1.5

• Nucleotides


  Monomers that make up DNA and RNA, containing a phosphate group, a nitrogen-containing organic base, and a pentose (5-carbon) sugar
• Organic bases found in DNA


  • Guanine (purine)
  • Cytosine (pyrimidine)
  • Adenine (purine)
  • Thymine (pyrimidine)
• In RNA, the pyrimidine uracil replaces thymine
• ATP (Adenosine triphosphate)


  A nucleotide with a ribose sugar joined to the adenine base, with three phosphate groups attached
• ATP hydrolysis


  1. High-energy bond between second and third phosphate group is broken by enzyme ATPase

  2. 30.6kJ of energy is released for use in the cell

  3. Adenosine diphosphate (ADP) is formed

  4. Reaction is reversible, requiring energy from respiration of glucose to reform the bond
• Advantages of ATP


  • Energy is released quickly from a one-step reaction involving just one enzyme
  • Energy is released in small amounts where it is needed
  • It is the 'universal energy currency' for all reactions in all living things
• Roles of ATP in cells


  • Used in many anabolic reactions (e.g. DNA and protein synthesis)
  • Active transport
  • Muscle contraction
  • Nerve impulse transmission
• DNA structure


  • Two polynucleotide strands arranged into a double helix
  • Formed by condensation reaction between deoxyribose sugars and phosphate groups
  • Strands run in opposite directions (anti-parallel)
  • Held together by hydrogen bonds between complementary nitrogenous bases
• DNA extraction


  1. Grind up sample in ice cold salt and washing up liquid solution

  2. Detergent dissolves lipids in phospholipid membranes, releasing DNA

  3. Cold temperature protects DNA from cellular DNAases

  4. Addition of protease digests remaining cellular enzymes and histones

  5. Adding ethanol causes DNA to precipitate out
• RNA structure


  • Usually a shorter, single-stranded molecule
  • Nucleotides have ribose sugar and uracil instead of thymine
• Types of RNA


  • mRNA (messenger RNA)
  • rRNA (ribosomal RNA)
  • tRNA (transfer RNA)
• mRNA


  Single-stranded molecule produced in nucleus using DNA as template during transcription
• rRNA


  Forms ribosomes with addition of protein
• tRNA


  Small molecule with anticodon at one end and amino acid at other, 'transfers' amino acid to growing polypeptide during translation
• Functions of DNA


  • Protein synthesis (determines amino acid sequence)
  • Replication (makes copies of DNA when cells divide)
• Theories of DNA replication


  • Conservative (original parent molecule conserved, new molecule synthesised)
  • Semi-conservative (parent strands separate, each acts as template for new strand)
  • Dispersive (new molecules contain fragments from parent and newly synthesised DNA)
• Semi-conservative DNA replication


  1. DNA helicase unwinds and separates DNA strands

  2. Free nucleotides bind to exposed bases

  3. DNA polymerase binds complementary nucleotides, forming new phosphodiester bonds

  4. New DNA molecule contains one parent strand and one newly synthesised strand
• Meselson-Stahl experiment confirmed semi-conservative DNA replication
• Genetic code


  • Triplet of bases (codon) codes for a specific amino acid
  • There are 64 possible codons but only 20 amino acids, so code is degenerate (multiple codons can code for same amino acid)
• Genetic code is universal, same in all living things
• One gene codes for one polypeptide (one gene, one polypeptide hypothesis)
• Transcription


  1. DNA helicase unwinds DNA, exposing bases

  2. RNA polymerase binds free RNA nucleotides to DNA template strand, forming phosphodiester bonds

  3. Continues until reaching STOP codon, then detaches
• In eukaryotes, pre-mRNA is spliced to remove non-coding introns before translation
• Translation


  1. tRNA molecules with anticodons bind to mRNA codons on ribosomes

  2. Ribosomal enzyme catalyses formation of peptide bonds between amino acids

  3. Ribosome moves along mRNA, adding new amino acids to polypeptide chain

  4. Stops when STOP codon reached and polypeptide is released
• Multiple ribosomes can bind to single mRNA strand at same time (polysome)