The structure of DNA
Francis Crick and James Watson first described the structure of DNA in 1953. They received the Nobel Prize for this discovery. They revealed that DNA has two strands and each one is made up of a chain of nucleotide bases.
There are four types of bases, which can appear in the chain in any order. The bases are called A, T, C and G - short for adenine, thymine, cytosine and guanine.
The nucleotide bases are attached to a supporting scaffold of sugar and phosphate molecules.
The two strands join together by weak bonds. These bonds always form in the same way:
As are attracted to Ts, so they always pair up. And Cs are attracted to Gs, so they always stick together.
This pairing means that one strand of DNA is the mirror image of the other - scientists call this complementary base pairing.
The paired bases are like the rungs of a flexible ladder. If you were to take hold of the two ends of the ladder and twist them in opposite directions you would end up with a spiral structure. In the case of DNA, this structure is known as a double helix.
When DNA needs to be copied the helix unwinds and the weak bonds between the nucleotide bases break, allowing the strands to separate. This means that the molecular 'machinery' that copies DNA can get access to the bases.
Each strand can then be used as a template to build a new complementary strand. In this way one DNA molecule becomes two. This is important when cells multiply.
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