$$ \text { Who revealed biochemical nature of the transforming principle? } $$
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.
Define a cistron. Giving examples differentiate between monocistronic and polycistronic unit.
A cistron is stretch of base sequences that codes for one polypeptide chain including adjacent control regions. It may also code for a tRNA, rRNA molecule or may perform other specific functions including regulating functions of other cistrons.
This term has replaced the definition of a gene. Monocistronic transcription unit will have all the regulatory and coding sequences for a single polypeptide, whereas polycistronic may have coding sequences for more than one polypeptide.
In eukaryotic cells almost all the messenger RNAs are monocistronic. In prokaryotes, lac operon coding sequence would be an example of polycistronic DNA region.
$$ \text { Give any six features of the human genome. } $$
Salient features of human genome
(i) The human genome contains 3164.7 million nucleotide bases.
(ii) The average gene consists of 30000 the largest know human gene being dystrophin at 2.4 Million bases.
(iii) The total number of genes is estimated to be 30000 and $99.9 \%$ nucleotide bases are exactly the same in all people.
(iv) The functions are unknown for over $50 \%$ of the discovered genes.
(v) Less than $2 \%$ of the genome codes for proteins.
(vi) The human genome contains large repeated sequences.
(vii) The repeated sequence is thought to have no direct coding functions but they throw light on chromosome structures, dynamics and evolution.
viii) Chromosome I has most genes (2968) and the Y has the fewest genes (231).
(ix) Scientists have identified about 1.4 million locations where single base DNA sequence differences called SNPs or Single Nucleotide Polymorphisms occur in humans.
During DNA replication, why is it that the entire molecule does not open in one go? Explain replication fork. What are the two functions that the monomers (dNTPs) play?
While replicating, the entire DNA molecule to keep the whole molecule stabilised does not open in one go because it would be highly expens energetically. Actually unwiding creates tension in the molecule as uncoiled parts.
Actually, unwinding creates tension in the molecule as uncoiled parts start forming super coils due to the interaction of exposed nucleotides.
Instead, helicase enzyme acts on the double strand at ori site (origin of replication) and a small stretch is unzipped. Immediately, it is held and stabilised by single strand binding proteins.
Slowly with the help of enzymes, exposed strands are copied as a point of unwinding moves and ahead in both directions.
It gives an appearance of Y-shaped structure which is called replication fork.
The two functions that the monomer units of NTPs play are
(i) They pair up with exposed nucleotides of the template strand and make phosphodiester linkages and release a pyrophosphate.
(ii) Hydrolysis of this pyrophosphate by enzyme pyrophosphatase releases energy that will facilitate making hydrogen bonds between free nucleotides and bases of the template strand.