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.
Retroviruses do no follow central dogma. Comment.
Retroviruses do not follow central dogma of biology (DNA $\rightarrow$ RNA $\rightarrow$ Protein) because their genetic material is not DNA. Instead they have RNA that is converted to DNA by the enzyme reverse transcriptase.
In an experiment, DNA is treated with the compound which tends to place itself amongst the stacks of nitrogenous base pairs. As a result of this, the distance between two consecutive base increases. From $0.34-0.44 \mathrm{~nm}$ calculate the length of DNA double helix (which has $2 \times 10^9 \mathrm{bp}$ ) in the presence of saturating of this compound.
$$ \text { The length of DNA double helix }=2 \times 10^9 \times 0.44 \times 10^{-9} / \mathrm{bp} . $$
What would happen if histones were to be mutated and made rich in acidic amino acids such as aspartic acid and glutamic acid in place of basic amino acids such as lysine and arginine?
If histones were mutated and made rich in acidic amino acids. They will not be able to serve the purpose of keeping the DNA coiled around them. This is because DNA is negatively charged molecule and histones are positively charged because of basic amino acids.
So, they are attracted to each other. If histones become negatively charged, instead of binding, they will rather repel DNA. The packaging of DNA in eukaryotes would not happen. Consequently, the chromatin fibre would not be formed.
Recall the experiments done by Frederick Griffith, Avery, MacLeod and McCarty, where DNA was speculated to be the genetic material. If RNA, instead of DNA was the genetic material, would the heat killed strain of Pneumococcus have transformed the R-strain into virulent strain? Explain.
RNA is more liable and prone to degradation (owing to the presence of $2^{\prime} \mathrm{OH}$ group in its ribose). Hence, heat-killed S-stain may not have retained its ability to transform the R-strain into virulent form if RNA was its genetic material.