DNA Replication Process Diagram

Let’s go through each question one by one, step by step.

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1. Why does DNA need to replicate?

DNA holds all the instructions for building and running a living thing — like a recipe book for your body. When a cell divides to make two new cells (like when you grow or heal), each new cell needs its own full set of instructions. So before the cell splits, it must copy its DNA so both new cells get a complete copy. That’s why DNA replicates — to make sure every new cell has the same genetic info as the original.

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2. In relation to the pictures below: Explain three main steps in the process of DNA replication. Name the enzymes that go with each step.

Looking at the images:

- Picture a: Shows the DNA double helix being “unzipped” at a point labeled “Helicase.” This is Step 1.
- Picture b: Shows the unzipped strands forming a Y-shape — this is called the replication fork. New bases are starting to pair up. This is Step 2.
- Picture c: Shows two new double helices formed — each with one old strand and one new strand. This is Step 3.

Now let’s explain each step with the enzyme involved:

Step 1: Unzipping the DNA
→ The enzyme helicase breaks the bonds between the base pairs and separates the two strands of DNA. It’s like opening a zipper down the middle of the DNA ladder.

Step 2: Building new strands
→ An enzyme called DNA polymerase adds new nucleotides (building blocks) to each separated strand. It matches A with T, and C with G, to build a perfect copy. Another enzyme, primase, helps start the process by laying down a small RNA primer first (but if we’re keeping it simple, focus on DNA polymerase).

Step 3: Finishing and sealing
→ After the new strands are built, another enzyme called ligase seals any gaps in the sugar-phosphate backbone — especially important on the lagging strand where pieces are made in chunks. Now you have two identical DNA molecules!

*(Note: Picture labels may vary slightly depending on textbook, but these are the standard 3 steps and enzymes.)*

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3. In which direction are new nucleotides added during replication?

New nucleotides are always added in the 5’ to 3’ direction. That means DNA polymerase can only add new bits to the 3’ end of the growing strand. Think of it like building a train — you can only add new cars to the back (the 3’ end), not the front.

This causes a problem on one strand (the lagging strand), because it has to be built in short backward pieces — but that’s covered in question 4.

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4. What is the difference between leading and lagging strand?

Both strands are copied at the same time, but they’re copied differently because DNA polymerase only works in one direction (5’ to 3’).

- Leading strand: This strand is copied smoothly and continuously in the same direction the replication fork is moving. It’s easy — just keep adding nucleotides as the helix opens.

- Lagging strand: This strand runs the opposite way, so DNA polymerase has to work backwards. It makes short pieces called Okazaki fragments, then ligase glues them together later. It’s like building a road in sections instead of one long stretch.

So: Leading = continuous; Lagging = chunky + glued later.

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5. Which enzyme is responsible for "unzipping" the DNA double helix?

That’s helicase. It breaks the hydrogen bonds between the base pairs and pulls the two strands apart — just like unzipping a jacket.

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Final Answer:

1. DNA replicates so each new cell gets a complete copy of genetic instructions when the cell divides.

2. Three main steps:
- Step 1: Helicase unzips the DNA double helix.
- Step 2: DNA polymerase builds new complementary strands by adding nucleotides.
- Step 3: Ligase seals gaps in the new strands (especially on the lagging strand).

3. New nucleotides are added in the 5’ to 3’ direction.

4. Leading strand is copied continuously; lagging strand is copied in short fragments (Okazaki fragments) that are later joined.

5. Helicase is responsible for unzipping the DNA double helix.