In Membrane, Cytoplasm and Nucleus you saw that the nucleus is the cell's control center. What is inside it? Instructions, written in a molecule called DNA. This is where inherited traits come from.
Key idea
DNA carries the instructions for building and running an organism. A gene is a section of DNA with the instructions for one protein. DNA is packed into chromosomes in the nucleus. A chromosome is one long DNA molecule carrying many genes.
Why it works
The order of the bases A, T, C and G spells out the instructions, the way the order of letters spells words. Because A always pairs with T and C with G, each side of the ladder can rebuild the other. That is how a cell copies its DNA before it divides, so nearly every cell of your body has a full set.
| Word | What it is | Size |
|---|---|---|
| Chromosome | one long DNA molecule, coiled tightly | largest |
| Gene | a section of a chromosome with instructions for one protein | middle |
| Base | one "letter": A, T, C or G | smallest |
Worked example: How many?
Most human body cells have 46 chromosomes in 23 pairs. How many chromosomes does a skin cell have? A liver cell?
- Body cells are made by copying the DNA and dividing.
- Each copy gets the full set.
Answer: Both have 46. (Red blood cells are an exception: they lose their nucleus as they mature.)
Common mistake
Students think genes and chromosomes are different things in different places, or that only some cells, like those in the blood, have DNA. A gene is part of a chromosome, the way a sentence is part of a book. And nearly every cell carries the whole book.