2- The Structure of DNA and RNA

• DNA


  • Consists of two polynucleotide strands
  • Sugar phosphate backbone on the outside with bases in the center
  • Strands held together by hydrogen bonds between complementary bases
• Complementary base pairing


  Guanine on one strand pairs with cytosine on the opposite strand, adenine pairs with thymine
• Purines


  • Adenine and guanine have a double ring structure
• Pyrimidines


  • Thymine and cytosine have a single ring structure
• Purines pair with pyrimidines, resulting in constant distance between sugar phosphate backbones
• DNA strands


  • Anti-parallel, running in opposite directions
• Hydrogen bonds


  Guanine-cytosine form 3, adenine-thymine form 2
• Calculating nucleotide proportions


  If 20% of nucleotides contain guanine, then 20% contain cytosine, and 30% each contain adenine and thymine
• DNA structure


  • Two polynucleotide strands twist to form a double helix
• RNA


  Also a polynucleotide, but differs from DNA in several ways
• RNA


  • Contains ribose sugar instead of deoxyribose
  • Contains uracil instead of thymine
  • Found in cytoplasm, not nucleus
  • Plays key role in protein synthesis
  • Shorter molecules than DNA, often only a few hundred nucleotides
  • Single-stranded, not double-helix