polynucleotides and DNA

• Nucleotides join together to form polynucleotides. The nucleotides join up between the phosphate group of one nucleotide and the sugar of another via a condensation reaction. This forms a phosphodiester bond (consisting of the phosphate group and two ester bonds). The chain of sugars and phosphates is known as the sugar-phosphate backbone. Polynucleotides can be broken down into nucleotides again by breaking the phosphodiester bonds using hydrolysis reactions.
• structure of a single polynucleotide strand

  A) sugar-phosphate backbone

  B) ester bond

  C) phosphodiester bond

  
• Each base can only join with one particular partner - this is called complementary base pairing. Adenine always pairs with thymine(A - T) and guanine always pairs with cytosine (G - C). This means that a purine (A or G) always pairs with a pyrimidine (Tor C). Two hydrogen bonds form between A and T, and three hydrogen bonds form between C and G.
• DNA is composed of two polynucleotide strands joined together to form a double-helix shape. The two strands join together by hydrogen bonding between the bases.
• complimentary base pairing in DNA molecules

  A) hydrogen bonds between the bases

  B) direction of strand

  C) base

  D) deoxyribose sugar

  E) phosphate group

  
• The two polvnucleotide strands are antiparallel - this means they run in opposite directions. Two antiparallel strands twist to form a DNA double-helix.
• DNA double-helix

  A) bases

  B) antiparallel strands

  C) polynucleotide strand with sugar-phosphate backbone

  D) hydrogen bonding between bases