DNA: The Code of Life

Mash a strawberry with soap and salt, strain it, and pour cold rubbing alcohol on top. A white, stringy clump collects where the alcohol meets the strawberry juice, and it contains many DNA molecules. You can wind the clump onto a stick. Strawberries, mushrooms, bacteria and people all have DNA. How can the same kind of molecule help make such different living things?

Explore: zoom into a gene and change a letter

The simulation starts with a whole living thing. A zoom slider takes you down step by step: organism, cell, nucleus, chromosome, gene, and then a ladder of letter blocks, A, T, C and G. You can tap a block on one side to change it. A “Read the gene” button shows the protein as a chain of coloured beads, and a trait, such as flower colour.

Try this.

1. Before you zoom, predict where in the cell you will find the DNA. Then zoom in one step at a time and check.
2. Predict what will happen to the partner block when you change one block on one side. Then tap it and see.
3. Change one block and predict whether the flower colour will change. Press “Read the gene.” Try a few different blocks.

Explain: a twisted ladder with a four-letter code

DNA is the molecule that carries the instructions for building proteins. Those proteins shape what a living thing is like and keep it alive.

Take a rope ladder and twist it along its length. That is the shape of DNA. A double helix is this shape, a ladder twisted around itself.

The two sides of the ladder are made of sugar and phosphate. These sides are the same in every living thing. A strawberry and a person build them from the same parts.

The code sits in the rungs. A base is one of four small units in the middle of the ladder. Scientists name them with letters: A, T, C and G. A base pair is two bases joined in the middle to make one rung. A always pairs with T, and C always pairs with G. So if one side reads A C G T, the other side reads T G C A.

The sequence is the order of the bases along the DNA. The sequence is the code. A different order gives different instructions. That is why scientists write DNA as letters only, such as ATTGCAC. The sides are the same everywhere, so writing them down would add nothing.

Think of the bases like letters in a word. The same letters in a new order spell a new word. The comparison breaks here: a cell does not understand words. Its molecules simply follow the bases in order.

People worked this out step by step, over many years. In the early 1950s, X-ray photos of DNA and careful model building showed the double helix. The model was published in 1953.

Explain: from chromosome to gene to protein to trait

DNA is very long and very thin. In plants, animals and fungi, most of it sits in the nucleus, which you met in Inside a Cell.

A chromosome is a strand of DNA wound up tightly with proteins. Before a cell divides, chromosomes pack down even more. Then they are thick enough to see with a microscope.

A gene is a section of a chromosome that carries the code for a particular protein. One chromosome carries many genes.

A protein is a molecule that builds cells, tissues and organs and does most of the work in cells. Some proteins form hair. Others carry oxygen in blood or speed up reactions in cells.

A trait is a feature of a living thing, such as flower colour or eye colour. Each gene controls the making of certain proteins, and those proteins affect traits.

chromosome → gene → protein → trait

How does a cell turn a gene into a protein? It reads the bases in order, and the order decides which protein is built. You learn how in high school Biology.

Here is an example. In pea plants, purple flowers need a working protein from one gene. A change in that gene can stop the protein from working, and the flowers come out white. Changes in the code are called mutations, as you see in Mutations and Variation. Many traits depend on several genes, and on the surroundings too.

Almost every cell in your body carries the same DNA. So why does a skin cell look so different from a muscle cell? Each kind of cell uses a different set of genes. So each makes a different mix of proteins. How chromosomes pass from parents to offspring comes in Reproduction and Meiosis.

Elaborate: how much DNA do living things have?

Scientists can count the base pairs in one full set of chromosomes. One million base pairs means 1,000,000 rungs.

Sources: seven published lab studies, not field samples. E. coli K-12, 1997 (Blattner and others). Yeast, 1996 (Goffeau and others). Fruit fly and thale cress, 2003 (Bennett and others). Woodland strawberry, 2011 (Shulaev and others). Rice, 2005 (rice sequencing project, an estimate). Human, 2022 (Nurk and others; 22 chromosomes plus X, rounded). Onion, 2021 (Finkers and others, quoting an earlier estimate). Each row is one strain or sample.

1. Does every group in the table (bacterium, fungus, plant, animal) have DNA?
2. Divide the onion amount by the human amount. About how many times as much DNA is in one onion set?
3. List the eight living things from least DNA to most. Does that list match how complex you think each one is?

Design problem

In the strawberry activity, does the amount of dish soap change how much white clump you collect? Plan an investigation.

Evaluate: check your thinking

The white clump held DNA from strawberry cells. Strawberries, mushrooms, bacteria and people all build DNA from the same sugar, phosphate and four bases. They differ because the order of the bases differs, so their genes code for different proteins. More DNA does not make a living thing more complex.

Science practices you used: making predictions and testing them with a model, naming where a model breaks, reading a data table, dividing to compare amounts, making a claim from evidence with reasoning, and planning a fair test with a control and repeated trials.