Cell Cycle and Cell Division

Cell Cycle Overview

• The cell cycle describes the timing of events in a eukaryotic cell
• It begins with cell division of a parent cell into two new cells
• It ends with the daughter cells also dividing
• The ability for organisms to reproduce distinguishes living things from nonliving matter

Eukaryotic Cell Cycle Regulation

• Tissue homeostasis requires a balance between differentiation, cell growth, and cell death
• Cells that stop dividing due to accumulated DNA damage are called senescent
• Neurons don't reenter the cell cycle after maturation
• Intestinal epithelial cells and bone marrow hematopoietic cells undergo continuous, rapid cell turnover and must be replaced
• Liver hepatocytes can reenter the active cell cycle for liver regrowth after injury or disease

Cell Division

• Eukaryotic cell division has two main stages: Mitotic (M) phase and Interphase
• Mitosis is the division of genetic material in the nucleus
• Cytokinesis is the division of the cytoplasm
• Interphase involves cell growth and chromosome copying
• Meiosis is a variation of cell division that produces gametes, yielding nonidentical daughter cells with half the chromosome number of the parent cell

Phases of the Cell Cycle

• Interphase occupies about 90% of the cell cycle and includes G1 (9 hours), S (10 hours), and G2 (4.5 hours) phases
• G0 phase is a special resting phase in G1
• Interphase describes the time between two cell divisions or mitoses
• Gene expression occurs throughout all stages of interphase
• Chromosomes are duplicated only during the S phase
• After G2, cells enter the complicated process of the M (mitotic) phase

Mitosis

• Mitosis or the M phase complex
• Mitosis is the shortest phase in the cell cycle, 1 hour
• Mitosis divides a cell to form two daughter cells
• The M phase has four main periods: prophase, metaphase, anaphase, and telophase
• Cytokinesis occurs during the M phase
• Mitosis distributes replicated chromosomes to two daughter cells
• The cell cycle is regulated primarily at the DNA replication and mitosis steps

Interphase: G1 Phase

• Collectively, the G1, S, and G2 phases are known as interphase
• Cells in the G0 phase have either postponed a division decision or made the decision never to divide again (e.g., nerve cells)
• During the G1 phase, a cell prepares to divide and reaches a restriction point
• The time is 9 hours with growth occurring before DNA synthesis
• It's the period between mitosis and initiation of nuclear DNA replication
• Cells synthesize RNAs and proteins and cytoplasmic organelles like mitochondria and endoplasmic reticulum

Interphase: S and G2 Phases

• During the S phase (10 hrs) DNA replication takes place
• DNA replication occurs and begins at the origin of replication
• At the end of the S phase, each chromosome doubles its DNA content and has two identical sister chromatids linked at the centromere
• During G2 (4.5 hrs) cellular growth occurs before mitosis.
• G2 occurs after DNA replication
• It's the period between completion of nuclear DNA replication and mitosis
• Replicated DNA is checked for errors

Chromosome Dynamics

• At the end of the S phase, a cell has twice as many chromatids as there are chromosomes in the G1 phase
• A human cell has 46 distinct chromosomes in G1 and 46 pairs of sister chromatids in S phase
• The term "chromosome" can refer to different structures depending on the cell cycle stage
• In G1 and late in the M phase, "chromosome" refers to the equivalent of one chromatid
• In G2 and early in the M phase, it refers to a pair of sister chromatids
• During the G2 phase, the cell collects the materials necessary for nuclear and cell division

Control System

• Eukaryotic cell division varies with the type of cell
• These differences result from regulation at the molecular level
• Cancer cells escape controls on the cell cycle
• The cell cycle has specific chemical signals present in the cytoplasm
• The cell cycle is directed by distinct cell cycle control system like a clock
• The cell cycle control system is regulated by both internal and external controls
• Checkpoints exist where the cell cycle stops until a go-ahead signal is received

Checkpoints

• For many cells, the G1 checkpoint is the most important
• A cell moves through the S, G2, and M phases and divides if it receives a go-ahead signal ate the C1 check point
• It will exit the cycle into a non dividing state called the G0 phase if the cells does not receive a go-ahead signal
• In the absence of regulation, cells replicate and divide uncontrollably - leads to cancer.
• Passage through 3 critical cell-cycle transitions: G1 to S, metaphase to anaphase, and anaphase to telophase/cytokinesis are needed
• These transitions are irreversible because they are triggered by regulated protein degradation, an irreversible process

Cell-Cycle Controllers

• Cells are forced to traverse the cell cycle in one direction only
• The master cell cycle controllers are: Siklinler ve Heterodimeric protein kinases, Regulatory phosphatases, and Two ubiquitin-protein ligases
• Cyclins and cyclin-dependent kinases (Cdks) control cell cycle progression
• Two types of regulatory proteins are involved in its control: cyclins and cyclin-dependent kinases (Cdks)
• Cyclins of types D, E, A, or B, are a family of cell cycle regulatory proteins
• Different cyclins regulate specific phases of the cell cycle
• Cyclin concentrations rise and fall throughout the cell cycle via synthesis and degradation

Maturation Promoting Factor (MPF)

• Cyclin-dependent kinases (CDKs) have a regulatory cyclical subunit and a catalytic cyclin-dependent Heterodimeric protein kinase (CDK) subunit
• Cdks activity fluctuates during the cell cycle
• Cyclins control Cdks' activity, so named due to their varying concentrations
• Cdks are enzymatically active during a narrow window of the cell cycle
• They're important for driving the cell cycle forward
• Growth factors stimulate cells to enter the cell cycle
• Growth factors activate specific cyclins belonging to the cyclin D family of G1 cyclins
• Cyclins and cyclin-Dependent Kinases (CDKs) regulate the 3 major classes of cyclin-CDK complexes that controls passage through the cell cycle

Cyclin-CDK complexes

• G1 phases (mid- and late stage) cyclin-CDKs including, Cyclin CDK4 and/or Cyclin D –CDK6
• S phase cyclin-CDKs activate DNA replication origins.
• Mitotic cyclin-CDKs (Maturation-promoting factor: MPF) regulate the process, including Cyclins A and B-CDK1: activate early steps in mitosis and is encoded by the CDC2 gene
• MPF is a cyclin-Cdk1 complex (cyclin B and CDK1 protein kinase)
• MPF triggers a cell's passage past the G2 checkpoint into the M phase and activates other proteins through phosphorylation
• In late anaphase, MPFs are inactivated by the degradation of mitotic cyclins

Regulation of Cyclin-Dependent Kinases (CDKs)

• Association with cyclins
• Activating phosphorylations
• Inhibitory phosphorylations
• Association with Cdk inhibitors (CKIs; tumor suppressors e.g. p21 protein)
• Active CDK2 is responsible for activating target proteins involved in S phase transition and DNA synthesis
• CDK1 targets proteins critical for the initiation of mitosis

Cell Cycle Inhibitors

• A cell has to pass the G1/S checkpoint to start the division process
• Cyclins and cyclin-dependent kinases (CDK) control the cell cycle
• The cyclin-CDK complex is regulated by cyclins, phosphorylation, and kinase inhibitors
• CKIs slow down cell cycle progression by binding and inhibiting the cyclin-CDK complexes
• CKIs are tumor suppressors
• Two classes of inhibitory proteins are Recognized:
• INK4A family inhibits D-type cyclins associating with and activating CDK4 and CDK6
• CIP/KIP family are potent inhibitors of CDK2 kinases with p21 (p21CIP1) being a member

Eukaryotic Cell Cycle Checkpoints

• Checkpoints ensure chromosomes are intact and critical stages are reached before the next stage initiates
• Included are the unreplicated-DNA, spindle-assembly, chromosome-segregation, and DNA-damage checkpoints
• The unreplicated-DNA checkpoint operates during S and G2 to prevent the activation of MPF before DNA synthesis is complete by inhibiting the activation of CDK1 by Cdc25C
• The spindle-assembly checkpoint prevents premature anaphase initiation and uses Mad2 and other proteins to regulate the APC/C specificity factor/ Cdc20 to target securin
• The chromosome-segregation checkpoint prevents telophase and cytokinesis until daughter chromosomes have been Properly segregated

DNA Damage Checkpoints

• Cdc14 phosphatase activates the APC specificity factor which targets cyclins for degradation, causing inactivation of MPF
• The DNA-damage checkpoint arrests the cell cycle in response to DNA damage
• Three tumor-suppressor proteins (ATM/ATR, Chk1, and p53) are critical to this checkpoint
• Activated p53 either induces cell cycle arrest or apoptosis if there is DNA damage
• Spindle-assembly monitors for chromosome attachment to microtubules, anaphase checkpoint and Unequal segregation
• The ATM protein kinase phosphorylates several key proteins (e.g. p53) that initiate activation of the DNA damage checkpoint, leading to cell cycle arrest, DNA repair or apoptosis

Cell Signals

• Kinetochores that are not attached to spindle microtubules sending a molecular signals that delays anaphase is an example of an internal cell signal
• Mitogens or growth factors are external signals that are released by certain cells that stimulate other cells to divide
• An example is platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture
• Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide
• Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence

Tumor Suppressor Genes

• ATM, ATR, Chk1, and p53 respond to DNA damage from replication errors, mutations from UV lights, irradiation, mutations, chemical exposure, oxidative insults, and cellular metabolism
• The usual response to DNA damage is to halt the cell cycle in G1
• S phase arrest allows time for damaged DNA to be repaired before replication
• Damage is halted at the G2 checkpoint as well

Master Regulator

• The DNA damage checkpoint arrests the cell cycle when DNA is damaged
• Three tumor-suppressor proteins (ATM/ATR, Chk1, and p53) are critical to this checkpoint
• Proteins encoded by tumor suppressor genes: directly/indirectly slow down cell division, control proteins of the cell-cycle checkpoints, signal transduction pathways, pro-apoptotic proteins
• The p53 gene is mutated across a wide variety of cancers
• It is the genetic guardian that checks for damaged DNA

Retinoblastoma (Rb) Gene

• Rb is the prototype tumor suppressor gene
• In non-proliferating cells, Rb binds to E2F, and activates histone deacetylases that lead to gene silencing
• Turning on G1 cyclin-CDKs prompts a mitogen to free E2F from RB to travel to G1/S, activating transcription for coding DNA
• Mutated Rb does not bind to E2F or has loss of functions will lead to cancer tumors