Step 1 of DNA replication: Unwinding of the DNA double helix by DNA helicase to create a replication fork.

You're looking at a DNA Replication Practice worksheet that walks through the 3 main steps of DNA replication, with diagrams and explanations. Since you asked to “solve the problem and explain the solution,” I’ll go step by step — answering each numbered question clearly and concisely, using the text and diagrams provided.

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## ✔ STEP 1: What is happening to the DNA molecule in the figure?
*(Explain the first step in DNA replication)*

🔍 Answer:

The DNA molecule is being unzipped or unwound into two separate single strands. This is the first step of DNA replication, called unwinding or initiation.

🧬 Explanation:

- DNA is normally a double helix made of two complementary strands held together by hydrogen bonds between base pairs: A–T and C–G.
- To replicate, these hydrogen bonds must be broken so each strand can serve as a template for building a new complementary strand.
- The enzyme DNA helicase breaks these hydrogen bonds, separating the two strands and forming a Y-shaped structure called the replication fork.
- The diagram shows the double helix opening up — this is the replication fork where replication will begin.

> 💡 Think of it like unzipping a zipper — the two sides (strands) are pulled apart so each can be copied.

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## ✔ STEP 2: What happens to the DNA molecule during the second step of DNA replication?

🔍 Answer:

During the second step, primers bind to the separated DNA strands, and new nucleotides begin to be added to build complementary strands.

🧬 Explanation:

- After the strands are separated, the enzyme DNA primase creates a short piece of RNA called a primer.
- The primer attaches to the 3’ end of each template strand and provides a starting point for DNA synthesis.
- The diagram shows small fragments (likely representing primers and initial nucleotides) being added near the ends of the separated strands.
- This step prepares the DNA for the next phase — elongation — where new strands are built.

> 💡 Primers are like “starter blocks” — DNA polymerase (the enzyme that builds new DNA) can’t start from scratch; it needs a primer to begin adding nucleotides.

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## ✔ STEP 3: What happens during the third step of DNA replication?

🔍 Answer:

In the third step, DNA polymerase adds complementary nucleotides to each template strand, creating two complete, identical DNA molecules.

🧬 Explanation:

- The enzyme DNA polymerase reads the template strand and adds matching nucleotides (A with T, C with G) in the 5’ to 3’ direction.
- On the leading strand, synthesis is continuous.
- On the lagging strand, synthesis occurs in short segments called Okazaki fragments, which are later joined together.
- The result is two identical double-stranded DNA molecules, each made of one original (parental) strand and one new strand — this is called semi-conservative replication.
- The final diagram shows two complete DNA ladders — each is a full double helix, identical to the original.

> 💡 Each new DNA molecule has one old strand and one new strand — preserving genetic information accurately.

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## 🧪 BONUS: Summary of All 3 Steps

| Step | Name | Key Enzyme(s) | What Happens |
|------|------------------|-----------------------|------------------------------------------------------------------------------|
| 1 | Unwinding | DNA Helicase | Double helix is unzipped into two single strands at the replication fork. |
| 2 | Priming | DNA Primase | Short RNA primers bind to each strand to provide a starting point. |
| 3 | Elongation | DNA Polymerase | New complementary strands are built; two identical DNA molecules form. |

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## ✍️ Final Notes for Student (Danica Camille Natividad):

You’ve done a great job following along! Remember:

- Base pairing rules: A–T, C–G — always!
- Enzymes matter: Helicase (unzips), Primase (makes primer), Polymerase (builds new strand).
- Semi-conservative: Each new DNA has one old + one new strand.
- Directionality: DNA polymerase only works 5’ → 3’.

This process ensures that when cells divide, each daughter cell gets an exact copy of the DNA — crucial for life!

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✔ You’re all set! If you need to fill this out on paper, just copy the answers above into your worksheet. Let me know if you want a printable version or quiz questions to test yourself!