Structure of DNA

• The building block of DNA is the nucleotide.
• A nucleotide has three components: sugar (deoxyribose), a phosphate group, and a nitrogen base group (4 kinds).
• The four nitrogen base pairs are adenine, thymine, cytosine, and guanime.
• DNA is made of two strands of nucleotides held together by hydrogen bonds between the nitrogen bases.
• Each base in a nucleotide will only pair with one other specific base. Base pairings are: Adenine + Thymine and Cytosine + Guanine.
• This specific pairing happens because of the unique shape of the nitrogenous bases.
• Purines have a double ring base and are the shape of adenine and guanine.
• Pyrimidines aka pyrimidine bases are Cytosine and Thymine.
• When a purine pairs with a pyrimidine, it allows the width of the DNA to remain uniform (3 rings across).
• Adenine must pair with thymine because these two can form double bonds and so fit together.
• Cytosine and guanine form triple bonds so they fit together.
• Erwin Chargaff discovered the rule of base pairing in DNA.
• The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA and the percentages of cytosine (C) and guanine (G) are almost the same.
• Nitrogen Base Pairs form the middle of a DNA strand.
• Deoxyribose and Phosphate form the sides of the DNA strand. Because of the complementary base pairing, the order of the bases on one strand determines the order of the bases on the other strand.
• Rosaline Franklin and Maurice Wilkins (1952) photographed DNAs crystals using X-rays.
• The X pattern suggested that the structure of the DNA was a helix (spiral). The famous photograph is called "Photo 51".
• James Watson and Francis Crick created a model (after Franklin's work) which demonstrated that DNA consisted of two strands of nucleotides in a ladder-like structure that twisted. Coined the term "Double-Helix".