Let’s go step by step to understand what happens during DNA replication.
DNA replication is how a cell makes an exact copy of its DNA before it divides. It happens in three main steps:
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Step 1: Unwinding the DNA
The double helix (twisted ladder shape) of DNA gets unwound and unzipped. An enzyme called helicase breaks the bonds between the base pairs (A-T, G-C), separating the two strands. This creates a “replication fork” — like opening a zipper.
In the first diagram you see, the DNA is still together as one double strand. That’s the starting point.
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Step 2: Building New Strands
Now, each separated strand acts as a template. Another enzyme called DNA polymerase adds new matching bases to each old strand. Remember:
- A always pairs with T
- G always pairs with C
So if one side has “A”, the new side will get “T”. If one side has “G”, the new side gets “C”.
This is shown in the second diagram — you can see the original strands are apart, and new complementary strands are being built alongside them. You might also notice little fragments on one side — that’s because one strand is built continuously, and the other in pieces (called Okazaki fragments). But for now, just know: new strands are being made using the old ones as guides.
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Step 3: Finishing Up
After all the new bases are added, another enzyme (ligase) glues any broken pieces together — especially those little fragments from Step 2. Now you have two complete double helices, each made of one original strand and one brand-new strand. This is called “semi-conservative” replication.
The third diagram shows two full DNA molecules — identical to each other and to the original.
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Final Answer:
1. The DNA molecule is unwound and unzipped into two separate strands. Enzymes break the bonds between base pairs so each strand can be used as a template.
2. New complementary strands are built onto each original strand. DNA polymerase adds matching bases (A with T, G with C) to create two new half-molecules.
3. The new strands are fully connected and sealed. Two identical DNA molecules are formed — each with one original strand and one new strand.
Parent Tip: Review the logic above to help your child master the concept of dna replication activity worksheet.
