Chapter 6 DNA Replication and Repair

Q: Why is DNA's structure considered "simple" for replication?

A: Because of specific base-pairing: Cytosine (C) always pairs with Guanine (G), and Adenine (A) always pairs with Thymine (T).

Image Description: Imagine a ladder. The sides are the "backbone" made of sugar and phosphate. The "rungs" are the bases (A, T, C, G) held together by hydrogen bonds in the middle.

Q: What is the main structural difference between a Purine and a Pyrimidine?

A:

• Purines: Have a double-ring structure (larger).

• Pyrimidines: Have a single-ring structure (smaller).

Image Description: Imagine Purines as a "double-wide" house and Pyrimidines as a "single-unit" apartment.

Q: Which DNA bases are Purines and which are Pyrimidines?

A:

• Purines: Adenine (A) and Guanine (G).

• Pyrimidines: Thymine (T) and Cytosine (C). (Note: Uracil in RNA is also a pyrimidine).

Q: Why must a Purine always pair with a Pyrimidine?

A: To keep the width of the DNA helix constant (exactly 2 nanometers wide).

• If two Purines paired, the "ladder" would be too wide and bulge.

• If two Pyrimidines paired, the "ladder" would be too narrow and the bases wouldn't reach each other to bond.

Q: What does it mean that DNA replication is "semiconservative"?

A: It means that after replication, each new DNA double helix consists of one original (parental) strand and one newly synthesized (daughter) strand

Q: How fast do human cells copy DNA?

A: They copy 3.1 × 10⁹ nucleotide pairs in just a few hours.

Q: How many origins of replication do bacteria typically have compared to eukaryotes (like humans)?

A: Bacteria typically have one single origin of replication, whereas eukaryotes have more than one on each chromosome.

Q: Approximately how many origins of replication are found in the human genome?

The human genome has about 10,000 origins, averaging roughly 220 origins per chromosome.

Q: What specific chemical characteristic is often found at origins of replication to make them easier to open?

A: They are often A-T rich.

Why? As you learned earlier, A-T pairs only have 2 hydrogen bonds, making them easier for the cell to pull apart than G-C pairs which have 3 bonds.

Q: What is the specific job of an "Initiator Protein"?

A: It is the "key" that starts the process. It binds to the origin of replication and physically opens the DNA helix by pulling the strands apart.

Image Description: Imagine a closed zipper. The Initiator Protein is like the hand that grabs the pull-tab and starts to unzip it so the "machinery" can get inside.

Q: Define a "Replication Bubble" and a "Replication Fork."

A:

• Replication Bubble: The opened-up area of the DNA where replication is occurring.

• Replication Fork: The Y-shaped junction at each end of the bubble where the DNA is actively being "unzipped".