Question

1) why is RNA "locked" into an A-form helix?

2) why is DNA synthesized 5' to 3', not 3' to 5'?

Answer 1

ANSWER 1 :-

1. The RNA also called as Ribose Nucleic Acid is made up of Ribose sugar and is always linked to other ribonucleotides to form a single stranded structure.
2. The Ribose sugar has a hydroxyl group present at the 2'-position which is absent in case of DNA. This in turn reduces the stability of the RNA molecule rendering it ineffective to undergo base pairing.
3. Apart from this, the presence of hydroxyl group also leads to the confirmatory state which suited the A-form and not the A-form which can be seen in case of the DNA molecule.
4. These are some reasons why the RNA molecule gets locked into the A-form.

ANSWER 2 :-

1. In case of the DNA strand, a new nucleotide usually forms a Phospholipase linkage with the 3'-hydroxyl group of the already synthesized DNA.
2. At the 5'-end of the incoming Nucleotide Triphosphate, there is presence of phosphate group linked to oxygen atom. There is a double bond which should be broken in order to initiate formation of the linkage.
3. Magnesium ion acts as a chelating agent as it accepts the electrons from the oxygen atom thereby enhancing susceptibility of the oxygen to form linkage with the 3'-hydroxyl group of the already synthesized DNA sequence.
4. This is basically due to nucelophilic reaction which in turn leads to shift in the electron position and thus leads to formation of the bond in 5'-3' direction. This process requires high input of energy which cannot be supplemented in case of reaction carried out in 3'-5' direction.

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