interphase

Meaning: Interphase is the stretch of the cell cycle between one M phase and the next, and in many cells it takes up most of the cycle. G1 is growth and normal function, with synthesis of proteins and organelles. S is DNA replication, so each chromosome becomes two sister chromatids joined at a centromere. G2 is further growth and preparation for mitosis, including checks that replication is complete and the DNA is undamaged. Cells that stop dividing often enter G0, which is usually drawn outside the cycle but is not metabolically inactive.

On the MCAT: A common figure is a flow cytometry graph of DNA content per cell. A peak at 2n is G1 (or G0), values between 2n and 4n are S, and a peak at 4n is G2 or M. Questions also ask where the checkpoints act (G1/S, G2/M, and the spindle checkpoint in M), how cyclins and CDKs drive progression, and what happens when p53 or Rb is lost or a drug blocks S phase. Chromatin is uncondensed during interphase, so individual chromosomes are not visible.

Common confusion: Interphase is not a resting phase, because the cell is growing and copying its DNA. After S phase a human cell still has 46 chromosomes, since sister chromatids count as one chromosome, but it has twice the DNA.