DNA Structure and Function
In 1953, Watson, Crick and Franklin revealed that DNA packs 3.2 billion base pairs into a nucleus just 6 micrometres wide.
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Q1 · What do you already know about the structure of DNA? Describe it in your own words.
Think about shape, building blocks and how the two strands fit together.
Q2 · Why do you think DNA needs to be copied before a cell divides? What might go wrong if it isn't?
Consider what would happen if a dividing cell did not have a complete copy of its genetic instructions.
● Know
- That DNA is a double helix made of nucleotides
- The three parts of a nucleotide: sugar, phosphate, nitrogenous base
- The base pairing rules: A pairs with T, G pairs with C
- That DNA stores genetic instructions in the sequence of bases
● Understand
- How the structure of DNA enables it to store and copy information
- Why complementary base pairing is essential for DNA's function
- How the sequence of bases forms a genetic "code"
● Can do
- Label the parts of a DNA molecule on a diagram
- Write the complementary strand for any DNA sequence
- Explain why DNA structure matters for its function
Hold a strand of human hair up to the light: invisible inside every cell of that hair is a twisted ladder carrying 3.2 billion coded instructions. DNA, short for deoxyribonucleic acidis the molecule that carries genetic instructions in every living thing. Its structure is a double helix: two long strands twisted around each other like a spiral staircase. Each strand is built from smaller units called nucleotides, and each nucleotide contains one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G).
The two strands are held together by specific pairing rules: A always pairs with T, and C always pairs with G. These pairs are called complementary base pairs, and they are the key to DNA's ability to copy itself. If you know the sequence of one strand, you automatically know the sequence of the other. This structure was discovered by Watson and Crick in 1953 using X-ray data from Rosalind Franklin, and it remains one of the most elegant findings in all of science.
Here is a short section of DNA: strand one reads A-T-C-G-G-A. Because of complementary pairing, strand two must read T-A-G-C-C-T. This pairing rule means that during replication, each strand can act as a template to build a new matching partner, like unzipping a zipper and using each side to build a new zipper.
Australian research: The Walter and Eliza Hall Institute in Melbourne uses knowledge of DNA structure to study how genes are switched on and off during immune responses. Understanding base pairing helps researchers design targeted therapies for diseases like cancer and malaria.
Many students think DNA does the work directly inside cells. In reality, DNA is an instruction manual. It does not move muscles, digest food or fight infections. Instead, DNA codes for proteins, long chains of amino acids that fold into molecular machines. These proteins do the actual work. DNA is the blueprint; proteins are the builders and the tools.
- Adenine (A)
- Thymine (T)
- Cytosine (C)
- Guanine (G)
- Pairs with Thymine (T)
- Pairs with Guanine (G)
- Pairs with Adenine (A)
- Pairs with Cytosine (C)
The sequence of bases in DNA is not random, it is a coded language. Every group of three bases, called a codon, specifies one of twenty possible amino acids. A gene is essentially a string of codons that spells out the order in which amino acids should be linked together. Once assembled, the amino-acid chain folds into a specific three-dimensional shape, and that shape determines what the protein can do.
Some proteins are structural, like collagen in skin and keratin in hair. Others are enzymesbiological catalysts that speed up chemical reactions. Haemoglobin, which carries oxygen in your blood, is a protein. Antibodies that fight infection are proteins. DNA stores the instructions, but proteins are the molecular workers that carry out almost every task a cell needs to survive.
Sickle cell disease is caused by a single base change in the haemoglobin gene. The DNA codon GAG (which codes for the amino acid glutamic acid) becomes GTG (which codes for valine instead). This one-amino-acid swap changes the shape of haemoglobin molecules, causing red blood cells to deform into a sickle shape under low oxygen. One letter in the DNA code produces a life-altering disease.
Australian biotech: CSL Limited, headquartered in Melbourne, is one of the world's largest producers of plasma-derived protein therapies. Their scientists use precise knowledge of how DNA codes for blood proteins to manufacture treatments for haemophilia and immune deficiencies.
DNA is a made of two strands. The sequence of bases along a strand determines the order of in a protein. Each group of three bases is called a and codes for one amino acid. do the actual work in cells, while DNA stores the instructions.
If you stretched out the DNA from a single human cell, it would be about two metres long. Yet it fits inside a nucleus roughly six micrometres across, a compression factor of about 10,000. How? DNA is wrapped around proteins called histones to form structures called nucleosomes, which are then coiled and folded into ever-tighter loops. At maximum compaction, the DNA becomes a visible chromosome.
This packing is not just about storage. It is also a control mechanism. When DNA is tightly wound, the cell's machinery cannot read the genes, they are effectively switched off. When DNA is loosely packed, genes are accessible and can be transcribed into proteins. This dynamic packing helps cells with the same DNA become different types: a nerve cell, a muscle cell or a skin cell.
During cell division, chromosomes become tightly condensed and visible under a light microscope. A human cell has 46 chromosomes arranged in 23 pairs. Each chromosome is a single, enormously long DNA molecule packed with histones. When division is complete, the chromosomes unwind again into looser chromatin so that genes can be read.
Australian research: Scientists at the Garvan Institute in Sydney study how histone modifications affect gene activity in cancer cells. By understanding the 'epigenetic' layer of control above DNA sequence, they hope to develop drugs that reactivate tumour-suppressor genes without altering the underlying genetic code.
Sort each structure into the correct category of DNA packaging.
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.
Sickle Cell Disease and a Single Base Change
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.
Build the Complementary Strand
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.
Analyse DNA Structure and Function
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?
Copy Into Your Book
▼DNA Structure
- DNA = deoxyribonucleic acid
- Shape = double helix (twisted ladder)
- Building block = nucleotide
- Each nucleotide = sugar + phosphate + base
The Four Bases
- Adenine (A) pairs with Thymine (T)
- Guanine (G) pairs with Cytosine (C)
- A-T = 2 hydrogen bonds
- G-C = 3 hydrogen bonds
Base Pairing
- Two strands are complementary
- Knowing one strand = knowing the other
- Complementarity enables replication
- Sequence of bases = genetic code
DNA Function
- Stores instructions for proteins
- Genes = segments of DNA
- Packaged into chromosomes
- Human genome = ~3 billion base pairs
At the start of this lesson you were asked how 2 metres of DNA manages to fit inside a nucleus less than 0.01 mm wide, a puzzle that sounds physically impossible. Now that you understand the double helix, nucleotides and how DNA is packed around histone proteins into chromosomes, revisit that question.
How well did your initial idea match what you have just learned? What was the most surprising thing about the structure of DNA and the way it stores such enormous amounts of information in such a tiny space?
Q1. Describe the structure of a DNA nucleotide. In your answer, name and explain the function of each of the three components. 3 MARKS
Q2. 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
Q3. 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
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly predict that DNA has a twisted, ladder-like structure?
- Did you identify that DNA is made of smaller units (nucleotides) with sugar, phosphate and bases?
- Write one sentence explaining why the double helix is the "secret of life."
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Build the Complementary Strand
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].
Activity 2, Analyse DNA Structure and Function
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 (you 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].
Multiple Choice
1. CAmino acids are the building blocks of proteins, not nucleotides. Nucleotides contain sugar, phosphate and a nitrogenous base.
2. BGuanine (G) always pairs with cytosine (C). Adenine pairs with thymine. Uracil is found in RNA, not DNA.
3. AComplementary 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. DIf A = 20%, then T = 20% (A-T pairing). Remaining 60% is split equally between G and C, so G = 30%.
5. BThe 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.
Short Answer Model Answers
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].
Jump Through DNA!
Climb platforms using your knowledge of the double helix, nucleotides and base pairing. Pool: Lesson 2.