prelecture 5-1

 Key points in simple words:

• Nucleotides are the building blocks of DNA. Each nucleotide has three parts:
  (1) a nitrogen base (like A, T, C, or G),
  (2) a phosphate group,
  (3) and a sugar molecule (called pentose).
• Nucleotides connect to each other by hydrogen bonds (A pairs with T, and C pairs with G).
• The backbone of DNA is made by strong bonds (called phosphodiester bonds) between the phosphate of one nucleotide and the sugar of the next one.
• Genotypes are about the DNA code (the exact sequence of A, T, C, G letters).
  Phenotypes are about what you can see — like eye color, height, or blood type.