BiologyISC Class 12 · Molecular Basis of Inheritance

DNA Replication: Semi-conservative Model and Enzymes, ISC Class 12

Updated 15 October 2026 · Fact-checked

DNA replicates semi-conservatively: the two strands separate and each acts as a template, so every daughter molecule has one old and one new strand. Meselson and Stahl proved this with ¹⁵N and ¹⁴N in E. coli, and Taylor with radioactive thymidine in Vicia faba. DNA polymerase builds new strands 5' to 3'; ligase joins Okazaki fragments.

Understand DNA Replication

The model. Watson and Crick (1953) noted that specific base pairing 'immediately suggests a possible copying mechanism'. The two strands separate, each acts as a template, and a complementary strand is built on each. Each daughter DNA then has one parental and one new strand: semi-conservative replication. (Alternatives: conservative, where the parent duplex stays intact and the daughter is entirely new; dispersive, where old and new pieces are mixed in both strands.)

Meselson and Stahl (1958): proof in E. coli.
1. E. coli were grown for many generations in ¹⁵NH₄Cl (¹⁵N, the heavy, non-radioactive isotope, as the only nitrogen source), so their DNA became heavy (¹⁵N/¹⁵N).
2. Cells were moved to ¹⁴NH₄Cl (normal) and sampled at intervals; E. coli divides every 20 minutes.
3. DNA was separated by caesium chloride (CsCl) density-gradient centrifugation, which separates molecules by density.

After one generation (20 min): all DNA of hybrid (intermediate) density (¹⁵N/¹⁴N).
After two generations (40 min): equal amounts of hybrid and light (¹⁴N/¹⁴N) DNA.
Later generations: hybrid stays at 2 molecules per original molecule while light DNA increases: after n generations, hybrid = 2/2ⁿ of all molecules.
The single hybrid band after one generation rules out conservative replication (which would give separate heavy and light bands); the result after two generations rules out dispersive replication.

Taylor and colleagues (1958): proof in chromosomes. Root tips of Vicia faba (faba bean) were allowed to replicate in radioactive (³H) thymidine and then transferred to non-radioactive medium. Autoradiography of the chromosomes showed that after one round in the label both chromatids were labelled, and after a further round in unlabelled medium only one chromatid of each chromosome was labelled: the chromosomal DNA also replicates semi-conservatively.

Machinery (ISC: role of DNA polymerase and ligase).

Origin of replication: a definite region where replication starts (this is why a vector, which supplies an origin, is needed to propagate a DNA piece in recombinant DNA work).
Because separating the whole length of a long DNA would need too much energy, replication occurs in a small opening, the replication fork.
Helicase unwinds the helix; single-strand binding proteins (SSBP) keep the separated strands apart; primase makes a short RNA primer, because DNA polymerase cannot start a chain on its own.
DNA-dependent DNA polymerase, the main enzyme, adds deoxynucleotides to the 3' end, so it works only in the 5' → 3' direction. It is fast (E. coli's 4.6 × 10⁶ bp are copied in about 18 minutes, about 2000 bp per second) and highly accurate; errors would be mutations.
Deoxyribonucleoside triphosphates serve a dual purpose: substrates, and the source of energy (their two terminal phosphates are high-energy bonds, as in ATP).
On the template running 3' → 5', the new strand is made continuously (leading strand). On the template running 5' → 3', it is made discontinuously in short pieces (Okazaki fragments) (lagging strand).
DNA ligase joins the Okazaki fragments into a continuous strand.
In eukaryotes replication occurs in the S phase; failure of cell division after replication leads to polyploidy.

Flow chart: origin → helicase opens fork → SSBP hold strands → primase lays RNA primer → DNA polymerase extends 5' → 3' (leading continuous, lagging as Okazaki fragments) → primers replaced → ligase seals → two daughter duplexes.

Draw and label: replication fork. Checklist: parental duplex splitting into a Y; label 5' and 3' ends on both parental strands; leading strand with a single long arrow towards the fork; lagging strand with short arrows (Okazaki fragments) pointing away from the fork; labels: helicase, SSBP, primer, DNA polymerase, DNA ligase, replication fork, template strands.

DNA Replication: key facts

Meselson-Stahl after n generations: $hybrid = \frac{2}{2 ^{n}}, light = 1 - \frac{2}{2 ^{n}} (n \geq 1)$; Starting from fully heavy DNA moved to ¹⁴N.
Direction of synthesis: $DNA polymerase adds to 3^{'} -OH \Rightarrow new strand grows 5^{'} \to 3^{'}$; Template read 3' → 5'.
Leading and lagging: $template 3^{'} \to 5^{'} : continuous; template 5^{'} \to 3^{'} : Okazaki fragments + ligase$; Both new strands still grow 5' → 3'.
Replication time with several origins: $t = \frac{bp}{2 \times fork speed \times number of origins}$; Two forks per origin (bidirectional), origins evenly spaced.

How to answer DNA Replication questions

ISC asks the Meselson-Stahl or Taylor experiment in steps, roles of enzymes, abbreviations, and density-band predictions.

1For an experiment, give organism, isotope, method of separation or detection, results after each generation, and conclusion.
2For band predictions, simulate: each duplex splits, each old strand pairs with a new one; count heavy, hybrid and light molecules.
3Say clearly that ¹⁵N is not radioactive and is separated by density; ³H-thymidine in Taylor's work is radioactive and detected by autoradiography.
4For enzyme roles, match: helicase unwinds; SSBP stabilises single strands; primase primes; DNA polymerase extends 5' → 3'; ligase joins.
5Explain discontinuity from polarity: polymerase works only 5' → 3' and the strands are antiparallel.
6Expand abbreviations exactly (SSBP = single-strand binding protein; dNTP = deoxyribonucleoside triphosphate).

DNA Replication in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Many origins make long genomes copy fast

Suppose a eukaryotic chromosome has 1.2 × 10⁸ bp and each replication fork moves at 50 bp per second. A student claims it could be copied from a single origin within one S phase of about 8 hours.

Test the claim, and work out how long replication takes with 1000 evenly spaced origins.

Try it first, then show the solutionSolution

One origin gives two forks moving in opposite directions, so 100 bp are copied per second.
Time with one origin = 1.2 × 10⁸ ÷ 100 = 1.2 × 10⁶ s ≈ 13.9 days: far longer than 8 hours, so the claim is wrong.
With 1000 origins, 2000 forks work together: 1.2 × 10⁶ s ÷ 1000 = 1200 s = 20 minutes.
So long eukaryotic chromosomes must replicate from many origins to finish within the S phase.

Answer: One origin: about 14 days; 1000 origins: about 20 minutes. Multiple origins are essential.

Memory hook: Many doors, quick copying.

Tricks to remember DNA Replication

Short memory aids. Say each one aloud once, then use it on a question the same day.

Semi = half old, half new
Each daughter keeps one old strand: like a sandwich with one old and one fresh slice.
Use it when: Defining semi-conservative replication.
Heavy, hybrid, half-half
Generation 0 heavy; generation 1 all hybrid; generation 2 half hybrid, half light.
Use it when: Meselson-Stahl results.
Lagging needs a Ligase
L for Lagging, L for Ligase: the pieces need gluing.
Use it when: Role of DNA ligase.

DNA Replication: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: SSBP and friends · 2 marks · Short answerBoard level

Expand SSBP and give its role. Name the enzyme that unwinds DNA and the one that joins Okazaki fragments.

Try it first, then show the solutionSolution

SSBP: single-strand binding protein; it binds to separated strands and keeps them apart (1 mark).
Helicase unwinds the helix; DNA ligase joins Okazaki fragments (1 mark).

Answer: Single-strand binding protein (keeps strands apart); helicase; DNA ligase.

Example 2: Why discontinuous? · 2 marks · Short answerBoard level

Explain why one new strand is synthesised continuously and the other discontinuously at a replication fork.

Try it first, then show the solutionSolution

DNA polymerase can add nucleotides only in the 5' → 3' direction, and the two template strands are antiparallel (1 mark).
On the 3' → 5' template the new strand grows towards the fork continuously (leading); on the 5' → 3' template it is made away from the fork in short Okazaki fragments, later joined by ligase (lagging) (1 mark).

Answer: Polymerase works 5' → 3' only and strands are antiparallel, so one strand is continuous and the other is made in fragments.

Example 3: Meselson and Stahl · 5 marks · Long answerHarder board level

Describe the Meselson and Stahl experiment and explain how its results support semi-conservative replication and rule out conservative replication.

Try it first, then show the solutionSolution

E. coli grown for many generations in ¹⁵NH₄Cl, so all DNA was heavy; ¹⁵N is non-radioactive and differs only in density (1 mark).
Cells transferred to ¹⁴NH₄Cl; samples taken every generation (20 min) and DNA separated by CsCl density-gradient centrifugation (1 mark).
After one generation all DNA was of hybrid density; after two generations equal amounts of hybrid and light DNA (1 mark).
Semi-conservative replication predicts exactly this: each heavy duplex gives two hybrids, which then give two hybrids and two light molecules (1 mark).
Conservative replication would give one heavy and one light band after the first generation, with no hybrid; this was not seen (1 mark).

Answer: Heavy → all hybrid (gen 1) → half hybrid, half light (gen 2): semi-conservative, not conservative.

Example 4: Eighty minutes later · 2 marks · Short answerHarder board level

In a Meselson-Stahl type experiment, heavy-DNA E. coli are grown in ¹⁴N medium for 80 minutes (cells divide every 20 minutes). What fraction of DNA molecules is hybrid and what fraction light?

Try it first, then show the solutionSolution

80 minutes = 4 generations: one heavy molecule becomes 2⁴ = 16 molecules (½ mark).
The two original heavy strands stay in two molecules, each paired with a light strand: 2 hybrid (1 mark).
Hybrid = 2/16 = 1/8; light = 14/16 = 7/8 (½ mark).

Answer: Hybrid 1/8; light 7/8.

Example 5: Taylor's experiment · 3 marks · Short answerHarder board level

Describe Taylor's experiment on Vicia faba and state what it showed.

Try it first, then show the solutionSolution

Root tips of Vicia faba were allowed to replicate in radioactive (tritiated) thymidine, which is incorporated only into new DNA, and were then moved to unlabelled medium (1 mark).
Autoradiography showed both chromatids labelled after replication in the label, and only one chromatid of each chromosome labelled after a further round in unlabelled medium (1 mark).
So each chromosome's DNA keeps one old strand and makes one new strand: replication in eukaryotic chromosomes is semi-conservative (1 mark).

Answer: Radioactive thymidine and autoradiography in Vicia faba root tips showed chromatid labelling patterns expected from semi-conservative replication.

Example 6: Reverse the media · 4 marks · Short answerHOTS / competency

Suppose E. coli with ordinary (¹⁴N) DNA are transferred to ¹⁵N medium. (a) Predict the bands after one and two generations. (b) What would a dispersive model predict after one generation, and how would the second generation distinguish it from the semi-conservative model? (c) Why must the cells be grown 'for many generations' in the first medium in the original experiment?

Try it first, then show the solutionSolution

(a) Generation 1: all hybrid; generation 2: half hybrid, half heavy (1 mark).
(b) Dispersive: generation 1 also all of intermediate density (1 mark); but in generation 2 dispersive replication gives a single band shifted further towards heavy, whereas semi-conservative gives two distinct bands, hybrid and heavy (1 mark).
(c) So that practically all the DNA is uniformly labelled at the start; otherwise mixed starting DNA would blur the bands (1 mark).

Answer: (a) Gen 1 hybrid; gen 2 half hybrid, half heavy. (b) Dispersive gives one shifting band; semi-conservative two bands. (c) To start with uniformly labelled DNA.

Where students lose marks in DNA Replication

The slips examiners see most often, and what to write instead.

Mistake: Wrong expansion of SSBP
In 2025 examiners reported invented expansions such as 'single synthetic bioprotein' and the misspelling 'stand' for strand.
Do this: SSBP = single-strand binding protein (single-stranded DNA binding protein).
Usually costs: 1 mark.
Mistake: Calling ¹⁵N radioactive
Students mix up Meselson-Stahl (density) with Taylor and Hershey-Chase (radioactivity).
Do this: ¹⁵N is a heavy, stable isotope separated by CsCl density gradient; ³H-thymidine (Taylor) is radioactive.
Usually costs: 1 mark.
Mistake: Saying the lagging strand grows 3' → 5'
Students assume the 'other' strand is made in the opposite direction.
Do this: Both new strands grow 5' → 3'; the lagging strand is made in pieces away from the fork.
Usually costs: 1 mark.
Mistake: Leaving out the separation method
Students describe heavy and light DNA without saying how they were distinguished.
Do this: Always write CsCl density-gradient centrifugation.
Usually costs: ½ to 1 mark.

Exam tips

Quote NCERT numbers: E. coli divides in 20 min, 4.6 × 10⁶ bp copied in about 18 min, about 2000 bp per second.
In a 5-mark experiment answer, add why conservative replication is ruled out.
Name the organism of Taylor's experiment: Vicia faba.
Draw the replication fork with polarity labels; arrows for Okazaki fragments must point 5' → 3'.
Use the full name DNA-dependent DNA polymerase at least once.

Practice questions on DNA Replication

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
In which direction does DNA polymerase synthesise a new strand?
(a) 3' → 5' (b) 5' → 3' (c) both directions (d) towards the origin only
Hint
It adds to the free 3'-OH.
Answer
(b) 5' → 3'.
Q2 · 2 marks · Short answerBoard level
Why are deoxyribonucleoside triphosphates said to have a dual role in replication?
Hint
Building blocks and fuel.
Answer
They are the substrates (building blocks) added to the growing chain, and their two terminal high-energy phosphate bonds provide the energy for polymerisation.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): One generation after transfer from ¹⁵N to ¹⁴N medium, E. coli DNA formed a single band of intermediate density in CsCl.
Reason (R): Each daughter duplex contained one ¹⁵N parental strand and one newly made ¹⁴N strand.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Semi-conservative replication.
Answer
(a).
Q4 · 2 marks · Short answerHarder board level
Why can DNA polymerase not start replication on its own, and what solves this?
Hint
It needs something to add to.
Answer
DNA polymerase can only add nucleotides to an existing 3'-OH end; it cannot begin a new chain. Primase makes a short RNA primer, which provides the 3'-OH, and replication begins at a definite origin of replication.
Q5 · 2 marks · Short answerHarder board level
Why does replication occur within a small opening (replication fork) rather than by separating the two strands along their whole length at once?
Hint
Energy.
Answer
Separating the two strands of a very long DNA along its entire length would need a very high amount of energy, so the helix is opened only locally at the fork, which moves along as replication proceeds.
Q6 · 2 marks · Short answerHarder board level
A lagging-strand stretch of 10 000 nucleotides is made as Okazaki fragments of about 1000 nucleotides each. How many fragments form, and how many joining reactions by ligase are needed to make it continuous?
Hint
n pieces in a line need n − 1 joins.
Answer
10 000 ÷ 1000 = 10 fragments; 9 ligase joins to link them into one strand.
Q7 · 3 marks · Short answerHOTS / competency
Suppose a mutant bacterium has a defective DNA ligase but normal DNA polymerase. Predict what happens to (a) the leading strand, (b) the lagging strand, and (c) the survival of the cell. Explain.
Hint
Which strand is made in pieces?
Answer
(a) The leading strand is made continuously and is largely unaffected. (b) Okazaki fragments of the lagging strand are made but cannot be joined, so the new strand remains fragmented. (c) The daughter DNA is broken and unstable; complete chromosomes cannot be produced, so the cell cannot divide normally and is likely to die.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose heavy-DNA (¹⁵N) bacteria are moved to ¹⁴N medium. Samples are taken at 0, 20 and 40 minutes (one division every 20 minutes) and spun in CsCl.
(a) Describe the band(s) at each time. (1½)
(b) Which observation rules out the conservative model? (½)
(c) What fraction of DNA molecules would be light at 60 minutes? (1)
Hint
Simulate strand by strand.
Answer
(a) 0 min: one heavy band; 20 min: one hybrid band; 40 min: two bands, hybrid and light, in equal amounts. (b) The single hybrid band at 20 min (conservative would give heavy and light bands, no hybrid). (c) At 60 min (3 generations): 8 molecules, 2 hybrid and 6 light, so 3/4 light.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.In which year did Meselson and Stahl perform their experiment?Show answer
1958.
2.Which compound was used to make DNA heavy?Show answer
¹⁵NH₄Cl.
3.What are Okazaki fragments?Show answer
Short DNA pieces made discontinuously on the lagging strand.
4.Which enzyme joins Okazaki fragments?Show answer
DNA ligase.
5.In which phase of the cell cycle does eukaryotic DNA replicate?Show answer
S phase.
6.What does an origin of replication provide?Show answer
A definite site where replication starts.

DNA Replication: frequently asked questions

What is semi-conservative replication?

Replication in which each daughter DNA molecule contains one strand from the parent and one newly made strand.

How did Meselson and Stahl prove semi-conservative replication?

They grew E. coli in ¹⁵N, moved them to ¹⁴N and separated DNA by CsCl density gradient: after one generation all DNA was hybrid, after two generations half hybrid and half light.

What is the role of DNA ligase?

It joins the Okazaki fragments of the lagging strand into a continuous strand.

Why is the lagging strand discontinuous?

DNA polymerase works only 5' to 3' and the strands are antiparallel, so on one template the new strand must be made in short pieces away from the fork.

What did Taylor's experiment show?

Using radioactive thymidine and autoradiography in Vicia faba root tips, it showed that DNA in chromosomes replicates semi-conservatively.