Nucleotides and Nucleic Acids

• RNA

  • Single-stranded polynucleotide
  • Ribose replaces deoxyribose as pentose (because extra oxygen at C2)
  • Uracil replaces thymine
  • Messenger RNA (mRNA):
  • In nucleus
  • From DNA template by transcription
  • Length and base sequence vary
  • Short life
  • Transfer genetic code from DNA to ribosome
  • Transfer RNA (tRNA):
  • Cloverleaf shape (64 forms)
  • Attaches to specific amino acid
  • Transfer to ribosome
  • Three bases make anticodon (determines carried amino acid)
  • Ribosomal RNA (rRNA):
  • Made in nucleolus
  • Forms half ribosomes mass
• Genetic Code

  • Gene
  • Sequence of nucleotides in DNA molecule
  • Codes for 1 polypeptide
  • Nucleotide order determines amino acids
  • Nucleotides (triplet codons) + amino acids relationship is genetic code
  • Same code by nearly all living organisms, near-universal
  • 20 amino acids, 4 nucleotide bases
  • If 1 base coded for 1 amino acid, only 4 amino acids have codes (4^1 = 4)
  • 2: 4^2 = 16
  • 3: 4^3 = 64; too many?
  • Each has more than one codon; degenerate
  • Some triplets “full stops”, indicating gene end
  • Non-overlapping code
  • Each base read once

  Remember

  • D egenerate
  • U niversal
  • N on-overlapped
  • T riple code
• Nucleic Acid

  • DNA: Deoxyribonucleic acid
  • RNA: Ribonucleic acid
  • Deoxyribose has H bonded to C2; has 1 less O_2 than ribose, which has OH group attached to C2
  • Long unbranched polymer chains
  • Polynucleotides
  • Nucleotide subunits joined by phosphodiester bonds
  • 5C phosphate group forms covalent bond with 3C OH group of other
  • Makes strong sugar-phosphate backbone
  • Macromolecules
  • Nucleotide:
  • Nitrogen-containing organic substance (8 types)
  • Pentose suage + phosphate group + nitrogenous base
  • Cellular roles:
  • Form nucleic acids + ATP
  • Metabolic reactions (NAD, NADP + coenzyme A)
• DNA

  • 2 anti-parallel polynucleotide strands
  • One 3'-5' direction, other 5'- 3' direction
  • Complementary base pair:
  • Purine (double-ring) - pyrimidine (single-ring)
  • Adenine - Thymine (A-T, two H-bonds)
  • Guanine - Cytosine (G-C, three H-bonds)
  • Twists to double helix
  • H-bonds individually weak but collectively stable
  • Long-term stable genetic store
  • Makes up chromosomes
  • DNA Extraction
  • Grind sample - break cell walls
  • Mix w/ detergent - break down cell membrane, releasing cell contents
  • Add salt - break H bonds between DNA + H_2O
  • Add protease enzyme - break down histones
  • Add alcohol/ethanol layer - cause DNA to precipate out solution
  • DNA seen as white strands between layer + solution
  • Spool onto glass rod to pick it up
  • Gene
  • DNA section
  • Codes for specific polypeptide
  • Codon (triplet code): Base sequence determining specific amino acid sequence
• DNA replication

  • S phase of interphase
  • Ensures 2 daughter cells have same chromosome number
  • Semiconservative
  • Each new DNA molecule has 1 old + 1 new strand
  • Original parent strand, template for new daughter strand

  Replication Errors

  • Sequences not always exact
  • Errors are random + spontaneous - Mutations

  Stages

  • Original molecule unwinds catalysed by DNA helicase
  • H bonds between nitrogenous bases
  • 2 strands separate (like zip), each now template for new strand
  • Free nucleotides join exposed complementary bases
  • DNA polymerase catalyse bond between nucleotides, 5’ to 3’ direction only + checks new bases
  • 3’ to 5’ strand formed in short segments (5’ to 3’), Okazaki fragments, by DNA ligase
  • DNA winds to 2 double helices