What was discovered?

The molecular structure of DNA, or in other words, what molecules DNA itself is made of and how they are fitted together into the amazing double helix that we are familiar with today.  

DNA is made of nucleotides which are joined together into a long chain, or strand, by strong covalent bonds (so that the chain cannot break easily).  Two of these chains make the "double helix".

Each nucleotide is made of a sugar called deoxyribose, joined to a phosphate group and a nitrogenous base.  The base could be Adenine (A) or Guanine (G) - these are known as purines - or Thymine (T) or Cytosine (C) - known as pyrimidines.

The nucleotides join together by linking the phosphate of one nucleotide with the deoxyribose of the next. Nucleotides of one strand of DNA link using hydrogen bonds from their bases to the bases of the other strand.  A pairs with T and G pairs with C. This is called "complementary base pairing" and it is how the strands are "stuck" together.

How?

Details of measurements of the overall structure of the DNA helix by X-ray crystallography were made by Rosalind Franklin and Maurice Wilkins.

Using these overall dimensions, and more data by Chargaff which showed that the proportion of purines (A and G) equals the proportion of pyrimidines (T and C), Watson and Crick built molecular models, like a fabulous 3-D jigsaw puzzle, to figure out the structure of DNA.

Why was it important?

Knowing the structure of DNA gave hints as to how the molecule could copy itself - to reproduce - to give a precise copy of the genetic information contained in the sequence of its bases to a new cell.

The strands "unzip" by breaking the weak hydrogen bonds between them - then each strand can be copied using complementary base pairing.  Result? 2 DNA molecules where there was just one!

Now that's important!!!   How we, and other living organisms, reproduce!

See for yourself how the news was published by the BBC back in 1953