Inheritance and DNA · Model
Chromosomes, genes and DNA
Four words that people use as though they meant the same thing — nucleus, chromosome, gene, DNA — and they are four different levels of one structure, nested inside each other like the levels of organisation you already know.
Start here
Two metres of DNA, in a nucleus you cannot see.
Unwind the DNA from a single one of your cells and lay it end to end and it measures about two metres. The nucleus holding it is around six thousandths of a millimetre across. Every cell in your body is doing this, right now.
How does two metres of anything fit into that?
It is unimaginably thin, and it is coiled — wound around proteins, then coiled, then coiled again, into 46 packages called chromosomes. A chromosome is not a different substance from DNA. It is DNA, packed. That is the single most useful sentence in this lesson.
At the bench · zoom in five times
From a whole person to four letters
level 1 of 6
Each step down is inside the one above it, and nothing new is added — you are looking at the same material, closer.
- A person1.6 mAround thirty trillion cells, almost every one of them carrying an identical copy of the same instructions.
- A cell0.02 mmAny body cell will do — a cheek cell, a liver cell, a root hair cell in a plant. All of them keep their instructions in the same place.
- The nucleus0.006 mmThe control centre of the cell, and where the DNA is kept. Red blood cells are the famous exception: they throw theirs away to make room for haemoglobin.
- A chromosome0.002 mm longDNA wound around proteins and coiled tightly. A human body cell has 46 of them, in 23 pairs — one of each pair from each parent. They are only visible under a microscope when a cell is about to divide.
- A genea section of the strandA length of the DNA in that chromosome, carrying the instruction for one characteristic. Not a separate object attached to the chromosome — a part of it, like a chapter in a book.
- The bases0.0000003 mm apartThe instruction is written in a sequence of four bases, known by their initials A, T, C and G. The order of those letters along the gene is the information.
Six levels, and the last three are all the same molecule seen at different magnifications. A chromosome is coiled DNA; a gene is a section of that DNA; the bases are the units the section is written in. Nothing was swapped for anything else on the way down.
Say it back — which one is which?
All the DNA in one cell — about two metres if you uncoiled it all. It is divided between 46 chromosomes, each packed down to a few thousandths of a millimetre.
The nucleus contains the chromosomes; each chromosome is one coiled DNA molecule; each gene is a section of that DNA; the bases are the units the section is written in. Five levels, each inside the last.
46 chromosomes in 23 pairs in a body cell, 23 single chromosomes in a sperm or egg cell, and around twenty thousand genes altogether.
Yes — every cell with a nucleus carries the same complete set. What differs is which genes are switched on, and that is what makes a nerve cell different from a skin cell.
The model in four sentences
A gene is an instruction for one job.
The molecule
DNA
A long, thin molecule made of two strands twisted round each other. It carries information in the order of its four bases, and it can be copied exactly.
The package
Chromosome
One DNA molecule, coiled with proteins so it can be stored and moved without tangling. 46 in a human body cell, in 23 matched pairs.
The instruction
Gene
A section of DNA that codes for one characteristic. Different versions of the same gene are why people differ — the gene for eye colour is in everybody.
The alphabet
Bases
Four of them, A, T, C and G, in a sequence. Everything a cell can be told is written with those four letters and nothing else.
Key fact
DNA is a long molecule found in the nucleus. It is coiled into chromosomes — 46 in a human body cell, in 23 pairs. A gene is a section of DNA carrying the instruction for one characteristic, and the instruction is written in a sequence of four bases.
Think again
“Chromosomes, genes and DNA are three different things in the nucleus.”
They are one thing described at three magnifications, and the levels-of-organisation ladder is the right way to hold it: cell, tissue, organ, organ system was one nesting, and DNA, gene, chromosome, nucleus is another. DNA is the molecule. A gene is a length of that molecule — a section, not a separate object sitting on it. A chromosome is the whole molecule coiled up tightly with proteins so that it can be moved around without tangling. Asking whether a nucleus contains chromosomes or DNA is like asking whether a library contains books or paper. The one place the analogy is worth pushing further: a chromosome is a single, extremely long DNA molecule, so a gene is not attached to a chromosome — it is part of it, the way a chapter is part of a book rather than a bookmark in one.
“Only the cells that need a gene contain it.”
Every cell with a nucleus carries the whole set: all 46 chromosomes, all of your genes, the complete instructions. A cell in your eye contains the genes for making liver enzymes and for growing hair, and a cell in your liver contains the genes for eye colour. What differs between cells is which genes are switched on, and that is what makes a nerve cell different from a skin cell despite the two carrying identical information — which is the mechanism behind the specialised cells. It is also the reason a single cell left at a crime scene identifies a person, and the reason a whole sheep could be built from one udder cell. If cells only carried the genes they were using, none of that would be possible.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Put them in order
Order these from largest to smallest: gene, nucleus, chromosome, base.
Rung 2 · The one that catches people
Does a cell in your foot contain the gene for eye colour?
Rung 3 · Explain the nesting
A student says "the nucleus contains chromosomes and DNA and genes". Rewrite the sentence so it is correct, and explain how the four things are related.
Rung 4 · Take it somewhere new
A single cheek cell left on a glass at a crime scene is enough to identify a person. Explain what makes that possible, using what you know about where genetic information is stored and which cells carry it.
Key note
DNA is a long molecule, coiled with proteins into chromosomes and stored in the nucleus. A human body cell has 46 chromosomes in 23 pairs, one of each pair from each parent. A gene is a section of DNA that carries the instruction for a characteristic, and the instruction is written as a sequence of four bases. Every cell with a nucleus carries the whole set; cells differ in which genes are switched on.
Going further
The number of chromosomes tells you almost nothing about an organism. A human has 46, a chimpanzee 48, a potato 48, a dog 78 and a particular fern over a thousand. Complexity does not track the count, and neither does the number of genes: humans have around twenty thousand, which came as an unwelcome surprise in 2003 when the Human Genome Project finished and the pre-project bets had run to a hundred thousand. A water flea has more genes than you do. What the count misses is that a gene can be read in several ways, that most of the genome is regulatory rather than instructional, and that the interesting differences between species are as much about when and where genes are switched on as about which genes exist. Chimpanzee and human DNA differ by something like one per cent of the letters, and that one per cent includes a great deal of switching.
Before this lesson
Connects to
At GCSE this becomes
- Alleles, genotype and phenotype, protein synthesis, and the genome as a whole.
Where to next
Ask Mr Badmus AI
Want to get the four words straight in your own wording?
The scale figures are rounded to the nearest order of magnitude to make the nesting readable. "One gene, one characteristic" is a deliberate simplification kept for KS3: most characteristics involve many genes and many genes affect more than one characteristic, as the height curve in Variation: continuous and discontinuous already implied.
Lesson content © MrBadmusAI.