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A look at the process of DNA replication and why it is essential for growth and reproduction.
Every single second, your body produces about 2 to 3 million new red blood cells. How does each new cell get the exact same 3-billion-letter instruction manual that the original cell had without making a single mistake?
Before a cell divides to create a new cell (a process called mitosis), it must ensure the new cell has a full set of instructions. Think of DNA as a massive architectural blueprint. If you are building a second house, you don't just give away half your blueprint; you make a perfect photocopy. This process is called DNA replication. It occurs in the nucleus of your cells and ensures that every new cell is a genetic twin of the original. Without this step, your body couldn't grow, heal wounds, or replace dead cells because the new cells wouldn't know how to function.
Quick Check
What would happen to a new daughter cell if DNA replication did not occur before the parent cell divided?
Answer
The new cell would be missing some or all of the genetic instructions needed to function and survive.
DNA is a double helix, shaped like a twisted ladder. To copy it, the cell must first open it up. An enzyme called helicase acts like a molecular zipper. It moves along the DNA molecule, breaking the weak hydrogen bonds that hold the nitrogenous bases together. This creates a Y-shaped structure called the replication fork. Once 'unzipped,' the two individual strands serve as templates or guides for creating the new matching sides.
1. Imagine a jacket zipper representing the DNA double helix. 2. The 'teeth' of the zipper are the nitrogenous bases (). 3. The slider of the zipper is the enzyme helicase. 4. As the slider moves down, it separates the teeth, leaving two open rows ready to be matched with new parts.
Quick Check
Which specific enzyme is responsible for 'unzipping' the DNA strand by breaking hydrogen bonds?
Answer
Helicase
Once the strands are separated, an enzyme called DNA polymerase steps in. Its job is to bring in free-floating nucleotides and match them to the exposed bases on the template strands. It follows the Base Pairing Rule: Adenine () always pairs with Thymine (), and Cytosine () always pairs with Guanine (). Because each original strand serves as a template, the result is two identical DNA molecules. This is called semi-conservative replication because each new DNA molecule consists of one 'old' original strand and one 'new' strand.
The DNA polymerase will build a matching strand by following the pairing rules: 1. pairs with 2. pairs with 3. pairs with 4. pairs with 5. pairs with 6. pairs with
Quick Check
If a template strand has the base sequence 'CCG', what sequence will DNA polymerase build to match it?
Answer
GGC
DNA replication is incredibly fast, adding about 50 nucleotides per second in humans. With that speed, mistakes happen. A mismatched base (like an paired with a ) is called a mutation. To prevent this, DNA polymerase also acts as a 'proofreader.' It checks its work as it goes, removing and replacing incorrect bases. This ensures an error rate of less than one mistake for every billion bases added! This accuracy is vital because even a small error can lead to diseases like cancer or genetic disorders.
Imagine a sentence in a manual: 'The cell should GROW.' 1. If a mutation changes 'G' to 'L', the instruction becomes 'The cell should LROW' (Nonsense). 2. If a mutation changes 'G' to 'K', it might change the meaning entirely. 3. DNA polymerase catches these 'typos' by feeling the physical shape of the DNA; a mismatch creates a 'bulge' in the helix because the bases don't fit together correctly.
Why is DNA replication described as 'semi-conservative'?
Which enzyme is responsible for matching new nucleotides to the template strand and proofreading the results?
DNA replication occurs after the cell has already finished dividing into two new cells.
Review Tomorrow
In 24 hours, try to sketch the 'replication fork' from memory and label helicase, DNA polymerase, and the base pairs and .
Practice Activity
Find a piece of string or a zipper. Use it to explain the process of 'unzipping' and 'base pairing' to a friend or family member to reinforce your understanding.