Question 2

During a study on mammalian cell cycle kinetics, a researcher synchronized a culture of fibroblasts and observed their biochemical activities as they progressed through the cycle. Immediately prior to entering nuclear division, the cells demonstrated:

A substantial increase in the synthesis of tubulin proteins (required for spindle fibers)
Active accumulation of ATP reserves
Detailed molecular scanning to check and repair newly replicated DNA

Based on these observations and the cell cycle schematic below, in which stage of the cell cycle are these cells performing these specific activities?

Cell cycle schematic

$G1\text{G}_1$ phase (first growth phase)

$G2\text{G}_2$ phase (second growth phase)

$S\text{S}$ phase (synthesis phase)

$M\text{M}$ phase (mitotic phase)

A substantial increase in the synthesis of tubulin proteins (required for spindle fibers)
Active accumulation of ATP reserves
Detailed molecular scanning to check and repair newly replicated DNA