What Is DNA and How Is It Structured?
DNA (deoxyribonucleic acid) is the molecule that stores genetic instructions for building and operating every living organism. Its double helix structure, made of nucleotide base pairs, allows it to store, copy and pass on information.
DNA is a double-helix molecule made of nucleotides (sugar, phosphate, and a nitrogenous base: A, T, C or G) that stores genetic information; adenine always pairs with thymine, and cytosine always pairs with guanine.
- •Double-stranded helix
- •Sugar: deoxyribose
- •Bases: A, T, C, G
- •Stores genetic information long-term, stays in the nucleus
- •Usually single-stranded
- •Sugar: ribose
- •Bases: A, U, C, G (uracil replaces thymine)
- •Carries instructions to build proteins (e.g., mRNA)
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Step-by-step worked examples
A DNA strand has 30 adenine (A) bases out of 100 total bases. Using complementary base pairing (A=T, C≡G), how many thymine (T) and guanine plus cytosine bases are there?
A = T, so T = 30 Remaining bases = 100 − 30 (A) − 30 (T) = 40 C = G, so C = 20 and G = 20
A gene segment reads 5'-ATG CGA TTC-3' on one strand. What is the complementary strand (3' to 5')?
Pair each base: A-T, T-A, G-C, C-G, G-C, A-T, T-A, T-A, C-G Complementary strand (3'→5'): TAC GCT AAG
A DNA molecule has 40% GC content and 200 total base pairs. How many adenine bases does it contain?
GC% = 40%, so AT% = 100% − 40% = 60% A% = AT% ÷ 2 = 30% A bases = 30% × 200 = 60 adenine bases
Flashcards
Quick quiz
Q1.What shape does DNA have?
Q2.Which base pairs with adenine?
Q3.What are the three parts of a nucleotide?
Q4.What is DNA's main function in a cell?
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Common mistakes
DNA and RNA are exactly the same molecule. — Correct: DNA is double-stranded with deoxyribose and thymine; RNA is usually single-stranded with ribose and uracil.
Adenine pairs with guanine. — Correct: Adenine always pairs with thymine (A-T); guanine pairs with cytosine (C-G).
DNA directly builds proteins itself. — Correct: DNA is transcribed into mRNA, which is then translated into proteins — DNA doesn't build proteins directly.
All of an organism's DNA is identical to its RNA sequence. — Correct: RNA is a transcribed copy that replaces thymine with uracil and is usually single-stranded, not identical to DNA.
FAQ
What is DNA?
Deoxyribonucleic acid — a double-helix molecule made of nucleotides that stores an organism's genetic instructions.
What is the rule for DNA base pairing?
Adenine (A) pairs with Thymine (T), and Cytosine (C) pairs with Guanine (G) — known as complementary base pairing.
What are examples of DNA's function?
Storing the genes for eye color, coding for enzymes, and passing traits from parents to offspring.
How do you calculate the number of each base in a DNA strand?
Since A=T and C=G, find the GC% first, then split the remaining AT% equally between A and T.