Step by Step
A
Adenine pairs with Thymine (DNA) — 2 hydrogen bonds
In DNA, adenine (A) always pairs with thymine (T). They form 2 hydrogen bonds. Memory trick: A-T = 2 letters, 2 bonds.
In RNA: adenine pairs with Uracil (U) instead — RNA has no thymine.
G
Guanine pairs with Cytosine — 3 hydrogen bonds
Guanine (G) always pairs with cytosine (C). They form 3 hydrogen bonds — stronger than A-T. Memory trick: G-C = 3 letters total, 3 bonds. Or: GC = "Got Caught" — stuck together tighter.
U
RNA uses Uracil instead of Thymine
RNA is single-stranded and uses uracil (U) wherever DNA uses thymine (T). So in RNA: A pairs with U, G pairs with C. This is critical for transcription questions.
Memory trick: U is in RNA. T is in DNA. "U" are a messenger (mRNA), "T" stays home (in DNA).
GC
High G-C content = more thermally stable DNA
Because G-C has 3 hydrogen bonds vs A-T's 2, regions with more G-C content require more heat to separate (denature). This matters in PCR — higher melting temperature for G-C-rich sequences.
Applied Walkthrough
1
A DNA strand reads: 5'-ATGCTA-3'. What is its complementary strand?
2
Apply the rules: A→T, T→A, G→C, C→G. Complementary strand: 3'-TACGAT-5'.
3
Now if this DNA is transcribed into mRNA: read the template strand (3'→5') and apply RNA rules (A→U, T→A, G→C, C→G). mRNA = 5'-AUGCUA-3'.
4
Notice: the mRNA matches the coding strand of DNA — just with U replacing T.
Exam Application
Exam questions give you one strand and ask for the complementary DNA or mRNA sequence. Always identify which strand you're given and which you need. Remember: mRNA uses U not T. The number of hydrogen bonds (A-T=2, G-C=3) appears in questions about DNA stability and PCR melting temperatures.
⚠ Common Trap
Students forget that RNA uses U instead of T — and write T in mRNA sequences. Also watch the direction: DNA and RNA are always synthesized 5'→3'. When writing a complementary strand, the antiparallel rule means the new strand runs opposite (3'→5' to 5'→3').
✓ Quick Self-Check
1. Which bases pair together in DNA?
A pairs with T (2 hydrogen bonds); G pairs with C (3 hydrogen bonds).
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2. What base replaces thymine in RNA?
Uracil (U).
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3. Which base pair has more hydrogen bonds — A-T or G-C?
G-C has 3 hydrogen bonds; A-T has only 2. G-C is therefore stronger/more stable.
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4. What is the mRNA sequence for the DNA template strand 3'-TACGAT-5'?
5'-AUGCUA-3' (A→U, T→A, C→G, G→C, A→U, T→A).
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5. Why does high G-C content make DNA more thermally stable?
G-C pairs have 3 hydrogen bonds vs A-T's 2, requiring more energy (heat) to break apart.
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