Biol 120: The Cell Cycle, Mitosis, and Meiosis

The Cell Cycle: Core Concepts & Processes

Introduction to The Cell Cycle

• Fundamental Principle: Living organisms must reproduce to remain viable and propagate.
• Cellular Reproduction: At the cellular level, 'reproduction' is synonymous with cell division or replication.
• Cell Cycle: Replication can occur once or many times in the life of a cell, comprising the cell cycle.
  • The cell cycle is composed of replicative (division) and quiescent (growth) stages.
• Four Coordinated Processes of Cell Division:
  1. Cell growth.
  2. DNA replication.
  3. Segregation of chromosomes to daughter cells.
  4. Cell cleavage.

The Prokaryotic Cell Cycle (Bacteria)

• Propagation: Bacteria grow and divide to propagate a population.
• Limitations for Replication: Primarily environmental factors such as nutrient shortage, unfavorable pH, presence of predators, and toxins.
• Proliferation Strategy: From a cell's perspective, continuous cell proliferation is advantageous as long as resources are available.
  • Positive regulation (proliferative stage) is typically 'ON' unless environmental conditions deteriorate, switching to 'OFF'.
• Stages of Bacterial Cell Life:
  • Lag phase: Initial period of adaptation and growth before rapid division.
  • Exponential phase (proliferation stages): Rapid cell proliferation under optimal conditions.
  • Stationary phase (non-proliferation stages): Growth rate slows due to resource depletion or waste accumulation.
  • Death phase: Cells begin to die off due to harsh conditions.
• Bacterial Cell Cycle Stages:
  • B period: Extends from the end of cell division to the beginning of DNA replication.
  • C period: The time required for chromosome replication.
  • D period: The interval between the completion of chromosome replication and the completion of the cell cycle.
• Sustained Replication: Replication of the bacterial circular chromosome consumes most of the cell cycle time.
  • Begins at a single site called the origin of replication (ori).
  • Once the ori is duplicated, each actively migrates to opposite ends of the cell.
• Cell Division (Binary Fission):
  • Division of the cytoplasm occurs through an inward growth and partition of cell wall material until the cell separates into two parts.

The Eukaryotic Cell Cycle: Interphase

• Complexity: The eukaryotic cell cycle is more complex than prokaryotic division.
• Multicellular Organisms:
  • Early Embryonic Development: Cells constantly replicate to generate all tissues and organs.
  • Cell Differentiation: Cells often differentiate as they develop, leading to specialized functions.
    • Some differentiated cells (e.g., neurons, muscle cells) stop dividing, potentially never dividing again throughout the organism's life.
    • Some cells remain undifferentiated (stem cells) and retain replicative capacity to replenish other cells and tissues later in life.
    • Some specialized cells (germ cells) undergo a different type of replication (meiosis) to make gametes.
  • Limitations for Replication: Come from signaling within the organism rather than solely environmental factors.
    • When cells lose the ability to restrain dividing, the organism suffers (e.g., cancer).
• Cell Senescence and Regenerative Medicine:
  • We unfortunately lose stem cells as we age, contributing to cell senescence (aging).
  • A major effort in regenerative medicine aims to reprogram differentiated cells back to a stem cell identity.
    • This could treat aging-related disorders like progressive neuro- and muscle degeneration by repopulating tissues with fresh, 'younger' cells.
    • Re-engineering the control of the cell cycle is a common target for these approaches.
  • Example: The number of circulating mesenchymal stem cells in human blood decreases with age, from approximately $1/10,000$ in newborns to $1/2,000,000$ in $80$-year-olds.
• Phases of the Eukaryotic Cell Cycle:
  • Interphase: Extends from the end of one mitosis to the beginning of the next mitosis. It comprises three phases:
    • G1 phase (Gap 1 / Growth 1 phase): The cell carries out its normal functions and, in some cases, grows in size.
    • S phase (Synthesis phase): Cells commit to replicating their DNA.
    • G2 phase (Gap 2 / Growth 2 phase): A brief gap where cell growth continues, and the cell prepares for mitosis and cytokinesis.
  • M phase (Mitosis and Cytokinesis): The period of actual cell division.
  • G0 phase (Quiescence): A resting phase where the cell has exited the cell cycle, stopped dividing, and differentiated.
• Duration of Interphase: Varies significantly among cell types.
  • Long Duration: Human mature neurons exit the cell cycle during brain development, differentiate, and stay in G0 for decades until they die.
  • Short Duration: In early Drosophila development, embryonic cells can skip G1 and G2 entirely, moving straight from mitosis to S phase.

Key Definitions in Cell Division

• Chromosomes: Nuclear units of genetic information that are divided and distributed during cell division. The name derives from Greek chroma (colour) and soma (body), describing their strong staining by particular dyes.
• Ploidy: The number of chromosome sets in a cell or species.
  • Diploid ($2n$): Cells containing two sets of chromosomes (e.g., humans have $23$ pairs, totaling $46$ chromosomes).
  • Haploid ($1n$): Cells containing one set of chromosomes.
• Chromatids: Replicated chromosomes after S phase, consisting of two identical chromosome copies (referred to as two sister chromatids) joined at the centromere.
  • DNA Packaging: DNA is packaged around histones. In G1 phase, a chromosome is a single DNA molecule. After S phase (G2 and early mitosis), it consists of two sister chromatids. After mitosis, daughter cells again have single chromosomes.
• Chromatin: A complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells.
• Chromosome Segregation: The equal distribution of daughter chromosomes into each of the two daughter cells that results from cell division.
• Spindle (Spindle Apparatus): The cytoskeletal structure that forms during cell division to separate sister chromatids between daughter cells. It is composed of hundreds of proteins, with microtubules being the most abundant component.
• Centromere: The constricted region of a chromosome where the kinetochore forms and attaches to spindle microtubules during cell division.
• Centrosome: The main microtubule organizing center (MTOC) of a cell, which organizes the microtubule cytoskeleton during interphase and positions many cytoplasmic organelles.
• Kinetochore: Large protein assemblies that connect chromosomes to microtubules of the mitotic and meiotic spindles to distribute the replicated genome from a mother cell to its daughters.
• Cleavage Furrow: In cytokinesis of animal cells, a groove that girdles the cell and gradually deepens until it cuts the cytoplasm into two parts.
• Cell Plate: A new cell wall that forms during cytokinesis in terrestrial plant cells. This process involves the delivery of Golgi-derived and endosomal vesicles carrying cell wall and cell membrane components to the plane of cell division, and the subsequent fusion of these vesicles within this plate.

Regulation of The Cell Cycle: Molecular Checkpoints

• Purpose: Cells tightly regulate entry and exit through the stages of the cell cycle to ensure proper division.
• Mechanism: Molecular checkpoints act as critical 'gates' or 'sensors' where cells can assess the preparations for cell division.
  • If the requirements for a checkpoint are not met, cells can arrest the process or abort it entirely.
  • This prevents the production of aneuploid daughter cells (cells with the wrong composition of chromosomes), which can lead to uncontrolled cell division and cancer.
• Key Checkpoints:
  • Restriction Checkpoint (G1/S Checkpoint):
    • Checks if all chromosomes are correctly copied.
    • Detects any DNA damage.
    • Assesses the availability of appropriate energetic resources.
  • Mitosis Entry Checkpoint (G2/M Checkpoint):
    • Verifies that all chromosomes have been correctly copied.
    • Checks for any DNA damage before entry into mitosis.
  • Spindle Checkpoint (Metaphase/Anaphase Checkpoint):
    • Ensures that all chromosomes are properly attached to the spindle microtubules.
• Control Mechanism: Cyclins and Cyclin-Dependent Kinases (CDKs):
  • Cells control the transition between stages via dynamic changes in the stability and concentration of stage-specific cyclins.
  • This stability depends on the action of Cyclin-dependent kinases (CDKs), which are enzymes that target specific cyclins for degradation.
  • The rise and fall of concentrations of different cyclins (e.g., G1/S cyclins, S cyclins, M cyclins) 'times' the transition between different stages of the cell cycle.
  • Importance: Regulating entry into mitosis in different cells ensures that cells adopt the correct fate in animals and plants (e.g., blood cells, muscle cells, nerve cells, stem cells exhibit different cell cycle behaviors).

Cancer: Uncontrolled Cell Proliferation

• Pathology: Loss of the ability to control the cell cycle leads to cancer, characterized by uncontrolled cell division.
• Origin of Cancer: Cancers typically originate from a single cell whose genetic material and its offspring mutate, causing cells to grow abnormally.
  • Tumor: An overgrowth of cells.
  • Benign/Pre-cancerous tumors: Do not spread to other tissues.
  • Invasive/Metastatic tumors: Evolve to spread to other tissues (e.g., lungs, blood vessels), which is a major cause of disease mortality.
• Cancer Statistics & Causes:
  • Approximately $1.5$ million people in North America are diagnosed with cancer each year, and over $0.5$ million will die from the disease.
  • About $10\%$ of cancers have a higher predisposition due to inherited traits.
  • Most cancers (about $90\%$) do not involve genetic changes passed from parent to offspring.
  • About $80\%$ of all human cancers are related to exposure to carcinogens (agents that increase the likelihood of developing cancer).
    • Most carcinogens (e.g., UV light, chemicals in cigarette smoke) are mutagens that promote genetic changes in somatic cells.
    • DNA alterations can lead to effects on gene expression that ultimately affect cell division, leading to cancer.
• Cancer Therapies: Usually target mitotic cells.
  • Side Effects: Chemotherapy and radiotherapy side effects are associated with their impact on non-cancerous dividing cells, which are also often killed during these treatments.
• Genetic Basis of Cancer:
  • Involves mutations in two main types of genes:
    1. Oncogenes (mutated proto-oncogenes): Genes that stimulate the cell cycle (positive regulation of cell division).
    2. Tumor Suppressor Genes: Genes that inhibit the cell cycle (negative regulation of cell division).
• Oncogenes:
  • Normally, cell division is regulated by hormones called growth factors that bind to cell surfaces and initiate signaling cascades leading to cell division.
  • Mechanism of Cancer Promotion: Mutations in genes producing cell growth signaling proteins can convert them into oncogenes, leading to abnormally high levels of activity in some proteins.
    • An oncogene may promote cancer by keeping the cell division signaling pathway in a permanent 'on' position.
    • This can result from an abnormally high amount of gene product or a functionally hyperactive protein.
  • Example: RAS Protein: Mutations in the GDP-GTP exchange protein RAS can lead to the activation of growth signals without the presence of the ligand (growth factor) in pathways like the Epidermal Growth Factor (EGF) pathway.
• Tumor Suppressor Genes:
  • Normal Role: Helps prevent cancerous growth.
  • Protein Functions:
    • Genome Integrity Maintenance: Proteins maintain the integrity of the genome by monitoring and/or repairing alterations in the DNA.
    • Checkpoint Proteins: Check the integrity of the genome and prevent a cell from progressing past a certain point in the cell cycle.
    • Negative Regulators of Cell Division: Their function is necessary to properly halt cell division; otherwise, cell division is abnormally accelerated.
  • Example: p53 Protein: A critical G1 checkpoint protein.
    • About $50\%$ of human cancers are associated with p53 mutations.
    • The expression of the p53 gene is induced when DNA is damaged.
    • p53 protein stops progression from the G1 to S phase of the cell cycle.
    • If DNA is repaired, the cell may later proceed. If repair is not possible, p53 will trigger programmed cell death (apoptosis).
• Mutations that Promote Cancer (Uncontrolled Proliferation):
  • Oncogenes: Gain-of-function mutations (e.g., boosting their function or expression) lead to unregulated proliferation.
  • Tumor Suppressor Genes: Loss-of-function mutations (e.g., decreasing their activity) result in uncontrolled proliferation.

Mitosis: Somatic Cell Division

• After interphase, mitosis proceeds through five distinct stages.
• Stages of Mitosis:
  • ### Prophase
    • Chromosomes: Condense into compact, rod-like structures that become visible under a light microscope.
    • Each chromosome is doubled as a result of prior DNA replication during S phase, consisting of two sister chromatids.
    • Sister chromatids are held together by the centromere and cohesin, a protein complex that 'zips' them together.
    • Centrosomes: The centrosome has divided into two parts and begins to move apart towards opposite poles of the cell.
    • Spindle Formation: The centrosomes, as they separate, begin generating the spindle (microtubules) in the cytoplasm.
    • Centriole Duplication: The original pair of centrioles duplicates during the S phase, producing two pairs. As prophase begins, the centrosome separates into two parts, each containing one 'old' and one 'new' centriole. These duplicated centrosomes continue to separate, reaching opposite sides of the nucleus, and the microtubules between them lengthen and increase in number to form the early spindle.
  • ### Prometaphase
    • Nuclear Envelope Breakdown: The nuclear envelope breaks down.
    • Spindle Entry: The spindle enters the former nuclear area.
    • Kinetochore Assembly: Kinetochores, protein assemblies, assemble on the centromeres of each chromosome.
    • Microtubule Attachment: Microtubules from opposite spindle poles attach to the two kinetochores of each chromosome, establishing bi-directional attachment.
    • Chromosome Movement: Microtubule motor proteins facilitate the 'walking' of kinetochores along the microtubules, which disassemble as the kinetochore passes over them.
  • ### Metaphase
    • Spindle Completion: The spindle is fully formed.
    • Chromosome Alignment: Chromosomes align at the spindle midpoint, known as the metaphase plate.
    • Equilibrium: Each sister chromatid pair is held in position by opposing forces: the kinetochore microtubules pulling towards the poles, maintaining tension.
  • ### Anaphase
    • Cohesin Removal: Separase, a protease enzyme, removes cohesin from the chromosomes, weakening the sister chromatid connection.
    • Centromere Division: The pulling force exerted by the spindle microtubules effectively breaks the centromere association.
    • Sister Chromatid Separation: Sister chromatids separate and move as individual chromosomes to opposite spindle poles.
    • Chromosome Segregation: Chromosome segregation is complete, with an equal set of chromosomes moving to each pole.
  • ### Telophase
    • Chromosome Decondensation: Chromosomes decondense and return to a less compact state.
    • Nuclear Envelope Re-formation: A new nuclear envelope forms around each set of chromosomes at the poles, creating two distinct nuclei.
    • Spindle Disassembly: The spindle apparatus disassembles.
• Cytokinesis: Cytoplasmic Division:
  • Cytokinesis is the physical process of cell division, which divides the cytoplasm of a parental cell into two daughter cells immediately following nuclear division.
  • ### In Animal Cells
    • Mechanism: Involves the formation of a cleavage furrow, a contractile ring of actin microfilaments that forms on the membrane between the two nascent cells.
    • This furrow constricts like a drawstring (an 'outside-in' process), gradually deepening until it pinches the cytoplasm into two parts, separating cell components.
  • ### In Plant Cells
    • Mechanism: A cell plate forms between the two plant cells. This structure arises from Golgi-derived and endosomal vesicles that move along microtubules to the center of the cell and coalesce.
    • These vesicles carry cell wall and cell membrane components (e.g., cellulose and callose). Their fusion partitions the cytoplasm and builds up a new cell wall between the daughter cells (an 'inside-out' process).

Therapeutic & Research Implications of Mitosis

• Microtubules as Chemotherapy Targets:
  • Microtubules are excellent targets for chemotherapies due to their critical role in spindle formation.
  • Taxol: Extracted from yew trees, used to treat breast and ovarian cancer.
    • Taxol molecules bind to tubulin (the protein monomer of microtubules) and prevent spindle polymerization.
    • Cells in mitosis (many of which are cancerous) cannot proceed with division and consequently die.
    • It achieves this by binding to tubulin and preventing its association into microtubule fibers, causing monomers to clump.
  • Colchicine: Extracted from crocuses, also binds to tubulin and inhibits microtubule formation (nucleation).
    • If added during mitosis, it will prevent spindle formation and stop the cell cycle in metaphase.
    • Condensed, metaphasic chromosomes will remain in the middle of the cell.
• Karyotyping: The collection of chromosomes (number and shape) that is specific to a species.
  • Researchers often use colchicine to arrest cells in metaphase (allowing chromosomes to condense fully) to collect condensed metaphasic chromosomes for karyotype analysis.

Meiosis: Gamete Formation and Genetic Diversity

• Purpose: Meiosis is the process by which sexually-reproducing organisms create gametes (sperm and egg cells) with half the required genetic material (haploid, $1n$) to create a zygote.
• Sexual Reproduction: Requires a fertilization event where two haploid gametes unite to create a diploid cell called a zygote.
• Overview: Like mitosis, meiosis is preceded by the replication of chromosomes during interphase (G1, S, G2 phases).
  • Meiosis reduces the number of chromosome sets from diploid ($2n$) to haploid ($1n$).
  • It involves two sets of cell divisions: Meiosis I and Meiosis II.
  • The two successive cell divisions result in four haploid daughter cells, each with un-replicated chromosomes and only half as many chromosomes as the parent cell.
• Stages of Meiosis:
  • ### Meiosis I (Reductional Division)
    • Purpose: Separates homologous chromosomes, reducing the ploidy from diploid ($2n$) to haploid ($n$).
    • Prophase I: Typically occupies more than $90\%$ of the time required for meiosis.
      • Chromosomes: Begin to condense.
      • Synapsis: Homologous chromosomes 'search' each other and loosely pair up, aligned gene by gene, along their entire length, forming bivalents (or tetrads).
        • The synaptonemal complex (SC), a protein and RNA structure, acts as a scaffold that builds between homologs, polymerizes, and holds bivalents together.
      • Crossing Over (Genetic Recombination): Non-sister chromatids exchange DNA segments, leading to novel forms.
        • This occurs during synapsis as homologs are physically connected and bound by the exchange of genetic information.
        • Each bivalent usually has one or more chiasmata (X-shaped regions) where crossing over occurred.
      • SC Disassembly: The synaptonemal complex disassembles, and bivalents are then held only by chiasmata.
    • Metaphase I:
      • Alignment: Bivalents (pairs of homologous chromosomes) line up at the metaphase plate in a double row, with one chromosome facing each pole.
      • Microtubule Attachment: Microtubules from one pole attach to the kinetochore of one chromosome (consisting of two sister chromatids) of each bivalent. This is a monopolar attachment (centromeres are left intact and sister chromatids remain attached).
      • Contrast with Mitosis: In mitosis, sister chromatids are subject to bipolar attachment, leading to centromere separation in anaphase.
    • Anaphase I:
      • Separation of Homologs: Pairs of homologous chromosomes separate.
      • One chromosome (which still consists of two sister chromatids) moves towards each pole, guided by the spindle apparatus.
      • Sister chromatids remain attached at their centromeres and move together as a single unit towards the pole.
    • Telophase I and Cytokinesis:
      • Ploidy Change: Each half of the cell now has a haploid set of chromosomes; however, each chromosome still consists of two sister chromatids (it is still duplicated).
      • Cytokinesis: Usually occurs simultaneously, forming two haploid daughter cells.
        • In animal cells, a cleavage furrow forms. In plant cells, a cell plate forms.
  • ### Meiosis II (Conservative Division)
    • Transition: The transition from Meiosis I to Meiosis II is not preceded by another S phase (no further DNA replication).
    • Purpose: Separates sister chromatids, similar to mitosis.
    • Prophase II:
      • Similarity to Mitosis: Meiosis II is very similar to mitosis.
      • A spindle apparatus forms.
      • Chromosomes (each still composed of two chromatids) begin to move toward the metaphase plate.
    • Metaphase II:
      • Alignment: The sister chromatids are arranged at the metaphase plate.
      • Genetic Variation: Because of crossing over in Meiosis I, the two sister chromatids of each chromosome are typically no longer genetically identical.
      • Microtubule Attachment: The kinetochores of sister chromatids attach to microtubules extending from opposite poles.
    • Anaphase II:
      • Sister Chromatid Separation: The sister chromatids separate.
      • The now individual chromosomes move towards opposite poles.
    • Telophase II and Cytokinesis:
      • Chromosome Arrival: Chromosomes arrive at opposite poles.
      • Nuclear Re-formation: Nuclei form, and the chromosomes begin decondensing.
      • Cytokinesis: Separates the cytoplasm.
      • Final Result: At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes.
      • Each daughter cell is genetically distinct from the others and from the parent cell, contributing to genetic diversity.
• Human Oocyte Arrest: Human oocytes (female gametes) remain arrested at Prophase I for years before finishing Meiosis I and then Meiosis II, typically only completing the process upon ovulation and fertilization, respectively.

Contrasting Mitosis and Meiosis

Key Differences and Similarities

• Chromosome Number: Mitosis conserves the number of chromosome sets (producing diploid cells from diploid parents), while meiosis reduces the number of chromosome sets from two (diploid) to one (haploid).
• Genetic Identity: Mitosis produces cells that are genetically identical to the parent cell and to each other. Meiosis produces cells that differ genetically from each other and from the parent cell due to crossing over and independent assortment.
• Sister Chromatid Separation: The mechanism for separating sister chromatids in Meiosis II is virtually identical to that in mitosis.

Events Unique to Meiosis I

• Prophase I: Synapsis and crossing over occur, where homologous chromosomes physically connect and exchange genetic information.
• Metaphase I: Paired homologous chromosomes (tetrads) align at the metaphase plate, rather than individual replicated chromosomes (sister chromatids) found in mitosis.
• Anaphase I: It is homologous chromosomes that separate and are carried to opposite poles of the cell, not sister chromatids (which separate in mitosis and Meiosis II).

Summary of Comparison (Table Format)

Contrasting the Two Meiotic Divisions

• Meiosis I:
  • Prophase I: Replicated chromosomes condense, and bivalents form as the nuclear membrane breaks down.
  • Prometaphase I: Spindle apparatus is complete, and the chromatids are attached to kinetochore microtubules.
  • Metaphase I: Bivalents are organized along the metaphase plate as a double row, a mechanism that promotes genetic diversity.
  • Anaphase I: Segregation of homologous chromosomes occurs. Connections between bivalents break, but the connections holding sister chromatids together do NOT. Each joined pair of chromatids migrates to one pole, and the homologous pair of chromatids moves to the opposite pole.
  • Telophase I: Sister chromatids (still duplicated) have reached their respective poles and decondense; nuclear membranes reform.
  • Cytokinesis: The original diploid cell has its chromosomes in homologous pairs, while the two cells produced at the end of Meiosis I are haploid; they do not have pairs of homologous chromosomes.
• Meiosis II:
  • No S phase between Meiosis I and Meiosis II.
  • The sorting events of Meiosis II are fundamentally similar to those of mitosis.
  • Sister chromatids are separated during Anaphase II, unlike Anaphase I where homologous chromosomes separate.

Take-Home Messages

• The cell cycle meticulously controls whether and when cells will enter cell division.
• Entry into mitosis and progression through the cell cycle is under surveillance by cell cycle checkpoints that ensure cells only divide when fully prepared.
• In mitosis, identical sister chromatids are segregated, resulting in daughter cells with the same ploidy ($2n \rightarrow 2n$).
• In meiosis, homologous chromosomes find each other, associate, and segregate from each other in the first division, thereby reducing the ploidy ($2n \rightarrow n$) and contributing significantly to genetic diversity.