The instructions for life
Every cell in your body (except red blood cells) contains about 2 metres of DNA, packed into a nucleus a few thousandths of a millimetre wide. That DNA holds the instructions to build and run your body. Those instructions are mostly used to build proteins, which do almost every job in the cell.
The flow of information is called the central dogma of molecular biology:
DNA → (transcription) → RNA → (translation) → protein
The structure of DNA
DNA is a double helix, like a twisted ladder. Its structure was described by James Watson and Francis Crick in 1953, using X-ray images made by Rosalind Franklin and Maurice Wilkins.
- Each strand is a chain of nucleotides.
- A nucleotide has three parts: a deoxyribose sugar, a phosphate group, and a nitrogen base.
- The four bases are adenine (A), thymine (T), guanine (G) and cytosine (C).
- Bases pair across the ladder: A with T, and G with C, held by hydrogen bonds.
- The two strands run in opposite directions (antiparallel).
DNA replication
Before a cell divides, it copies its DNA.
- The enzyme helicase unzips the double helix.
- DNA polymerase adds matching nucleotides to each original strand.
- Each new DNA molecule has one old strand and one new strand. This is called semi-conservative replication.
DNA vs. RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Strands | Double | Usually single |
| Bases | A, T, G, C | A, U (uracil), G, C |
| Location | Mostly nucleus | Nucleus and cytoplasm |
| Job | Long-term storage | Carries and uses instructions |
Main types of RNA: messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA).
Step 1: Transcription
Transcription happens in the nucleus.
- RNA polymerase binds to the start of a gene (the promoter).
- It unwinds the DNA and builds an mRNA strand that matches the template strand (with U instead of T).
- In eukaryotes, the mRNA is processed: non-coding sections called introns are cut out, leaving exons, and a cap and tail are added.
- The mRNA leaves the nucleus through a nuclear pore.
Example: DNA template TAC GGA TTC → mRNA AUG CCU AAG
Step 2: Translation
Translation happens at ribosomes in the cytoplasm.
- The ribosome reads the mRNA three bases at a time. Each group of three is a codon.
- tRNA molecules carry amino acids. Each tRNA has an anticodon that pairs with a codon.
- The ribosome links the amino acids into a chain, forming a protein.
- Translation starts at the start codon AUG (methionine) and ends at a stop codon (UAA, UAG or UGA).
There are 64 possible codons but only 20 amino acids, so most amino acids have more than one codon. The code is nearly universal: the same codons mean the same amino acids in bacteria, plants and people.
| mRNA codon | Amino acid |
|---|---|
| AUG | Methionine (start) |
| CCU | Proline |
| AAG | Lysine |
| UAA | Stop |
Mutations
A mutation is a change in the DNA sequence.
| Type | What happens | Possible effect |
|---|---|---|
| Substitution (point mutation) | One base swapped for another | Silent, missense (different amino acid) or nonsense (early stop) |
| Insertion | Extra base added | Frameshift: every codon after it changes |
| Deletion | A base removed | Frameshift |
Sickle cell disease is caused by a single substitution that changes one amino acid in hemoglobin. Most mutations are neutral, some are harmful, and a few are helpful, providing variation for evolution.
Gene expression is controlled
Every cell has the same DNA, but a nerve cell and a skin cell use different genes. Cells switch genes on and off using regulatory proteins and chemical tags on DNA. This control is how specialized cells develop.
Biotechnology applications
- PCR copies DNA for testing and forensics.
- Recombinant DNA puts human genes into bacteria to make insulin.
- CRISPR-Cas9 lets scientists edit genes precisely. In 2023, the first CRISPR-based therapy was approved to treat sickle cell disease.
- mRNA vaccines give cells instructions to make a harmless virus protein that trains the immune system.
Common mistakes
- Forgetting that RNA uses U instead of T.
- Mixing up codons and anticodons. Codons are on mRNA; anticodons are on tRNA.
- Thinking all mutations are harmful.
Try it yourself
DNA template strand: TAC CAT GGG ATT
- Write the mRNA.
- Use a codon chart to find the amino acids.
- What happens if the first G in GGG is deleted?
(mRNA: AUG GUA CCC UAA → Met–Val–Pro–Stop)
Quick recap
- DNA is a double helix of nucleotides with A–T and G–C pairs.
- The central dogma: DNA → RNA → protein.
- Transcription makes mRNA in the nucleus; translation builds proteins at ribosomes.
- Each codon of three bases codes for one amino acid.
- Mutations change DNA and can alter proteins.