Summary OCR A/AS-Level Biology 2.6.1 Cell Cycle

Cell Cycle
Cell Cycle
 Consists of two main phases: m-phase and interphase
 M-phase is division and interphase is the growth stages and synthesis stage
 Checkpoints verify if processes have been completed correctly before ethe cell moves
onto the next stage
 Molecule evets that control the cycle occur in a specific order to ensure the cycle
cannot be reversed and that DNA is only replicated once per cycle
 Cells normally undergo about 50 cycles (Hayflick constant)
M-Phase G₁-Phase
 Cell growth stops  Cell grows
 Nuclear division (mitosis or meiosis)  Synthesis of enzymes and RNA for DNA
 Cytoplasmic Division (Cytokinesis) replication
 Checkpoint mechanism ensures cell is
ready for DNA replication
S-Phase G₂-Phase
 DNA replication  Cell grows
 Rapid because exposed DNA is  Checkpoint mechanism checks cell is
susceptible to mutation ready for division


G₀-Phase
 Resting phase triggered if the G₁ checkpoint detects damage or if the cell cannot
continue the cell cycle
 Attempts to repair damage then continue the cell cycle
 If damage cannot be fixed, cell undergoes apoptosis (programmed cell death)
 If a cell undergoes differentiation, it becomes specialised and can no longer continue
the cell cycle so remains carrying out its function indefinitely in G₀-phase

Mitosis
 Nuclear division
 DNA is moved into two separate nuclei
 Forms two identical daughter cells
 Basis of asexual reproduction
 All multicellular organisms grow and repair using mitosis