🎓 Prepared by students from Boğaziçi University

What is DNA Structure?

DNA (deoxyribonucleic acid) is the double-helix molecule that stores genetic instructions in every living cell. Its structure — two antiparallel strands held together by base pairing — enables accurate copying (replication) each time a cell divides.

Short answer

DNA is a double helix of two complementary strands linked by hydrogen-bonded base pairs (A-T, G-C); it replicates semiconservatively, with each new molecule keeping one original strand and one newly synthesized strand.

Steps of DNA Replication

1
Unwinding
Helicase unwinds the double helix, separating the two strands at the replication fork.
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2
Priming
Primase lays down a short RNA primer to give DNA polymerase a starting point.
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3
Leading strand synthesis
DNA polymerase builds the new strand continuously toward the replication fork.
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4
Lagging strand synthesis
The other strand is copied in short Okazaki fragments, away from the fork.
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5
Ligation
DNA ligase joins the Okazaki fragments into one continuous strand.

Try it: interactive calculator

Melting temperature (Tm)

64°C

= 4*12+2*8

G+C base count12 basesA+T base count8 bases

Step-by-step worked examples

A short DNA primer has 12 G/C bases and 8 A/T bases. Estimate its melting temperature using the Wallace rule.

Tm = 4(G+C) + 2(A+T)
Tm = 4(12) + 2(8) = 48 + 16 = 64°C

A 20-base primer has 8 G/C bases and 12 A/T bases. Find its Tm.

Tm = 4(G+C) + 2(A+T)
Tm = 4(8) + 2(12) = 32 + 24 = 56°C

Estimate the Tm of the sequence ATCGGCTA (8 bases).

Count bases: A,T,C,G,G,C,T,A → G/C = 4 (C,G,G,C), A/T = 4 (A,T,T,A)
Tm = 4(4) + 2(4) = 16 + 8 = 24°C

Flashcards

Quick quiz

Q1.Which base pairs with adenine in DNA?

Correct answer: C. Adenine pairs with thymine via two hydrogen bonds.

Q2.A primer has 10 G/C and 10 A/T bases. What is its Tm by the Wallace rule?

Correct answer: C. Tm = 4(10) + 2(10) = 40 + 20 = 60°C.

Q3.What is DNA replication called because each new molecule has one old and one new strand?

Correct answer: B. Semiconservative replication preserves one parental strand in each new double helix.

Q4.Which enzyme joins Okazaki fragments together?

Correct answer: D. DNA ligase seals the gaps between Okazaki fragments into a continuous strand.

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Common mistakes

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DNA replication produces two entirely new molecules from scratch. — Correct: Replication is semiconservative — each new molecule keeps one original strand and adds one new strand.

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Adenine pairs with guanine. — Correct: Adenine pairs with thymine (A-T); guanine pairs with cytosine (G-C).

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Both DNA strands are synthesized continuously. — Correct: Only the leading strand is continuous; the lagging strand is made in short Okazaki fragments.

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More G-C base pairs make DNA melt at a lower temperature. — Correct: G-C pairs have three hydrogen bonds versus two for A-T, so more G-C content raises the melting temperature.

FAQ

What is DNA structure?

DNA is a double helix of two antiparallel strands connected by complementary base pairs (A-T, G-C), each strand a sugar-phosphate backbone with attached bases.

What is the formula for DNA melting temperature?

The Wallace rule: Tm = 4(G+C) + 2(A+T), estimating melting temperature in °C from base composition.

What are examples of DNA replication in cells?

Every time a cell divides — in skin regeneration, embryonic growth, or bacterial reproduction — its DNA is fully replicated first.

How do you calculate DNA Tm (melting temperature)?

Count the G/C and A/T bases in a short sequence, then apply Tm = 4×(G+C count) + 2×(A+T count).