Body 101 · Level Two · Lesson 10 · ages 11 to 12

DNA, genes and how cells divide

Almost every cell in your body keeps a complete manual of that body inside it. If you unwound the DNA from a single cell and laid it out end to end, it would be about 2 meters long, and if you joined up the DNA from every cell in your body it would be long enough to reach the Sun and back several times. This lesson looks at what letters the manual is written in, how it is read, and how it passes from parents to their children.

A blue DNA double helix twisting diagonally across the frame; each rung is a pair of small pieces in four colors, orange, green, blue and yellow, always matched the same way.
1 · StructureA twisted ladder

DNA is written with four letters

DNA is a double strand twisted into a spiral. The two rails of the ladder are made of sugar and phosphate, and the rungs are four bases that always pair in a fixed way: A with T, and C with G. Tap a dot on the picture to see the details.

Left strandRight strand

Try it: pair the bases correctly

The top strand is the template. Pick the base that pairs correctly, one at a time, to build its partner strand. Just remember the simple rule: A goes with T, and C goes with G.

Pick the base that pairs with the first one

The route from gene to protein: on the left a partly unzipped DNA helix, a golden single strand peeling off and travelling right through a two-lobed blue-purple ribosome, then emerging as a chain of colored beads that folds into a compact protein on the far right.
2 · Reading itA gene becomes a protein

From code to a real protein

A gene is a stretch of DNA that holds the recipe for one protein. Using it takes two steps. First the recipe is copied out as RNA. Second, a ribosome reads the RNA three letters at a time and joins amino acids in the order the code gives.

DNA in the nucleus RNA Amino acid chain CopyTranslate Ribosome
Press to watch

Three-letter code translator

Pick three RNA bases and see which amino acid the code calls for. One set of three letters is called a codon.

Codon chosen
AUG
Translates to
Methionine

One code for the whole planet Bacteria, trees, fish and people all use almost exactly the same code table. This is one of the biggest pieces of evidence that every living thing shares a common ancestor, and it is the reason scientists can put a human gene into bacteria to make insulin.
Twenty-three pairs of human chromosomes laid out in karyotype rows on a black background, each an X-shaped blue structure with golden bands, arranged from the largest pair to the smallest.
3 · Chromosomes46 chromosomes, 23 pairs

The whole manual is folded into 46 chromosomes

Two meters of DNA is wound around proteins and coiled up on itself until it becomes 46 chromosomes in a human cell, arranged as 23 pairs, with one chromosome of each pair from your mother and one from your father. Tap to switch and look at pair 23.

Genes in total
about 20,000 genes
Letters in total
about 3,200 million base pairs
The part that is really genes
about 2 percent
Shared between any two people
about 99.9 percent
The 0.1 percent that differsTwo people who are not identical twins have DNA that differs in about one letter in a thousand. That sounds tiny, but out of 3,200 million letters it works out to several million positions. That is where all the differences between us come from.
A cell in mid-division, its membrane pinching in at the middle while golden spindle fibres pull two sets of blue chromosomes to opposite poles; to the right sit four small round cells, each with half as many chromosomes.
4 · Cell divisionMitosis and meiosis

Dividing to repair yourself, and dividing to make new life

Our bodies divide cells all the time to repair and to grow. That is called mitosis, and it makes new cells that are the same in every way. But making sex cells uses a different kind of division called meiosis, which halves the number of chromosomes and swaps pieces between them as well. Press to go step by step.

TopicMitosisMeiosis
Where it happensAlmost every kind of body cellOnly in the testes and the ovaries
New cells made2 cells4 cells
Number of chromosomes46, the same as beforecut down to 23
Same as the parent cell?Identical in every wayNo, pieces are swapped between the pairs
What it is forGrowing, repairing, replacing old cellsMaking sperm and eggs
Why brothers and sisters are not alike Because meiosis swaps pieces between paired chromosomes at random, and then deals out which chromosome of each pair goes where at random too. The number of possible combinations from one pair of parents is more than 8 million, before you even count the swapping.
Two purple-flowered pea plants at the top left and right, and below them four seedlings in pots, three with purple flowers and one with white, joined to the parents by thin golden lines of light.
5 · InheritanceThe Punnett square

Predicting the chances for the next generation

At each gene position we carry two copies, one from our mother and one from our father. If we know which copies the parents have, we can work out the chances for their child. Try it. This example uses thalassemia carriers, which are very common in Thailand.

Choose the parents' genes

Thalassemia and Thailand

Thalassemia is the most common inherited blood disorder in Thailand. About 30 to 40 Thai people in every 100 are carriers, meaning they have one hidden copy of the gene but no symptoms. So when two carriers marry, there is a 1 in 4 chance of a child with the disease in every pregnancy.

One in four does not mean that exactly one child out of four will have the disease. Each pregnancy is a fresh roll of the dice. Four children might have none, or several.

This is why hospitals offer blood tests to couples who plan to have children, so they know ahead of time and can plan. It is not to stop anyone from having children.

What older books often get wrongTraits like tongue rolling, earlobes or dimples are often given as easy examples of dominant and recessive genes, but modern research finds that these traits come from many genes working together, not one gene. Better examples are blood groups and inherited diseases such as thalassemia.
A blue and gold DNA helix with a magnifying glass held over one rung in the middle that is a different color from the rest and glows.
6 · MutationWhat happens when there is a mistake

A mutation does not always mean something bad

Every time a cell copies its DNA there is a chance of a small typo. Most are repaired right away. Some have no effect at all. Some cause disease. And some turn out to be an advantage. Mutations are the raw material of evolution.

Some change nothing

Because several codes translate to the same amino acid, changing a letter at some positions does not change the protein at all. Geneticists call this a silent mutation.

One letter can cause a disease

Sickle cell disease comes from changing a single letter in the hemoglobin gene. That changes one amino acid, and the red blood cells change shape until they block the smallest blood vessels.

Sometimes it is an advantage

Carriers of thalassemia and of sickle cell get a milder form of malaria. That is why genes like these are common in places where malaria used to be everywhere, including many parts of Thailand.

Genes are not a fate you cannot change

Genes set what is possible, but what you eat, how you sleep, how you exercise and the world around you decide how much each gene gets switched on. Two people with the same genes can still end up different.

A word of caution about gene testsAdvertised gene test kits that claim to tell you how good you will be at sport or at school have no scientific evidence behind them. The gene tests that really do help are tests for clear inherited diseases, and those should be done through a doctor and always come with genetic counseling.
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