Q.Which moieties of nucleosides are involved in the formation of phosphodiester linkages present in dinucleotides? What does the word diester in the name of linkage indicate? Which acid is involved in the formation of this linkage?

Concept: Nucleic Acid Components — The phosphodiester linkage joins nucleotides in a nucleic acid chain.

Reasoning:

1. A nucleoside consists of a nitrogenous base attached to a sugar (ribose or deoxyribose). The sugar has a $5'$ carbon and a $3'$ carbon, each bearing a hydroxyl ($-\text{OH}$) group.
2. In a dinucleotide, the $5'$‑hydroxyl of one nucleoside and the $3'$‑hydroxyl of the next nucleoside each lose a hydrogen atom, while a phosphate group ($\text{H}_3\text{PO}_4$) loses two hydroxyl groups. This forms two ester bonds — one at the $5'$ end and one at the $3'$ end — hence the name diester. …

1. What are the moieties involved?

A nucleoside is made of a nitrogenous base attached to a sugar (ribose in RNA, deoxyribose in DNA). The sugar has two key hydroxyl groups: one at the 3′ carbon and one at the 5′ carbon.

When two nucleosides join to form a dinucleotide, a phosphate group bridges them. The phosphate attaches to the 3′-OH of the first nucleoside and the 5′-OH of the second nucleoside.

So the moieties are:

• The 3′-hydroxyl group of the first nucleoside’s sugar.
• The 5′-hydroxyl group of the second nucleoside’s sugar.

These two hydroxyl groups each lose a hydrogen atom, and the phosphate group provides the oxygen bridges.

2. What does “diester” mean in the name?

An ester is formed when an alcohol reacts with an acid, releasing water. Here, the alcohol is the hydroxyl of a nucleoside, and the acid is phosphoric acid.

In a monoester, the phosphate would be attached to only one alcohol. But in a diester, the same phosphate group has formed two ester bonds — one with the 3′-OH of the first nucleoside and another with the 5′-OH of the second nucleoside.

So the word “diester” tells you that the phosphate is doubly esterified: it is linked to two different sugar molecules.

3. Which acid is involved?

The acid that provides the phosphate group is phosphoric acid ($H_3PO_4$).

In biological systems, it is often in the form of a phosphate ion ($PO_4^{3-}$) or a hydrogen phosphate ($HPO_4^{2-}$), but the parent acid is always phosphoric acid. …

Concept: Structure of Nucleosides and Phosphodiester Linkages in Nucleic Acids

Method: Functional Group Identification in Nucleotide Polymerisation

Step 1 — Recall the components of a nucleoside

A nucleoside consists of:

• A nitrogenous base (purine or pyrimidine)
• A pentose sugar (ribose in RNA, deoxyribose in DNA)

The sugar has a $3'$ hydroxyl group ($-OH$) and a $5'$ hydroxyl group ($-OH$).

Step 2 — Identify the moieties that form the phosphodiester linkage

In a dinucleotide, two nucleosides are joined by a phosphate group that links:

• The $5'$ hydroxyl of one nucleoside’s sugar
• The $3'$ hydroxyl of the next nucleoside’s sugar

Thus, the moieties involved are the $5'$‑OH and $3'$‑OH groups of the pentose sugars.

Step 3 — Interpret the term “diester”

The word diester means two ester bonds are formed by the same phosphate group:

• One ester bond with the $5'$‑OH of the first nucleoside
• One ester bond with the $3'$‑OH of the second nucleoside …

Here are the common mistakes students make on this question, along with how to avoid each.

Mistake 1: Confusing Nucleosides with Nucleotides

• The Error: Students often say the linkage is between the bases or between the sugar and the base. They also sometimes say the linkage involves the "phosphate group of a nucleotide" but then forget that the question specifically asks about nucleosides.
• Why it happens: The terms are very similar. A nucleoside = sugar + base (no phosphate). A nucleotide = sugar + base + phosphate. The phosphodiester linkage is formed after a phosphate is added to a nucleoside, but the question asks which moieties of the nucleosides are involved.
• How to avoid: Memorize the hierarchy:
  1. Nucleoside: Base + Sugar (Ribose or Deoxyribose).
  2. Nucleotide: Nucleoside + Phosphate.
  3. Linkage: The phosphate connects the $3'$ carbon of one nucleoside's sugar to the $5'$ carbon of the next nucleoside's sugar.
• Correct Answer: The sugar moieties (specifically the $3'$ and $5'$ carbon atoms of the pentose sugar) are involved.

Mistake 2: Forgetting the "Di" in Diester

• The Error: Students think "diester" just means "two esters" in a vague sense, or they confuse it with a simple ester bond (like in lipids).
• Why it happens: They don't visualize the structure. A single phosphate group forms two ester bonds.
• How to avoid: Draw the structure. The phosphate group ($\text{H}_3\text{PO}_4$) loses two hydroxyl ($-\text{OH}$) groups. One ester bond links it to the $3'$ carbon of one sugar, and the other ester bond links it to the $5'$ carbon of the next sugar. The phosphate is the "bridge."
• Correct Answer: The word diester indicates that the same phosphoric acid molecule forms two ester bonds (one on each side).

Mistake 3: Naming the Wrong Acid

• The Error: Students answer "Sulfuric acid" or "Nitric acid" or just "Phosphate." They might say "Phosphoric acid" but forget to specify that it is orthophosphoric acid ($\text{H}_3\text{PO}_4$).
• Why it happens: They know "phosphate" is involved, but they don't connect it to the specific acid that provides the $-\text{OH}$ groups for esterification. …