Part 1: DNA Replication Basics

WALT: We Are Learning To explain how DNA is copied accurately before a cell divides.

Success criteria: I can identify complementary bases, describe the stages of replication, and explain the roles of key enzymes.

Dyslexia-friendly options: Read each question aloud, use a ruler or coloured strip to track each line, and ask for the word bank if needed.

Support: Use these key words: helicase, DNA polymerase, complementary, leading strand, lagging strand, Okazaki fragments.

1. Which enzyme unwinds and separates the two DNA strands?

DNA polymerase

Helicase

Ligase

Amylase

2. Which statement best describes complementary base pairing?

Any base can join to any other base.

A always pairs with G, and C always pairs with T.

A pairs with T, and C pairs with G.

The bases are joined in random pairs.

3. Complete the complementary DNA strand.

Original strand: A T C G G A T C

New strand:

4. Number these stages from 1 to 4 to show the correct order of DNA replication.

_____ DNA polymerase adds complementary nucleotides.

_____ The DNA molecule rewinds into two identical DNA molecules.

_____ Helicase separates the two DNA strands.

_____ Each original strand acts as a template.

Part 2: Replication Fork Investigation

5. Match each enzyme to its role. Write the correct letter beside each enzyme.

1. Helicase

2. DNA polymerase

3. Ligase

4. Primase

A. Joins short DNA sections together

B. Adds complementary DNA nucleotides

C. Separates the DNA strands

D. Makes a short RNA primer

6. Draw and label a DNA replication fork. Include: the original DNA strands, helicase, new DNA strands, DNA polymerase, the leading strand, the lagging strand and at least two Okazaki fragments.

7. Explain the difference between the leading strand and the lagging strand.

8. What are Okazaki fragments, and which enzyme joins them together?

9. Application: A DNA section has the base sequence A–C–T–G–A. A copying error changes the third base in the new strand. Explain how this could affect a gene or the protein it codes for.

10. Extension challenge: DNA polymerase can add nucleotides only in one direction. Explain why this causes one strand to be copied continuously and the other strand to be copied in fragments.

Challenge option: Use arrows on your diagram to show the direction in which DNA polymerase builds each new strand.

Answer Key

1. Helicase.

2. A pairs with T, and C pairs with G.

3. T–A–G–C–C–T–A–G.

4. DNA polymerase adds complementary nucleotides = 3; the DNA rewinds into two identical molecules = 4; helicase separates the strands = 1; each original strand acts as a template = 2.

5. 1–C, 2–B, 3–A, 4–D.

6. Diagram should show a separated DNA double strand, complementary bases, helicase at the replication fork, DNA polymerase, a continuously built leading strand, a discontinuously built lagging strand and Okazaki fragments.

7. The leading strand is made continuously towards the replication fork. The lagging strand is made in short sections away from the fork.

8. Okazaki fragments are short sections of newly made DNA on the lagging strand. Ligase joins them together.

9. A changed base may alter a codon. This could change the amino acid sequence and possibly the shape or function of the protein. Some changes may have no effect.

10. Because DNA polymerase builds in only one direction, the leading strand can be made continuously, while the lagging strand must be made in separate Okazaki fragments that are later joined by ligase.