Modes of Transfer: Before starting the main conversation, previously, we have seen how DNA replicates and all the DNA replication enzymes required for DNA replication. As we have seen, helicase, ligase, and DNA polymerase III are the most important enzymes used in the replication process.
The synthesis of two different daughter DNA from the parent DNA is the overall process in DNA replication. Now the question is how the parent DNA distribute in two daughter DNA? What is the mode of transfer of DNA?
At a Glance
Now coming to the main point today in this blog, I will discuss the Modes of Transfer in DNA. Since our childhood, we always listened to one quote is “SHARING IS ALWAYS CARING.” This quote is not only heard by our ears, but our genetic material DNA is the only start point of this quote. You might say this writer is crazy and summing up some funny story, but the reality is our DNA is the combination of our parent’s DNA. And that is the reason behind our genetic and morphological resemblance with our parents.
What do you think about how this sharing of DNA has happened? Is it like tearing one strand in one shot, or is it something like joining many pieces of DNA together? The answer to all these questions is the “SEMICONSERVATIVE MODE OF DNA TRANSFER.”
Different Models for the Modes of Transfer in DNA Replication
As we know, DNA replication results in two DNA molecules. Before confirmation of the primary transfer mode of DNA, there are three types of possible modes, namely
- Conservative
- Semiconservative and
- Dispersive
Immediate after the structural conformation of DNA using X-ray crystallography. Watson and Crick proposed the mode of transfer is semiconservative. And this is proved by the Meselson and Stahl experiments.
Description of Different Modes of Transfer
1. Conservative Modes of Transfer
In the Conservative model of DNA replication, whole parent DNA acts as a template for the new double helix. So, in each round of cell division, one new daughter cell with new DNA is formed. And another daughter cell is produced with an old parent DNA. Simply you can imagine it as you have two strings attached in helix form. Now, these helices get replicate with the cell cycle in each cell division.
During each cell division, one cell divides into two daughter cells. Then, those two daughter cells get divided into four cells (each into two). So, according to the conservative model, when a cell divides first, its DNA is completely transferred into only one daughter cell. And the second daughter cell is formed with completely new DNA. For fun, you can imagine a kitten baby to the doggy couple, which is practically impossible until and unless one parent is not a cat.
2. Semiconservative Modes of Transfer
According to the semiconservative model, every single strand of double-stranded DNA acts as a template for synthesizing a new strand. Thus, during each cell division, every daughter cell contains a combination of one parent and one newly synthesized strand in the form of a double helix.
Similarly, two strings are separated and transferred into each daughter cell, half in one daughter cell and half in the second daughter cell. Thus, simply both parental DNA shares their genetic material equally in the next generation. Therefore, a semiconservative model is quite a convincing model and gets proved further.
3. Dispersive Modes of Transfer
In the dispersive model, the parental DNA strands get distributed in each newly formed DNA. It means each strand of the freshly synthesized double helix contains a mixture of both parental DNA. So in every round of replication, there is a formation of hybrid. And that hybrid is partial parental DNA and partial new DNA.
We can imagine it as a patch of original and new DNA in single strands of the double helix. No equal sharing of parental DNA occurs in the daughter cells.
Meselson’s And Stahl’s Experiment
Matthew Meselson and Franklin Stahl are both well versed with all three predictions about the DNA replication mode. They have known the technique for the separation of new and old DNA. A similar technique they thought to apply for the isolation and detection of parent and progeny DNA.
This experiment takes place with the help of two isotopes of Nitrogen N14 (natural and lighter form) and N15 (rare and heavier form). Their technique is CsCl (cesium chloride) equilibrium density gradient centrifugation. And the whole experiment was concluded on the sedimentation method. Nitrogen is the key atom present in the DNA bases, and the common N14 form is naturally present in it.
They performed experiments on around 14 bacterial generations with the medium containing heavy N15 and lighter N14 isotopes. The separation was performed based on bands obtained using a CsCl gradient.
The primary function of the Cesium chloride gradient is to separate particles based on their buoyancy density or their rate of sedimentation.
Experimental Steps
This experiment was performed on DNA replication using E. coli bacteria as a model organism. Initiation of the experiment started with the growth of bacterial generation in the heavy N15 isotope of nitrogen.
As bacteria are growing in N15, media takes up the nitrogen and synthesize new strands using it. Hence all the generated bacteria are grown with the heavy isotope, and their DNA gets labeled with N15. Because it already contains all the required enzymes and conditions for the replication.
After several generations, the bacteria were shifted to the lighter isotope N14 containing nutrient media and allow to grow in that for many generations.
Now the heavy isotope containing DNA has only a lighter N14 isotope to grow on. So it starts taking up a lighter isotope for the new DNA strand synthesis.
Isolation and identification of the sample DNA take place on each division of bacteria simultaneously with the series of experiments. On each phase, the sedimentation rate and the obtained bands were different. The different positions of bands on the centrifugation show the separation of parent and newly formed DNA.
Sample Collection Phases and Description
The model organism was E. coli; hence the cell division timing is always 20 minutes. First, scientists collect a sample on each division and examine it under equilibrium density gradient centrifugation using CsCl. Then, in four different phases, they examine the generation.
Phase 0
DNA isolated in the very initial stage, that is, before switching to the N14 media. On centrifugation, it shows the single band in the bottom. As initially, it has only a heavy N15 isotope.
Phase 1
Here, the fraction of DNA sample removes after the first division, after the first round of replication—this time, the band occurs in the intermediate level in the middle. But again, a single band intermediate in between N14 and N15. That is, the band is a mixture of heavy and light isotopes. The intermediate band gives a prediction about the dispersive and the semiconservative mode of replication. And not about the conservative way.
Phase 2
DNA from the second round of replication, now as the conservative mode eliminate from the above two results. So, the competition is between the semiconservative and dispersive methods. Here it produced two bands; one is on the intermediate position, same as in phase 1 result, while the second one is higher, and that is of lighter N14. This pattern of the band shows the semiconservative mode of replication.
As one band is at the mixture, that is an intermediate position, and one is at a lighter position which is the only expectation of semiconservative mode.
Phase 3 & 4
Now, in the next generations, the hybrid DNA from phase 2 was expected to produce hybrid molecules and light bands in the third generation. And further, light molecules will give rise to light molecules and a faint band than before.
Concluding all phases, the experiment proves the mode of replication is semiconservative. Thus, each separated strand acts as a template for synthesizing a new strand.
