Inside every cell of your body is a molecule so elegant it has been called the "secret of life." Twisted into a double helix, built from simple repeating units, DNA stores the instructions that make you uniquely you — and connects every living thing on Earth to a shared molecular heritage.
If you unzipped a single molecule of DNA from one of your cells and stretched it out, it would be about 2 metres long. Your body has roughly 37 trillion cells. How does something so tiny store so much information?
Now answer: What do you think DNA might look like at the molecular level? Draw or describe what you imagine, and list any parts you think it might be made of.
If you could zoom into the nucleus of any cell in your body and peer at the chromosomes, you would see long threads of a molecule wound into a spiral staircase — the famous double helix.
DNA (deoxyribonucleic acid) is not a single mysterious substance. It is a polymer — a large molecule built from many repeating smaller units called nucleotides. Each nucleotide has three parts:
The four nitrogenous bases are adenine (A), thymine (T), guanine (G) and cytosine (C). It is the sequence of these bases along the DNA strand that encodes genetic information — just as the sequence of letters in a sentence encodes meaning.
The two strands of DNA are held together not by strong chemical bonds, but by weaker hydrogen bonds between the bases. This is deliberate — it allows the strands to unzip when the cell needs to read or copy the genetic code.
The base pairing rules are absolute:
These pairings are not arbitrary. The shapes of A and T fit together like puzzle pieces, as do G and C. A cannot pair with G, and C cannot pair with T — the shapes simply do not match.
This strict pairing rule means the two strands are complementary: if you know the sequence of one strand, you automatically know the sequence of the other. This is the key insight that makes DNA replication possible — which you will explore in Lesson 3.
Ancient DNA research in Australia is revealing remarkable stories about the deep past. At Griffith University, researchers have extracted DNA from ancient Aboriginal Australian remains up to 1,600 years old, confirming continuous occupation and deep genetic connections to the land. This work respects strict Cultural and Intellectual Property protocols and demonstrates how understanding DNA structure — especially how it degrades over time — is essential for interpreting ancient genetic evidence. Aboriginal and Torres Strait Islander knowledge of inheritance and kinship has long recognised patterns of heredity that Western science now explains through DNA.
The structure of DNA is beautiful, but its true power lies in what it does: it stores instructions for building and running a living organism.
Think of the four bases (A, T, G, C) as an alphabet with only four letters. The "words" written in this alphabet are typically three bases long — called codons. Each codon specifies a particular amino acid, and chains of amino acids fold into proteins. Proteins do almost everything in a cell: they build structures, speed up chemical reactions, transport materials and fight infections.
A single gene might contain anywhere from a few hundred to over a million base pairs. The human genome contains approximately 3 billion base pairs, organised into about 20,000–25,000 genes. The exact sequence matters enormously: changing just one base can alter the amino acid inserted into a protein, potentially changing its function — this is the basis of genetic variation and mutation.
DNA profiling has transformed Australian criminal investigations. In 1994, New South Wales became the first Australian state to establish a DNA database. One of the most remarkable cases involved the 2005 conviction of a man for a 1984 murder in Victoria, solved when cold-case investigators matched DNA from the crime scene to a sample taken years later for an unrelated offence. Because DNA base sequences are unique to each individual (except identical twins), analysing 13–20 specific regions can identify someone with odds of billions to one. Australian forensic scientists now process over 20,000 DNA samples annually, making it one of the most powerful tools in modern justice.
If the DNA from a single human cell were stretched out, it would measure about 2 metres. Yet it fits inside a nucleus roughly 6 micrometres across — about 300,000 times smaller. How?
DNA is wrapped around proteins called histones, which coil and supercoil into increasingly dense structures. The final packaged form is a chromosome — visible under a microscope during cell division. In humans, there are 46 chromosomes in most body cells, arranged as 23 pairs.
Between cell divisions, chromosomes uncoil into a looser form called chromatin, which allows the cell to access and read genes. This packaging system is dynamic: genes that are actively being used are kept looser, while genes that are not needed are packed away more tightly.
Sickle cell disease is caused by changing just one base in the gene for haemoglobin (the protein that carries oxygen in red blood cells). In the DNA, a T is replaced with an A at position 6 of the beta-globin gene. This changes one amino acid in the protein from glutamic acid to valine. The result? Red blood cells deform into a sickle shape, clogging blood vessels and causing severe pain. This single-letter change in a 3-billion-letter genome demonstrates how precise the genetic code is — and how powerful even tiny mutations can be.
1 Original: 5'- A G C T A T G C -3'
2 Original: 5'- T T A G C C G A T A -3'
3 A strand contains 30% adenine (A). What percentage of the strand is cytosine (C)? Explain your reasoning.
1 Explain why the double helix structure of DNA is essential for its ability to store genetic information.
2 The base pairing rules state that A pairs with T and G pairs with C. Explain how these rules make DNA replication possible. Refer to the concept of "complementary strands."
3 A mutation changes one base in a gene from G to A. The original codon was GGC (codes for glycine). The new codon is GAC. Research or predict: what amino acid does GAC code for, and why might changing one amino acid alter a protein's function?
1. Which of the following is not a component of a DNA nucleotide?
2. According to the base pairing rules, which base pairs with guanine (G)?
3. If one DNA strand has the sequence 5'-A G C T T A-3', what is the sequence of the complementary strand?
4. A DNA molecule contains 20% adenine (A). What percentage of the molecule is guanine (G)?
5. Why is the double helix structure of DNA advantageous for storing genetic information?
6. Describe the structure of a DNA nucleotide. In your answer, name and explain the function of each of the three components. 3 MARKS
7. Explain why the base pairing rules (A-T, G-C) are essential for DNA's ability to store and transmit genetic information. Use the concept of complementary strands in your answer. 4 MARKS
8. A scientist discovers that a particular organism has DNA with a G-C content of 60%. The organism lives in hot thermal vents where DNA is more likely to unzip. Analyse why a high G-C content might be advantageous in this environment. 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. Original: 5'-A G C T A T G C-3' → Complementary: 3'-T C G A T A C G-5'. Every A pairs with T, every G pairs with C, and the strands run antiparallel.
2. Original: 5'-T T A G C C G A T A-3' → Complementary: 3'-A A T C G G C T A T-5'.
3. If A = 30%, then T = 30% (because A pairs with T) [1 mark]. That leaves 40% for G + C combined [1 mark]. Since G = C, each must be 20% [1 mark].
1. The double helix allows long DNA molecules to be compactly coiled and packaged into chromosomes that fit inside the nucleus [1 mark]. The two strands can unzip along the hydrogen bonds between bases, allowing the cell to read or copy the genetic code [1 mark]. The twisted structure also provides physical stability and protects the bases inside [1 mark].
2. Because A always pairs with T and G always pairs with C, the two strands are complementary [1 mark]. This means if you know one strand's sequence, you automatically know the other's [1 mark]. When DNA replicates, each strand serves as a template for building a new complementary strand, ensuring accurate copying [1 mark]. Without base pairing rules, there would be no reliable way to copy or read genetic information [1 mark].
3. GAC codes for aspartic acid (students may look this up) [1 mark]. Changing one amino acid can alter the protein's shape because amino acids have different chemical properties [1 mark]. Protein function depends on precise three-dimensional folding [1 mark]. Even a single amino acid change can disrupt folding, change binding sites or alter the protein's activity — as seen in sickle cell disease [1 mark].
1. C — Amino acids are the building blocks of proteins, not nucleotides. Nucleotides contain sugar, phosphate and a nitrogenous base.
2. B — Guanine (G) always pairs with cytosine (C). Adenine pairs with thymine. Uracil is found in RNA, not DNA.
3. A — Complementary strand must have T opposite A, C opposite G, G opposite C, A opposite T. The strands run antiparallel (5' to 3' vs 3' to 5').
4. D — If A = 20%, then T = 20% (A-T pairing). Remaining 60% is split equally between G and C, so G = 30%.
5. B — The double helix enables compact packaging and allows strand separation for replication and transcription. It does not prevent mutations or make DNA immune to UV damage.
Q6 (3 marks): A nucleotide consists of three components: a sugar (deoxyribose), a phosphate group and a nitrogenous base [1 mark]. The sugar and phosphate form the structural backbone of the DNA strand, linking nucleotides together [1 mark]. The nitrogenous base (A, T, G or C) projects inward and pairs with a complementary base on the opposite strand, encoding genetic information [1 mark].
Q7 (4 marks): The base pairing rules ensure that the two DNA strands are complementary — each base on one strand has a predictable partner on the other [1 mark]. This complementarity is essential because it allows each strand to serve as a template during DNA replication [1 mark]. When the double helix unzips, new nucleotides match up according to the pairing rules, producing two identical DNA molecules [1 mark]. Without these rules, genetic information could not be copied accurately from one generation of cells to the next [1 mark].
Q8 (5 marks): G-C base pairs are held together by three hydrogen bonds, whereas A-T pairs have only two [1 mark]. In hot thermal vents, higher temperatures provide more thermal energy that can break hydrogen bonds [1 mark]. A higher G-C content means more triple-bonded pairs, making the DNA more stable and less likely to unzip at high temperatures [1 mark]. This is an example of how DNA structure adapts to environmental conditions [1 mark]. Organisms in extreme environments often show this pattern, demonstrating the relationship between molecular structure and function [1 mark].
Test your knowledge of DNA structure, nucleotides and base pairing in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of the double helix, nucleotides and base pairing. Pool: Lesson 2.
Tick when you have finished all activities and checked your answers.