Semiconservative Replication of DNA – Mechanism

• DNA is just one way that humans and other living things carry their genetic information.
• The information, which has been turned into a code, is stored in the DNA bases adenine, guanine, cytosine, and thymine. This is the plan that helps figure out what the organism looks like.
• DNA helps an organism store the information it needs to stay alive, grow, change, and reproduce.
• To do all of these things, the DNA sequence must be turned into a message that will be used to make proteins, which are much more complicated molecules that do most of the body’s work.
• There are three kinds of DNA, and they are all double-stranded and linked by interactions between base pairs that are opposite each other. They are called the B-form, A-form, and Z-form of DNA.
• DNA is important for three things, including immunity, structure, and genetics.
• DNA copies itself during a process called DNA replication, which happens when a cell divides. This way of copying DNA is only half as strict as the other way.
• Beginning, growing, and ending are all parts of the process of DNA replication.
• It starts with finding the site where the DNA will be copied, goes through a few steps, and ends with the joining of DNA fragment molecules.
• Double-stranded DNA is made by putting together two separate strands of DNA.
• DNA helps the genetic code be passed on so that we can grow, work, and develop.
• It also helps cells grow new copies of themselves by keeping their original DNA. There is this kind of nucleic acid.

Mode of DNA replication

Imagine yourself in 1953, right after the discovery of the double helix structure of DNA. What are some burning questions you and other scientists might have?

One big question concerned DNA replication. The double helix shape of DNA gave a tantalising hint about how copying might happen. It seemed likely that the two complementary strands of the helix would break apart during replication, with each strand becoming a pattern for making a new, matching strand.

But did this really happen? Yes, but you already knew that! In this article, we’ll look at a famous experiment that has been called “the most beautiful experiment in biology.” It showed that the basic mechanism of DNA replication is semi-conservative, meaning that it makes DNA molecules with one new strand and one old strand.

Three models for DNA replication

After the structure of DNA was figured out, scientists came up with three basic models for how DNA could be copied.

In the diagram below, you can see these models:

• Semi-conservative replication: In this model, the two strands of DNA unwind from each other, and each acts as a template for making a new, complementary strand. This makes two molecules of DNA, one with the old strand and one with the new strand.
• Conservative replication: In this model, DNA replication makes one molecule with both of the original DNA strands (the same as the original DNA molecule) and another molecule with two new strands (with exactly the same sequences as the original molecule).
• Dispersive replication: In this model, DNA replication makes two molecules that are a mix of parent and daughter DNA. These molecules are called “hybrids.” In this model, each strand is made up of pieces of old and new DNA.

At the time, most biologists probably would have bet on the semi-conservative model.

Given that the two strands of DNA in a double helix are perfectly and predictably complementary to each other, this model made a lot of sense (where one has a T, the other has an A; where one has a G, the other has a C; and so forth).

Because of this relationship, it was easy to see how each strand could be used as a model for making a new partner.

But biology is also full of situations where the “obvious” answer turns out to be wrong.(Has anyone heard of protein as the genetic material?)

So, it was important to do experiments to find out which model cells really used when they copied their DNA. 

The Meselson-Stahl experiment

Matt Meselson and Franklin Stahl first met in the summer of 1954, a year after Watson and Crick published their paper on the structure of DNA. Even though the two researchers had different research interests, they were both interested in the question of how DNA copies itself. They decided to work together to try to figure out how DNA copies itself.

Meselson and Stahl did their famous DNA replication experiments with E. coli. E. coli bacteria as an example. At first, they grew E. coli in medium, or nutrient broth, that had a “heavy” isotope of nitrogen, 15N. An isotope is just a version of an element that differs from other versions by the number of neutrons in its nucleus. When grown on medium with heavy 15N, the bacteria took up the nitrogen and used it to make new biological molecules, including DNA.

After many generations of growth in 15N, all of the nitrogenous bases in the DNA of the bacteria were marked with heavy 15N. The bacteria were then put in a medium with a “light” 14N isotope and given time to grow for several generations. Since 14N was the only nitrogen that could be used to make DNA after the switch, 14N would have to be used to make DNA.

Meselon and Stahl knew how often E. Since E. coli cells could divide, they could take small samples from each generation and use them to extract and clean the DNA. Then, they used density gradient centrifugation to measure the DNA’s density and, in turn, its 15Nt and 14N content.

This method separates molecules like DNA into bands by spinning them at high speeds in the presence of another molecule, like cesium chloride, that makes a density gradient from the top to the bottom of the spinning tube. The difference between 15N-labeled DNA and 14N-labeled DNA can be found by using density gradient centrifugation.

Results of the experiment

When the DNA from the first four generations of E. coli was analysed, it showed the pattern of bands in the figure below:

What did Meselson and Stahl learn from this? Let’s look at the first few generations, which contain the most important information.

Generation 0

After centrifuging, the DNA taken from cells at the beginning of the experiment (“generation 0,” right before the switch to 14N medium) made a single band. This result made sense because at that time, the DNA should have only had the heavy 15N.

Generation 1

When DNA was separated after one generation, or one round of DNA replication, there was also only one band. But this band was higher and had a density that was somewhere between the dense 15N DNA and the less dense 14N DNA.

The intermediate band told Meselson and Stahl that the first round of replication made DNA molecules that were a mix of light and heavy DNA. The dispersive and semi-conservative models fit this result, but not the conservative model.

The conservative model would have said that this generation would have two different bands (a band for the heavy original molecule and a band for the light, newly made molecule).

Generation 2

Meselson and Stahl used information from the second generation to figure out which of the two remaining models (semi-conservative or dispersive) was the right one.

When the DNA from the second generation was put through a centrifuge, it made two bands. One was in the same place as the first generation’s intermediate band, and the other was higher (appeared to be labelled only with 14N).

Meselson and Stahl could tell from this result that the DNA was being copied in a semi-conservative way. The pattern of two distinct bands—one where a hybrid molecule should be and one where a light molecule should be—is exactly what we’d expect from semi-conservative replication (as illustrated in the diagram below). In dispersive replication, on the other hand, all of the molecules should have bits of both old and new DNA. This makes it impossible to make a “purely light” molecule.

Generations 3 and 4

In the semi-conservative model, each hybrid DNA molecule from the second generation would be expected to produce a hybrid molecule and a light molecule in the third generation, while each light DNA molecule would only produce more light molecules.

So, over the third and fourth generations, we’d expect the hybrid band to get weaker (since it would represent less of the total DNA) and the light band to get stronger (because it would represent a larger fraction).

As the figure shows, Meselson and Stahl’s results showed the same pattern, which confirms a semi-conservative replication model.

Why DNA replication is called semiconservative? – What is semiconservative dna replication

Now, tell me what the semiconservative DNA replication process is and how it works. Starting, growing, and ending are all parts of the process of DNA replication. It starts with figuring out where the replication will happen and ends when the DNA fragment molecules are joined together.

We know that DNA replication is a process in which one strand copies itself and makes copies of itself.

DNA is a double strand that has a nitrogenous base (A, T, G, and C), a pentose sugar molecule called deoxyribose sugar, and a highly negative phosphate molecule. So, when the first strand of DNA is copied, one strand stays the same and the other strand is made by adding the right nucleotides.

One is the new strand of DNA that was made by adding nucleotides, and the other is the old strand of DNA that was kept. This is called the template strand.

The strand of DNA called the “template strand” is

A G G G G C C T T T A A C C T G G C A T A G G

The new strand of DNA that matches the old one will be

T A A C G T A T C C C C G G A A A T T G G A C C G T A T C C A A A T T T G G G C C C C T C C C G G A A A T T G G A C C G T A T C C

Steps in the semiconservative DNA replication process

The semiconservative DNA replication process includes 3 steps.

• Initiation
• Elongation
• Termination

Initiation

• This is the process that starts the process of DNA replication.
• The enzyme helicase, which finds the ORIC—Origin of replication, binds to the DNA strand and unwinds or splits the double-stranded DNA molecule.
• At the end of the step, the two strands of double-stranded DNA become two separate strands of DNA, but not completely.
• Half of the strand stays as a double strand because it doesn’t take part in the positive moment. This makes a structure that looks like a fork, which is called the replication fork.

Elongation

• This is where RNA nucleotides are added to the template DNA strand by the primer.
• Another enzyme called Primase makes a short piece of RNA that the next enzyme finds and attaches to.
• The part of the strand that goes from 3′ to 5′ is called the leading strand.
• The part of the strand that goes from 5′ to 3′ is called the lagging strand.
• Nucleotides are added by an enzyme called DNA polymerase. This enzyme adds the nucleotide that goes with the template DNA.
• The DNA polymerase can’t make a new strand from scratch; it can only add to one that’s already there.
• In the end, enzymes called exonucleases FEN1 and RNase H will replace the RNA nucleotides with DNA nucleotides.
• So, when there is adenine, thymine is added, and when there is guanine, cytosine is added.

Termination

• Now, the process of making new strands must come to an end. DNA polymerase stops adding nucleotides when the termination point is reached.
• DNA polymerase can’t join the two pieces together because it can’t make a bond between them.
• The enzyme ligase joins the strands together by adding a phospho-di-ester bond to the DNA molecule. This brings the strands together.

Summary of steps in the semiconservative DNA replication process

In semiconservative DNA replication, the actual and well-known steps are initiation, elongation, and termination. Recognition of the site or origin of replication is a much simpler version.

• The DNA strands separate or unwind.
• Putting the nucleotides together
• A new strand of DNA is made.
• The end of the process of synthesis
• Putting the strands together through an enzyme process

So, one double strand of DNA splits into two double stranded DNA molecules. Each strand of DNA has one template and one piece of newly made DNA that matches the template.

Enzymes responsible for the Semiconservative DNA replication

• DNA Helicase Enzyme – The enzyme DNA Helicase helps unwind the two strands of double-stranded DNA.
• Primase Enzyme – With primase enzyme, a short RNA primer is added.
• DNA polymerase Enzyme – DNA polymerase is an enzyme that adds new nucleotides to DNA.
• Exonucleases Enzyme FEN1 Enzyme  and RNase H Enzyme – Exonucleases Enzyme FEN1 Enzyme and RNase H Enzyme get rid of the nucleotides that were made when the RNA was first made.
• Enzyme Ligase– Joins two separate strands of DNA into one long strand. This makes double-stranded DNA. 

What if the dispersive model had been correct?

The dispersive model says that each copy of DNA should be a patchwork of DNA from the previous generation and DNA from the next generation (newly synthesised during replication). From a mechanical point of view, it’s as if the old and new DNA were cut up, swapped, and then put back together to make helices. So, under the dispersive model, each round of replication would make patchwork molecules with both heavy and light parts. With each replication, the patchwork DNAs would have more and more 14N. As a result, the density of the DNA would decrease over time, making a band (or fuzzy band or smear) that moved higher with each generation.

What if the conservative model had been correct?

Under the conservative model, if we started with one “heavy” 15N) DNA molecule, we would have the original heavy molecule and one new light molecule after one round of replication. What would happen if these DNAs went through a second round of replication? There would be one big piece of DNA and three small pieces. And after a third round of replication, we’d end up with one heavy and seven light molecules.

This pattern shows that, according to the conservative model, the heavy DNA never goes away completely, but the number of light DNA molecules that are made quickly outnumbers the heavy DNA molecules. You can also see that the hybrid molecules seen in the other two models never happen when conservative replication is used. Because of this difference, Meselson and Stahl were able to get rid of the conservative model after one generation.

Applications

The advantages of semiconservative DNA replication are numerous and are very appealing for DNA. It is quick, accurate, and simple for repair of DNA. The mechanism responsible for the phenotypic diversity of a few prokaryotic species also involves the production of a new strand from a template strand. The production of the new strand allows for the old strand to be methylated at a separate time. This lets repair enzymes check the new strand for mistakes or mutations and fix them.

DNA could be able to turn on or off certain parts of the newly made strand, which would change the phenotype of the cell. This could be good for the cell because the DNA could turn on a better phenotype that would help it stay alive. Because of natural selection, the phenotype that is best for the species as a whole would stay that way. This brings up the idea of inheritance, or why some phenotypes are more likely to be passed down than others.

Difference between Conservative and Semiconservative Replication

Conservative Replication

During this process, the DNA copies itself to make many copies. In conservative replication, two copies of DNA are made from the same original piece of DNA, which acts as a template. One is made of brand-new DNA, and the other is made of old DNA strands. This kind of DNA replication doesn’t mean anything in terms of biology.

Semiconservative Replication

It makes two copies of DNA, one of which is made from the original DNA and the other from a new piece of DNA. Most of the time, the new strand changes based on the template strand. The way DNA is fixed is helped by this method. Watson and Crick came up with this replication model, which is now widely used.

Difference Table between Conservative and Semiconservative Replication

Conservative Replication	Semiconservative Replication
It makes one new DNA, and the old one.	It creates two DNA, that have a new strand as well as another strand derived from old DNA.
It isn’t biologically significant.	This replication is significant for biology.
The function of DNA as an underlying strand of DNA isn’t certain.	Each strand of the initial DNA serves as a model for the creation of a new strand.

FAQ

References

• https://embryo.asu.edu/pages/categorization-conservative-semi-conservative-and-dispersive-dna-replication-theories-1953.
• https://lambdageeks.com/what-is-conservative-dna-replication/
• https://www.nature.com/scitable/topicpage/semi-conservative-dna-replication-meselson-and-stahl-421/
• https://royalpitch.com/sickle-cell-anemia-is-an-example-of-codominance/
• https://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/27-dna-replication-transcri/semi-conservative.html
• https://www.ibiology.org/genetics-and-gene-regulation/semi-conservative-replication/
• https://www.bionity.com/en/encyclopedia/Semiconservative_replication.html
• https://www.learnpick.in/questions/details/13897/what-is-semiconservative-mode-of-dna-replication
• https://www.ck12.org/book/ck-12-biology-advanced-concepts/section/8.5/
• https://socratic.org/questions/what-does-semiconservative-mean-in-the-context-of-dna
• https://www.khanacademy.org/science/biology/dna-as-the-genetic-material/dna-replication/a/mode-of-dna-replication-meselson-stahl-experiment