DNA and Other Naturally Occurring Polymers | Chemistry

📖 In-Depth Theory

DNA as a Polymer

DNA (deoxyribonucleic acid) is a naturally occurring polymer essential for life.

STRUCTURE:

Two polymer chains made from four different NUCLEOTIDE monomers.

Arranged in a DOUBLE HELIX — two strands twisted around each other.

The four nucleotide monomers differ in their base (A, T, G or C).

Bases pair complementarily: A–T, G–C.

FUNCTION:

Encodes GENETIC INSTRUCTIONS for the development and functioning of all living organisms.

The sequence of bases determines which proteins are made.

Copied when cells divide — ensuring daughter cells inherit the same genetic information.

POLYMER CHEMISTRY PERSPECTIVE:

DNA is a condensation polymer — nucleotide monomers join with loss of water molecules.

This is chemistry-only content — the polymer chemistry of DNA.

The sequence of monomers (bases) determines the polymer's function.

Other Naturally Occurring Polymers

Many important biological molecules are natural polymers:

PROTEINS:

Monomers: amino acids (20 different types).

Joined by peptide bonds (condensation polymerisation — water released).

Function: enzymes, structural proteins (collagen, keratin), antibodies, haemoglobin.

Different sequences of amino acids → different protein shapes → different functions.

STARCH AND CELLULOSE:

Monomers: glucose molecules.

Starch: glucose monomers joined in a branched or helical chain — energy storage in plants.

Cellulose: glucose monomers in straight chains with different linkages — structural, forms plant cell walls.

Same monomer, different polymer structure → very different properties.

NATURAL RUBBER:

Monomer: isoprene (2-methylbuta-1,3-diene).

Addition polymer — the double bonds polymerise.

Elastic properties — used in tyres, gloves, seals.

Comparing Natural and Synthetic Polymers

SIMILARITIES:

Both are large molecules made from repeating monomer units.

Both have properties determined by their monomer sequence/structure.

Both can be addition or condensation polymers.

KEY DIFFERENCES:

Natural polymers: produced by living organisms, often biodegradable, complex 3D structures.

Synthetic polymers: manufactured from chemicals (often from crude oil), often non-biodegradable, simpler structures.

BIODEGRADABILITY:

DNA, proteins, starch, cellulose: broken down by enzymes in living systems.

Poly(ethene), PVC, PTFE: non-biodegradable — persist in environment.

APPLICATIONS:

Natural polymers inspire synthetic design — spider silk proteins inspire strong synthetic fibres.

Biopolymers (e.g. PLA — polylactic acid from corn starch) are biodegradable alternatives to petroleum-based plastics.

⚠️ Common Mistake

DNA is a CONDENSATION polymer — nucleotides join with loss of water. This is different from addition polymers where no by-product forms. Starch and cellulose both have glucose as a monomer but have DIFFERENT structures and properties due to different types of linkage between the glucose units.

📌 Key Note

DNA: condensation polymer of nucleotide monomers, double helix, encodes genetic information. Proteins: condensation polymers of amino acids. Starch/cellulose: glucose monomers, different linkages → different properties. Natural rubber: addition polymer of isoprene. Natural polymers usually biodegradable; synthetic often not.

🎯 Matching Activity — Natural Polymers

Match each natural polymer to its monomer and function. — drag the symbols on the right to match the component names on the left.

DNA

Drop here

Proteins

Drop here

Starch

Drop here

Cellulose

Drop here

Amino acid monomers — form enzymes, structural proteins, antibodies

Glucose monomers in straight chains — structural, forms plant cell walls

Nucleotide monomers — encodes genetic instructions in a double helix

Glucose monomers in helical/branched chains — energy storage in plants

⭐ Higher Tier Only

Describe condensation polymerisation of amino acids to form proteins (peptide bonds, water lost). Explain DNA as a condensation polymer of nucleotides. Distinguish addition from condensation polymerisation using examples. Explain why condensation polymers are potentially biodegradable.

🔬 Triple Science Only

DNA and naturally occurring polymers (4.7.3.4) is chemistry-only — not in Combined Science.

🎯 Test Yourself

Question 1 of 2

1. Why do starch and cellulose have very different properties even though both are made from glucose monomers?

2. What type of polymerisation forms DNA?

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🧪 DNA and Other Naturally Occurring Polymers

DNA as a Polymer

Other Naturally Occurring Polymers

Comparing Natural and Synthetic Polymers

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