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MAKING THE MODERN WORLD
Stories about the lives we've made

story:What is life?

scene:The Double Helix

The Double Helix
Biologically, DNA is the common language of every living thing. By opening up the cells of any living thing - bacteria, plants, moulds or ourselves - we find DNA controlling every activity. A close look at DNA shows that humans are remarkably similar to the rest of the living world - sharing about 98% of our DNA with chimpanzees.
While the DNA of almost all organisms is distinct in its fine detail, the overall structure of the DNA found in every living organism is the same.
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Crick and Watson look at the double helix shortly after the announcement of its discovery.
Perspective
Life covers an immense range of organisms. Plants, flies and people look very different. Yet behind the differences are many similarities. Animal cells are very similar in structure although plant cells are somewhat different. Among the many chemicals within these cells are molecules of DNA. Its overall structure is always exactly the same but in detail it is capable of subtle variety.
By increasing the magnification of the image of a human, a fruit fly and a plant cell we can gain a deeper level of understanding about what they are made of.
Human
The human face is easily distinguished from other forms of life. But what makes us work? How do we repair ourselves? What sorts of things are we made of?
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Human face
Human eye.
Using the magnification of an ordinary magnifying glass, we can see a lot more detail. For example, the pupil is not a black dot but a hole into the centre of the eye, surrounded by a coloured area, the iris.
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Human eye
Human cell
Even though a microscope is needed to see this much detail, we have known for hundreds of years that all animals are made up of cells. In fact, apart from bacteria, all living creatures are made of cells that look relatively similar.
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Human cell
Human cell nucleus
At the heart of each cell is a nucleus. If the nucleus is removed the cell cannot survive for very long and cannot reproduce at all.
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Human cell nucleus
Human DNA
DNA is generally to be found in chromosomes usually coiled up very tightly, like the tape in a cassette. Its general structure is shared among living creatures. However each species has its own characteristics and among humans only identical twins share the same DNA.
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Human DNA
Fruit fly
The fruit fly is the geneticist's ideal subject - small, easy to keep and it breeds quickly. It also has many amazing mutant forms: flies with legs on their heads or extra sets of wings. What causes these bizarre changes?
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Fruit fly
Fly legs
Unlike humans, the fly has an 'exoskeleton' - bones on the outside, not the inside. Its blood is not red and it usually has six legs instead of two. So what can it possibly have in common with us?
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Fly legs
Fly cell
At the level of fine detail, all animals share many similarities. Even though a microscope is needed to see this much detail, we have known that all animals are made up of cells for hundreds of years. In fact, all living creatures apart from bacteria are made of cells that look relatively similar.
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Fly cell
Fly cell nucleus
At the heart of each cell is a nucleus. If the nucleus is removed, the cell cannot survive for very long and cannot reproduce at all.
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Fly cell nucleus
Fly DNA
The DNA of fruit flies shares many details with human DNA. DNA is generally to be found in chromosomes usually coiled up very tightly, like the tape in a cassette.
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Fly DNA
Plant
Without plants, there would be no life on Earth. They produce oxygen which we need to breathe and all the food we eat can be traced back, eventually, to plants.
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Plant
Leaf
The structure of a plant is remarkably different to our own bodies and other animals: it cannot move, it does not feed and it absorbs energy directly from the sun. Yet at some level it is related to us.
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Leaf
Plant cell
If we use a microscope we can see that all plants are composed of cells and in fact they were the first cells ever seen by scientists. Unlike human cells, each plant cell is surrounded by a box-like wall. Many of the other components are exactly the same, such as the nucleus.
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Plant cell
Plant cell nucleus
At the heart of each cell is a nucleus. If the nucleus is removed the cell cannot survive for very long and cannot reproduce at all.
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Plant cell nucleus
Plant DNA
Even the DNA of plants is similar to that of humans. We share 60% of our DNA with a banana. DNA is generally to be found in chromosomes usually coiled up very tightly, like the tape in a cassette.
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Plant DNA
Representing DNA
The double helix of DNA is among the iconic images of our time. It is the symbol of controversies over the nature of the human being, industrial development and of the beauty of science. Each molecule looks like a twisted ladder composed of two helical backbones made of a sugar and an acid linked by rungs of four related bases.
You can't actually see DNA with the naked eye but a simplified model can give you a flavour of the structure and symmetry.
Sugar-phosphate backbone
The curving edge of the beautiful double helix is made up of an unusual sugar (deoxyribose) alternating with phosphoric acid. This sugar phosphate 'backbone' of the molecule is remarkably strong. In genetic engineering, restriction enzymes are used to break the backbone.
Base-pair rungs
The 'rungs' of the twisted 'ladder' of DNA are made up of pairs of bases. There are four different bases: adenine (A), thymine (T), cytosine (C) and guanine (G). An A will only form a complete 'rung' if it can be matched with a T and vice versa. In the same way, a C will only match up with a G, or a G with a C. It is the bonds holding these A-T and C-G pairs together which are quite weak and easily split down the middle. This makes it easy to split the molecule and is the basis of replication.
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Animation of DNA.

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