2.1.3

• Nucleotides


  Consist of a phosphate group, a pentose sugar, and an organic base
• Nucleic acids


  RNA and DNA are nucleic acids made from nucleotides
• RNA


  • Non-helical, usually single-stranded and much shorter than DNA
  • Contains only the small portion(s) of the genetic code necessary for its function
  • Uses ribose sugar, not deoxyribose
  • Bases used include adenine, cytosine, guanine and uracil
  • Types: mRNA, tRNA, rRNA
• DNA


  • Found in the nucleus
  • Longer than RNA
  • Two polynucleotide strands lie in opposite directions (antiparallel)
  • Twists to form a double helix
  • Has two sugar-phosphate backbones attached by complementary base pairs
• Gene


  A sequence of DNA nucleotides that codes for a protein
• DNA cannot leave the nucleus of the cell as it is too big to fit through any given nuclear pore in the nuclear envelope
• The fact that all organisms have similar amounts of A in relation to T and G in relation to C acts as evidence for complementary base pairing
• Polynucleotide


  Formed when nucleotides bind together in a long chain, with phosphodiester bonds
• ADP and ATP


  Phosphorylated nucleotides containing a pentose sugar, a nitrogenous base (adenine), and two or three inorganic phosphates
• Semi-conservative DNA replication


  Double-stranded molecule untwists and hydrogen bonds break

  2. Polynucleotide chains separate, exposing bases

  3. Each strand used as template to make new double strands

  4. New nucleotides pair to exposed bases using complementary shapes

  5. New chains bonded together by DNA polymerase

  6. Enzyme checks base pairing is correct

  7. New molecules twist to form double helix
• Mutations are random and spontaneous
• Genetic code


  Universal - all organisms use the same code

  Sequence of bases, with three consecutive bases (triplets) coding for one amino acid

  64 possible triplet codes but only 20 amino acids used

  Some amino acids coded for by more than one triplet (degenerate codons)

  Triplets are non-overlapping
• Synthesis of polypeptides


  Transcription - reading the code and producing mRNA

  2. Translation - converting the code to a sequence of amino acids
• Transcription


  DNA molecule unwound and split by RNA polymerase

  2. Coding strand of DNA used as template to build mRNA

  3. mRNA leaves nucleus and enters cytoplasm
• Translation


  mRNA joins ribosome in cytoplasm

  2. Amino acids activated and attached to complementary tRNA

  3. Amino acids aligned with mRNA by codon-anticodon pairing

  4. Amino acids bonded together by enzymes to form polypeptide chain

  5. Complete polypeptide chain released and folds into protein