What is DNA Replication?
DNA replication is the process by which a cell copies its DNA before dividing, so each daughter cell receives a complete, identical set of genetic instructions. It is semiconservative — each new double helix contains one original strand and one newly made strand.
DNA replication is the semiconservative copying of a DNA molecule, in which helicase unwinds the double helix and DNA polymerase synthesizes two new complementary strands, producing two identical DNA molecules from one.
- 1↓InitiationHelicase binds the origin of replication and unwinds the double helix, forming a replication fork.
- 2↓Primer synthesisPrimase lays down short RNA primers to give DNA polymerase a starting point.
- 3↓ElongationDNA polymerase adds nucleotides 5'→3'; the leading strand is made continuously, the lagging strand in Okazaki fragments.
- 4↓Primer removal & fill-inRNA primers are removed and replaced with DNA by DNA polymerase I (or equivalent).
- 5↓LigationDNA ligase seals the gaps between fragments, forming a continuous new strand.
- 6TerminationReplication complexes disassemble once the entire molecule is copied, yielding two identical double helices.
Step-by-step worked examples
A DNA template strand reads 3'-TACGGCAT-5'. What is the sequence of the newly synthesized complementary strand?
DNA polymerase reads the template 3'→5' and synthesizes the new strand 5'→3'. Base pairing: A-T, T-A, C-G, G-C. Template 3'-TACGGCAT-5' → New strand 5'-ATGCCGTA-3'
Why is one new strand (leading) synthesized continuously while the other (lagging) is made in fragments?
DNA polymerase only adds nucleotides in the 5'→3' direction. The leading strand template runs 3'→5' toward the fork, so synthesis is continuous, following the fork. The lagging strand template runs 5'→3' toward the fork, so polymerase must work in short bursts (Okazaki fragments) away from the fork, later joined by ligase.
A bacterial chromosome has 4.6 million base pairs and replicates from a single origin bidirectionally at about 1,000 bp/sec per fork. Roughly how long does replication take?
Bidirectional replication means two forks share the work, each copying half the genome. Bases per fork ≈ 4,600,000 / 2 = 2,300,000 bp Time = 2,300,000 bp ÷ 1,000 bp/sec ≈ 2,300 seconds ≈ 38 minutes
Flashcards
Quick quiz
Q1.DNA replication is described as semiconservative because:
Q2.Which enzyme unwinds the DNA double helix at the origin?
Q3.Why is the lagging strand synthesized in fragments?
Q4.What does DNA ligase do?
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Common mistakes
Thinking both new DNA molecules are entirely new. — Correct: Each new molecule keeps one original strand — this is the semiconservative model.
Believing DNA polymerase can start synthesis from scratch. — Correct: DNA polymerase needs a primer (made by primase) to begin adding nucleotides.
Assuming both strands are copied identically in direction. — Correct: The leading strand is continuous; the lagging strand is made in Okazaki fragments due to polymerase's 5'→3' restriction.
Forgetting that RNA primers must be removed. — Correct: Primers are later removed and replaced with DNA, then sealed by ligase.
FAQ
What is DNA replication?
The biological process of copying a DNA molecule so each daughter cell gets an identical, complete set of genetic material, occurring semiconservatively.
What is the DNA replication formula for direction of synthesis?
There's no numeric formula, but the key rule is: DNA polymerase always synthesizes new strands in the 5'→3' direction, reading the template 3'→5'.
What are examples of DNA replication in action?
Every time a cell divides — from bacteria doubling every 20 minutes to human skin cells replacing themselves — DNA replication copies the full genome first.
How to calculate DNA replication time or speed?
Divide total base pairs to be copied (per fork, if bidirectional) by the polymerase's synthesis rate in base pairs per second.