"How many types of amino acids?"
Kana was reading a book about proteins.
Milia answered. "20 standard types."
"Only 20?"
Rei supplemented. "But their combinations create countless proteins."
"Combinations?"
"Connected by peptide bonds," Milia drew a diagram.
"The carboxyl group of one amino acid reacts with the amino group of the next."
Rei wrote the equation. "-COOH + H2N- → -CO-NH- + H2O"
"One water molecule is removed, a bond is formed."
Kana wrote in her notebook. "Condensation reaction."
"Correct," Milia acknowledged. "This bond repeats, creating a chain."
"Where?"
"Ribosome. The protein synthesis factory."
Rei assembled a model. "Two subunits, large and small. Made of RNA and protein."
"RNA is an enzyme?" Kana was surprised.
"Ribozyme. RNA with catalytic activity."
Milia explained. "Ribosome reads mRNA while connecting amino acids."
"How does it choose the right amino acids?"
"tRNA. Transfer RNA," Rei drew a diagram.
"Cloverleaf structure. Amino acid on one end, anticodon on the other."
Kana understood. "Codon and anticodon correspond?"
"Yes. Complementary base pairing. A with U, G with C."
Milia continued. "Reading mRNA codons, corresponding tRNA comes."
"tRNA carries the correct amino acid."
Rei supplemented. "Aminoacyl-tRNA synthetase. This attaches the correct amino acid to tRNA."
"Doesn't it make mistakes?"
"Surprisingly accurate. Has proofreading mechanisms too."
Kana asked. "What happens inside the ribosome?"
Milia explained in detail. "Ribosome has A site, P site, E site."
"New tRNA enters A site. P site has tRNA holding the growing chain."
Rei continued. "Peptidyl transfer reaction. Peptide from P site transfers to amino acid in A site."
"Then, ribosome moves three nucleotides."
Kana imagined. "Conveyor belt?"
"Good metaphor," Milia laughed. "mRNA is the belt. tRNA carries parts."
"How fast?"
Rei answered. "About 20 amino acids per second. Fast."
"But accurate."
Milia added. "Error rate about one in ten thousand."
Kana was surprised. "Amazing precision."
"Life's precision," Rei said quietly.
"Amino acid order determines protein function. One mistake can cause disease."
Kana listened seriously. "Disease?"
"Sickle cell anemia. Just one amino acid changed in hemoglobin."
Milia explained. "Glutamic acid changed to valine. Just that changed red blood cell shape."
"One amino acid is that important?"
"Affects protein three-dimensional structure. Structure changes, function changes."
Rei summarized. "That's why ribosome accuracy is important."
"The place where amino acids meet. Mistakes there throw everything off."
Kana murmured. "Amino acids have fateful meetings."
Milia smiled. "Poetic. But also scientifically correct."
"Each amino acid meets in order and holds hands. That's a protein."
Rei added. "Hundreds, thousands of amino acids line up precisely. That's life's miracle."
Kana stared at the ribosome diagram. In the tiny factory, countless amino acids are meeting even now.
"Meetings create life."
"And life continues," Milia said quietly.
The three fell silent. Invisible meetings repeat inside cells.