Name a few enzymes involved in DNA replication other than DNA polymerase and ligase. Name the key functions for each of them.
The enzymes involved in DNA replication other than DNA polymerase and ligase are listed below with their functions.
(i) Helicase - Opens the helix
(ii) Topoisomerases - Removes the super coiling of DNA
(iii) Primase - Synthesises RNA primer
(iv) Telomerase - To synthesis the DNA of telomeric end of chromosomes.
$$ \text { Name any three viruses which have RNA as the genetic material. } $$
In some viruses, RNA is the genetic material.
e.g., Tobacco mosaic viruscs, QB bacteriophage, HIV, influenza virus, etc.
Define transformation in Griffith's experiment. Discuss how it helps in the identification of DNA as the genetic material.
In Griffith's experiment, transformation can be defined as a change in the genetic constitution of an organism by picking out up DNA from the environment (from dead organisms).
Transformation helps in identification of DNA as a genetic material. When heat was used to kill the virulent bacteria, they died but not their genetic material (DNA). This DNA when picked up by non-virulent bacteria made them capable of causing infection.
Since, ability to cause infection could be passed on by these organisms to their progeny, it was concluded that DNA was the material that was inherited.
$$ \text { Who revealed biochemical nature of the transforming principle? } $$
Oswald, Avery, Colin MacLeod and Maclyn McCarty revealed biochemical nature of the transforming principle.
They reported Griffith's experiment in an in vitro system in order to determine biochemical nature of transforming principle.
They reported that DNA from the heat-killed S-type bacteria caused the transformation of non-virulent R-type bacteria into virulent S-type bacteria. They also discovered that proteases and RNase did not affect transformation while DNase inhibited the process. They concluded that DNA is the hereditary material.
Discuss the significance of heavy isotope of nitrogen in the Meselson and Stahl's experiment.
They performed experiments on $E$. coli to prove that DNA replication is semi-conservative. They first grew the bacteria in a medium containing ${ }^{15} \mathrm{NH}_4 \mathrm{Cl}$ (in which ${ }^{15} \mathrm{~N}$ is the heavy istope of nitrogen) for many generations.
Then they transferred the cells into a medium with normal ${ }^{14} \mathrm{NH}_4 \mathrm{Cl}$ (in which ${ }^{14} \mathrm{~N}$ is the lighter isotope) and took the samples at various definite time intervals as the cells multiplied. The extracted DNAs were centrifuged and measured to get their densities.
The DNA extracted from the culture after one generation of transfer from then ${ }^{15} \mathrm{~N}$ mediun to ${ }^{14} \mathrm{~N}$ mediun, (i.e., after 20 minutes E.coli divides every 20 minutes) showed an intermediate hybrid density, i.e., both heavy and light nitrogen, which proved the semi-conservative nature of DNA.