Cell Cycle

Definitions

• Chromatin: the combination of nucleic acids and proteins that makes up chromosomes
• Histones:
  • Most common protein in chromatin
  • Organize DNA, allowing it to be more tightly or loosely wound
• Chromosomes:
  • DNA and protein-containing subunits in which genes are organized
  • Usually string-like and diffuse, during mitosis condense into discrete rod-like structures
• Sister chromatid: 1 of 2 identical halves of a replicated chromosome
• Centrioles: cylindrical organelles that function as 1 of the attachment poles of the mitotic spindle in a replicating cell (the other end being a kinetochore)
• Centrosomes: organelle containing centrioles
• Kinetochore: protein structure that attaches to the central zone of a chromosome (centromere) to which 1 of the ends of the mitotic spindle attaches in a replicating cell (the other end being the centrioles)
• Centromeres: the region of the chromosome to which the microtubules of the mitotic spindle attach via the kinetochore
• Mitotic spindle: the microtubule structure created from the disassembled cytoskeleton of a cell along which chromosomes are pulled when they are separated during mitosis

Overview

The cell cycle describes the cyclic sequence of events through which a eukaryotic, proliferating parent cell splits into 2 identical daughter cells.

G0

• Mature, differentiated cells outside the cell cycle are in this reversible, resting state.
• Low concentration of growth factor and high population density keep cells in G0.
• Normal metabolism occurs, but there is no cell division.
• Specific triggers, such as growth factors, induce some cells to return to G1.

Interphase

G1, S, and G2 phases are grouped together as interphase.

• G1 (gap 1): duration = approximately 12 hours
  • Occurs immediately after mitosis (cell is half its normal size)
  • Period of cell-volume growth 
  • RNA and protein synthesis take place.
  • Proteins and cellular constituents required for mitosis (e.g., microtubules) are created.
• S (synthesis): duration = 8–12 hours
  • Chromosomes double, creating identical pairs of sister chromatids (diploid → tetraploid set)
  • Quality assurance phase:
    • Ensures the accuracy of replication
    • DNA mismatch repair occurs
  • Machinery for mitosis is prepared:
    • Centrosomes duplicate
    • Centriole separate
    • Histones synthesized
• G2 (gap 2): duration = 1.5–3 hours
  • Replication of certain organelles
  • Preparation of structures used to move chromosomes (cytoskeleton is dismantled)
  • Specific proteins that are required for cell division (e.g., RNA molecules) are synthesized.
  • Replication is checked using repair mechanisms.
  • The cell completes the interphase and is ready for mitosis.

Cell Division (Mitosis)

During mitosis, the cell’s duplicated DNA is separated and distributed to 2 identical daughter cells. The process lasts approximately 1 hour.

• Prophase:
  • Chromosomes condense, becoming visible.
  • Microtubules assemble, creating the mitotic spindle attached to the centrioles.
  • Centrosomes migrate toward opposite poles of the cell.
  • Nucleolus is no longer visible. 
  • Nuclear envelope begins to dissolve.
• Prometaphase:
  • Kinetochores assemble at the chromosome centromeres.
  • Free ends of microtubules connect to the kinetochores.
  • Centrosomes reach the opposite poles.
• Metaphase:
  • Chromosomes are aligned along the equatorial region (metaphase plate) of the cell. 
  • Each sister chromatid is attached to a mitotic spindle originating from opposite poles.
  • The nuclear envelope completely dissolves.
• Anaphase: 
  • Kinetochores break down.
  • Sister chromatids become single chromatids or chromosomes.
  • Chromatids migrate to opposite poles along the mitotic spindle. 
  • Spindle fibers opposite the chromatids elongate, stretching the cell into an oval.
• Telophase:
  • New nuclear envelopes form around the decondensing chromosomes.
  • The spindle apparatus dissolves. 
  • “Cleavage furrow” splits the cell in 2 (cytokinesis)
  • Each of the daughter cells develops a complete cell membrane.
  • Both daughter cells are completely separated from each other.

Regulation of the Cell Cycle

The cell cycle is tightly regulated. Not passing a checkpoint should trigger programmed cell death (apoptosis). Apoptosis failure results in mutation accumulation, leading to disease.

The cell cycle begins when the cell has:

• A sufficient minimum size
• Appropriate nutritional conditions
• Been stimulated by growth factors, in addition to the absence of anti-mitogenic signals

Molecules regulating the cell cycle

• Positive regulators: Presence at appropriate concentrations permits the cell cycle to progress.
  • Cyclins:
    • Regulatory proteins released in response to internal and external stimuli at specific stages of the cell cycle.
    • 4 different cyclin concentrations fluctuate predictably during the cycle.
  • Cyclin-dependent kinases (CDK) are protein kinases that are:
    • Activated by cyclins and protein kinases (via phosphorylation)
    • Inhibited by protein phosphatases (via dephosphorylation) and cyclin-dependent kinase inhibitors (CKIs)
  • Nuclear localization sequence (NLS) allows cyclins to enter the cell nucleus as a heterodimer with CDKs.
• Negative regulators: Activation of these factors blocks the cell cycle. 
  • p53 and p21: halt cell cycle if DNA damage is detected
  • Retinoblastoma protein: halts the G1/S transition
  • If hyperphosphorylation occurs due to cyclin-D/CDK 4/6 dimers → transcription of cyclin E and other proteins for progression to S phase begins.

Restriction points

• Later G1 phase (G1S checkpoint): irreversible 
  • Regulated by CDK2 and cyclin E
  • Transition between the G1 and S phases determines cell progression to cell division or dormancy in G0 phase.
• End of the G2 phase (G2M checkpoint)
  • Regulated by CDK1 and cyclin B
  • Checkpoint for favorable environmental conditions and correct and complete replication of DNA 
    • Incomplete genome at this stage will result in cell death.
    • M-phase promoting factors (MPF), which are cyclin-CDK heterodimers, play an important role in this checkpoint.
• During metaphase (spindle checkpoint): irreversible
  • Ensures that all chromosomes adhere to the spindle microtubules prior to separation in anaphase
  • Tension on the spindle fibers → production of anaphase-promoting complex (APC) 
  • APCs promote breakdown of connexin (kinetochore) proteins between sister chromatids.
• G1 and G2 phases have further DNA damage checkpoints.
  • The cell cycle can be arrested until damage has been repaired.
  • If the DNA is irreparable, apoptosis is triggered.