Question 1

Which tautomeric shift in cytosine is depicted in the first image, and why is this shift significant in DNA replication?

Accepted Answer

Amino to imino; it can cause mispairing and lead to mutations during DNA replication.

Question 2

What is the structural difference between β-D-ribose and 2-deoxy-β-D-ribose as shown in the second image, and how does this difference affect the properties of DNA and RNA?

Accepted Answer

β-D-ribose has a hydroxyl group at C2, while 2-deoxy-β-D-ribose lacks this group.

Question 3

At physiological pH, which ionization state of the phosphate group predominates in nucleotides, as shown in the third image?

Accepted Answer

Dihydrogenphosphate ion ($$\mathrm{H_2PO_4^-}) $$and hydrogenphosphate ion ($$\mathrm{$$HPO_4$$^{2-}})$$

Question 4

Identify the nucleotide shown in the fourth image and explain its components.

Accepted Answer

Adenosine monophosphate (AMP); composed of adenine, ribose, and a phosphate group.

Question 5

In the fifth image, which type of bond links the phosphate group to the pentose sugar, and what is its significance?

Accepted Answer

Phosphoester bond; it links the phosphate group to the 5' carbon of the sugar, forming the backbone of nucleic acids.

Question 6

Based on the sixth image, what reaction occurs during the formation of a dinucleotide from UMP and CMP, and what is the byproduct?

Accepted Answer

Condensation; water is released.

Question 7

Referencing the seventh image, what is the significance of the 3' and 5' ends in a nucleotide, and how does this relate to nucleic acid directionality?

Accepted Answer

The 5' end contains a phosphate group, and the 3' end contains a hydroxyl group; nucleic acids are synthesized from 5' to 3'.

Question 8

In the eighth image, which base is NOT present in RNA, and what is its equivalent in DNA?

Accepted Answer

Thymine; its equivalent in RNA is uracil.

Question 9

The ninth image shows a DNA polynucleotide chain. What type of linkage connects the nucleotides, and what is the functional significance?

Accepted Answer

Phosphodiester linkage; it forms the backbone and imparts directionality to the DNA strand.

Question 10

In the tenth image, which torsion angle defines the rotation of the base relative to the sugar, and why is this important?

Accepted Answer

Angle χ; it defines the syn and anti conformations of the nucleoside.

Question 11

The eleventh image shows shorthand notation for DNA and RNA. What does the 'p' symbol represent in these diagrams?

Accepted Answer

Phosphate group, indicating the presence of a phosphodiester bond.

Question 12

According to the twelfth image, which modification of nucleoside analogues is used to increase their hydrophobicity and cellular uptake?

Accepted Answer

Protection by polar groups on the phosphate moiety.

Question 13

In the thirteenth image, which nucleoside analogue is used for the treatment of HIV and what is its structural modification?

Accepted Answer

Zidovudine (AZT); it has an azido group at the 3' position.

Question 14

Based on the fourteenth image, which phosphorus functional group is present in phosphoramidate prodrugs, and what is its chemical bond type?

Accepted Answer

Phosphoramidate; contains P-N bonds.

Question 15

Which property of the nitrogenous bases allows nucleic acids to be analyzed by UV spectrophotometry at 260 nm?

Accepted Answer

Their aromatic ring structure.

Question 16

Why are L-nucleoside analogues often less toxic to mammalian cells but still effective against viruses?

Accepted Answer

L-nucleosides are not recognized by mammalian enzymes but are recognized by virus-encoded enzymes.

Question 17

Which cyclic nucleotide is a key regulator of cell metabolism and is formed by a phosphodiester bond between the 3' and 5' hydroxyl groups of ribose?

Accepted Answer

Cyclic adenosine monophosphate (cAMP)

Question 18

What is the main reason that the anti conformation is more prevalent than the syn conformation in nucleosides?

Accepted Answer

Anti conformation is energetically favored due to reduced steric clashes between the base and sugar.

Nucleosides, Nucleotides, and Polynucleotides: Structure, Properties, and Analogues

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