Interphase

Interphase

has three main subphases, often remembered as G1 → S → G2

G1 Phase

First Gap

G1 Phase

• Cell grows in size and carries out normal metabolic functions.

• Organelles duplicate (mitochondria, ribosomes, etc.).

• Proteins needed for DNA synthesis are produced.

G1 Phase

Duration varies widely:

• Rapid in embryonic cells.

• Can be very long in adult cells (e.g., neurons).

Checkpoint (G1/S)

Cell checks for DNA damage, adequate nutrients, and growth signals.

G0 phase

If conditions aren’t favorable, cell may enter _____ (a resting, non-dividing state).

G0 phase

Example: most nerve and muscle cells remain in this phase permanently.

S Phase

Synthesis

S Phase

DNA replication occurs.

S Phase

Each chromosome is duplicated into two sister chromatids, held together by a centromere.

S Phase

Histone proteins are produced, and DNA wraps around them to form nucleosomes → helps with packaging.

S Phase

Centrosomes (microtubule-organizing centers) also duplicate, preparing for spindle formation in mitosis.

G2 Phase

Second Gap

G2 Phase

• The cell continues to grow.

• Final preparations for cell division:

  • Synthesis of spindle proteins (tubulin).

  • Repair of any replication errors.

Checkpoint (G2/M)

• Ensures DNA is completely and correctly replicated.

• Checks for DNA damage; repair enzymes act if necessary.

• If errors remain, cell cycle is halted to prevent defective cells from dividing.

Chromosomes

During interphase, these are not visible under a light microscope → they exist as extended chromatin.

Interphase

Ensures genetic stability: only accurate DNA copies are passed on.

Interphase

Provides multiple checkpoints that prevent uncontrolled cell division (loss of control = cancer).

Interphase

Allows for cell growth and specialization between divisions.

G0 Phase

• Some cells exit the cycle permanently or temporarily.

• Permanent G0: neurons, cardiac muscle (don’t divide once mature).

• Temporary G0: liver cells can re-enter the cycle if tissue repair is needed.