Nucleotides and nucleic acids

• Pentose sugars in DNA & RNA:

  • DNA: deoxyribose
  • RNA: ribose
• Polynucleotide strands are formed and broken down through:

  • Condensation reactions between nucleotides forming strong phosphodiester bonds (sugar-phosphate backbone)
  • Hydrolysis reactions using a molecule of water to break these bonds
  • Enzymes catalyse these reactions
• Structure of DNA:

  • Molecule twists to form a double helix of 2 deoxyribose polynucleotide strands (2 sugar-phosphate backbones)
  • H-bonds form between complementary base pairs (AT & GC) on antiparallel strands
• Purine bases and their structure:

  • Adenine
  • Guanine
  • Two-ring molecules
• Pyrimidine bases and their structure:

  • Thymine
  • Cytosine
  • Uracil
  • One-ring molecules
• Complementary base pairs in DNA and RNA:

  • DNA: 2 H-bonds between adenine (A) + thymine (T), 3 H-bonds between guanine (G) + cytosine (C)
  • RNA: 2 H-bonds between adenine (A) + uracil (U), 3 H-bonds between guanine (G) + cytosine (C)
• DNA replication is semiconservative because:

  • Strands from original DNA act as templates
  • New DNA contains 1 old strand & 1 new strand for accurate conservation of genetic material
• Role of DNA helicase in semiconservative replication:

  • Breaks H-bonds between base pairs to form 2 single strands acting as templates
• Formation of a new strand during semiconservative replication:

  1. Free nucleotides attach to exposed bases by complementary base pairing

  2. DNA polymerase joins adjacent nucleotides in a 5’ → 3’ direction via condensation reactions to form phosphodiester bonds

  3. H-bonds reform
• Features of the genetic code:

  • Non-overlapping: each triplet is read once
  • Degenerate: more than one triplet codes for the same amino acid
  • Universal: same bases and sequences used by all species
• Gene determines amino acid sequence in a protein through:

  • Base triplets that code for specific amino acids
• DNA purification by precipitation involves:

  • Adding ethanol & salt to aqueous solution
  • Nucleic acids precipitate out, centrifuge to obtain pellet, wash with ethanol, and centrifuge again
• Transcription produces mRNA and occurs in the nucleus:

  1. RNA polymerase binds to gene promoter region

  2. DNA uncoils into 2 strands with exposed bases, antisense strand acts as template

  3. Free nucleotides attracted to complementary bases

  4. RNA polymerase joins adjacent nucleotides to form phosphodiester bonds
• After mRNA transcription:

  • RNA polymerase detaches at terminator region
  • H-bonds reform, DNA rewinds
  • Splicing removes introns from pre-mRNA in eukaryotic cells
  • mRNA moves out of nucleus via nuclear pore and attaches to ribosome
• Translation produces proteins and occurs in the cytoplasm on ribosomes:

  • Ribosome moves along mRNA until ‘start’ codon
  • tRNA anticodon attaches to complementary bases on mRNA
  • Condensation reactions between amino acids on tRNA form peptide bonds with energy from ATP hydrolysis
  • Process continues until ‘stop’ codon is reached
• Structure of adenosine triphosphate (ATP) and adenosine diphosphate (ADP):

  • ATP: nucleotide derivative of adenine with 3 inorganic phosphate groups
  • ADP: nucleotide derivative of adenine with 2 inorganic phosphate groups
• Mutation is an alteration to the DNA base sequence, often arising spontaneously during DNA replication