Connecting DNA Structure to Biological Function
The DNA-RNA-Protein Connection (Central Dogma)
DNA → (Transcription) → RNA → (Translation) → Protein
Why DNA uses Thymine (not Uracil):
- Cytosine (C) can spontaneously deaminate to form uracil (U)
- In DNA, this would be recognized as a mutation (U not normally in DNA)
- DNA repair enzymes recognise U in DNA as aberrant and remove it
- If DNA used U originally, repair would be impossible — T (thymine = 5-methyluracil) serves as the "correct" base
Why RNA uses Ribose (not Deoxyribose):
- Ribose has 2'-OH group, making RNA more reactive
- This reactivity suits RNA's transient roles (mRNA) and catalytic roles (ribozymes)
- DNA's deoxyribose lacks this reactive group, making DNA more chemically stable (appropriate for long-term genetic storage)
Chargaff's Rules — What They Reveal:
- A = T and G = C (in double-stranded DNA) → COMPLEMENTARY BASE PAIRING
- Consequence 1: AT-GC ratio varies between species (species-specific DNA composition)
- Consequence 2: G-C content predicts thermal stability (higher G-C = higher Tm)
- Consequence 3: During replication, each strand serves as a template for its complement
Hydrogen Bond Numbers — Why They Matter:
| Base Pair | H-bonds | Significance |
|---|---|---|
| A-T | 2 | Weaker — AT-rich DNA melts at lower temperature |
| G-C | 3 | Stronger — GC-rich DNA melts at higher temperature |
| A-U (RNA) | 2 | Same as A-T but in RNA context |
Key Connections to Remember:
- Ribose in RNA → can form ribozymes (connects nucleic acids to enzyme biochemistry)
- tRNA connects nucleotide sequence (anticodon) to amino acid sequence (primary protein structure)
- Disruption of DNA by mutagens → changes in primary protein structure → disease