Skip to main content

Renin Angiotensin Mechanism.

DNA Synthesis (Semiconservative Model).

 

Introduction:

  • DNA replication is similar to transcription in its most general idea: a polymerase enzyme reads a strand of DNA one nucleotide at a time, it takes a random nucleotide from the nucleoplasm, and if it is complementary to the nucleotide in the DNA, the polymerase adds it to the new strand it is creating. 

  • Although, there are significant differences between replication and transcription too, not the least of which is that both strands of DNA are being read simultaneously in order to create two new complementary strands that will eventually result in a complete and nearly perfect copy of an entire organismal genome.

  • Models proposed for DNA Synthesis:

    • Conservative Model.

    • Semiconservative Model.

    • Dispersive Model.

  • One of the most important concepts of DNA replication is that it is a semi-conservative process. 

  • It means that every double helix in the new generation of an organism consists of one complete “old” strand and one complete “new” strand wrapped around each other. 

  • This is in contrast to the two other possible models of DNA replication, the conservative model, and the dispersive model. 

  • A conservative mechanism of replication proposes that the old DNA is used as a template only and is not incorporated into the new double-helix. 

  • Thus the new cell has one completely new double-helix and one completely old double-helix. 

  • The dispersive model of replication proposes a final product in which each double helix of DNA is a mixture of fragments of old and new DNA. 

  • In light of current knowledge, it is difficult to imagine a dispersive mechanism, but at the time, there were no mechanistic models at all. 

  • The Meselson-Stahl experiments (1958) clearly demonstrated that the mechanism must be semi-conservative, and this was confirmed once the key enzymes were discovered and their mechanisms elucidated.

Meselson-Stahl experiments

  • In these experiments, E. coli were first incubated with 15N, a heavy isotope of nitrogen. 

  • Although it is only a difference in mass of one neutron per atom, there is a great enough difference in mass between heavy nitrogen-containing DNA (in the purine and pyrimidine bases) and light/normal nitrogen-containing DNA that they can be separated from one another by ultracentrifugation through a CsCl concentration gradient.

  • Over 14 generations, this led to a population of E. coli that had heavy nitrogen incorporated into all of the DNA (shown in blue). 

  • Then, the bacteria are grown for one or two divisions in “light” nitrogen, 14N. 

  • When the DNA from the bacterial populations was examined by centrifugation, it was found that instead of light DNA and heavy DNA, as would be expected if DNA replications was conservative, there was a single band in and intermediate position on the gradient. This supports a semi-conservative model in which each strand of original DNA not only acts as a template for making new DNA, it is itself incorporated into the new double-helix.

Commonly Asked Questions.

  1. Write a short note on Conservative model of DNA replication.

  2. Write a short note on Meselson-Stahl experiments on DNA replication.
Labels:

Popular posts from this blog

Rate of Drying Curve.

  Definition Drying is defined as the removal of liquid from a product usually with application of heat. Rate of Drying Curve. Drying process can be divided into three periods Initial Adjustment Period. Constant drying rate period. First falling drying rate period. Second falling rate period. Initial Adjustment Period (A-B): Also called the “ Heating up” period . In this period the substance gets heat and increases in temperature. Drying has not yet started. Constant drying rate period (B-C): During this period the temperature of the solid and the rate of drying remain constant. The moisture evaporating from the surface is replaced by water diffusing from the interior of the solid at a rate equal t o the rate of evaporation.  The moisture content at the end of constant rate (point C) is referred to as the critical moisture content (CMC).  At CMC, dry spots start appearing and drying rate starts falling . First falling drying rate period (C-D): This period is also called ...
Read more

Heat Exchangers and Heat Interchangers.

  In pharmaceutical industries many types of equipments are used for transfer of heat, they can be classified as follows, Heat Exchangers. Heat Interchangers. Heat Exchangers: These devices are used for transferring heat from a fluid (Hot Gas or Steam) to another fluid (Liquid) through a metal wall. Heat Interchangers: These devices are used for transferring heat from a One liquid to another liquid or one gas to another gas through a metal wall. HEAT EXCHANGERS; The equipment used for heat transferring are known as heat exchangers. Some of the processes that involves heat transfer in pharmaceutical industries are: Preparation of starch paste (in steam jacketed kettle). Crystallization. Evaporation. Distillation.  Classification of heat exchangers On the basis of transfer of heat, heat exchangers are classified as: Direct transfer type:  The hot and cold fluids are separated by a metal wall through which the heat is transferred from hot fluid to cold fluid. E.g. shell and ...
Read more

Flash Distillation.

  Principle: When a hot mixture is allowed to enter from a high-pressure zone into a low pressure zone, the entire liquid mixture is suddenly vaporized. This process is known as flash vaporization .  During this process, the chamber is cooled.  The less volatile fraction is condensed and the more volatile component remains in the vapor phase .  This process requires time, hence liquid and vapor are kept in intimate contact until equilibrium is achieved. Flash distillation is also called equilibrium distillation because separation of two liquids takes place when liquid and vapor phases are at equilibrium. Equipment used for Flash Distillation: Construction: It consists of a pump, which is connected to a feed reservoir.  Pumps help in pumping the feed into the heating chamber.  The heating chamber is heat supplied by steam.  The other end of the pipe is directly introduced into the vapor-liquid separator through a reducing valve.  The vapor outlet...
Read more