DNA (Biology Link)

DNA Structure

DNA (deoxyribonucleic acid) is a natural polymer found in all living cells. It has a distinctive double helix shape, which looks like a twisted ladder.

The structure of DNA is made up of repeating units called nucleotides. Each nucleotide consists of three parts:

• a sugar molecule (deoxyribose)
• a phosphate group
• a nitrogenous base

The sugar and phosphate groups form the backbone of the DNA strand, while the bases stick out from the backbone and pair with bases on the opposite strand.

There are four types of bases in DNA:

• Adenine (A)
• Thymine (T)
• Cytosine (C)
• Guanine (G)

Base pairing rules mean that A always pairs with T, and C always pairs with G. These pairs are held together by hydrogen bonds, which help stabilise the double helix. These hydrogen bonds are relatively weak, allowing the two strands to separate during DNA replication.

For example, if one strand has the sequence A-T-G-C, the complementary strand will be T-A-C-G.

This complementary base pairing is crucial for DNA replication and for storing genetic information accurately.

DNA as a Natural Polymer

DNA is a natural polymer, meaning it is made by living organisms by joining many small units (nucleotides) together in a long chain.

Its main biological function is to store and transmit genetic information. The sequence of bases along a DNA strand forms a code that carries instructions for building proteins.

Proteins are made from chains of amino acids. The order of bases in DNA determines the order of amino acids in a protein, which in turn determines the protein’s shape and function.

This connection between DNA and proteins is fundamental to all life processes, as proteins perform many vital roles in cells, including enzymes, structural components, and hormones.

For instance, a gene is a segment of DNA that codes for a specific protein. The process of protein synthesis involves copying the DNA code into messenger RNA (mRNA), which then guides amino acid assembly.

Synthetic Polymers Overview

Unlike natural polymers such as DNA, synthetic polymers are man-made materials created by joining many small molecules called monomers into long chains. Many synthetic polymers are hydrocarbons or derived from hydrocarbons, linking this topic to organic chemistry.

Synthetic polymers are produced through chemical processes and are widely used in everyday products.

There are two main types of synthetic polymers:

• Addition polymers: formed by joining monomers with double bonds (alkenes) without losing any small molecules.
• Condensation polymers: formed when monomers join together with the loss of small molecules like water.

Examples include:

• Addition polymer: Poly(ethene) made from ethene monomers.
• Condensation polymer: Polyester made from dicarboxylic acids and diols.

Synthetic polymers often have different properties from natural polymers, such as being more durable or resistant to chemicals, but they are usually less biodegradable.

For example, DNA is biodegradable and breaks down naturally in the environment, whereas many synthetic plastics can persist for hundreds of years.

Comparison of Natural and Synthetic Polymers

Property	Natural Polymers (e.g. DNA)	Synthetic Polymers
Source	Produced by living organisms (biological origin)	Man-made, produced chemically from petroleum or other raw materials
Structure	Polymers of nucleotides with precise base sequences	Polymers of simple monomers, often repeating units
Biodegradability	Biodegradable and broken down by enzymes	Usually non-biodegradable, can cause pollution
Uses	Genetic information storage, biological functions	Packaging, clothing, containers, medical devices
Environmental Impact	Natural recycling in ecosystems	Plastic waste pollution, microplastics in oceans

The environmental impact of synthetic polymers is a major concern. Many plastics do not break down easily, leading to waste accumulation. Efforts are ongoing to develop biodegradable synthetic polymers to reduce pollution and environmental harm.

Natural polymers like DNA are essential for life and are recycled naturally by living organisms, so they do not cause long-term environmental harm.

For example, cotton (a natural polymer made of cellulose) biodegrades naturally, while synthetic polyester can take decades to break down.

Learning Example: Understanding DNA Base Pairing

If one strand of DNA has the base sequence G-T-A-C, what is the sequence of the complementary strand?

Using base pairing rules:

• G pairs with C
• T pairs with A
• A pairs with T
• C pairs with G

So, the complementary strand is C-A-T-G.