Cells and DNA
One very important cell structure is the nucleus. It is the control centre of the cell and sends and receives messages from other cells. It also makes important decisions about how a cell behaves. The nucleus holds a cell's DNA or deoxyribonucleic acid.
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.
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:
A's are attracted to T's, so they always form bonds and C's are attracted to G's, so they always form bonds.
This means that one strand of DNA is the mirror image of the other and the paired bases are described as being complementary. 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 the 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.
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.
