Cells & DNA

Why do Biscuit's cells act so differently?

Imagine a Labrador named Biscuit, nose to the ground, then sprinting across the yard.

A skin cell on her nose builds a tough protective coat. A muscle cell in her leg contracts so she can run.

Notice how differently the two cells are built, even though both came from the same single egg.

Yet both cells started from the same single fertilized egg.

So how does a skin cell know not to act like a muscle cell? To answer that, we first need to look inside a cell.

Every cell carries an instruction book

Biscuit is made of trillions of cells, the smallest living units of life.

Inside nearly every cell, in a compartment called the nucleus, sits DNA: a long, coiled molecule written in a four-letter code (A, T, C, G).

Stretches of that code are called genes. Each gene holds the recipe for one specific protein.

Proteins do most of the body's work, from building hair and muscle fibers to sending signals between cells.

So which recipes does each cell get?

Does each cell carry only its own manual?

Many people guess that a skin cell holds a skin manual and a muscle cell holds a muscle manual.

It seems logical. The cells look and act so differently that different instructions feel like the only explanation.

But Biscuit's cells all came from one egg, and DNA is copied each time a cell divides. Almost every cell carries the same complete DNA, roughly 20,000 genes.

So if the book is the same, what makes the cells differ?

Cells copy only the recipes they need

Each cell type switches on only the genes it needs.

To use a gene, the cell copies its DNA into a short, temporary note called messenger RNA (mRNA).

The mRNA leaves the nucleus for ribosomes, the cell's protein builders, which link amino acids in the order the note spells out.

Soon the mRNA is broken down, so the cell stays in control of how much protein it makes.

So what does this look like in Biscuit's nose and leg?

Same DNA, different genes switched on

Suppose we follow two of Biscuit's cells. Both hold the same full set of roughly 20,000 genes.

The nose skin cell switches on keratin genes, for a tough protein found in skin, hair, and nails.

The leg muscle cell switches on genes for actin and myosin, the proteins that let muscle contract.

Ribosomes build those proteins from the mRNA. Same DNA with different genes switched on gives two very different cells.

So what does that mean for the original question?

Which genes a cell reads decides its job

Back to Biscuit. Her nose cell never switches on muscle genes. It copies keratin genes into mRNA instead.

Every cell carries the whole instruction book; which genes it copies into mRNA and turns into proteins decides its job.

This flow, from DNA to mRNA to protein, runs every cell. It explains how DNA mistakes can lead to disease.

It even lets scientists deliver mRNA on purpose, teaching cells to make useful proteins.

A cell in a dog's ear and a cell in its stomach look and act very differently. These two cells _____.

• carry the same complete set of DNA
• each carry only the DNA instructions for their own job

Answer: carry the same complete set of DNA

Almost every cell in a dog carries the same complete DNA instruction book. The cells differ because each one switches on different genes, not because each holds only its own 'manual'.

Tap the compartment where the cell stores its DNA instruction book.

Answer: nucleus

The nucleus houses the cell's complete set of DNA, where active genes are transcribed into messenger RNA.

A cell copies a gene into a temporary _____ message. Because this message is soon _____, the cell keeps control over how much _____ it makes.

Answer: A cell copies a gene into a temporary mRNA message. Because this message is soon broken down, the cell keeps control over how much protein it makes.

mRNA is a short-lived copy of a gene. Since it is broken down quickly, the cell can control how much of each protein gets built.