DNA (deoxyribonucleic acid) and other naturally occurring polymers
What you'll learn
- What DNA is and why it is essential for life.
- How DNA is a natural polymer made from nucleotide monomers.
- The monomers that make proteins, starch and cellulose.
- How to avoid mixing up biological polymers in exam questions.
Starting point: monomers and polymers
Before DNA, proteins, starch and cellulose make sense, you need two key words: monomer and polymer.
Monomer and polymer
A monomer is a small molecule that can join to other similar molecules. A polymer is a large molecule made when many monomers join together in a long chain.
You have already met synthetic polymers, such as poly(ethene), which humans make from alkene monomers. In this chemistry-only part of the course, the focus is on naturally occurring polymers.
Naturally occurring polymer
A naturally occurring polymer is a polymer made by living organisms rather than manufactured in a chemical factory. Examples include DNA, proteins, starch and cellulose.
The main exam skill here is not doing a calculation — it is recognising the polymer and naming the type of monomer it is made from.
DNA: a large molecule essential for life
DNA stands for deoxyribonucleic acid. It is a very large molecule found in cells and is essential for life.
DNA carries the genetic instructions needed for the development and functioning of living organisms. The spec also mentions viruses: viruses are not usually described as living organisms, but they still contain genetic instructions.
What DNA does
DNA encodes genetic instructions. The information is stored in the order, or sequence, of its monomers.
In simple terms, DNA is like the chemical instruction set used to help build and control an organism. You do not need to know detailed genetics for this chemistry topic, but you do need to know that DNA is a polymer.
DNA is made from nucleotide monomers
DNA is a polymer because it is made from many smaller monomers joined together.
The monomers in DNA are called nucleotides.
Nucleotide
A nucleotide is a monomer used to make DNA. DNA contains four different types of nucleotide.
Each DNA nucleotide contains three parts: a sugar, a phosphate group and a base. For GCSE Chemistry, the key point is simpler than the full structure: DNA is made from nucleotide monomers.
Most DNA molecules consist of two polymer chains. These chains twist around each other to form a shape called a double helix.
Double helix
A double helix is a structure made from two chains twisted around each other, like a twisted ladder.
DNA is not made from amino acids
Do not say DNA is made from amino acids. Proteins are made from amino acids. DNA is made from nucleotides.
Why four different nucleotides matter
DNA is not just a long chain of identical monomers. It contains four different nucleotides.
The order of these nucleotides along the DNA chain stores information. That is what the phrase encodes genetic instructions means: the sequence of monomers acts as a chemical code.
You do not need to memorise the detailed names of the four bases for this specification point, but you may see them written as A, T, C and G in diagrams or wider biology work.
A quick memory hook
Link the monomer to the polymer: DNA → nucleotides, proteins → amino acids, starch and cellulose → glucose/sugars.
Other naturally occurring polymers
DNA is not the only important natural polymer. The specification also names proteins, starch and cellulose.
Each of these is a large molecule made from smaller monomer molecules.
| Naturally occurring polymer | Type of monomer | What to remember |
|---|---|---|
| DNA | Nucleotides | Contains four different nucleotide monomers and usually forms a double helix |
| Proteins | Amino acids | Different amino acids join to make protein chains |
| Starch | Glucose, a simple sugar | A plant storage carbohydrate |
| Cellulose | Glucose, a simple sugar | A structural carbohydrate in plant cell walls |
Proteins are polymers of amino acids
A protein is a naturally occurring polymer made from amino acid monomers.
Amino acid
An amino acid is a small molecule that can join with other amino acids to form protein chains.
Proteins are incredibly important in living organisms. For example, many enzymes are proteins. In this topic, though, the main chemistry point is the monomer-polymer link:
amino acids join together → proteins form
Different amino acids can be joined in different orders. This means there are many possible protein structures and many different protein functions.
Protein is the polymer, amino acid is the monomer
Avoid writing that “proteins make amino acids” when asked about the polymer. The monomers are amino acids; the polymer is the protein.
Starch and cellulose are polymers of glucose
Starch and cellulose are both naturally occurring polymers made from glucose monomers. Glucose is a simple sugar.
Glucose
Glucose is a simple sugar molecule. In this topic, it is the monomer used to make starch and cellulose.
This can feel surprising because starch and cellulose have very different jobs:
- Starch is used by plants as an energy store.
- Cellulose is used to make plant cell walls strong.
They can have different properties even though they are made from the same type of monomer, because the glucose units are joined and arranged differently.
Same monomer, different polymer
Starch and cellulose are both made from glucose monomers, but they are different polymers with different structures and uses.
Comparing the key natural polymers
The easiest way to learn this section is to keep matching each polymer to its monomer.
DNA
DNA is a large natural polymer made from nucleotide monomers. Most DNA molecules are two chains twisted into a double helix. DNA stores genetic instructions.
Proteins
Proteins are natural polymers made from amino acid monomers. The order of amino acids affects the protein made.
Starch
Starch is a natural polymer made from glucose monomers. It is used by plants to store energy.
Cellulose
Cellulose is a natural polymer made from glucose monomers. It helps form strong plant cell walls.
Identifying natural polymers from monomer evidence
A question gives four descriptions. Work out the polymer or monomer in each case.
- If a molecule is described as a double helix and contains four different monomers, match this to DNA, because DNA is made from four different nucleotide monomers.
- If a polymer is made from amino acid monomers, identify it as a protein, because amino acids are the building blocks of proteins.
- If a natural polymer is made from glucose and used as an energy store in plants, identify it as starch, because starch is the plant storage carbohydrate.
- If a natural polymer is made from glucose and used in plant cell walls, identify it as cellulose, because cellulose is the structural carbohydrate in plant cell walls.
What exam questions often ask
Questions on this section are usually short, but they can be very precise. You may be asked to:
- state what DNA stands for;
- describe the structure of most DNA molecules;
- name the monomers that make DNA;
- name the monomers that make proteins;
- name the monomers that make starch or cellulose;
- explain what is meant by a naturally occurring polymer.
For structure, write something like:
Most DNA molecules are made from two polymer chains of nucleotide monomers twisted into a double helix.
That sentence includes the key marking points: two chains, polymer, nucleotides and double helix.
Use the word monomer carefully
If the question asks for the monomer, give the small building block: nucleotide, amino acid or glucose. If it asks for the polymer, give the large molecule: DNA, protein, starch or cellulose.
The big picture
This topic links chemistry to biology. The chemistry idea is simple but powerful: large biological molecules are built from smaller units.
A small change in the monomer sequence or arrangement can make a huge difference to the final molecule. That is why DNA can store information, proteins can have many different functions, and glucose can form both starch and cellulose.
In the exam
- If you see DNA, write nucleotide monomers and mention double helix if structure is asked for.
- If you see protein, write amino acid monomers — not nucleotides or glucose.
- If you see starch or cellulose, write glucose monomers or simple sugar monomers, and use the context to tell them apart.
Check yourself
- What are the monomers in DNA called?
- Which natural polymer is made from amino acid monomers?
- How can starch and cellulose be made from the same monomer but have different uses?