DNA Replication: Semiconservative Replication, Meselson–Stahl Experiment & Mechanism
DNA Replication: Types, Semiconservative Replication, Meselson–Stahl Experiment & Mechanism
DNA replication is the biological process by which a DNA molecule produces two daughter DNA molecules. It is an essential process for growth, development, cell division, heredity, and transmission of genetic information.
What Is DNA Replication?
Replication is an autocatalytic function of DNA, in which one molecule of DNA synthesizes two molecules of DNA.
DNA replication takes place during the S-phase (Synthesis phase) of the cell cycle.
In rapidly growing bacteria such as Escherichia coli, DNA replication can proceed at a very high rate, approximately 2,000 base pairs per second.
Types of DNA Replication
DNA replication can be described on the basis of several features:
- Semiconservative replication
- Semi-discontinuous replication
- Bidirectional replication
- Monorepliconic replication
- Multirepliconic replication
1. Semiconservative Replication
In semiconservative DNA replication, both parental DNA strands act as templates for the synthesis of new complementary strands.
As a result, each daughter DNA molecule contains:
- One parental (old) DNA strand
- One newly synthesized DNA strand
Therefore, each daughter DNA molecule retains one strand of the original DNA molecule and contains one newly formed strand.
2. Semi-discontinuous Replication
DNA synthesis occurs in a semi-discontinuous manner because the two DNA strands are antiparallel and DNA polymerase synthesizes DNA only in the 5′ → 3′ direction.
- The leading strand is synthesized continuously.
- The lagging strand is synthesized discontinuously in the form of Okazaki fragments.
3. Bidirectional DNA Replication
In bidirectional replication, DNA synthesis proceeds in two opposite directions from the origin of replication.
When replication begins at an origin, two replication forks generally move away from the origin in opposite directions.
4. Monorepliconic and Multirepliconic Replication
A. Monorepliconic Replication
In monorepliconic replication, replication starts from a single origin of replication.
This is commonly associated with circular DNA molecules, such as bacterial chromosomes and many plasmids.
ORI stands for Origin of Replication.
B. Multirepliconic Replication
In multirepliconic replication, replication starts from multiple origins of replication.
This arrangement is characteristic of large, linear eukaryotic chromosomes, where multiple replication origins allow the long DNA molecule to be replicated efficiently.
Semiconservative Replication: Historical Background
The semiconservative model of DNA replication was proposed by James Watson and Francis Crick in 1953 based on the structure of DNA.
The model was experimentally demonstrated by the famous Meselson and Stahl experiment in Escherichia coli.
Experimental Verification of Semiconservative DNA Replication
Meselson and Stahl Experiment
Matthew Meselson and Franklin Stahl experimentally demonstrated the semiconservative nature of DNA replication in E. coli.
They used different nitrogen isotopes to distinguish old DNA from newly synthesized DNA.
- ¹⁵N = heavy isotope of nitrogen
- ¹⁴N = normal/light isotope of nitrogen
Step 1: Growth in ¹⁵N Medium
E. coli bacteria were first grown for several generations in a culture medium containing ¹⁵N, supplied in the form of ¹⁵NH₄Cl.
As a result, the bacterial DNA became labelled with heavy nitrogen and formed heavy DNA (¹⁵N–¹⁵N).
Step 2: Transfer to ¹⁴N Medium
The bacteria containing heavy DNA were transferred to a normal culture medium containing ¹⁴N.
The cells were then allowed to undergo DNA replication.
First Generation
After one generation in the ¹⁴N medium, all DNA molecules were found to have an intermediate density.
Each DNA molecule contained:
¹⁵N–¹⁴N
These molecules were called hybrid DNA molecules.
Second Generation
After the second generation, two types of DNA molecules were observed:
- 50% hybrid DNA (¹⁵N–¹⁴N)
- 50% light DNA (¹⁴N–¹⁴N)
Density Gradient Centrifugation
DNA samples from each generation were separated using density-gradient centrifugation in cesium chloride (CsCl).
The different DNA molecules formed distinct bands according to their density.
Conclusion of the Meselson–Stahl Experiment
The experimental results demonstrated that:
DNA replication is semiconservative.
During each round of replication, each daughter DNA molecule receives one parental strand and one newly synthesized strand.
Other Experimental Evidence
Cairns Experiment
J. Cairns studied DNA replication in E. coli using tritiated thymidine and autoradiography.
Taylor Experiment
Taylor and colleagues demonstrated semiconservative replication in eukaryotic chromosomes using tritiated thymidine in an in vivo experiment.
Mechanism of DNA Replication
DNA replication involves several coordinated steps and enzymes. The major events include DNA unwinding, stabilization of single strands, relief of torsional strain, primer formation, and synthesis of new DNA strands.
1. Initiation at the Origin of Replication
DNA replication begins at a specific DNA sequence called the origin of replication (ORI).
Initiator proteins recognize the origin and initiate the process of DNA unwinding.
2. Unwinding of DNA
The enzyme helicase unwinds the DNA double helix by breaking the hydrogen bonds between complementary nitrogenous bases.
This results in the separation of the two parental DNA strands.
3. Relief of Torsional Strain
Unwinding of DNA produces torsional strain ahead of the replication fork.
Topoisomerase enzymes relieve this strain by temporarily cutting and rejoining DNA strands.
4. Stabilization of Single DNA Strands
After separation, the exposed single DNA strands tend to pair again.
Single-strand binding proteins (SSB proteins) bind to the exposed single-stranded DNA and prevent the strands from reannealing.
5. Formation of the Replication Fork
The unwinding of DNA produces a characteristic Y-shaped structure known as the replication fork.
DNA synthesis then proceeds at the replication fork on both template strands.
Key Points for NEET Biology
- DNA replication occurs during the S-phase of the cell cycle.
- DNA replication is semiconservative.
- Each daughter DNA contains one old strand and one new strand.
- The Meselson–Stahl experiment demonstrated semiconservative replication.
- ¹⁵N is the heavy isotope of nitrogen.
- ¹⁴N is the lighter, naturally abundant isotope used as the normal nitrogen source.
- Meselson and Stahl used CsCl density-gradient centrifugation.
- The first generation produces 100% hybrid DNA.
- The second generation produces approximately 50% hybrid DNA and 50% light DNA.
- Helicase unwinds the DNA double helix.
- Topoisomerase relieves torsional strain.
- SSB proteins stabilize single-stranded DNA.
- The replication fork is a Y-shaped structure.
- The leading strand is synthesized continuously.
- The lagging strand is synthesized discontinuously.
Frequently Asked Questions
When does DNA replication occur?
DNA replication occurs during the S-phase of the cell cycle.
What is semiconservative DNA replication?
Semiconservative replication is a mode of DNA replication in which each daughter DNA molecule contains one parental strand and one newly synthesized strand.
Who experimentally proved semiconservative DNA replication?
Meselson and Stahl experimentally demonstrated the semiconservative nature of DNA replication in E. coli.
What isotope was used in the Meselson–Stahl experiment?
They used ¹⁵N (heavy nitrogen) and later transferred the bacteria to a medium containing ¹⁴N.
What enzyme unwinds DNA during replication?
Helicase unwinds the DNA double helix at the replication fork.
What is the function of topoisomerase?
Topoisomerase relieves the torsional strain generated during DNA unwinding.
What is the replication fork?
The replication fork is the Y-shaped region where parental DNA strands are separated and new DNA strands are synthesized.
Quick Revision
DNA Replication → S-phase → Origin of replication → DNA unwinding → Replication fork → Leading & lagging strands → New DNA synthesis
Meselson–Stahl → ¹⁵N → ¹⁴N → CsCl density-gradient centrifugation → Hybrid DNA → Semiconservative replication
