# From DNA to Protein

> Genes, RNA, transcription, translation, the genetic code, how proteins fold and work, and how genes are switched on and off.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/protein-synthesis/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: A recipe book you cannot take out of the library
Think of the nucleus as a library with one precious recipe book: your DNA. The book can never leave the library. So when the kitchen (a ribosome) needs to make a dish, a librarian copies just one recipe onto a scrap of paper and sends it out.

The kitchen reads the copy three letters at a time and adds the right ingredient for each one. When the recipe is done, the dish (a protein) is finished, and the paper copy is thrown away. Every protein in your body, from the keratin in your hair to the insulin in your blood, is made this way.

**In short:** Protein synthesis is how a cell uses a gene to make a protein. In transcription, the gene is copied into mRNA in the nucleus, and in translation, a ribosome reads the mRNA three bases (a codon) at a time and joins the matching amino acids into a polypeptide.

## Starting point: what you already know

- DNA is a double helix with bases A, T, C and G (see the previous chapter).
- Ribosomes make proteins (see Cell Organelles).
- Enzymes and many hormones are proteins.

## Genes, proteins and RNA

A **gene** is a section of DNA that codes for a **polypeptide** (a chain of amino acids, which folds into a protein). There are 20 different amino acids, and the order of bases in the gene sets the order of amino acids.

**RNA** is a single-stranded copy molecule. It differs from DNA:

| | DNA | RNA |
|---|---|---|
| Strands | Double | Single |
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, **U** (uracil), C, G |
| Where | Nucleus (and mitochondria, chloroplasts) | Made in the nucleus, works in the cytoplasm |

Three kinds of RNA help make proteins: **mRNA** (messenger, the copy), **tRNA** (transfer, brings amino acids) and **rRNA** (ribosomal, part of the ribosome).

## Step 1: Transcription (in the nucleus)

1. The DNA double helix unwinds at the gene.
2. **RNA polymerase** reads one DNA strand (the template) and builds a matching **mRNA** strand. In RNA, A pairs with **U**.
3. The mRNA leaves the nucleus through a nuclear pore.

## Step 2: Translation (at a ribosome)

1. The ribosome attaches to the mRNA and reads it three bases at a time. Each group of three is a **codon**.
2. Each **tRNA** carries one amino acid and has three bases called an **anticodon**, which pairs with a codon.
3. The ribosome joins the amino acids with **peptide bonds**.
4. A **stop codon** ends translation, and the polypeptide is released.

### Worked example: Reading the code
DNA template strand: **TAC GGA TTT ACT**

mRNA (A to U, T to A, C to G, G to C): **AUG CCU AAA UGA**

Codons: AUG = methionine (the **start** codon), CCU = proline, AAA = lysine, UGA = **stop**.

Polypeptide: methionine, proline, lysine.

### Key idea: Features of the genetic code
- **Triplet**: three bases code for one amino acid.
- **Universal**: the same code in almost all living things, which is why a human gene works in a bacterium.
- **Degenerate** (redundant): most amino acids have several codons, so some mutations change nothing.
- There is one **start** codon (AUG) and three **stop** codons.

## From polypeptide to protein

A polypeptide folds into a 3D shape held by bonds between its amino acids. Its shape decides its job. Some proteins are made of more than one polypeptide (haemoglobin has four).

| Type of protein | Example |
|---|---|
| Enzymes | Amylase, DNA polymerase |
| Structural | Keratin in hair, collagen in skin |
| Transport | Haemoglobin, membrane channels |
| Hormones | Insulin |
| Defence | Antibodies |

**Mind map**

```text
DNA to protein
  Gene
    Codes for a polypeptide
    Base order sets amino acid order
  RNA
    Single strand
    Uracil instead of thymine
    mRNA, tRNA, rRNA
  Transcription
    In nucleus
    RNA polymerase makes mRNA
  Translation
    At ribosome
    Codon and anticodon
    Peptide bonds
    Stop codon
  Code
    Triplet
    Universal
    Degenerate
```

### Year 11: Processing and gene expression
In eukaryotes, the first mRNA copy contains **introns** (non-coding sections) and **exons** (coding sections). Before leaving the nucleus, the introns are cut out and the exons joined (**splicing**). A cap and a tail are added to protect the mRNA. Different splicing of the same gene can make different proteins, so humans make far more than 20 000 kinds of protein from about 20 000 genes.

Every cell has the same genes, but only some are **expressed** (switched on). **Promoters** and **transcription factors** (proteins that bind DNA) control when a gene is transcribed. Environmental factors like temperature, diet and stress can affect expression too. Siamese cats have dark fur on their cooler ears and paws because their pigment enzyme only works at lower temperatures.

**Strands and anticodons.** The mRNA is complementary to the **template** strand, so it has the same sequence as the **coding** strand, with U for T. Coding strand ATG GCA gives mRNA AUG GCA. A tRNA anticodon is complementary to the codon: codon AUG is read by anticodon UAC.

**Counting bases.** Each amino acid needs three mRNA bases, so 150 amino acids need at least $150 \times 3 = 450$ bases (plus 3 for the stop codon). If a gene is 1200 bases with introns of 400 and 250 bases, the mature mRNA has $1200 - 650 = 550$ bases.

**Same protein, different genes.** Because the code is degenerate, two slightly different insulin genes can still code for exactly the same amino acids.

**Environment and phenotype.** Hydrangeas of one genotype are blue in acid soil and pink in alkaline soil. Himalayan rabbits, like Siamese cats, have black fur only on cooler parts (ears, nose, feet, tail), where their pigment enzyme can work.

### Year 12: Epigenetics
**Epigenetics** is change in gene expression that does not change the DNA sequence. Chemical tags such as **methyl groups** on DNA, or changes to how tightly DNA is wound on histones, switch genes off or on. Some tags can be passed from cell to cell and even, in some cases, to offspring. Honeybee larvae fed royal jelly become queens rather than workers, even though their DNA is the same, because of epigenetic changes.

**Levels of protein structure.** The amino acid order is the **primary** structure. Folding into coils and sheets is **secondary**. The full 3D shape is **tertiary**. Several polypeptides joined together, like the four chains of haemoglobin, is **quaternary** structure.

**Shape is everything.** An enzyme's **active site** fits its substrate exactly. Changing one amino acid there can change the shape so the substrate no longer fits, and the enzyme stops working.

**Cracking the code.** In 1961 Nirenberg added mRNA made only of uracil (UUU UUU ...) to a cell-free system and got a chain of only **phenylalanine**. This showed that UUU codes for phenylalanine and that the code is read in triplets.

**One faulty tRNA, many proteins.** If the anticodon of a tRNA changes, it puts the wrong amino acid in at that codon in **every** protein the cell makes.

## Quick summary

- A gene codes for a polypeptide. The base order sets the amino acid order.
- Transcription in the nucleus: RNA polymerase copies a gene into mRNA (U replaces T).
- Translation at a ribosome: codons on mRNA pair with tRNA anticodons, and amino acids are joined by peptide bonds.
- The code is triplet, universal and degenerate, with start (AUG) and stop codons.
- Protein shape decides function. Gene expression is controlled, so cells make different proteins.

## Quiz

Try these questions from the chapter quiz.

1. Which base is in RNA but not DNA?
   - A. Thymine
   - B. Uracil
   - C. Adenine
   - D. Guanine
   - Answer: B. Uracil. RNA has uracil in place of thymine.

2. Where does transcription happen in a eukaryotic cell?
   - A. The nucleus
   - B. The ribosome
   - C. The cell membrane
   - D. The Golgi body
   - Answer: A. The nucleus. DNA is copied into mRNA in the nucleus.

3. Where does translation happen?
   - A. The nucleus
   - B. At ribosomes
   - C. In the vacuole
   - D. In lysosomes
   - Answer: B. At ribosomes. Ribosomes read mRNA and build polypeptides.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/protein-synthesis/#quiz