The regulated sequence of events that occurs between one cell division and the next
Phases of the cell cycle
Interphase
Nuclear division (mitosis)
Cell division (cytokinesis)
Interphase
The longest and most active phase of the cell cycle
Phases of interphase
G1 phase
S phase
G2 phase
G1 phase
The gap between the previous cell division and the S phase, where the cell makes the RNA, enzymes and other proteins required for growth
S phase
The DNA in the nucleus replicates, resulting in each chromosome consisting of two identical sister chromatids
G2 phase
The cell continues to grow and the new DNA that has been synthesised is checked and any errors are usually repaired. Other preparations for cell division are made.
Cyclins
A group of proteins that control the cell cycle
Types of cyclins
Cyclin D
Cyclin E
Cyclin A
Cyclin B
Cyclin D
Triggers cells to move from G1 to S phase
Cyclin E
Highest concentration at the start of S phase, prepares the cell for DNA replication
Cyclin A
Highest concentration in G2 phase, activates kinases that trigger DNA replication and prepare the cell for mitosis
Cyclin B
Highest concentration at the beginning of mitosis, promotes the formation of the mitotic spindle
Mechanism of cell cycle control by cyclins
1. Cyclins bind with cyclin-dependent kinases to form a complex
2. Complex phosphorylates target proteins, activating them
3. Activated proteins trigger specific functions (e.g. DNA replication)
4. Phosphate is released, cyclin breaks down, kinases become inactive
The discovery of cyclins was serendipitous (occurred by chance)
