Every cell needs instructions. A cell must know which substances to make, how to grow and how to carry out its specialised functions. Much of the information needed to produce these cellular components is stored in a remarkable molecule called DNA.
DNA may seem complicated at first, but its basic structure follows a small number of simple rules. It is made from repeating nucleotides, there are only four possible bases, and those bases pair in predictable combinations. Once these ideas are connected, the relationship between DNA, genes and chromosomes becomes much easier to understand.
Try these five questions before reading the chapter. They test the basic relationships between DNA, nucleotides, bases, genes and chromosomes.
- 5 diagnostic questions
- Core DNA structure and terminology
- Instant explanation for every answer option
How DNA, genes and chromosomes fit together
Before looking closely at the DNA molecule, it helps to place DNA in its biological context. In a typical animal or plant cell, most of the DNA is found in the nucleus, where it is organised into chromosomes.
Chromosomes contain DNA associated with proteins. Along the DNA are particular sequences called genes. A gene is therefore not a separate substance floating beside the DNA; it is a specific region within the DNA molecule itself.
The relationship to remember:
Nucleus → contains chromosomes → chromosomes contain DNA →
particular sequences of DNA are genes.
This distinction is important because the words are sometimes used as though they mean the same thing. They do not. A chromosome is a larger structure, DNA is the molecule carrying the genetic information, and a gene is a particular nucleotide sequence within that DNA.
DNA is built from repeating nucleotides
A DNA molecule is extremely long, but it is assembled from many smaller repeating units called nucleotides. The easiest way to understand DNA is therefore to understand one nucleotide first.
Every DNA nucleotide has three components.
| Component | What you need to know |
|---|---|
| Phosphate group | Forms part of the repeating outer backbone of a DNA strand. |
| Deoxyribose sugar | The five-carbon sugar found in DNA. It also forms part of the backbone. |
| Nitrogenous base | The part that can vary between nucleotides. The base may be A, T, C or G. |
One DNA nucleotide = phosphate + deoxyribose sugar + one nitrogenous base.
A common source of confusion is to use the words base and nucleotide as though they mean the same thing. A base is only one component of a nucleotide. The nucleotide also includes the sugar and phosphate.
The four nitrogenous bases in DNA
The phosphate group and deoxyribose sugar are repeated throughout DNA. What changes from one nucleotide to another is the nitrogenous base.
DNA contains four types of nitrogenous base:
| Base | Symbol | Complementary partner |
|---|---|---|
| Adenine | A | Thymine |
| Thymine | T | Adenine |
| Cytosine | C | Guanine |
| Guanine | G | Cytosine |
These four bases provide an elegant way of storing information. DNA does not need hundreds of different types of building block. Instead, information is stored in the order in which A, T, C and G occur.
How nucleotides form a DNA strand
Nucleotides join together to form long chains. Along each chain, the sugars and phosphate groups repeat, creating what is called the sugar-phosphate backbone.
The bases project from this backbone. Because the sugar-phosphate arrangement repeats, it is not the backbone itself that provides the changing genetic message. The information comes from the sequence of bases attached along the strand.
Think of the two parts separately:
Sugar-phosphate backbone: repeated structural framework of the DNA strand.
Sequence of bases: variable order that carries genetic information.
DNA consists of two strands forming a double helix
A complete DNA molecule contains two nucleotide strands. The strands lie alongside one another, with their sugar-phosphate backbones on the outside and the bases directed towards the centre.
The two strands twist around each other. This produces the characteristic double helix structure of DNA.
Double helix: two strands of nucleotides twisted around each other, with sugar-phosphate backbones on the outside and paired bases between the strands.
Textbooks sometimes compare DNA with a twisted ladder. Used carefully, the comparison is useful: the two sugar-phosphate backbones form the sides, while paired bases connect the two strands across the centre. The important point is that the real molecule is twisted rather than flat.
Complementary base pairing joins the two strands
The bases on opposite DNA strands do not pair randomly. Each base has a specific complementary partner:
A pairs with T
C pairs with G
This is known as complementary base pairing. The bases of each pair are held together by hydrogen bonds, helping to hold the two DNA strands together.
Because the pairing rules are fixed, the sequence on one strand determines the sequence on the other.
| Position | 1 | 2 | 3 | 4 | 5 | 6 |
|---|---|---|---|---|---|---|
| One DNA strand | A | G | T | C | C | A |
| Complementary strand | T | C | A | G | G | T |
Notice that the second strand is complementary to the first. It is not simply an identical copy. Every A requires a T opposite it, while every C requires a G.
How to work out a complementary DNA sequence
Take the sequence one base at a time. Replace each base with its complementary partner.
| If you see | Write |
|---|---|
| A | T |
| T | A |
| C | G |
| G | C |
For example:
DNA strand: A–T–G–C–C–A
Complementary strand: T–A–C–G–G–T
The safest approach in an examination is to move from left to right and apply the pairing rule to each position individually.
How DNA carries genetic information
The sugar and phosphate groups repeat throughout DNA, so they do not provide the changing message. What varies is the sequence of nitrogenous bases.
Consider two short sequences:
A–T–G–C–C–A
A–C–G–T–C–A
Both contain the same four types of base, but their order is different. That difference in sequence represents different genetic information. In real genes, the sequences are far longer than these simple examples.
The key idea is therefore not simply that DNA contains A, T, C and G. Almost all DNA does. The important information lies in which bases occur and the order in which they occur.
What exactly is a gene?
A gene is a particular sequence of nucleotides within DNA. At O-Level, there are three important parts to the definition.
| Feature of a gene | Meaning |
|---|---|
| A sequence of nucleotides | A gene consists of a particular order of nucleotides within DNA. |
| Codes for a polypeptide | The nucleotide sequence contains information used to determine the amino acid sequence of a polypeptide. |
| A unit of inheritance | Genes can be passed from parents to offspring. |
Exam definition: A gene is a sequence of nucleotides, as part of a DNA molecule, that codes for a polypeptide and is a unit of inheritance.
A polypeptide is a chain of amino acids. Polypeptides fold and may form proteins, which can then contribute to the structure and functioning of cells.
This explains why the order of bases matters. Changing the sequence of DNA can change the genetic information and may affect the polypeptide produced.
Putting the entire DNA structure together
We can now build the complete picture from the smallest unit upwards.
| Level | Key idea |
|---|---|
| Nucleotide | Contains phosphate, deoxyribose sugar and one nitrogenous base. |
| DNA strand | A long chain of nucleotides with a sugar-phosphate backbone. |
| DNA molecule | Two complementary nucleotide strands twisted into a double helix. |
| Gene | A particular nucleotide sequence within DNA that codes for a polypeptide. |
| Chromosome | A structure containing DNA associated with proteins and carrying many genes. |
Instead of memorising these as five unrelated definitions, see them as parts of the same story. Nucleotides build DNA. DNA contains genes. DNA is organised into chromosomes. The sequence of bases within the DNA carries the genetic information.
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Saying a nucleotide is just a nitrogenous base | A base is only one component of a nucleotide. | State that a DNA nucleotide contains phosphate + deoxyribose sugar + a nitrogenous base. |
| Pairing A with C or T with G | DNA follows specific complementary base-pairing rules. | Remember A–T and C–G. |
| Saying the two DNA strands have identical base sequences | The strands are complementary, not normally identical. | Use the pairing rules to derive one strand from the other. |
| Saying the sugar-phosphate backbone carries the genetic code | Sugar and phosphate repeat along the strand. | State that genetic information is carried by the sequence of bases. |
| Treating DNA, genes and chromosomes as the same thing | They describe different levels of organisation. | A chromosome contains DNA, while a gene is a particular sequence within DNA. |
| Saying a gene is made of protein | A gene is DNA. It carries information used in making a polypeptide. | Define a gene as a sequence of nucleotides in DNA. |
| Saying a gene directly “is” a characteristic | A gene is genetic information, not the visible characteristic itself. | State that a gene codes for a polypeptide, which can influence cell structure or function and therefore phenotype. |
| Calling a single base a gene | A gene consists of a sequence of many nucleotides. | Think of a gene as a particular stretch of DNA with a specific nucleotide sequence. |
What you should be able to do
Describe a DNA nucleotide
State that it contains a phosphate group, deoxyribose sugar and a nitrogenous base.
Name the four DNA bases
Recall adenine, thymine, cytosine and guanine and their symbols A, T, C and G.
Apply complementary base pairing
Use A–T and C–G to determine the complementary sequence of a DNA strand.
Describe the double helix
Explain that DNA consists of two nucleotide strands twisted around each other.
Explain how DNA stores information
State that genetic information is determined by the sequence of bases along DNA.
Define a gene
Describe a gene as a sequence of nucleotides in DNA that codes for a polypeptide and is a unit of inheritance.
Relate genes, DNA and chromosomes
Explain that genes are sequences within DNA and that DNA is organised into chromosomes.
Avoid nucleotide–base confusion
Recognise that A, T, C and G are bases, while a complete nucleotide also contains sugar and phosphate.
These ten questions test whether you can connect nucleotide structure, complementary base pairing, base sequences, genes and chromosomes rather than simply memorising individual definitions.
- 10 mastery questions
- Sequence, structure and inheritance reasoning
- Instant explanation for every answer option
Frequently asked questions
The Structure of DNA: questions students often ask
Once the overall structure is clear, most mistakes come from mixing up nucleotides with bases or treating DNA, genes and chromosomes as though they were interchangeable terms.
Is DNA the same thing as a chromosome?
No. DNA is the molecule that carries genetic information. A chromosome is a structure containing DNA associated with proteins. Each chromosome carries many genes along its DNA.
Is a gene the same thing as DNA?
Not exactly. DNA is the long molecule, while a gene is a particular sequence of nucleotides within that DNA. A chromosome therefore contains DNA carrying many different genes.
What is the difference between a base and a nucleotide?
A nitrogenous base is only one part of a nucleotide. A complete DNA nucleotide contains a phosphate group, deoxyribose sugar and one nitrogenous base. The base may be A, T, C or G.
Why is DNA called a double helix?
DNA contains two nucleotide strands, which explains the word “double”. These strands twist around each other in a helical shape, giving the molecule its double-helix structure.
What forms the sides of the DNA molecule?
Each DNA strand has a repeating sugar-phosphate backbone. The nitrogenous bases are attached to the sugars and face towards the bases on the opposite strand.
What holds the two DNA strands together?
Complementary nitrogenous bases on the two strands pair with one another and are held together by hydrogen bonds. Adenine pairs with thymine, while cytosine pairs with guanine.
Are the two DNA strands identical?
No. They are complementary. If one strand contains A at a particular position, the other has T at that position. If one has C, the other has G.
How does DNA store so much information using only four bases?
The information depends on the sequence of the bases, not simply on which four types exist. A very long DNA molecule can contain an enormous number of possible sequences of A, T, C and G.
Why must adenine pair with thymine and cytosine with guanine?
These are the specific complementary base-pairing relationships in DNA. Their molecular structures allow A to form the appropriate interactions with T and C with G. At O-Level, the essential pairing rules to remember are simply A–T and C–G.
If I know one DNA strand, can I work out the other?
Yes. Apply complementary base pairing one base at a time. For example, if one strand is A–C–G–T, the complementary bases are T–G–C–A.
What does it mean when we say a gene codes for a polypeptide?
It means the sequence of nucleotides in the gene contains information that determines the amino acid sequence of a polypeptide. A polypeptide is a chain of amino acids that folds and may form all or part of a protein.
Why is a gene called a unit of inheritance?
Genes are carried in DNA on chromosomes and can be passed from parents to offspring. They therefore transmit genetic information from one generation to the next.
If 25% of the bases in double-stranded DNA are adenine, what percentage are thymine?
Thymine would also make up 25%. Every adenine on one strand pairs with a thymine on the other, so double-stranded DNA contains equal numbers of A and T. For the same reason, the numbers of C and G are equal.
What is the most important DNA definition to memorise for an exam?
Rather than memorising only one sentence, make sure you can connect the key ideas: DNA is made from nucleotides; each nucleotide contains phosphate, deoxyribose and a base; DNA has two strands in a double helix; A pairs with T and C with G; and the sequence of bases carries genetic information.
DNA becomes much less mysterious once you build it from the nucleotide upwards. Each nucleotide contains phosphate, deoxyribose and one of four bases. Nucleotides join to form strands with sugar-phosphate backbones, two complementary strands come together through A–T and C–G base pairing, and the two strands twist into a double helix. Most importantly, the changing sequence of those bases stores genetic information. A gene is simply a particular stretch of that DNA whose nucleotide sequence carries the information for a polypeptide and can be inherited from one generation to the next.