AQA A-Level Biology (7402)

## Learning Outcomes

- Define nucleotide, nucleoside, polynucleotide, phosphodiester bond and complementary base pairing accurately
- Describe the structure of DNA nucleotides and RNA nucleotides
- Explain how nucleotides join together to form polynucleotide strands
- Compare DNA and RNA using precise paired structural differences
- Explain how the double helix is formed and why complementary base pairing matters
- Avoid common exam errors such as confusing hydrogen bonds with phosphodiester bonds

## AQA A-Level Biology (7402) Syllabus

For the AQA A-Level Biology (7402), you are expected to understand DNA and RNA structure, with a focus on the following syllabus points:

- Know that DNA and RNA are polymers made from nucleotide monomers
- Identify the three components of a nucleotide and distinguish a nucleotide from a nucleoside
- Explain how a condensation reaction between nucleotides forms a phosphodiester bond
- Describe the sugar-phosphate backbone of a polynucleotide
- Know that DNA consists of two antiparallel polynucleotide strands twisted into a double helix
- Understand that hydrogen bonds form between specific complementary base pairs in DNA
- Compare DNA and RNA in terms of sugar, bases, strand number and length
- Recall that ribosomes contain RNA and proteins, and that RNA transfers genetic information from DNA to ribosomes

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What are the three components of a nucleotide?
2. What is the difference between a nucleoside and a nucleotide?
3. How are adjacent nucleotides joined together in one strand?
4. Why are the two strands of DNA described as antiparallel?
5. Give four structural differences between DNA and RNA.

## Introduction

DNA and RNA are nucleic acids, and both are central to the storage and transfer of genetic information. In exams, this topic often looks simple because the structures can be listed in short bullet points. However, many students lose marks through imprecise wording, especially when they confuse bases with nucleotides, or hydrogen bonds with phosphodiester bonds.

This subtopic is synoptic because it links directly to replication, transcription, translation, mutation and inheritance. If your structural knowledge is weak, later topics become much harder. A strong answer needs exact biological language and clear distinctions between similar terms.

> **Key Term: nucleic acid**
> A biological molecule involved in storing or transferring genetic information; DNA and RNA are the two main nucleic acids.

### Nucleotides, nucleosides and polynucleotides

The starting point is the **nucleotide**, because both DNA and RNA are polymers built from nucleotide monomers. A nucleotide has three parts:

- a **pentose sugar**
- a **phosphate group**
- a **nitrogen-containing organic base**

The sugar is called pentose because it has five carbon atoms. In DNA the sugar is **deoxyribose**. In RNA the sugar is **ribose**. That single difference matters because it is one of the standard paired contrasts examiners expect.

> **Key Term: nucleotide**
> A monomer of DNA or RNA consisting of a pentose sugar, a phosphate group and a nitrogen-containing organic base.

A very common confusion is the word **nucleoside**. A nucleoside is not the same as a nucleotide. It has only two parts:

- a pentose sugar
- a nitrogen-containing base

It does **not** include a phosphate group. If the question asks for the difference, that missing phosphate is the whole point.

> **Key Term: nucleoside**
> A molecule made of a pentose sugar and a nitrogen-containing base, with no phosphate group attached.

The organic bases are:

- **adenine**
- **guanine**
- **cytosine**
- **thymine** in DNA
- **uracil** in RNA

These bases can also be grouped into two families. **Adenine** and **guanine** are **purines**, which are larger double-ring bases. **Cytosine**, **thymine** and **uracil** are **pyrimidines**, which are smaller single-ring bases. This is not just extra detail. Purine-pyrimidine pairing helps keep the DNA molecule at a constant width. If two purines paired, the molecule would be too wide; if two pyrimidines paired, it would be too narrow.

> **Key Term: polynucleotide**
> A long chain polymer formed when many nucleotides join together.

Nucleotides join by a **condensation reaction**. The phosphate group of one nucleotide bonds to the pentose sugar of the next nucleotide. This forms a **phosphodiester bond**. Because this bond is covalent, it is strong and helps create a stable backbone for the strand.

> **Key Term: phosphodiester bond**
> The covalent bond formed between the phosphate group of one nucleotide and the pentose sugar of another nucleotide in a polynucleotide strand.

The repeated chain of sugar-phosphate-sugar-phosphate is called the **sugar-phosphate backbone**. The bases project from this backbone. Students often state that the bases form the backbone, but that is wrong. The bases are attached to the backbone; they do not make it.

> **Key Term: sugar-phosphate backbone**
> The repeating chain of sugars and phosphates that forms the structural outer part of a polynucleotide strand.

Why does this matter biologically? The backbone gives the molecule strength and order, while the sequence of bases provides information. The sugar and phosphate are repetitive; the information changes only when the base sequence changes. That is why different DNA molecules can all share the same general structure but still store different genetic instructions.

> **Test Tip:** When asked to describe a nucleotide, do not just write “sugar, base, phosphate.” Name the correct sugar and, if needed, state that the base is nitrogen-containing. Precise terminology gains marks.

### DNA structure: how the double helix is built

DNA is made of two polynucleotide strands. Each strand has its own sugar-phosphate backbone, and the two strands are joined together by interactions between bases. When these two strands twist around each other, they form the **double helix**.

> **Key Term: double helix**
> The shape of a DNA molecule formed by two polynucleotide strands twisted around each other.

The bases in DNA are **adenine, thymine, cytosine and guanine**. Pairing between these bases is specific:

- adenine pairs with thymine
- cytosine pairs with guanine

This is called **complementary base pairing**.

> **Key Term: complementary base pairing**
> The specific pairing of bases where adenine pairs with thymine in DNA, and cytosine pairs with guanine.

The two strands are held together by **hydrogen bonds** between the paired bases. These are different from phosphodiester bonds. Hydrogen bonds act **between the two strands**. Phosphodiester bonds act **within each strand**. This distinction is one of the most frequent exam traps.

> **Key Term: hydrogen bond**
> A weak bond that forms between complementary bases on opposite DNA strands.

There are **two hydrogen bonds** between adenine and thymine, and **three hydrogen bonds** between cytosine and guanine. This means DNA with a higher proportion of cytosine-guanosine pairings is more stable, because more hydrogen bonds must be broken to separate the strands. In longer synoptic questions, this can be linked to melting temperature or stability of different DNA regions.

The two DNA strands are **antiparallel**, meaning they run in opposite directions. At A-level, the main point is the opposite orientation. A more exact statement is that one strand runs 5′ to 3′ and the other runs 3′ to 5′. You do not always need the numbers, but you do need the term **antiparallel** if the question asks for full structure.

> **Key Term: antiparallel strands**
> Two polynucleotide strands that run in opposite directions.

Why is complementary pairing so useful? It does more than hold the molecule together.

- It keeps the width of the DNA molecule constant because one purine always pairs with one pyrimidine.
- It allows accurate copying of DNA because each strand can act as a template.
- It means the sequence on one strand determines the sequence on the other.

A good exam answer explains these links rather than just listing pairings. For example, if you know one strand has the sequence ACGT, the complementary DNA strand must be TGCA. The second sequence is not random; it is fixed by base-pairing rules.

Worked example:

A DNA molecule contains 68 bases in total, and 22 are adenine. How many are thymine, cytosine and guanine?

Step 1: In double-stranded DNA, adenine pairs with thymine.  
So if adenine = 22, then thymine = 22.

Step 2: Total bases = 68.  
Bases already accounted for = 22 + 22 = 44.

Step 3: Remaining bases = 68 - 44 = 24.  
These must be cytosine and guanine.

Step 4: Cytosine pairs with guanine, so they are equal.  
24 ÷ 2 = 12.

So:

- thymine = 22
- cytosine = 12
- guanine = 12

Students often go wrong by forgetting that the total given is total bases, not base pairs.

> **Test Tip:** In base-ratio questions, first decide whether the total refers to bases or base pairs. Then use A = T and C = G only for double-stranded DNA.

### RNA structure and how it differs from DNA

RNA is also a polymer of nucleotides, so it shares some features with DNA. It has a sugar-phosphate backbone, nucleotides are linked by phosphodiester bonds, and each nucleotide contains a sugar, a phosphate and a nitrogen-containing base. But the differences are essential.

RNA usually differs from DNA in four main ways:

- RNA contains **ribose** instead of deoxyribose
- RNA contains **uracil** instead of thymine
- RNA is usually **single-stranded**
- RNA molecules are usually **shorter**
  > **Key Term: RNA**
  > A nucleic acid that is usually single-stranded, contains ribose sugar and uses uracil instead of thymine.

Many exam questions simply ask for differences between DNA and RNA, but weak answers often give unpaired facts such as “DNA is in the nucleus” or “RNA helps make proteins.” Those may be true in context, but they are not structural differences. A stronger answer uses matched contrasts:

- DNA has **deoxyribose**; RNA has **ribose**
- DNA has **thymine**; RNA has **uracil**
- DNA is **double-stranded**; RNA is **single-stranded**
- DNA is **very long**; RNA is **relatively short**

That pairing shows the examiner you are comparing directly rather than listing facts.

RNA is often described as information transfer molecule, but that statement needs unpacking. One type, **messenger RNA (mRNA)**, carries a copy of base information from DNA to ribosomes. Ribosomes themselves are made from **RNA and proteins**, showing that RNA is not only a messenger but also part of cell structure. Other forms of RNA are involved in protein synthesis too. Even though this article is about structure, examiners like students to connect structure to role. A single-stranded, relatively short molecule is easier to produce quickly and move around the cell than a huge double helix.

A further structural point is that RNA can still form complementary base pairs, but where adenine pairs in RNA, it pairs with **uracil**, not thymine. So in RNA:

- adenine pairs with uracil
- cytosine pairs with guanine

This matters later in transcription and translation, but it is already part of sound structural knowledge.

### Comparing DNA and RNA with exam precision

The best way to revise this topic is to build from monomer to full molecule.

First, identify the monomer:

- DNA and RNA are made of **nucleotides**

Then define the monomer correctly:

- nucleotide = sugar + phosphate + base
- nucleoside = sugar + base

Then explain polymer formation:

- nucleotides join by **condensation reactions**
- this creates **phosphodiester bonds**
- many joined nucleotides form a **polynucleotide**

Then move to whole-molecule structure:

- DNA = two antiparallel polynucleotide strands in a double helix
- strands held together by **hydrogen bonds** between complementary bases
- RNA = usually one relatively short polynucleotide strand

A side-by-side comparison is often the clearest approach:

- **Sugar:** DNA has deoxyribose; RNA has ribose
- **Bases:** DNA has adenine, thymine, cytosine, guanine; RNA has adenine, uracil, cytosine, guanine
- **Strands:** DNA is double-stranded; RNA is usually single-stranded
- **Length:** DNA is very long; RNA is relatively short

Notice what should not be mixed into these structural comparisons unless asked: function, location and enzymes. Students often drift away from structure and lose focus.

There are also classic wording mistakes to avoid:

- Do not say DNA is “made of bases”  
  DNA is made of **nucleotides**; bases are only one part of each nucleotide.

- Do not say phosphodiester bonds join the two DNA strands  
  **Hydrogen bonds** join the strands; phosphodiester bonds join nucleotides within one strand.

- Do not say hydrogen bonds form the sugar-phosphate backbone  
  The backbone is built from **phosphodiester bonds**.

- Do not forget the word **antiparallel** when describing full DNA structure.

- Do not write only letters if the question is clearly asking for named bases  
  Write **adenine**, **thymine**, **cytosine**, **guanine**, **uracil**.

A final synoptic point: DNA was once thought too chemically simple to carry genetic information. That seems surprising now, but it makes sense if you think about it. Proteins contain 20 different amino acids and have varied shapes, so early scientists assumed hereditary material would need similar complexity. The key insight was that DNA does not rely on varied chemistry in its backbone. Instead, its informational power comes from the **sequence** of its four bases. A simple repeating structure can still store huge amounts of information if the order of the bases changes.

> **Test Tip:** For a 3- or 4-mark “compare DNA and RNA” question, aim for paired statements. Four matched contrasts usually scores better than four isolated facts.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- DNA and RNA are nucleic acids and both are polymers of nucleotides
- A nucleotide consists of a pentose sugar, a phosphate group and a nitrogen-containing organic base
- A nucleoside contains a pentose sugar and a base, but no phosphate group
- DNA nucleotides contain deoxyribose and one of the bases adenine, thymine, cytosine or guanine
- RNA nucleotides contain ribose and one of the bases adenine, uracil, cytosine or guanine
- Adjacent nucleotides join by condensation reactions that form phosphodiester bonds
- Phosphodiester bonds form the sugar-phosphate backbone of each polynucleotide strand
- DNA consists of two antiparallel polynucleotide strands twisted into a double helix
- The two DNA strands are held together by hydrogen bonds between complementary base pairs
- In DNA, adenine pairs with thymine and cytosine pairs with guanine
- RNA is usually single-stranded and relatively short compared with DNA
- Ribosomes are made of RNA and proteins, and RNA transfers genetic information from DNA to ribosomes

## Key Terms and Concepts

- nucleic acid
- nucleotide
- nucleoside
- polynucleotide
- phosphodiester bond
- sugar-phosphate backbone
- double helix
- complementary base pairing
- hydrogen bond
- antiparallel strands
- RNA


Biological molecules - DNA and RNA structure