Nucleotides and amino acids

• What is the structure of a nucleotide?

  A nucleotide consists of a phosphate group, a pentose sugar, and a nitrogenous base.

  
• What are the pentose sugars found in DNA and RNA?

  DNA contains deoxyribose, while RNA contains ribose.

  
• How are polynucleotide strands formed and broken down?

  • Formed by condensation reactions between nucleotides.
  • Strong phosphodiester bonds create a sugar-phosphate backbone.
  • Broken down by hydrolysis reactions using a molecule of water.
  • Enzymes catalyze these reactions.

  
• Describe the structure of DNA.

  • Twists to form a double helix.
  • Composed of 2 deoxyribose polynucleotide strands.
  • Contains 2 sugar-phosphate backbones.
  • Hydrogen bonds form between complementary base pairs (A-T & G-C).
  • Strands run antiparallel.

  
• What are the purine bases and their structures?

  The purine bases are adenine (C₅H₅N₅) and guanine (C₅H₅N₅O).

  
• What are the pyrimidine bases and their structures?

  The pyrimidine bases are thymine (C₅H₆N₂O₂), cytosine (C₄H₅N₃O), and uracil (C₄H₄N₂O₂).

  
• What are the complementary base pairs in DNA and RNA?

  • DNA: Adenine (A) pairs with Thymine (T) using 2 H-bonds.
  • RNA: Adenine (A) pairs with Uracil (U) using 2 H-bonds.
  • Both: Guanine (G) pairs with Cytosine (C) using 3 H-bonds.

  
• Why is DNA replication described as semiconservative?


  Because each new DNA molecule contains one old strand and one new strand, conserving genetic material accurately.
• What is the role of DNA helicase in semiconservative replication?

  DNA helicase breaks hydrogen bonds between base pairs to form two single strands that can act as templates.

  
• How is a new strand formed 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.
  3. Phosphodiester bonds are formed, and hydrogen bonds reform.

  
• What are the features of the genetic code?

  • Non-overlapping: each triplet is read once.
  • Degenerate: multiple triplets can code for the same amino acid.
  • Universal: same bases and sequences used by all species.
• How does a gene determine the sequence of amino acids in a protein?

  A gene consists of base triplets that code for specific amino acids.
• How can DNA be purified by precipitation?

  1. Add ethanol and salt to an aqueous solution.
  2. Nucleic acids precipitate out of solution.
  3. Centrifuge to obtain a pellet of nucleic acid.
  4. Wash the pellet with ethanol and centrifuge again.