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DNA Replication Worksheet with Answers (Grade 11 Biology)

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📄 DNA Replication Worksheet with Answers (Grade 11 Biology)

A free printable DNA structure and replication worksheet for Grade 11–12 biology.

This free dna replication worksheet takes students from the structure of a single nucleotide to how a whole chromosome gets copied. They label a drawn DNA segment, write complementary strands with 5′ and 3′ ends, and use Chargaff's rule to work out base percentages.

The second half covers the scientists behind the double helix, the six key replication enzymes, the order of replication steps, Okazaki fragments and the Meselson–Stahl experiment. Sequences, percentages and replication results in the answer key were all calculated and checked.

Grade 11–12 Biology
22 marks
40–50 minutes
Full answer key

What's in This DNA Replication Worksheet

Part	Skill practised	Marks
A	Label the DNA molecule	3
B	Complementary strands, Chargaff's rule and hydrogen bonds	6
C	Scientists who discovered DNA's structure	2
D	Replication enzymes	3
E	Steps of replication, Okazaki fragments and Meselson–Stahl	6
F	Spot the mistake	2

Parts A–C fit the Ontario Grade 11 Genetic Processes unit, and Parts D–E preview the Grade 12 Molecular Genetics unit, so the sheet works well for review in either course.

The Structure of DNA

DNA is a double helix built from nucleotides. Each nucleotide has a phosphate group, a deoxyribose sugar and one of four nitrogenous bases. The sugars and phosphates form the backbone, and pairs of bases joined by hydrogen bonds form the rungs. This is the diagram from Part A of the DNA replication worksheet:

DNA labels (Part A)
1 phosphate group · 2 deoxyribose sugar · 3 nitrogenous base · 4 hydrogen bond · 5 nucleotide · 6 sugar-phosphate backbone

The two strands are antiparallel: one runs 5′ to 3′ and the other 3′ to 5′, which matters a lot once replication begins. For more detail, see DNA replication on Wikipedia.

Base Pairing and Chargaff's Rule

Adenine always pairs with thymine and guanine with cytosine. So the complement of 5′-ATGCCGTA-3′ is 3′-TACGGCAT-5′. G–C pairs are held by three hydrogen bonds and A–T pairs by two, so that eight-base stretch is held together by 20 hydrogen bonds.

Because bases pair one-to-one, %A = %T and %G = %C (Chargaff's rule). If DNA is 30% adenine, it is also 30% thymine, leaving 20% each for guanine and cytosine.

How DNA Replication Works

Helicase unwinds and unzips the double helix; topoisomerase relieves the twisting ahead of it.
Primase lays down a short RNA primer.
DNA polymerase III adds nucleotides in the 5′ → 3′ direction. The leading strand grows continuously; the lagging strand is built in Okazaki fragments.
DNA polymerase I replaces the RNA primers with DNA.
DNA ligase seals the fragments together.

Enzyme	Job
Helicase	Unwinds the double helix by breaking hydrogen bonds between bases
Topoisomerase (gyrase)	Relieves the twisting strain ahead of the replication fork
Primase	Builds a short RNA primer so DNA polymerase can start
DNA polymerase III	Adds new nucleotides to the 3′ end of a growing strand (5′ → 3′)
DNA polymerase I	Replaces RNA primers with DNA nucleotides
DNA ligase	Joins Okazaki fragments into one continuous strand

Students match these enzymes and order the steps in Parts D and E of the DNA replication worksheet.

Semi-Conservative Replication

In 1958, Meselson and Stahl grew bacteria on heavy ¹⁵N, then moved them to light ¹⁴N. After one generation every DNA molecule was a hybrid, and after two generations half were hybrid and half were light. Starting from one molecule, two rounds give 4 molecules, 2 of them hybrid; after three rounds the hybrid fraction falls to 1/4. This showed that each new molecule keeps one original strand.

DNA Replication Worksheet Practice Questions

These questions match the DNA replication worksheet PDF, which has the full-size diagram, word bank and ruled answer space.

Part A – Label the DNA Molecule

Questions 1–6: label the six numbered parts of the DNA diagram above.

Part B – Base Pairing and Chargaff's Rule

Write the complementary strand for 5′-ATGCCGTA-3′. Label the 5′ and 3′ ends.
Write the complementary strand for 5′-GGATCCTA-3′. Label the 5′ and 3′ ends.
A DNA sample is 30% adenine. Find the percentages of thymine, guanine and cytosine. Show your reasoning. (2 marks)
Another sample is 18% guanine. What percentage is adenine?
How many hydrogen bonds hold the double strand of question 7 together?

Part C – Discovering DNA's Structure

Questions 12–15: match Chargaff, Franklin, Watson and Crick, and Meselson and Stahl to their discoveries.

Part D – Replication Enzymes

Questions 16–21: match each enzyme to its job.

Part E – How DNA Replicates

Put the steps of DNA replication in order: P dna polymerase iii adds complementary nucleotides in the 5′ → 3′ direction; Q helicase unwinds and unzips the double helix at the origin of replication; R dna ligase joins the okazaki fragments; S primase adds a short rna primer; T dna polymerase i replaces the rna primers with dna.
Why is DNA replication called semi-conservative?
Why is the lagging strand made in short pieces (Okazaki fragments)?
Bacteria with only ¹⁵N DNA are moved to a ¹⁴N medium. One DNA molecule replicates twice. How many of the 4 molecules still contain a ¹⁵N strand?
After a third round of replication, what fraction of the molecules are hybrid?

Part F – Spot the Mistake

A student writes: “In DNA, adenine pairs with uracil.” Explain the error and correct it.
A student writes: “DNA polymerase builds both new strands continuously in the same direction as the fork.” Explain the error and correct it.

DNA Replication Worksheet Answer Key

Open each part of the DNA replication worksheet answers to check your work. The printable key is at the end of the PDF.

Part A answers (Q1–6)

Phosphate group
Deoxyribose sugar
Nitrogenous base
Hydrogen bond
Nucleotide
Sugar-phosphate backbone

Part B answers (Q7–11)

3′-TACGGCAT-5′
3′-CCTAGGAT-5′
A = T, so T = 30%. G + C = 100 − 60 = 40%, so G = C = 20% each.
C = 18%, so A + T = 64% and A = 32%
4 A–T pairs × 2 + 4 G–C pairs × 3 = 20 hydrogen bonds

Part C answers (Q12–15)

Erwin Chargaff: c
Rosalind Franklin: d
Watson and Crick: a
Meselson and Stahl: b

Part D answers (Q16–21)

Helicase: b
Topoisomerase (gyrase): d
Primase: f
DNA polymerase III: e
DNA polymerase I: c
DNA ligase: a

Part E answers (Q22–26)

Q → S → P → T → R (helicase, primase, DNA polymerase III, DNA polymerase I, ligase)
Each new DNA molecule keeps one original strand and has one newly made strand.
DNA polymerase can only add nucleotides in the 5′ → 3′ direction. The lagging strand runs the opposite way to the fork, so it is built backwards in short pieces as the fork opens.
2 (both are hybrid ¹⁵N/¹⁴N molecules)
2 of 8 = 1/4

Part F answers (Q27–28)

Uracil is only found in RNA. In DNA, adenine pairs with thymine (two hydrogen bonds).
DNA polymerase only works 5′ → 3′, so only the leading strand is continuous; the lagging strand is made in Okazaki fragments.

5 Common DNA Replication Mistakes

Pairing A with U in DNA. Uracil belongs to RNA; DNA uses thymine.
Forgetting to flip the 5′ and 3′ labels. A complementary strand always runs the opposite direction.
Saying polymerase works 3′ → 5′. New strands only grow 5′ → 3′.
Mixing up the two polymerases. Polymerase III builds; polymerase I swaps primers for DNA.
Assuming Chargaff means all four bases are 25%. Only A = T and G = C; the A:G ratio varies by species.

How to Use This DNA Replication Worksheet in Class

Lesson 1: Parts A–C of the DNA replication worksheet, alongside a paper or candy DNA model.
Lesson 2: Parts D–F with an animation of the replication fork.
Extension: Have students act out the replication fork, with each student playing one enzyme.

Next, follow DNA into protein with the protein synthesis quiz, or see how DNA is divided in the meiosis worksheet. For applications, try the biotechnology quiz.

Frequently Asked Questions

Is this dna replication worksheet free?

Yes. The worksheet and its answer key are free to download and print as a PDF, with no account needed.

What are the base pairing rules in DNA?

Adenine pairs with thymine (two hydrogen bonds) and guanine pairs with cytosine (three hydrogen bonds). In RNA, uracil replaces thymine.

What does semi-conservative replication mean?

Each new DNA molecule contains one original strand and one newly built strand. Meselson and Stahl proved this in 1958 using heavy (15N) and light (14N) nitrogen.

Why does the lagging strand form Okazaki fragments?

DNA polymerase can only add nucleotides in the 5′ to 3′ direction. Because the strands are antiparallel, one new strand must be built in short pieces that DNA ligase later joins together.

What grade is this dna replication worksheet for?

It suits Grade 11 and Grade 12 biology. In Ontario, DNA structure appears in Grade 11 and the replication enzymes are covered in depth in Grade 12 Molecular Genetics.

More Free Grade 11 Biology Resources

Ready to start? Download the free DNA replication worksheet PDF (use the Download button above), print it double-sided, and keep the answer key pages for marking.