DNA, chromosomes, genes and the genome

Biology · Inheritance

tap a part of the picture ↓

Level 4: zoom into a gene and the DNA is made of smaller units called A, T, C and G.

Tap any part of the picture, or use the Explore buttons. The View buttons rebuild the zoom one step at a time.

Biology · Cell structure

tap the nucleus, then the circle of DNA

A bacterium: no nucleus. Its DNA is one big circle in the cytoplasm.

Compare the two cells, then tap the nucleus, the chromosomes or the circle of DNA.

Predict, then check

Two stretches of DNA. Read the letters, then decide.

Stretch 1 reads A T C G G A. Stretch 2 reads G A T G A C. They use exactly the same units, just in a different order. Do they carry the same code?

What you need to know

• Heredity is how genetic information gets passed from one generation to the next.
• DNA is a long chemical molecule that stores the genetic code, built from smaller units called A, T, C and G.
• The code depends on the order of those units, so different combinations give different features and control different processes.
• In animal, plant and fungi cells, the DNA is stored in the nucleus, packaged into chromosomes of tightly wound DNA.
• Have a goYou need to fit a really long piece of string into a small pocket. What would you do with it? Then: what do you think the cell does with its long DNA?

  Wind it up tightly. The cell packages its DNA as chromosomes: tightly wound DNA in the nucleus.

  A chromosome is a package of tightly wound DNA, not a loose strand floating about.

• Chromosomes come in pairs, one of each pair from each parent. Human body cells have 23 pairs.
• Have a goYour cousin says: "23 pairs of chromosomes? So that is 23 chromosomes in total." Is she right?

  No. 23 pairs means two chromosomes in every pair, so there are more than 23.

  The number counts pairs, and each pair is two chromosomes, one from each parent.

• Bacteria have no nucleus, so no pairs. They have one circular chromosome in the cytoplasm, and still have a genome.
• A gene is a section of the DNA in a chromosome that codes for a feature or a process, such as growth.
• Have a goYour classmate Sam says, very confidently: "Genes are made of chromosomes, like bricks make a wall." Sam is building it backwards. Put it the right way round.

  A gene is a section of the DNA in a chromosome.

  The chromosome is the package of wound DNA, and the gene is one section of that DNA, so genes are not made of chromosomes.

• Non-coding DNA sits between genes. It is not a gene, but it is not junk: it helps control when genes are used.
• The genome is all of a cell's genetic material, genes plus non-coding DNA, storing the code for its inherited features and processes.
• All genomes share similarities because of common life processes, and differences make individuals unique.

The big picture

DNA, chromosomes, genes and the genome are levels of the same thing. DNA is a long molecule that stores the genetic code. It is packaged into chromosomes, which are stored in the nucleus. Genes are sections of that DNA that code for features and processes. The genome is all of the genetic material, genes and non-coding DNA together.

⚠ Watch out

Saying that genes are made of chromosomes, or that a gene and the genome are the same thing. Fix it with the zoom: a gene is a section of the DNA in a chromosome, and the genome is all of the DNA, genes and non-coding DNA together.

Memory hook

Cats Never Chase Dogs Gently: Cell, Nucleus, Chromosome, DNA, Gene. Each one sits inside the one before. The genome is not another step down. It is all of the DNA put together, genes and the bits between them.

Check yourself

Cover the screen and zoom in from memory: say what sits inside what, from the cell down to a gene. Then say where non-coding DNA sits, and what the genome is.