DNA Polymerization Revision Notes | A Level AQA Chemistry

1. Why DNA Polymerization Matters

Every instruction your cells follow — from building an enzyme to replicating before division — is encoded in DNA. To understand how that information is stored, you need to understand how DNA is built at the chemical level.

DNA is a polymer, which means it is a long chain of repeating units called monomers. In DNA, those monomers are nucleotides. The way nucleotides link together — and the specific bonds that hold them — determines the structure of the entire strand. This is what AQA calls DNA polymerization.

At A-level, you are expected to go beyond naming the components. You need to explain precisely which part of one nucleotide bonds to which part of the next, and what kind of bond forms. That level of chemical precision is what separates a 4-mark answer from a 2-mark one.

2. The Building Block: Anatomy of a DNA Nucleotide

Before you can understand how nucleotides join, you need a clear picture of what one nucleotide actually contains. Picture a nucleotide as having three distinct regions bonded together: a phosphate group on the outside, a five-carbon sugar in the middle, and a nitrogenous base projecting inward.

The four possible bases — adenine (A), thymine (T), guanine (G), and cytosine (C) — attach to the 1' carbon of the deoxyribose. The phosphate group attaches to the 5' carbon. This numbering matters because it defines the directionality of the chain once nucleotides polymerise.

Notice how the phosphate group sits at one end of the nucleotide and the sugar occupies the centre. When two nucleotides come together, it is these two regions — the phosphate of one and the sugar of the next — that form the covalent bond holding the chain together.

3. Forming the Chain: The Phosphodiester Bond

Here is where the chemistry becomes precise. When two nucleotides join, a condensation reaction occurs: a water molecule is eliminated, and a covalent bond forms between the phosphate group of one nucleotide and the 3' carbon of the 2-deoxyribose of the adjacent nucleotide.

The name 'phosphodiester' tells you the chemistry directly: 'phospho' refers to the phosphate group, and 'diester' means the phosphate forms two ester linkages — one to the 3' carbon of the preceding sugar and one to the 5' carbon of the following sugar. Each phosphate group therefore bridges two deoxyribose units.

This creates a directional chain. One end of the strand has a free 5' phosphate group (the 5' end), and the other has a free 3' hydroxyl group (the 3' end). Chemists describe this as the strand running 5' to 3'. This directionality is fundamental to DNA replication and transcription, though the detailed mechanisms of those processes sit beyond this particular spec point.

4. The Resulting Structure: Sugar-Phosphate Backbone

Once many nucleotides have been linked by phosphodiester bonds, the result is a polynucleotide strand. Zoom out from the individual bond and you see the pattern: the backbone of the strand alternates continuously between deoxyribose sugar units and phosphate groups — sugar, phosphate, sugar, phosphate — repeating along the entire length of the chain.

Projecting from each sugar, at the 1' carbon, is one of the four nitrogenous bases. The bases are not part of the backbone — they hang off it like pendants on a necklace. This arrangement is critical: the backbone provides structural integrity through covalent bonds, while the bases carry the genetic information through their sequence.

• The backbone consists of alternating 2-deoxyribose and phosphate units, linked by covalent phosphodiester bonds.
• Each phosphate bridges the 3' carbon of one sugar to the 5' carbon of the next.
• Each deoxyribose carries one nitrogenous base at its 1' carbon — either adenine, thymine, guanine, or cytosine.
• The strand has a defined directionality: a 5' end (free phosphate) and a 3' end (free hydroxyl group).

It is worth pausing on the distinction between the backbone bonds and the base-attachment bonds. The phosphodiester bonds in the backbone are the ones that polymerise the chain — they are the bonds formed during DNA synthesis. The glycosidic bond between each base and its sugar is a separate bond, already present within the nucleotide monomer before polymerization begins.

Worked example: Tracing the Backbone of a Trinucleotide

A trinucleotide contains the bases adenine, guanine, and cytosine (in that order, 5' to 3'). Describe the complete covalent backbone connecting all three nucleotides, naming every component in sequence.

1. 5'-phosphate group attached to the 5' carbon of deoxyribose-1 (carrying adenine at its 1' carbon) — The free phosphate at the 5' end is the starting point of the strand. It is not bridging two sugars here — it has only one ester linkage.
2. 3' carbon of deoxyribose-1 — O — phosphate — O — 5' carbon of deoxyribose-2 (carrying guanine) — This phosphate bridges the first and second sugars. It forms two ester bonds: one to the 3'-O of sugar-1 and one to the 5'-O of sugar-2. Water was eliminated when this bond formed.
3. 3' carbon of deoxyribose-2 — O — phosphate — O — 5' carbon of deoxyribose-3 (carrying cytosine) — The same pattern repeats. Each internal phosphate group is a diester — bonded to two different sugars simultaneously.
4. Free 3'-OH group on deoxyribose-3 — The 3' end has no further phosphate group attached. This free hydroxyl is where the next nucleotide would be added during DNA synthesis.
5. 5'-phosphate — sugar(A) — phosphate — sugar(G) — phosphate — sugar(C) — 3'-OH — The repeating pattern is sugar-phosphate-sugar-phosphate. Bases project from each sugar but are not part of the backbone connectivity.

Answer: The backbone runs: 5'-phosphate — deoxyribose(A) — phosphodiester bond — deoxyribose(G) — phosphodiester bond — deoxyribose(C) — 3'-OH. Two phosphodiester bonds link the three nucleotides, each bridging the 3' carbon of one sugar to the 5' carbon of the next.

5. Chemical Precision: Why These Bonds Matter

The phosphodiester bond is a covalent bond — one of the strongest bond types in chemistry. This is not incidental. The genetic information encoded in the base sequence must be preserved with extraordinary fidelity across billions of cell divisions. A covalent backbone provides the chemical stability required for that long-term storage.

Contrast this with the hydrogen bonds that hold the two strands of the double helix together. Those are weak, non-covalent interactions — strong enough to maintain the double-stranded structure under normal conditions, but weak enough to be broken during replication and transcription. The covalent backbone, by contrast, remains intact throughout those processes.

The use of 2-deoxyribose rather than ribose also contributes to stability. The absence of the 2'-OH group makes the backbone less susceptible to hydrolysis under physiological conditions. RNA, which uses ribose, is considerably less stable — a chemical consequence of that single structural difference.

6. Visualising the Full Polynucleotide Strand

Earlier, we built up the bond-by-bond picture of how nucleotides connect. Now step back and see the whole strand at once. The molecule-viewer below shows a short DNA single strand — look for the repeating sugar-phosphate units along the outside of the structure, with the bases projecting inward (in the context of the double helix) or outward (as a free single strand).

When you look at this structure, focus on the backbone. You should be able to trace the alternating pattern: phosphate — sugar — phosphate — sugar — running continuously from the 5' end to the 3' end. Each sugar has a base attached, but those bases do not contribute to the chain's connectivity — they are passengers on the backbone, not load-bearing members of it.

7. Exam Technique and Common Mistakes

AQA questions on DNA structure frequently use command words such as 'describe', 'explain', and 'state'. Each demands a different level of response. 'State the type of bond between nucleotides' requires only the term 'phosphodiester bond' — no further explanation. 'Explain how nucleotides are joined' requires you to name the bond, identify the reacting groups (phosphate and 3'-OH of deoxyribose), and state that a condensation reaction occurs with elimination of water.

Mark schemes for 'describe the structure of a DNA strand' questions typically award separate marks for: (1) identifying nucleotides as the monomers, (2) naming the phosphodiester bond, (3) specifying that the bond forms between the phosphate of one nucleotide and the deoxyribose of the next, and (4) describing the resulting sugar-phosphate backbone with bases attached. Missing any one of these loses a mark — precision at every step is rewarded.

To consolidate the full picture: a single strand of DNA is a covalently bonded polymer in which phosphodiester bonds link the phosphate group of each nucleotide to the 2-deoxyribose of the next, generating a continuous sugar-phosphate backbone with bases projecting from each sugar. This precise molecular architecture — covalent backbone, projecting bases, defined directionality — is the foundation on which all of DNA's biological functions depend.