Biology 11–12 · Year 12

Modelling DNA replication and the Meselson-Stahl test of three models

Module 5: Heredity (Cell Replication)

Practical, model not builtLow risk

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The idea

Each strand of a DNA molecule is a template for a new complementary strand, so every daughter molecule keeps one old strand, and the pattern of heavy and light DNA across generations distinguishes this semi-conservative model from its rivals.

What you need

Paper nucleotide cut-outs in two colours (dark for 'heavy' nitrogen-15 parental strands, light for new nitrogen-14 strands), each marked A, T, G or C, about 160 per group
A written 10 base pair starting sequence; sticky tape; a results table for three generations

How to do it

Build one double-stranded molecule of 10 base pairs from dark nucleotides, pairing A with T and G with C.
Replicate it: separate the two strands and build a complementary light strand along each, checking every pair.
Replicate every molecule again, and then a third time, keeping each strand's colour.
After each generation count the molecules that are dark-dark (heavy), dark-light (hybrid) and light-light (light).
Use the model to predict the band pattern after each generation for semi-conservative, conservative and dispersive replication, and compare your counts with each.
Compare the predictions with Meselson and Stahl's 1958 result as OpenStax describes it: one band of hybrid density, halfway between heavy and light, after one generation in nitrogen-14, and a hybrid and a light band after two.

What you should see

After one generation both molecules are hybrid; after two, 2 of 4 are hybrid and 2 light; after three, 2 of 8 are hybrid and 6 light (fraction hybrid 0.25), and 140 light nucleotides have been added to the 20 dark ones (160 in 8 molecules; computed). Only semi-conservative replication gives one hybrid band then a hybrid and a light band; conservative replication would show heavy and light bands from the first generation, and dispersive replication a single band drifting toward light. The learner knows it worked when every daughter molecule contains exactly one strand from its parent and the counts match the semi-conservative prediction.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Replication makes a brand-new copy and keeps the original intact; each daughter molecule keeps one original strand.
Any base can pair with any other; A pairs only with T and G only with C, which is what makes the copy exact.
One experiment can show which model is right after a single generation; the first generation cannot separate semi-conservative from dispersive, the second can.

Safety card

Low riskLearners carry it out

Hazards

No hazard is listed.

Controls

No control is listed.

Note

No hazards: paper model and computed model.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-12 BIO11/12-4 BIO11/12-6
Biology 11–12 Syllabus (2025), Year 11 focus area Cells as the basis of life; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-01
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-04 BI-11WS-06
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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