DNA vs RNA Comparison:

(a) Sugar Component:

DNA: Deoxyribose (lacks OH on 2' carbon)

• Formula: C₅H₁₀O₄
• 2'-H (hydrogen at position 2)

RNA: Ribose (has OH on 2' carbon)

• Formula: C₅H₁₀O₅
• 2'-OH makes RNA more chemically reactive

(b) Nitrogenous Bases:

DNA:

• Purines: Adenine (A), Guanine (G)
• Pyrimidines: Cytosine (C), Thymine (T)

RNA:

• Purines: Adenine (A), Guanine (G)
• Pyrimidines: Cytosine (C), Uracil (U)

Key difference: Thymine (DNA) vs Uracil (RNA)

• Thymine = methylated uracil (extra -CH₃ group)

(c) Structure:

DNA:

• Double-stranded (double helix)
• Antiparallel strands (5'→3' and 3'→5')
• Base pairing: A-T (2 H-bonds), G-C (3 H-bonds)
• Very stable, long-term storage
• Width: ~2 nm, 10 bp per turn

RNA:

• Usually single-stranded
• Can fold into secondary structures (hairpins, loops)
• Some regions may base pair (A-U, G-C)
• More flexible, temporary

(d) Primary Biological Functions:

DNA:

1. Long-term genetic storage

   • Contains hereditary information
   • Passed from parent to offspring

2. Template for replication

   • Makes identical copies during cell division

3. Template for transcription

   • Genes transcribed into RNA

RNA:

1. mRNA (messenger RNA):

   • Carries genetic information from DNA to ribosomes
   • Template for protein synthesis

2. rRNA (ribosomal RNA):

   • Structural and catalytic component of ribosomes
   • Catalyzes peptide bond formation

3. tRNA (transfer RNA):

   • Brings amino acids to ribosome during translation
   • Has anticodon that pairs with mRNA codon

4. Other RNAs:

   • miRNA, siRNA (gene regulation)
   • snRNA (splicing)
   • Ribozymes (catalytic RNA)

Summary Table:

$\boxed{\text{DNA: stable storage (T, deoxyribose); RNA: functional (U, ribose)}}$

Given: 5'-ATGCGATACG-3'

(a) Complementary strand:

Rules:

• Strands are antiparallel
• A pairs with T (2 H-bonds)
• G pairs with C (3 H-bonds)

Original: 5'-ATGCGATACG-3' Complement: 3'-TACGCTATGC-5'

Or written in conventional 5' to 3' direction:

$\boxed{\text{Complement: } 5\text{'-CGTATCGCAT-}3'}$

(b) Chargaff's Rules:

Statement: In double-stranded DNA:

1. Amount of adenine (A) = Amount of thymine (T)
2. Amount of guanine (G) = Amount of cytosine (C)
3. Amount of purines (A+G) = Amount of pyrimidines (T+C)
4. The ratio (A+T)/(G+C) varies by species but is constant within species

Verification for this segment:

Count bases in both strands:

Original strand: A=3, T=2, G=3, C=2 Complement: A=2, T=3, G=2, C=3

Total (double-stranded):

• A = 3 + 2 = 5
• T = 2 + 3 = 5 ✓ (A = T)
• G = 3 + 2 = 5
• C = 2 + 3 = 5 ✓ (G = C)
• Purines (A+G) = 5 + 5 = 10
• Pyrimidines (T+C) = 5 + 5 = 10 ✓

$\boxed{\text{Chargaff's rules verified: A=T=5, G=C=5}}$

(c) Percentage of G-C base pairs:

Total base pairs = 10 bp (double-stranded segment)

G-C base pairs = 5

$\%GC = \frac{\text{G-C pairs}}{\text{Total pairs}} \times 100\%$

$\%GC = \frac{5}{10} \times 100\%$

$\boxed{\%GC = 50\%}$

Biological Significance:

• G-C content affects DNA stability
• 3 H-bonds (G-C) vs 2 H-bonds (A-T)
• Higher GC% → higher melting temperature (T_m)
• This segment: 50% GC = moderate stability

Calculation for melting temperature: $T_m \approx 81.5 + 0.41(\%GC)$ $T_m \approx 81.5 + 0.41(50) = 102°C$

(For longer DNA; short oligos use different formula)