The instruction book of life
Every living thing, from a bacterium to a blue whale, carries instructions for how to build and run its body. Those instructions are written in a molecule called DNA (deoxyribonucleic acid).
If you could stretch out the DNA from just one of your cells, it would be about 2 metres long. Yet it fits inside a nucleus far too small to see without a microscope.
The shape of DNA
DNA is shaped like a twisted ladder, called a double helix.
| Ladder part | DNA part |
|---|---|
| The two sides (rails) | Sugar and phosphate molecules |
| The rungs | Pairs of bases |
Each unit of DNA is a nucleotide, made of a sugar, a phosphate and one of four bases:
- A (adenine)
- T (thymine)
- G (guanine)
- C (cytosine)
Base pairs
The bases always pair the same way to make each base pair:
- A pairs with T
- G pairs with C
So if one side of the ladder reads A-T-G-C-C-A, the other side must read T-A-C-G-G-T. This rule is the secret to how DNA copies itself.
How DNA is packed
| Level | Description |
|---|---|
| DNA | The long double-helix molecule |
| Gene | A section of DNA with instructions for one protein or trait |
| Chromosome | DNA wound tightly around proteins into a compact package |
| Genome | All the DNA in an organism |
Humans have 46 chromosomes (23 pairs) in most cells, about 20 000 genes, and a genome of about 3 billion base pairs. One set of chromosomes comes from each parent.
What does DNA do?
The order of the bases is a code, like letters spelling words. Cells read the code three letters at a time to build proteins. Proteins do most of the work in your body: they make up muscles, carry oxygen in your blood, fight germs and control chemical reactions.
Small differences in DNA explain why people have different eye colours, hair textures and heights.
Copying DNA
Before a cell divides, it copies its DNA:
- The double helix unzips down the middle.
- Each side is used as a pattern.
- New bases are added, following the A–T and G–C rules.
- Two identical DNA molecules are made, one for each new cell.
Sometimes a copying mistake happens. This is called a mutation. Most mutations are harmless, some cause diseases, and a few are helpful.
The discovery of DNA
| Year | Scientist(s) | Discovery |
|---|---|---|
| 1869 | Friedrich Miescher | First isolated DNA from white blood cells |
| 1944 | Oswald Avery and colleagues | Showed DNA carries genetic information |
| 1950 | Erwin Chargaff | Found that amounts of A equal T and G equal C |
| 1952 | Rosalind Franklin and Raymond Gosling | Took X-ray image "Photo 51" showing a helix |
| 1953 | James Watson and Francis Crick | Built the double-helix model using Franklin’s data |
| 2003 | Human Genome Project | Finished reading nearly all the human genome |
Watson, Crick and Maurice Wilkins won the 1962 Nobel Prize. Franklin had died in 1958, and Nobel Prizes are not given after death. Today her key role is widely recognized.
DNA in the real world
- Medicine: genetic tests can find disease risks; gene therapies treat some inherited diseases.
- Forensics: DNA left at a crime scene can identify a person.
- Ancestry: DNA tests show family relationships and where ancestors lived.
- Evolution: comparing DNA shows how species are related. Humans and chimpanzees share about 98–99% of their DNA.
- Agriculture: scientists breed or modify crops to resist pests and drought.
Common mistakes
- Thinking only humans have DNA. All living things do.
- Thinking genes and DNA are different substances. A gene is a section of DNA.
- Mixing up the pairs. Remember: A–T and G–C.
Try it yourself
- Write the matching strand for: A-G-T-C-C-A-T-G.
- Extract DNA from a strawberry: mash it with salty, soapy water, strain it, and gently pour cold rubbing alcohol on top (with an adult). White strands of DNA will appear.
- Build a DNA model using candy or beads, with a different colour for each base.
(Answer to 1: T-C-A-G-G-T-A-C)
Quick recap
- DNA holds the instructions for all living things.
- It is a double helix made of nucleotides.
- Bases pair A–T and G–C.
- Genes are sections of DNA; chromosomes are packages of DNA.
- DNA copies itself before cells divide.