๐ 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 โ two strands twisted around each other.
The four nucleotide monomers differ in their base (A, T, G or C).
Bases pair complementarily: AโT, GโC.
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 โ ensuring daughter cells inherit the same genetic information.
POLYMER CHEMISTRY PERSPECTIVE:
DNA is a condensation polymer โ nucleotide monomers join with loss of water molecules.
This is chemistry-only content โ 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 โ water released).
Function: enzymes, structural proteins (collagen, keratin), antibodies, haemoglobin.
Different sequences of amino acids โ different protein shapes โ different functions.
STARCH AND CELLULOSE:
Monomers: glucose molecules.
Starch: glucose monomers joined in a branched or helical chain โ energy storage in plants.
Cellulose: glucose monomers in straight chains with different linkages โ structural, forms plant cell walls.
Same monomer, different polymer structure โ very different properties.
NATURAL RUBBER:
Monomer: isoprene (2-methylbuta-1,3-diene).
Addition polymer โ the double bonds polymerise.
Elastic properties โ 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 โ persist in environment.
APPLICATIONS:
Natural polymers inspire synthetic design โ spider silk proteins inspire strong synthetic fibres.
Biopolymers (e.g. PLA โ polylactic acid from corn starch) are biodegradable alternatives to petroleum-based plastics.
โ ๏ธ Common Mistake
DNA is a CONDENSATION polymer โ 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.