Key points
DNA is the genetic code which makes up genes, which are responsible for giving an organism a specific characteristic.
Watson and Crick, with help from Franklin and Wilkins, discovered the double helix structure of DNA in 1953.
Video - The importance of DNA
Dr Andrew Hart: My name is Dr Andrew Hart and I'm a senior forensic scientist.
DNA stands for deoxyribonucleic acid, it's a complex chemical. It's found in the nucleus of cells.
DNA is fundamental to our job. And what it is, it codes for your sort of genetic makeup, such as things like your eye colour and your hair colour, sort of your physical and chemical characteristics. Everyone's DNA is almost unique apart from that of identical twins.
With the police, we're using forensic science to help with their criminal investigations, using things like bloodstain pattern analysis, we're looking at body fluids.
DNA can transfer in many different ways, my saliva could be deposited on a surface or I might have cut myself and left my blood or a hair might have fallen out.
DNA can survive thousands of years, so we can go back to old crime scenes.
If you're thinking of applying for this job, is to get a good grounding in science, chemistry, physics, biology. The job I do as a forensic scientist, you know, it's different every day, and that one particular case, it could be that, you know, the breakthrough that the police need. That gives you such a buzz and a rush and you just feel that, you know, you've really made a difference.
Can you answer these questions based on the video?
1. What does DNA stand for?
2. Whose DNA is not unique?
Deoxyribonucleic acid
Identical twins
The structure of DNA
The store of genetic information for all living things, passed from parents to offspring. stands for deoxyribonucleic acid. It is a chemical made up of two long strands, arranged in a spiral. This is the double-helix structure. DNA carries genetic information - the genetic code. It has all the instructions that a living organism needs to grow, reproduce and function. DNA is passed on from parents to their offspring during fertilisation.
DNA is made from four A pair of DNA bases, A-T, T-A, G-C and C-G.. These are made from the The four letters that make up the genetic code: A and T, G and C.: A and T, G and C. These can be either way around, but A always pairs with T and G with C. The base pairs are held together by weak chemical hydrogen bonds.
Did you know?
There are over three billion different DNA base pairs in each person.Everyone鈥檚 DNA is unique - apart from an identical twin who has the same DNA as their twin.
DNA is present in the A cell component found in most cells which contains the genetic material (DNA) of the organism and controls the cell鈥檚 activities. of almost every cell in your body. In fact, there are around two metres of DNA packed into each nucleus of your body cells. Your sex cells or gametes, sperm or ova, only have half of this, because they join the other cell during The joining of male and female gametes (sperm and ova or eggs)..
All life on Earth has the same genetic code. The same four letters make up bacteria, fungi, animals - including humans -, plants and single-celled A single-celled organism with characteristics of animals, plants or both..
From DNA to the genome
A small section of DNA that gives a characteristic. are small sections of DNA that are the genetic code for an inherited characteristic such ear shape or eye colour. You inherit pairs of genes for most characteristics from each of your parents. So you have two genes for ear shape and two for eye colour. Pairs of genes that are inherited. One gene is inherited from each parent. are pairs of genes that are inherited 鈥 one from each parent.
There are around 23 000 genes that make up a human.
Genes are arranged into larger coiled structures called A coiled structure of DNA found in the nucleus of cells made from many genes.. Humans have 46 chromosomes in total. We inherit 23 from each of our parents. Other organisms have different amounts. The fruit fly has only 8 chromosomes and is often used to study patterns of inheritance, while some ferns have over one hundred.
All of the chromosomes of an organism add up to make its genome. This is one copy of all its DNA. Every human on Earth has a unique genome unless they are identical twins who were made from the same fertilised ovum (egg cell). Non-identical twins were made from two separate sperm fertilising two different ova (egg cells) and so have different genomes.
DNA makes up genes which make up chromosomes which make up the genome. A good model for this is thinking about money. Pennies make up 拢1 coins which make up 拢5 notes which make up 拢20 notes.
The discovery of DNA
Scientists worked out the structure of DNA in the 1950s. Rosalind Franklin made 鈥榅-ray diffraction鈥 images of DNA. James Watson and Francis Crick used information from one of her images to work out a model for the structure of DNA. Work by Maurice Wilkins, a colleague of Franklin, supported their model.
Video - Brian Cox explains DNA
Professor Brian Cox explains the discovery of DNA.
Professor Brian Cox: It's here in Cambridge that Francis Crick and James Watson discovered the structure of DNA, the molecule that passes biological information from generation to generation.
Crick and Watson's approach to finding that structure was to build physical models of the molecule. But it was proving unsuccessful. They desperately needed more and better data. And it came from a branch of physics called 'X-ray crystallography.'
This is a very famous photograph. It's called Photograph 51. It was actually taken by another scientist, Rosalind Franklin and it's what's called an X ray diffraction photograph, so Franklin shone X rays through a sample of DNA molecules and the way they scatter or diffract off the molecules, the pattern they leave on the photographic plate, allows you to deduce the structure of those molecules.
The key piece of evidence is the X that allowed Franklin to suggest that the molecule must be helical, and in fact, must have that famous double helix. So this photograph along with Franklin's suggestions, her interpretation of the pattern, allowed Watson and Crick to go away and build their model of DNA.
When they published the structure of DNA in 1953, Crick said "we have discovered the secret of life." Crick was right. The discovery of the structure of DNA was one of the great moments in modern scientific history.
By the early 1970s, the genetic code had been translated, making it possible to identify individual genes and study their function. We now had access to the workings of life itself.
Can you answer these questions based on the video?
1. Which two scientists discovered the structure of DNA?
2. Who helped them by taking Photograph 51?
Francis Crick and James Watson
Rosalind Franklin
Test your knowledge
Quiz - What is DNA?
Test questions
1. Describe how genetic information is stored in living cells.
2. Describe how the structure of DNA was discovered.
- DNA is made from a four-letter code made of A, T, G and C.
- The DNA bases pair together: A-T, T-A, G-C and C-G.
- DNA is arranged in a double helix structure.
- A gene is a short section of DNA that is the genetic code for a characteristic.
- A chromosome is a coiled structure of DNA found in the nucleus of cells made from many genes.
- A genome is one copy of an organism鈥檚 DNA.
- James Watson and Francis Crick discovered the structure of DNA in the 1950s.
- They used 鈥榅-ray diffraction images of DNA taken by Rosalind Franklin.
- She was helped by Maurice Wilkins.
- Watson, Crick and Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine for their discovery.
- Franklin had died before then and so could not be awarded it with them.
- Now the Nobel Prize is able to be awarded posthumously (after the recipient has died).
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