"How is DNA copied?"
Kana stared at the model.
Milia answered. "Replication. A process like a precision machine."
"Machine?"
Rei explained. "Multiple enzymes work in coordination. Remarkably accurate."
"First, unwind the helix," Milia opened the model.
"Unwind?"
"An enzyme called helicase breaks hydrogen bonds. Double strand becomes single strands."
Kana questioned. "Unwind everything?"
"No, little by little. A region called replication fork forms."
Rei drew a diagram. "Y-shaped structure. Replication progresses here."
"Then?"
"Primase makes RNA primers," Milia continued.
"Primers?"
"Short RNA sequences. Become scaffolding for DNA polymerase."
"DNA polymerase?"
Rei answered. "The main player in DNA synthesis. Links new nucleotides."
Kana wrote in her notebook. "How?"
"Reads the template strand. If A, matches T, if G, matches C."
"Complementarily?"
"Yes. One base at a time, accurately."
Milia supplemented. "But DNA polymerase has constraints."
"Constraints?"
"Can only synthesize in 5' to 3' direction."
Kana looked confused. "Direction?"
Rei explained. "DNA has directionality. Determined by sugar carbon numbering."
"Two strands are opposite directions?"
"Yes. Called antiparallel."
Milia drew a diagram. "So one side can synthesize continuously, but the other becomes fragmented."
"Fragmented?"
"Okazaki fragments. Make short pieces, join later."
Rei continued. "Leading strand and lagging strand. Different synthesis methods."
"Complex..." Kana murmured.
"But this is the mechanism that maintains precision."
Milia asked. "What if there's a mistake?"
"There's proofreading," Rei answered. "DNA polymerase itself checks mistakes."
"How?"
"When base pairs are wrong, the shape distorts. Detects and removes it."
Kana was surprised. "It corrects itself?"
"3' to 5' exonuclease activity. Can cleave in reverse direction."
Milia supplemented. "Thanks to this, error rate is below one in a billion."
"One in a billion?"
"Nearly perfect. But mistakes still sometimes remain."
Rei said. "Those mistakes become mutations."
"Mutations..."
"Also the driving force of evolution. Too perfect, can't adapt."
Kana questioned. "When replication finishes?"
"Ligase joins Okazaki fragments," Milia answered.
"Joins?"
"Chemically bonds the sugar-phosphate backbone."
Rei completed the diagram. "Result, two double helices form."
"Two?"
"Semiconservative replication. One old strand and one new strand pair up."
Milia showed the model. "Parent DNA information is inherited by two daughter DNAs."
Kana was moved. "Information passes across generations."
"Yes," Rei nodded. "Three billion characters of information copied in hours."
"Every time, with each cell division?"
"Yes. Happening in your body right now."
Kana looked at her hand. "Unbelievable precision."
Milia said quietly. "Life is an information copier. But doesn't aim for perfect copies."
"Why?"
"Small changes create diversity. That's the key to survival."
Rei continued. "Precision and flexibility. Balance of both supports life."
Kana nodded. "A machine, but not perfect."
"That's why it's beautiful," Milia smiled.
The three gazed at the model.
DNA replication continues in the body right now.
Accurately, but occasionally making mistakes.
That is the essence of life.