The cell cycle

The cell cycle is the sequence of events that take place to enable DNA replication and cell division. It can be divided into two phases: interphase and mitosis. Interphase is further divided into the G1 (gap 1), S (synthesis), and G2 (gap 2) phases, which prepare the cell for division. In mitosis, the single cell is dividing into two identical daughter cells. The cell cycle is subject to strict controls that prevent cells with damaged or faulty DNA from further dividing and passing on defects to daughter cells. Controlled cell death (apoptosis) is initiated if the DNA damage is irreparable. Disorders of these regulatory mechanisms play an important role in carcinogenesis.

The mechanisms for apoptosis are discussed in “Cellular changes and adaptive responses:”

Abnormalities of the cell cycle that lead to the development of cancer are discussed in detail in “General oncology.”

Overview

• Definition: the sequence of events through which cell growth, DNA replication, and cell division occur
• Characteristics
  • The cell involves two phases: interphase and mitosis.
  • Interphase further involves the G1 phase (gap 1), S phase (synthesis), and G2 phase (gap 2), which prepare the cell for division.

G stands for Gap/Growth and S for Synthesis.

Interphase

• Definition: the interval between cell divisions in which the cell prepares for the next division
• Duration: variable
• 3 phases of interphase (excluding the G0 phase)
  1. G1 phase (several hours to months)
     • Synthesis of RNA, proteins, and cell organelles
     • Occurs after mitosis
     • There is one chromatid present per chromosome.
     • The cell grows during this phase
     • Nucleotide excision repair takes place.
     • G1 checkpoint before entering S phase
  2. S phase (∼ 8 h)
     • DNA replication results in two sister chromatids per chromosome.
     • Synthesis of proteins required for DNA packaging (especially histones)
     • Most mismatch repair takes place during the S phase.
     • Once the S phase is initiated, the cell cycle must be completed.
  3. G2 phase (∼ 2–5 h)
     • Further synthesis of proteins required for mitosis
     • Repair of DNA replication errors
     • G2 checkpoint before entering mitosis

G0 phase (resting phase)

• Definition: a resting phase which a cell enters after exiting the cell cycle from the G1 phase
• Duration: variable
• Characteristics
  • Cells that enter the G0 phase are differentiated, have specific functions, and are no longer undergoing cell division.
  • Most mature tissue cells are in the G0 phase.
  • Certain cell types reenter the G1 phase after the G0 phase when exposed to certain stimuli (e.g., hepatocyte proliferation after hepatectomy).

Most cells in mature tissue are in the resting phase (G0 phase). Mitosis is rare in mature differentiated tissue.

One of the features of malignant tumors is the high mitotic rate and dedifferentiation of tumor cells (i.e., reversal to less differentiated cells with a high mitotic rate). Multiple mitotic figures seen on microscopy are indicative of a malignant process.

M phase

• Definition: the process of cell division from the distribution of DNA to the budding of a cellular body
• Duration: ∼1 h (shortest phase of the cell cycle)

Mitosis

• Definition: the final phase of the cell cycle, following the replication of DNA
• 5 phases of mitosis
  1. Prophase
     • Chromosome condensation
     • Centrosome separation: The centrosome is the point of origin of the mitotic spindle. It consists of two centrioles and a surrounding matrix, from which the microtubules emerge.
     • Formation of the mitotic spindle
  2. Prometaphase
     • Degradation of the nuclear membrane into small vesicles and storage of intercellular vesicle
     • Completion of the mitotic spindle formation
  3. Metaphase: maximal condensation of the chromosomes, which are aligned along the equatorial plane of the cell
  4. Anaphase
     • Separation of sister chromatids due to cohesion dissolution at the centromere by the enzyme separase
     • Cell elongation
  5. Telophase
     • Decondensation of the chromosomes
     • Disintegration of the mitotic spindle
     • Formation of a new nuclear membrane
     • Cell bodies division at the equatorial plane
     • Ribosomal RNA (rRNA) synthesis
• Mitotic index: the ratio of the number of cells undergoing mitosis to the total number of cells in the given population (e.g., per 1,000 cells or per microscopic area in the specimen)

Proper functioning of the mitotic spindle is a prerequisite for chromosome transportation. Inhibition with spindle poisons leads to arrest of mitosis and cessation of cell division. Spindle poisons include colchicine, which inhibits microtubule polymerization, as well as vinca alkaloids and taxanes.

Cytokinesis

• Definition: the division of a cell's cytoplasm into two new cell bodies
• Characteristics
  • Usually begins at the end of anaphase
  • Cytoplasm and cell organelles are distributed between the two new cells.
  • Each of the new cells contains one copy of the sister chromatids.

Basic principles of cell cycle regulation

• S phase initiation
  • Certain growth factors (e.g., insulin, EGF, EPO, PDGF) stimulate the cell to go from G1 phase into the S phase by binding to tyrosine kinase receptors.
  • Epidermal growth factor inhibitors (EGFRi) such as erlotinib (used in nonsmall cell lung cancer) and cetuximab (used in colorectal cancer and head/neck cancer) block the progression of a cell from the G1 phase to the S phase.
• Cyclin-dependent kinase (CDK)
  • A type of inactive kinase that must be activated to enable the transition from one phase of the cell cycle to the next
  • Present throughout the entire cell cycle
  • Activated via binding of cyclins to form cyclin-CDK complexes
  • Inhibited by cyclin-dependent kinase inhibitor proteins (CDKIs) if any errors in the genome are detected
• Cyclins
  • A group of regulatory proteins that activate CDKs and, therefore, play an important role in controlling the cell cycle
  • There are specific cyclins for specific phases of the cell cycle.
    • Cyclin A: a protein that regulates cell cycle transition by binding to cyclin-dependent kinases (CDKs) 1 and 2.
      • The transcription of cyclin A begins at the G1 restriction point and peaks in the middle of the S phase.
      • The cyclin A-CDK2 complex allows the transition from the G1 to the S phase, and the cyclin A-CDK1 complex allows the transition from the S to the G2 phase.
    • Cyclin B: a component of mitosis promoting factor, a cyclin-CDK complex that regulates the G2 checkpoint of the cell cycle
    • Cyclin C
      • A protein that regulates cell cycle transition by binding to cyclin-dependent kinases (CDKs) 3 and 8
      • The cyclin C-CDK3 complex allows the transition from the G0 to the G1 phase, and the cyclin C-CDK8 complex regulates gene transcription by phosphorylating transcription factors and RNA polymerase II.
    • Cyclin D
      • A protein that regulates the transition from G1 to S phase in the cell cycle via activation of cyclin-dependent protein kinases
      • Forms a complex with CDK4, which initiates DNA replication by inactivating retinoblastoma protein
    • Cyclin E: a protein that regulates the transition from the G1 to the S phase of the cell cycle by binding to cyclin-dependent kinase 2 (CDK2) and activating it
• Cyclin-CDK complexes
  • A type of protein complex with an enzymatic function that phosphorylates other proteins to regulate the progression of the cell cycle
  • The physiological progression of the cell cycle depends on the appropriate activation and deactivation of cyclin-CDK complexes at specific checkpoints.
• Tumor suppressors
  • A group of proteins that arrest and modulate (e.g., repair or induce apoptosis) the cell cycle of cells with an abnormal genome
  • DNA mutations can lead to defective tumor suppressor genes allowing cells to divide uncontrollably.
  • Examples
    • BRCA-1/BRCA-2 (DNA repair proteins): defect associated with breast cancer
    • NF1: defect associated with neurofibromatosis type 1
    • p53: defect associated with Li-Fraumeni syndrome
    • APC: defect associated with familial adenomatous polyposis
    • pRb: defect associated with retinoblastoma
  • See “Tumor suppressor genes” for more information.

The p53 tumor suppressor prevents cells with genetic errors from entering the S phase of the cell cycle.

Mutations in tumor suppressor genes result in uncontrolled proliferation. An important example is Li-Fraumeni syndrome, resulting from a mutation in the p53 tumor suppressor gene.

Important checkpoints and transition points

• Definition: : A cell cycle checkpoint is a specific point in time that marks the transition from one cell cycle phase to another during which the current condition of a cell is revised (i.e., if all requirements for the transition to the next phase are met)
  • If DNA is faulty or damaged, the cell cycle can be stopped in order to repair the DNA.
  • Apoptosis is initiated if the DNA damage is not repaired.

G1 checkpoint

• Definition: a cell division checkpoint during the G1 phase that restricts entry into the S phase
• Characteristics
  • Controls the cell's nuclear-cytoplasmic ratio, sufficient nutrient levels, and DNA damage
  • Growth signals lift the checkpoint.
  • Cells that pass this checkpoint become committed to division.
  • Dysfunction of this checkpoint (e.g., from loss of p53 function) leads to unregulated cell division.
• Regulation
  • Cyclin D/Cdk4 complex
    • A regulatory protein complex that initiates DNA replication by inactivating pRb
    • Cyclin D/CDK4 complex phosphorylates pRb → pRb inactivation → release of previously bound transcription factor E2F → transcription of genes that are required for DNA replication
  • p53 tumor suppressor
    • A protein that inhibits DNA replication by activating pRb and initiates apoptosis of the cells with irreparable DNA damage
    • DNA damage → activation of protein kinases → phosphorylation of p53 → activation of p21 → inhibition of Cdks → inhibition of Cdk-mediated phosphorylation of pRb → pRb activation and binding of transcription factor E2F → cell arrest in the G1 phase (no entry into the S phase)
    • Activation of the p27 protein (a phosphoprotein that prevents cell cycle progression by inhibiting cyclin/cyclin-dependent kinase complexes) leads to the same sequence of events.
    • Present in every cell but undergoes continuous ubiquitylation and degradation
    • Phosphorylated p53 can no longer be ubiquitinylated and degraded, leaving it free to act as a transcription factor.
  • Proapoptotic active proteins of the Bcl-2 family such as Bax and Bak

Mutations in the Rb gene lead to the dysregulation of the cell cycle and, thus, unrestrained growth of retinal tumor cells (e.g., retinoblastoma).

G2 checkpoint

• Definition: a cell division checkpoint during the G2 phase
• Characteristics
  • Checks for DNA damage and completeness of DNA replication
  • Initiates mitosis by phosphorylation of various proteins (e.g., histones)
  • Also an important regulator for synchronized cell division of multiple cells
• Regulation: mitosis promoting factor (MPF), which is composed of Cdk1 and cyclin B

M checkpoint (spindle checkpoint)

• Definition: a checkpoint between metaphase and anaphase in mitosis
• Characteristics: ensures correct alignment of the chromosomes and sister chromatids at the equatorial plane before the separation of sister chromatids

Cell types