Pyrimidines (single ring):

• Cytosine (C) - DNA & RNA
• Thymine (T) - DNA only
• Uracil (U) - RNA only (replaces thymine)

DNA Structure

Double helix:

• Two antiparallel polynucleotide strands
• Sugar-phosphate backbone (outside)
• Bases paired in middle
• Complementary base pairing:
  • A pairs with T (2 hydrogen bonds)
  • G pairs with C (3 hydrogen bonds)

Chargaff's Rules:

• Amount of A = amount of T
• Amount of G = amount of C

Directionality:

• 5' end (phosphate group)
• 3' end (OH group on sugar)
• Strands run antiparallel (5'→3' and 3'→5')

RNA Structure

Single-stranded (can fold on itself)

Types of RNA:

1. mRNA (messenger) - carries genetic code from DNA to ribosomes
2. tRNA (transfer) - brings amino acids to ribosomes
3. rRNA (ribosomal) - component of ribosomes

DNA vs RNA

Key Concepts

1. Nucleotides are monomers of nucleic acids
2. DNA stores genetic information; RNA transfers it
3. Complementary base pairing: A-T and G-C (DNA); A-U and G-C (RNA)
4. DNA is double helix; RNA is usually single-stranded
5. Antiparallel strands in DNA (one 5'→3', other 3'→5')
6. Chargaff's rules: amount of purines = amount of pyrimidines