DNA and RNA Structure

• DNA and RNA belong to a class of macromolecules called nucleic acids.
• Nucleic acids are polynucleotides which means they contain many nucleotides joined together.
• A nucleotide consists of:
  • One cyclic five-carbon sugar (The carbons found in this sugar are numbered 1' through to 5')
  • One phosphate
  • One nitrogenous base
• The sugar is deoxyribose in DNA and ribose in RNA. The only difference between the sugars is that ribose has a hydroxyl group (OH) on the 2' carbon and deoxyribose does not. This makes deoxyribose more stable than ribose.
• The phosphate is linked to the 5' carbon of the sugar in both RNA and DNA.
• The nitrogenous bases are adenine(A), guanine(G), cytosine(C), thymine(T), and uracil(U).
  • Adenine and guanine are purines (contains a six membered ring of carbon and nitrogen fused to a five membered ring).
  • Adenine and guanine are both found in DNA and RNA.
  • Cytosine, thymine and uracil are pyrmidines (contains a six membered ring of carbon and nitrogen).
  • Cytosine is found in both DNA and RNA, but thymine is only found in DNA and uracil is only found in RNA.
• A nucleotide is formed when a phosphate attahes to the 5' carbon of the sugar and one of the nitrogenous baseses attaches to the 1' carbon of the sugar.
• A strand of DNA or RNA consists of nucleotides linked together by phosphodiester bonds.
  • A phosphodiester bond exists between the phosphate of one nucleotide and the sugar 3' carbon of the the next nucleotide.
  • This forms a backbone of alternating sugar and phosphate molecules known as the "sugar-phosphate backbone".
• RNA, in most cases, consists of one strand of nucleic acids linked together by phosphodiester bonds.
• A DNA molecule consists of two strands of nucleotides twisted together to form a double helix.
  • The sugar-phosphate backbone is found on the outside of this helix and the bases are found braching towards the middle.
  • Hydrogen bonds join the the nitrogenous bases and hold the two strands together.