
several generations of growth, the bacteria were transferred to a medium
containing the normal, lighter isotope of nitrogen, 14N.
- DNA
Extraction and Centrifugation:
- Bacterial
DNA was extracted at various time points after the shift to the 14N-containing
medium.
- The
extracted DNA was then subjected to ultracentrifugation in a caesium
chloride (CsCl) density gradient.
- Centrifugation
Results:
- In the
first generation after the shift to 14N, the DNA formed a band
with an intermediate density between pure 14N DNA and pure 15N
DNA.
- In
subsequent generations, the density of the DNA band shifted toward the
density of pure 14N DNA.
Interpretation: The results of the Meselson-Stahl
experiment provided strong support for the semiconservative model of DNA
replication. The observed intermediate density in the first generation after
the shift indicated that each DNA molecule was composed of one parental 15N
strand and one newly synthesized 14N strand. This supported the idea
that, during replication, each original DNA strand serves as a template for the
synthesis of a new strand.
Conclusions: The Meselson-Stahl experiment
demonstrated that DNA replication is semiconservative, meaning that each newly
synthesized DNA molecule consists of one strand from the original DNA and one
newly synthesized strand. This groundbreaking work significantly contributed to
our understanding of DNA replication and laid the foundation for subsequent
research in molecular biology
LET’S EXPLAIN THIS IN SIMPLY
Goal of the Experiment: Meselson and Stahl wanted to
figure out how DNA gets copied when cells divide. They were testing different
ideas about how this process might work.
What They Did:
- They grew
bacteria in a special liquid containing a heavier form of nitrogen (15N).
This made the bacteria's DNA a bit heavier than usual.
- After some
time, they switched the bacteria to a normal nitrogen (14N) liquid.
What They Found: When they looked at the DNA after one
round of cell division:
- They saw a
band of DNA that was kind of in the middle — not as heavy as the 15N-only
DNA and not as light as the 14N-only DNA.
- This meant
that each new DNA molecule had one heavy strand (from the original DNA)
and one light strand (newly made).
In the next rounds of cell division:
- The DNA
bands shifted more towards the lighter side, showing that with each round,
the new DNA was getting lighter.
What It Means: The results supported the idea that when
cells divide, the DNA gets copied in a semi-conservative way. Each new DNA
molecule is made up of one strand from the old DNA and one newly made strand.
This was a big deal because it confirmed a key part of how genetic information
is passed on from one generation of cells to the next.
Bottom Line: The Meselson-Stahl experiment provided
strong evidence that DNA replication is like making a copy of a book where each
new book is created by using one old page and adding a new page. It showed that
this process helps maintain the integrity of genetic information during cell
division.