DNA, Genes, and Chromosomes
Inside almost every one of your cells sits a complete set of instructions for building and running you. Those instructions are written in DNA, packaged into chromosomes, and read in units called genes. Getting these three words straight is the doorway to all of genetics, and it clears up a surprising number of test questions.
This lesson sorts out DNA, genes, and chromosomes and shows how they fit together.
DNA is the molecule that carries genetic instructions. A gene is a segment of DNA that codes for a particular trait or protein. A chromosome is a long, tightly coiled strand of DNA. In order of size: DNA makes up genes, genes sit along chromosomes, and chromosomes are stored in the cell’s nucleus.
What is DNA?
DNA, short for deoxyribonucleic acid, is shaped like a twisted ladder called a double helix. Its rungs are made of four bases, often written A, T, C, and G, that always pair the same way: A with T, and C with G. The order of these bases is a code, and that code spells out the instructions for building proteins, which do most of the work in your body.
How do genes and chromosomes relate?
A gene is a section of DNA that carries the instructions for one trait or protein, such as eye color. Genes are lined up along chromosomes, which are DNA wound tightly around proteins so a huge amount can fit inside a tiny nucleus. Humans have 46 chromosomes, arranged in 23 pairs, with one of each pair coming from each parent.
| Term | What it is | Size |
|---|---|---|
| DNA | The instruction molecule | Smallest |
| Gene | A segment of DNA for one trait | Middle |
| Chromosome | Coiled strand holding many genes | Largest |
How do instructions become traits?
The base sequence of a gene tells the cell how to build a specific protein, and proteins shape your traits, from the color of your eyes to the enzymes that digest your food. Change the sequence and you can change the protein, which is why small differences in DNA lead to the huge variety of living things. This link from DNA to protein to trait is the core idea of molecular genetics.
Watch: A Short Video Lesson
Amoeba Sisters walks through this skill clearly in a few minutes. It is a helpful companion to the reading above:
A routine for DNA questions
- Remember the size order: DNA builds genes, genes sit on chromosomes.
- Recall the base pairing: A with T, C with G.
- A gene codes for a trait or protein.
- Chromosomes come in pairs, one from each parent.
- Trace the path: DNA sequence to protein to trait.
Practice questions
- What is the shape of a DNA molecule called?
- Which base pairs with A?
- What is a gene?
- How many chromosomes do humans have?
- Put in order from smallest to largest: chromosome, DNA, gene.
- True or false: proteins are built using instructions in DNA.
Answers:
- A double helix.
- T (thymine).
- A segment of DNA that codes for a trait or protein.
- 46, in 23 pairs.
- DNA, gene, chromosome.
- True.
Where this fits
DNA and genes are the foundation of heredity, so this lesson connects directly to heredity and DNA and to how traits pass to offspring in genes, alleles, and Punnett squares. Find every topic on the ASVAB General Science Learning Hub.
Recommended Prep Books
These study guides and practice books help you keep building momentum as you prepare:
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