In which stage of the cell cycle do copying errors in DNA bases occur?

• A. G1
• B. S-phase
• C. G2
• D. Mitosis

Answer: (B)

Copying errors in DNA bases happen during DNA replication, which occurs in the S phase of the cell cycle. During S phase, the cell’s DNA is duplicated by DNA polymerases as they pair each base with its complementary partner. These polymerases can occasionally insert the wrong nucleotide, creating a mispair. The cell has proofreading and repair systems to fix most of these mistakes, but some errors can slip through and become mutations if not corrected. The other stages don’t involve copying the genome: G1 is a growth and preparation period before replication, G2 is a second checkpoint after replication to repair any damage, and mitosis is the division of chromosomes. So the stage where copying errors arise is the S phase.