Cell Cycle and Cell Division

• The cell cycle is an ordered series of events leading to the replication of a cell

Cell Division

• Cell division is a part of the cell cycle
• The main purpose of the cell cycle is to properly duplicate DNA and then segregate it into two identical daughter cells

Eukaryotic Cell-Cycle Times

• Early frog embryo cells: 30 minutes
• Yeast cells: 1.5 hours
• Mammalian intestinal epithelial cells: ~12 hours
• Mammalian fibroblasts in culture: ~20 hours
• The life of a cell consists of a long interphase and a shorter mitotic phase
• Some cells don't undergo cell division after terminal differentiation (nerve cells, skeletal muscle cells, erythrocytes)
• Cells grow continuously in interphase

Cell Cycle Phases

• G1, S, G2, and M (mitosis) are the 4 sequential phases
• In interphase, cells transcribe genes, synthesize proteins, and grow in mass and size
• Interphase consists of G1 (gap1), S (synthesis), and G2 (gap2)
• The two gap phases serve to provide time for cell growth and monitor internal/external environment

G1 Phase

• G1 corresponds to the interval between mitosis and initiation of DNA replication
• Cells are metabolically active and continuously growing in G1, but do not replicate DNA
• Cells accomplish most of their growth during this phase and make proteins and organelles needed for DNA synthesis
• Proteins and RNAs are synthesized; the centromere and other centrosome components are made
• If extracellular conditions are unfavorable, cells delay progress through G1 or enter a resting state known as G0
• G1 is followed by S phase (synthesis), during which DNA replication takes place
• To finish S phase and enter the G2 phase, DNA replication has to be completed

G2 Phase

• Cell growth continues and proteins are synthesized in preparation for mitosis
• Cells produce proteins intensively before they divide successfully
• New microfilaments form the mitotic spindles
• Early embryonic cell cycles have no G1 or G2 phases
• DNA replication occurs very rapidly in early embryonic cell cycles
• Early embryonic cell cycles consist of very short S phases alternating with M phases
• Nonproliferating cells exit the cell cycle; instead of staying in G1 phase they enter the G0 phase
• Cells in G0 can enter a quiescent (inactive) stage, not actively preparing to divide
• Some cells enter G0 temporarily and re-enter the cell cycle
• Some cells enter G0 permanently and never divide (mature cardiac muscle and nerve cells)

Cell-Cycle Control

• The cell-cycle control system ensures that key processes occur in the proper sequence
• Entry into cell division cycle is dependent on the extracellular signals
• Most extracellular signals are growth factors in mammalian cells
• Phases in cell cycle and extracellular signals must be tightly regulated
• This regulation is controlled by cell cycle check points

Cell Cycle Checkpoints

• G1 checkpoint: growth factors, nutrients, DNA damage
• G2 checkpoint: cell size, DNA replication
• M checkpoint: spindle assembly
• Cell division is blocked if problems are detected in the checkpoints
• Intracellular control of the cell cycle is controlled by regulator molecules that either promote or stop the process

Cell Cycle Regulators

• Cyclins are regulatory subunits of the protein kinases that control the cell cycle
• Cyclin-Dependent Kinases (Cdks) are the catalytic subunits of the protein kinases
• Cyclin-Dependent Kinases must be associated with a cyclin to be activated
• Progression through the cell cycle depends on cyclin-dependent protein kinases (Cdks)
• Cyclin proteins are normally inactive and when they bind to Cdk, then Cdk enzymes are activated
• Cdk enzymes phosphorylate and activate other cellular proteins
• Different cyclin-Cdk complexes trigger different steps of the cell-cycle
• Cyclin concentrations vary during a cell cycle
• Accumulation of cyclins helps regulate the activity of Cdks
• Cyclins oscillate throughout the cell cycle
• The formation of active cyclin-Cdk complexes drives various cell-cycle events, including entry into S or M phase
• Four basic groups of cyclins are found in humans
• Each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle
• Cdk levels remain relatively constant across the cell cycle
• Cdk activity and target proteins change as levels of the various cyclins rise and fall
• Each Cyclin-CDK complex phosphorylates a different set of target proteins in the cell
• G1-Cdk: cyclin D*, Cdk4, Cdk6
• G1/S-Cdk: cyclin E, Cdk2
• S-Cdk: cyclin A, Cdk2
• M-Cdk: cyclin B, Cdk1
• The activity of some Cdks is regulated by cyclin degradation
• Ubiquitylation of S or M cyclin tags the protein for degradation in proteasomes
• Because Cdks must bind to cyclins to be enzymatically active, the loss of cyclin returns its Cdk partner into an inactive state
• The activity of cyclin-Cdk complexes depends on phosphorylation and dephosphorylation
• The Cdk and M cyclin complex is phosphorylated as soon as it is formed (inactive)
• When the complex is dephosphorylated, it becomes active
• Cdk activity can be regulated by a Cdk inhibitor
• Binding of a Cdk inhibitor (p27) blocks the activity of cyclin-Cdk complex
• Various mechanisms are used to pause the cell cycle, including:
  • Inhibition of activating phosphatase (Cdc25): Blocks entry to mitosis
  • Inhibition of APC activation: Delays exit from mitosis
  • Cdk inhibitors: Blocks entry to S phase

DNA Damage & Cell Division

• DNA damage can arrest the cell cycle in G1
• G1/S-Cdk and S-Cdk complexed with p21 is inactive
• S-Cdk initiates DNA Replication and Blocks Re-Replication, taking place in 2 steps
• M-Cdk drives entry into M phase and mitosis

Mitosis

• Activated M-Cdk indirectly activates more M-Cdk, creating a positive feedback loop
• Cdk inhibitors have been developed for treatment of cancer
• Cell cycle dysregulation is a hallmark of all cancers, causing cells to divide without control
• Cdk proteins are frequently overexpressed or mutated in cancer; these are oncogenes
• p53 is a master regulator of cell division
• Cdk proteins are frequently overexpressed or mutated in cancer, this is oncogenes
• p53 is the most commonly mutated gene in all human tumors (60%)
• Mutations in p53 mediated signaling pathways are present in over 90% of all human tumors
• p53 is a transcription factor that binds to DNA in a sequence specific manner.
• p53 has been shown to activate or repress the expression of downstream target genes
• p53 is involved in growth arrest, apoptosis, DNA repair, and angiogenesis
• Negative regulators: Rb & p53
• Tumor suppressor genes code for a signaling protein in an inhibitory pathway
• If a tumor suppressor gene mutates, the end result can be active cell division
• RB(retinoblastoma) protein prevents cell from moving into S phase; binding to a transcription factor
• When RB is phosphorylated it can not bind so cell can move into S phase
• P53 prevents damaged cells from dividing(by inhibiting RB pathway)
• Mutations that affect the pRb-signaling pathway:
  • have been documented in nearly every type of adult tumor
  • these include deletion of point mutations in the pRb gene
  • the constituitive activation/overexpression of tyrosine;
  • kinase receptors, Ras family members, growth stimulatory;
  • transcription factors, cyclin D1, and cdk4
  • the loss of other tumor suppressors, including p53,PTEN
• All of these events allow the unscheduled activation of E2F transcription factors and entry in S-phase

Growth Factors

• Mutations in k-Ras occur in 19% of small cell lung carcinomas and 60% of all pancreatic carcinomas
• HER2 Tyrosine Kinase Receptor is overexpressed in 30% of breast cancers
• Genetic amplification of Cyclin D1 occurs in breast, colon and hepatocellular, carcinomas as well as gliomas

Cell Division

• Cell division in prokaryotes is called binary fission
• Cell division in eukaryotes includes somatic cells:
  • Diploid cells (46 chromosomes) that are found around everywhere in the body and divide with mitosis
  • Gamete cells: Haploid cells (23 chromosomes), produced by meiosis and give rise to diploid zygote after fusing
• The division of a cell into two daughters occurs in the M phase of the cell cycle
• M phase consists of nuclear division, or mitosis, and cytoplasmic division, or cytokinesis
• M phase = mitotic cell division + cytokinesis in eukaryotes

Prometaphase & Kinetochores

• Kinetochores attach sister chromatids to the spindle at prometaphase
• At anaphase, the sister chromatids synchronously separate and are pulled slowly toward the spindle pole to which they are attached
• The kinetochore microtubules get shorter, and the spindle poles also move apart
• At telophase, the two sets of chromosomes arrive at the poles of the spindle
• A new nuclear envelope reassembles around each set, completing the formation of two nuclei

Cytoskeletal structures in Animal Cells

• Two transient cytoskeletal structures mediate M phase in animal cells:
  • Microtubules of the mitotic spindle
  • Actin and myosin filaments of the contractile ring
• Centrosomes duplicate to help form the two poles of the mitotic spindle and occurs before mitosis
• Three classes of microtubules form the mitotic spindle in an animal cell:
  • Aster microtubules
  • Kinetochore microtubules
  • Interpolar microtubules
• Cleavage furrow is formed by the action of the contractile ring underneath the plasma membrane
• The contractile ring divides the cell into two
• Cytokinesis in plant cells involves the formation of a new cell wall
• Asymmetric cell division is a type of division that gives rise to two daughter cells that have different fates
• Asymmetric cell division is used by stem cells and progenitor cells
• Meiosis is one round of DNA replication with two rounds of nuclear division involving one round of DNA replication
• Meiosis occurs similarly in a diploid germ-line cell with a somatic cell mitosis:
  • Before the first meiotic division, random segregation of homologous chromosome pairs happens
  • At the end of the second meiotic division, 4 haploid cells that are genetically dissimilar with each other are produced

Comparing Mitosis and Meiosis

• Mitosis vs Meiosis:
  • Number of divisions: 1 vs 2
  • Number of daughter cells: 2 vs 4
  • Identical daughters: Yes vs No
  • Chromosome #: Same as parent vs Half of parent
  • Where: Somatic cells vs Germ cells
  • When: Throughout life vs At sexual maturity
  • Role: Growth & repair vs Sexual Reproduction