What Are DNA and RNA?
DNA and RNA are the nucleic acids that store and use genetic information in every living cell. DNA holds the long-term genetic blueprint, while RNA carries and executes those instructions to build proteins.
DNA (deoxyribonucleic acid) is a double-stranded molecule that stores genetic information; RNA (ribonucleic acid) is typically single-stranded and helps read and express that information, especially during protein synthesis.
- •Double-stranded helix
- •Sugar: deoxyribose
- •Bases: A, T, G, C
- •Stores genetic information long-term
- •Stable, stays in the nucleus
- •Usually single-stranded
- •Sugar: ribose
- •Bases: A, U, G, C
- •Carries and expresses genetic instructions
- •Short-lived, moves to cytoplasm
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Step-by-step worked examples
A 100-base DNA segment has 30 guanine (G) and 25 cytosine (C) bases. What is its GC content?
GC% = (G + C)/L × 100 GC% = (30 + 25)/100 × 100 GC% = 55%
A DNA strand reads 5'-ATG CGT ACG-3'. What is the complementary strand?
Pair each base: A↔T, T↔A, G↔C, C↔G ATG CGT ACG → TAC GCA TGC Complementary strand (3'→5'): 3'-TAC GCA TGC-5'
An mRNA strand is transcribed from the DNA template 3'-TAC GCA TGC-5'. What is the mRNA sequence?
RNA polymerase pairs A↔U, T↔A, G↔C, C↔G (uses U instead of T) Template 3'-TAC GCA TGC-5' → mRNA 5'-AUG CGU ACG-3'
Flashcards
Quick quiz
Q1.Which sugar is found in RNA?
Q2.Which base replaces thymine in RNA?
Q3.DNA with 40% guanine has what percentage of cytosine?
Q4.What is the main structural difference between DNA and RNA?
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Common mistakes
Thinking DNA and RNA use the same four bases. — Correct: They share A, G, C, but DNA uses thymine (T) while RNA uses uracil (U).
Assuming RNA is always single-stranded and never folds. — Correct: RNA is single-stranded but often folds into complex secondary structures (like tRNA's cloverleaf).
Confusing base pairing rules (thinking A pairs with G). — Correct: A pairs only with T/U, and G pairs only with C — never A with G.
Believing RNA is only found in the nucleus. — Correct: RNA moves out of the nucleus into the cytoplasm to be used at ribosomes.
FAQ
What is DNA and RNA?
DNA is the double-stranded molecule that stores genetic information; RNA is usually single-stranded and helps express that information as proteins.
What is the formula for GC content in DNA?
GC content (%) = (G + C) ÷ total bases × 100, a common way to describe a DNA sequence's stability.
What are examples of DNA and RNA differences?
Sugar (deoxyribose vs ribose), strandedness (double vs single), and bases (thymine vs uracil) are the three classic differences.
How do you calculate GC content in a DNA sequence?
Count the guanine and cytosine bases, divide by the total sequence length, and multiply by 100.