Ch 1 · 6.1 Structure of nucleic acids and replication of DNA · AS & A Level Biology 9700

Topic 1 of 2 in Nucleic acids and protein synthesis — AS & A Level Biology 9700 — 67 lessons.

Introduction

DNA and RNA carry and express the genetic information of every organism. This lesson builds from a single nucleotide (including ATP, the cell's energy currency), through the antiparallel double helix, to how DNA polymerase and DNA ligase copy DNA one strand at a time, and finishes with the structure of messenger RNA. (Syllabus section 6.1.)

Key ideas — Nucleotides and ATP

A nucleotide has three parts: a pentose sugar, a phosphate group and a nitrogenous base.

🧑‍🏫 Teacher's explanation

Think of DNA as a long, twisted ladder where the side rails are made of repeating sugar-phosphate units and the rungs are made of nitrogenous bases. To make this ladder, you need specific building blocks called nucleotides. If you want to build a second ladder (replication), you don't build it from scratch; you take the first ladder apart and use the existing rungs as a template to build new ones. This is 'semi-conservative'—each new ladder is half-old and half-new.

📖 Definitions

🔍 Worked example — Easy

Identify the components of a nucleotide and classify the bases A, G, C, and T.

🔍 Worked example — Standard

Compare the structure of DNA and mRNA.

🔍 Worked example — Exam-style

Explain why DNA replication is described as semi-conservative during the S phase.

⚠️ Common errors

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🎯 Exam question

Key ideas — Purines and pyrimidines

The bases fall into two groups by size:

🧑‍🏫 Teacher's explanation

Think of DNA as a long, twisting ladder. The 'sides' of the ladder are made of sugar and phosphate, but the 'rungs' are made of nitrogenous bases. To keep the ladder perfectly straight and stable, every rung must be exactly the same width. If you tried to pair a large purine with another large purine, the ladder would bulge; if you paired a small pyrimidine with another small pyrimidine, the ladder would narrow.

📖 Definitions

🔍 Worked example — Easy

Identify the type of nitrogenous base (purine or pyrimidine) for the following set: Adenine, Cytosine, Guanine, Thymine.

🔍 Worked example — Standard

A single strand of DNA has the following sequence of bases: 5' ext-A-G-C-T-T-A-3'. Determine the sequence of the complementary strand in the 3' ext to 5' direction.

🔍 Worked example — Exam-style

A DNA molecule contains 20\% Adenine. Calculate the percentage of Guanine in this DNA molecule.

⚠️ Common errors

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🎯 Exam question

(a) State the percentage of Guanine in the sample. [1] (b) Calculate the percentage of Adenine in the sample. [2] [3 marks]

Key ideas — The DNA double helix

Nucleotides are joined by phosphodiester bonds, formed by condensation between the phosphate of one nucleotide and the sugar of the next, to give a sugar–phosphate backbone. Each strand has a direction: one end has a free phosphate on carbon 5 of the sugar (the 5′ end) and the other a free carbon 3 (the 3′ end).

🧑‍🏫 Teacher's explanation

Think of DNA as a long, twisted ladder. The side rails of the ladder are the sugar-phosphate backbones, which are incredibly strong because they are held together by covalent phosphodiester bonds. The rungs of the ladder are the nitrogenous bases. To keep the ladder stable, the rungs must fit perfectly; this is 'complementary base pairing'.

📖 Definitions

🔍 Worked example — Easy

A single strand of DNA has the following sequence of bases: 5' - A - T - G - C - C - G - 3'. Determine the sequence of the complementary strand in the 3' to 5' direction.

🔍 Worked example — Standard

Calculate the total number of hydrogen bonds present in a double-stranded DNA fragment that contains 10 A-T base pairs and 5 C-G base pairs.

🔍 Worked example — Exam-style

Explain why a DNA molecule with a high percentage of Cytosine (C) requires more thermal energy to denature (separate the strands) than a DNA molecule with a high percentage of Adenine (A).

⚠️ Common errors

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🎯 Exam question

(a) Calculate the percentage of Cytosine in this DNA segment. [2] (b) Explain the role of DNA polymerase during the S phase of the cell cycle in relation to this DNA segment. [2] [4 marks]

Key ideas — Semi-conservative replication

DNA replicates during the S phase of interphase:

🧑‍🏫 Teacher's explanation

Imagine you have a long zipper. To make a copy of it, you don't build a whole new zipper from scratch; instead, you pull the two sides of the existing zipper apart and use each side as a guide to build a new matching side. By the end, you have two zippers, and each one is half 'old' and half 'new'.

📖 Definitions

🔍 Worked example — Easy

A single strand of DNA has the following base sequence: 5' ext - A - T - G - C - C - G - 3'. Determine the sequence of the complementary strand.

🔍 Worked example — Standard

Calculate the percentage of nitrogenous bases in a double-stranded DNA molecule that are Guanine, given that the molecule contains 20\% Adenine.

🔍 Worked example — Exam-style

Explain why the lagging strand is synthesized in short fragments (Okazaki fragments) rather than a single continuous strand.

⚠️ Common errors

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🎯 Exam question

Key ideas — Leading and lagging strands

DNA polymerase can only add nucleotides to the 3′ end of a growing strand, so new DNA is always made in the 5′ to 3′ direction. Because the template strands are antiparallel, the two new strands are made differently:

🧑‍🏫 Teacher's explanation

Imagine you are painting a long, winding road. If you can only paint moving forward, but the road is being built in the opposite direction of your movement, you have a problem. You can paint one side of the road in one long, smooth stroke (the leading strand). But on the other side, you have to wait for a new section of road to appear, run back to the start of that section, paint a small bit, and then repeat the process every time a new section is revealed (the lagging strand).

📖 Definitions

🔍 Worked example — Easy

A DNA template strand has the sequence 3'- ext{TAC-GGC-AAA}-5'$. If the DNA polymerase synthesizes a new strand in the $5'$ to $3'$ direction, what is the sequence of the new strand?

🔍 Worked example — Standard

During DNA replication, a single strand of DNA is being synthesized. If the replication fork is moving in the 5' to 3' direction of the template strand, is the new strand being synthesized as a leading or lagging strand?

🔍 Worked example — Exam-style

A researcher observes DNA replication and identifies short fragments of DNA on one strand. These fragments are eventually joined into a continuous strand. Identify the enzyme responsible for joining these fragments and the type of bond it creates.

⚠️ Common errors

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🎯 Exam question

Key ideas — Messenger RNA

mRNA is a single-stranded polynucleotide made of RNA nucleotides (ribose, phosphate, and the bases A, U, C, G) joined by phosphodiester bonds. It is a copy of one gene, much shorter than DNA, so it can leave the nucleus through the nuclear pores. Its bases are read in triplets (codons) at the ribosome. mRNA is broken down quickly once used, so the cell can change which proteins it makes.

🧑‍🏫 Teacher's explanation

Think of DNA as a massive, permanent master blueprint kept safely in a vault (the nucleus). Because the blueprint is too large and precious to move to the construction site (the ribosome), the cell makes a portable, single-stranded photocopy called mRNA. This photocopy carries the exact instructions for one specific protein.

Ch 1 · 6.1 Structure of nucleic acids and replication of DNA · AS & A Level Biology 9700

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