Gen Bio - Cell Cycle PDF

Gen Bio: Cell cycle Kaya natin to’ artemis tayo e Kung may notes kayu n di ko nalagay padagdag nalang para malay niyo, perfect nating lahat SHEESH Overview of the Cell Cycle Introduction to the Cell Cy...

Gen Bio: Cell cycle Kaya natin to’ artemis tayo e Kung may notes kayu n di ko nalagay padagdag nalang para malay niyo, perfect nating lahat SHEESH Overview of the Cell Cycle Introduction to the Cell Cycle - The cell cycle is a crucial biological process that drives the development and maintenance of multicellular organisms, starting from a single fertilized egg. This cycle involves a series of tightly regulated events ensuring that cells grow, replicate, and divide at precise times and locations. - Disruptions in the cell cycle, such as uncontrollable cell division, can lead to pathological conditions like cancer, highlighting the importance of its regulation. -Cells are all identical except for Sperm and Egg cells who are unique to each other -Cells divide for replacement and healing -Cells are not always active in the sense of continuously growing or dividing. Their activity depends on their specific role in the organism, the cell type, and the stage they are in within the cell cycle. -Cells need a Complete clone such as the WBC as they die after eating bacteria Activities in the Cell Cycle - Copying cellular components: Before a cell can divide, it must duplicate its essential components, including DNA, organelles, and other critical structures, ensuring that each daughter cell receives a complete set. - Cell Division:*Following duplication, the cell undergoes division, where these components are equally distributed between the two daughter cells. - The cell cycle is composed of alternating phases: growth (Interphase) - the cell grows, duplicates its DNA, and prepares all necessary components for division. division (M phase) - the cell undergoes division, distributing duplicated genetic material and cellular components evenly between two daughter cells.  > Cells need Duplication off cellular cells before division to ensure that the 2 daughter cells receive even organelles > Cells like WBC don’t always undergo cell division, 10% of cell activity is division > Cells only divide if all requirements are met and the environment warrants them to do so -Cell cycle has cell division but cell division is not the cell cycle itself The cell cycle Interphase: Is the Longest phase and is subdivided into G1, S, and G2 phases - In yeast “Start” is at the end of G1; at this point the cell is committed to DNA synthesis. - In mammals, this is called the “restriction point”. This point late in G1 is a “checkpoint”; a cell will exit the cell cycle if certain requirements to proceed to synthesis are not met. - A second restriction point occurs in G2 before entry into mitosis. 1. G1 Phase (Gap 1 Phase) Cell Growth: ○ The cell increases in size and synthesizes various enzymes and nutrients necessary for DNA replication and cell division. Preparation for chromosomes for Replication: ○ Chromosomes are prepared for duplication through the synthesis of nucleotide building blocks and replication machinery components. Duplication of Cellular Components: ○ Organelles such as mitochondria and ribosomes are replicated to ensure each daughter cell inherits sufficient cellular machinery. The cell assesses environmental conditions, nutrient availability, DNA ans size G1 Checkpoint (Restriction Point): The cell is checked for damage, nutrient availability, and DNA integrity. Cells past this phase are inevitable for division Located at the End of G1 phase just before S phase Decisions Made: ○ Proceed to S Phase: If conditions are favorable and DNA is undamaged. ○ Delay Division: If conditions are suboptimal, the cell may pause to repair or acquire necessary resources. ○ Exit to G0 Phase: The cell may enter a quiescent state where it remains metabolically active but does not divide. - Examples are neutrons, Rbc, Cardiac muscles - Small intestines are also in G0 however they still divide after a long time - S Phase (Synthesis Phase): The DNA is replicated during this phase, and the centrosome, a key structure near the nucleus that organizes the spindle during cell division, is also duplicated. - This phase is the shortest but most important DNA Replication: The entire genome is duplicated precisely, ensuring each daughter cell receives an exact copy of genetic information. Mechanism: ○ DNA strands unwind, and replication forks form to synthesize complementary strands. Centrosome Duplication: The centrosome, the microtubule-organizing center crucial for spindle formation during mitosis, is replicated, It contains two centrioles that migrate to the poles before cell division and serve to organize the spindle Importance: ○ Ensures proper segregation of chromosomes during cell division. - G2 Phase( Gap 2/Final growth phase): The cell continues to grow and prepare for mitosis. Another checkpoint at the end of G2 ensures that all processes are complete and the cell is ready to enter the M phase. Further Cell Growth: ○ The cell continues to grow and produces proteins and organelles necessary for mitosis. Preparation for Mitosis: ○ Final preparations are made, including the synthesis of microtubules for the mitotic spindle. G2 Checkpoint:  ○ The cell verifies that DNA replication is complete and checks for DNA damage. ○ Decisions Made: Proceed to M Phase: If DNA is fully replicated and intact. Repair Mechanisms Activated: If errors or damage are detected, allowing time for correction before division. Regulation of the Cell Cycle The cell cycle is tightly regulated by a complex network of proteins and signaling pathways to ensure accurate and timely progression through each phase. Intrinsic and extrinsic regulations are two fundamental mechanisms through which cells manage their activities and interactions within an organism. Intrinsic Regulation - operates internally within a cell, controlling its functions based on its own conditions. It encompasses: a- Gene Expression Regulation: Managing the production of proteins through transcription factors, RNA processing, and translation to fulfill cellular functions. - Cell Cycle Control: Utilizing internal checkpoints to monitor cell size, DNA integrity, and other critical factors to regulate the progression of the cell cycle. - Metabolic Regulation Adjusting metabolic activities in response to internal energy levels, nutrient availability, and waste accumulation to maintain cellular homeostasis. Extrinsic Regulation - on the other hand, involves external signals that influence a cell's behavior. These signals can originate from other cells, hormones, growth factors, or environmental conditions and include: - Cell Signaling Pathways: Communication between cells via signaling molecules like hormones and neurotransmitters, which bind to cell surface receptors and initiate intracellular signaling cascades to regulate various cellular activities. - Cell Adhesion: Interactions with the extracellular matrix and adjacent cells through adhesion molecules, affecting cell shape, migration, proliferation, and differentiation. - Response to Physical Stimuli: Cells' ability to respond to physical stimuli such as light, temperature, and pressure by adjusting their activities accordingly. Cyclins and Cyclin-Dependent Kinases (Cdks) These molecules serve as the primary regulators of cell cycle progression: Cyclins - Cyclins only appear at specific phases Function: ○ Activate Cdks by forming cyclin-Cdk complexes. Regulation: ○ Cyclin levels fluctuate throughout the cell cycle, synthesized and degraded at specific stages to control progression. Classes of Cyclins: ○ G1 Cyclins (Cyclin D): Regulate progression through the G1 phase and passage through the G1 checkpoint. ○ G1/S Cyclins (Cyclin E): Bind to Cdk2 at the end of G1, committing the cell to DNA replication. ○ S Cyclins (Cyclin A): Associate with Cdk2 during the S phase, necessary for initiation and completion of DNA replication. ○ M Cyclins (Cyclin B): Bind to Cdk1 before the M phase, initiating mitosis by triggering events like nuclear envelope breakdown and spindle assembly. Cyclin-Dependent Kinases (Cdks) - CDK levels are constant, Cyclins however are changing - CDK are different per phase - They are not always active, no matter how many they are - Cdks are a family of serine/threonine protein kinases that are activated by binding to cyclins. - Once activated, Cdks phosphorylate target proteins that drive the cell through the cell cycle phases. - The activity of Cdks is regulated by their association with cyclins and by phosphorylation and dephosphorylation events. - For example, Cdk1, when bound to Cyclin B, initiates the M phase (mitosis). - CDK’s are not direct regulators, instead they command enzymes through phosphates Function: ○ Enzymes that phosphorylate specific target proteins, leading to activation or deactivation of processes necessary for cell cycle progression. Regulation Mechanisms:  ○ Cyclin Binding: Activation requires binding to specific cyclins. ○ Phosphorylation: Activation: Phosphorylation by activating kinases enhances Cdk activity. Inhibition: Phosphorylation by inhibitory kinases (e.g., Wee1 kinase) or binding of Cdk inhibitor proteins can suppress Cdk activity. ○ Degradation: Ubiquitin-mediated proteolysis ensures timely degradation of cyclins, thus inactivating Cdks at appropriate times. Cell Cycle Checkpoints Checkpoints act as surveillance mechanisms that monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before progression to the next phase. Wee1 Kinase and cdc25 Phosphatase: Wee1 Kinase inhibits Cdk activity by phosphorylating Cdks at specific inhibitory sites, preventing premature entry into mitosis. cdc25 Phosphatase removes these inhibitory phosphates, activating Cdks and allowing the cell to proceed into mitosis. Rb (Retinoblastoma) Protein: The Rb protein is another tumor suppressor that controls the G1 checkpoint by regulating the activity of E2F transcription factors, which are necessary for the transition from G1 to S phase. When Rb is phosphorylated by Cdk-cyclin complexes, it releases E2F, allowing the transcription of genes required for DNA synthesis. APC/C (Anaphase-Promoting Complex/Cyclosome): APC/C is an E3 ubiquitin ligase that marks specific proteins for degradation by the proteasome, regulating the exit from mitosis and the transition to the next cell cycle. It plays a critical role in triggering the separation of sister chromatids during anaphase by degrading securin, an inhibitor of separase, which then cleaves cohesin, the protein complex holding sister chromatids together. 1. G1 Checkpoint (Restriction Point) Purpose: ○ Determines whether the cell has adequate size, energy reserves, and undamaged DNA to proceed to DNA synthesis. Key Players: ○ Retinoblastoma Protein (Rb): Inhibits transcription factors necessary for S phase entry. Phosphorylation by the Cyclin D-Cdk4/6 complex inactivates Rb, allowing progression. ○ p53 Protein: Acts as a tumor suppressor by detecting DNA damage and halting the cell cycle to allow for repair or inducing apoptosis if damage is irreparable. Outcome: ○ Proceed to S Phase: If conditions are optimal and DNA is intact. ○ Cell Cycle Arrest or Apoptosis: If DNA damage is detected and cannot be repaired. 2. DNA Replication Checkpoint (G2 Checkpoint) Purpose: ○ Ensures that all DNA has been accurately replicated without errors or damage before entering mitosis. Mechanism: ○ Detects incomplete or damaged DNA and inhibits activation of M phase-promoting factors. Key Players: ○ Cdc25 Phosphatase: Activates Cdk1 by dephosphorylation; inhibition delays entry into mitosis. Outcome: ○ Proceed to M Phase: When DNA replication is complete and accurate. ○ Delay M Phase Entry: Allows time for completion and repair of DNA replication. 3. Spindle-kinetochore Checkpoint Purpose: ○ Ensures that all chromosomes are correctly attached to the spindle apparatus before separation during anaphase. Mechanism: ○ Unattached kinetochores generate signals that inhibit the anaphase-promoting complex/cyclosome (APC/C), preventing progression.  Key Players: ○ Mad and Bub Proteins: Monitor kinetochore attachment and tension. ○ Cdc20-APC/C Complex: When activated, triggers degradation of securin, allowing separase to cleave cohesin and initiate chromatid separation. Outcome: ○ Proceed to Anaphase: When all chromosomes are properly attached. ○ Delay Anaphase: Prevents missegregation of chromosomes, reducing the risk of aneuploidy. 4. Growth Checkpoints Purpose: ○ Monitor cell size and environmental conditions to ensure cells are prepared for division. Mechanism: ○ Assess availability of nutrients, growth factors, and space. Examples: ○ In Budding Yeast: Daughter cells must reach a certain size before proceeding past G1. ○ In Animal Cells: External growth factors (e.g., hormones, cytokines) signal through pathways like MAPK to promote progression. Outcome: ○ Proceed with Cell Cycle: Under favorable growth conditions. ○ Cell Cycle Arrest or Differentiation: If conditions are not conducive to division. Role of p53 in Cell Cycle Regulation Function: ○ Acts as a guardian of the genome by preserving DNA integrity. Mechanisms: ○ DNA Damage Response: Upon DNA damage, p53 is stabilized and activates transcription of genes involved in DNA repair, cell cycle arrest (e.g., p21), and apoptosis. ○ Cell Cycle Arrest: p21, induced by p53, inhibits Cyclin E-Cdk2 and Cyclin D-Cdk4/6 complexes, halting the cell cycle to allow DNA repair. Implications in Cancer: ○ Mutations in the p53 gene disrupt these protective mechanisms, leading to uncontrolled cell proliferation and tumor development. ○ Statistics:  Over 50% of human cancers involve mutations in p53, highlighting its critical role in preventing malignancy. Negative Feedback Loops: The cell cycle is also regulated by feedback mechanisms that prevent the overactivation of cyclins and Cdks. These loops ensure that once a phase is completed, the cell cycle machinery is reset for the next cycle. For example, the degradation of cyclins by the ubiquitin-proteasome pathway is a key feedback mechanism that ensures the timely inactivation of Cdks, allowing the cell to progress through the cycle in an orderly manner. M Phase-Promoting Factor (MPF) and Exiting Mitosis M Phase-Promoting Factor (MPF) Composition: ○ A complex of Cyclin B and Cdk1. Function: ○ Triggers the cell's entry into mitosis by phosphorylating various substrates leading to: Chromosome Condensation Nuclear Envelope Breakdown Spindle Formation Regulation: ○ Activation: Accumulation and activation of Cyclin B-Cdk1 complex lead to initiation of mitosis. ○ Inactivation: Degradation of Cyclin B through ubiquitin-mediated proteolysis reduces MPF activity, allowing exit from mitosis. Role in Early Development: ○ Critical during early embryogenesis where rapid cell divisions occur with minimal interphase periods. Exiting Mitosis Mechanism: ○ Cyclin Degradation: The anaphase-promoting complex/cyclosome (APC/C), activated by Cdc20, tags Cyclin B for degradation via ubiquitination. ○ Consequences: Decrease in MPF activity leads to reversal of mitotic processes: Spindle Disassembly Reformation of Nuclear Envelopes  Chromosome Decondensation Regulatory Complexes: ○ Cdc20-APC/C Complex: Initiates anaphase by targeting securin and Cyclin B for degradation. ○ Cdh1-APC/C Complex: Maintains low Cyclin B levels post-mitosis and during G1 phase. Importance: ○ Ensures accurate completion of cell division and proper resetting of the cell cycle for the next round of division. Chromatid: Each chromosome consists of two identical halves known as sister chromatids, which are formed after DNA replication. These chromatids are joined together at a specific region called the centromere, will only bind in cell division. Centromere: This is the constricted region of the chromosome where the two sister chromatids are held together. The centromere plays a crucial role during cell division by serving as the attachment point for spindle fibers, which pull the chromatids apart. Telomeres: These are the protective caps at the ends of each chromosome. Telomeres consist of repetitive nucleotide sequences and protect the chromosome from degradation and from fusion with neighboring chromosomes. Chromatin: The DNA in a chromosome is not naked but is packaged with proteins called histones to form chromatin. Chromatin can be further classified into: Kinetochores: These are protein complexes associated with the centromere. Kinetochores are critical for chromosome movement during cell division as they serve as the attachment points for the microtubules of the spindle apparatus. P Arm (Short Arm): The shorter section of the chromosome above the centromere is called the p arm. Q Arm (Long Arm): The longer section below the centromere is known as the q arm. 1. Lamins Description: Lamins are a type of intermediate filament protein that forms a network lining the inside of the nuclear envelope. This network, known as the nuclear lamina, provides structural support to the nucleus and helps organize chromatin. Functions: ○ Nuclear Stability: Lamins help maintain the shape and integrity of the nucleus. ○ Chromatin Organization: They play a role in the spatial arrangement of chromatin, influencing gene expression. ○ Nuclear Assembly/Disassembly: During cell division, lamins are phosphorylated, leading to the disassembly of the nuclear envelope in prophase, and dephosphorylated to reassemble the nuclear envelope during telophase. 2. Cohesins Description: Cohesins are protein complexes that hold sister chromatids together after DNA replication until they are separated during mitosis or meiosis. Functions: ○ Sister Chromatid Cohesion: Cohesins form a ring-like structure that encircles the sister chromatids, keeping them together until the appropriate time for separation. ○ Chromosome Segregation: By ensuring sister chromatids stay paired, cohesins ensure that each daughter cell receives the correct number of chromosomes during cell division. ○ DNA Repair and Regulation: Cohesins also play a role in DNA repair and the regulation of gene expression by affecting chromatin structure. 3. Separase Description: Separase is a protease enzyme that plays a critical role in the separation of sister chromatids during cell division. Functions: ○ Cleavage of Cohesins: During the anaphase of mitosis, separase cleaves the cohesin proteins holding the sister chromatids together, allowing them to be pulled apart toward opposite poles of the cell. ○ Triggering Anaphase: The activation of separase is a key event that triggers the onset of anaphase, marking the start of chromatid separation. 4. Securin Description: Securin is a regulatory protein that inhibits the activity of separase until the proper time during cell division. Functions: ○ Inhibition of Separase: Securin binds to separase, preventing it from cleaving cohesins prematurely. ○ Regulation of Anaphase Onset: At the correct stage of mitosis, securin is ubiquitinated and degraded by the anaphase-promoting complex/cyclosome (APC/C), releasing separase to initiate chromatid separation. ○ Protection Against Premature Chromatid Separation: By inhibiting separase until all chromosomes are correctly attached to the spindle apparatus, securin ensures accurate chromosome segregation. 5. Anaphase-Promoting Complex/Cyclosome (APC/C) Description: The APC/C is a multi-subunit E3 ubiquitin ligase that regulates the progression of the cell cycle by targeting specific proteins for degradation. Functions: ○ Degradation of Securin: APC/C marks securin for destruction, allowing separase to become active and initiate anaphase. ○ Regulation of Cyclins: APC/C also targets cyclins, particularly cyclin B, for degradation, helping to control the exit from mitosis. ○ Mitotic Exit: By degrading key proteins, APC/C helps the cell transition from mitosis to the next interphase. 6. Condensins Description: Condensins are protein complexes that play a crucial role in chromosome condensation during mitosis and meiosis. Functions: ○ Chromosome Condensation: Condensins help compact chromatin into the tightly packed, visible chromosomes during cell division. ○ Structural Maintenance of Chromosomes: They contribute to the structural organization and stabilization of chromosomes, ensuring they are correctly segregated during cell division. ○ Topological Changes in DNA: Condensins introduce supercoils into DNA, aiding in the structural changes necessary for mitosis. 7. Kinetochore Description: The kinetochore is a protein complex that assembles on the centromere of a chromosome during cell division. Functions: ○ Attachment Point for Spindle Fibers: The kinetochore is the site where spindle fibers attach to pull chromosomes apart.  ○ Chromosome Movement: It plays a critical role in the movement of chromosomes during mitosis and meiosis, ensuring they are evenly distributed to the daughter cells. ○ Checkpoint Control: The kinetochore is involved in the spindle assembly checkpoint, which ensures that chromosomes are correctly attached to the spindle before anaphase begins. Cell division: Mitosis and Meiosis Mitosis Prophase Early Prophase: ○ Centrioles begin to move apart, which is crucial as they organize the microtubules that form the spindle apparatus. ○ Chromosomes appear as long, thin threads, indicating the beginning of chromatin condensation into visible chromosomes. ○ The nucleolus becomes less distinct, signifying the halting of ribosome production as the cell prepares for division. ○ Aster begin to move ○ Centrioles move to opposite poles ○ Only chromatids are visible ○ Nucleolus begin to disappear Middle Prophase: ○ Centrioles continue moving farther apart, eventually reaching opposite poles of the cell.  ○ Aster formation begins, with microtubules radiating out from each centriole, forming a star-like structure. ○ Twin chromatids become visible, connected by a centromere, representing duplicated DNA that will be separated during mitosis. ○ Condensins become active 2. Prometaphase Late Prophase/Prometaphase: ○ Centrioles almost reach opposite sides of the nucleus, establishing the two poles of the spindle. ○ The spindle begins to assemble, and kinetochore microtubules extend from the centromeres toward the spindle poles, attaching to the chromosomes at the kinetochores. ○ The nuclear membrane starts to disintegrate, allowing the spindle fibers to interact with chromosomes. ○ The nucleolus disappears, marking the end of the nucleus as an intact structure until after mitosis. ○ Lamins become active Aster - An aster is a star-shaped structure formed around centrosomes in animal cells during cell division. It consists of microtubules that organize the spindle apparatus, ensuring chromosomes are correctly aligned and distributed to daughter cells during mitosis and meiosis. 3. Metaphase - the Shortest face The nuclear membrane has completely disappeared, allowing the chromosomes to be fully accessible to the spindle fibers. Kinetochore-spindle moves each twin-chromatid chromosome to the midline (metaphase plate) of the cell, ensuring that each daughter cell will receive one copy of each chromosome. Other spindle microtubules interact with those from the opposite pole, stabilizing the spindle apparatus. 4. Anaphase Early Anaphase: ○ The centromeres split, and the sister chromatids (now individual chromosomes) are pulled toward opposite poles of the cell by the shortening of kinetochore microtubules. Late Anaphase: ○ The two sets of chromosomes approach their respective poles, and cytokinesis begins, marking the start of the physical separation of the cell’s cytoplasm. ○ Cytokinesis may begin here or at telophase 5. Telophase New nuclear membranes begin to form around each set of chromosomes, re-establishing the nucleus in each daughter cell. Chromosomes start to de-condense, becoming longer and thinner, and less distinct under a microscope. The nucleolus reappears in each new nucleus, indicating the resumption of ribosomal RNA synthesis. Cytokinesis progresses, completing the formation of two genetically identical daughter cells. 6. Cytokinesis Differences in Animal and Plant Cells Animal Cells: ○ A cleavage furrow forms at the end of anaphase, initiated by a contractile ring of actin and myosin filaments that slowly constricts, dividing the cell. ○ The constriction eventually leads to the complete separation of the two daughter cells. Plant Cells: ○ Due to the rigid cell wall, plant cells cannot undergo cleavage furrowing. ○ Instead, the Golgi apparatus releases vesicles that form a cell plate at the center of the dividing cell. ○ This plate grows outward until it fuses with the cell membrane, forming two separate cells, each with its own new cell wall strengthened by cellulose fibers. Meiosis Meiosis Meiosis Overview: ○ Meiosis is the process by which gametes (sperm and egg cells) are formed, reducing the chromosome number by half (from diploid 2n to haploid n). ○ This ensures that when fertilization occurs, the resulting zygote has the correct diploid number of chromosomes. ○ Meiosis involves two rounds of cell division: Meiosis I and Meiosis II. ○ Locus/loci - where genes are located `Gamete Formation Gametes: ○ These are the sex cells that carry half the genetic information of an organism. ○ Meiosis occurs in the gonads (testes in males, ovaries in females). ○ The process is called spermatogenesis in males (produces sperm) and oogenesis in females (produces eggs). ○ Gametogenesis is the production of Gametes Interphase I Interphase I: ○ Chromosomes replicate during the S phase, leading to duplicated chromosomes consisting of two sister chromatids. ○ The centriole pairs also replicate, setting the stage for spindle formation in meiosis. ○ The nucleus and nucleolus remain visible, indicating that the cell is still in a preparatory phase before actual division begins. Meiosis I Meiosis I Overview: ○ This division reduces the chromosome number by half. ○ Four main stages: Prophase I, Metaphase I, Anaphase I, and Telophase I. Prophase I Prophase I: ○ This is the longest and most complex phase. ○ Chromosomes condense and homologous chromosomes pair up in a process called synapsis, forming tetrads (groups of four chromatids). ○ Genetic recombination occurs here through crossing over between non-sister chromatids, which is crucial for genetic diversity. Homologous Chromosomes Homologous Chromosomes: ○ These are chromosome pairs, one from each parent, that are similar in shape, size, and genetic content but are not the same. ○ Homologs carry genes controlling the same traits at identical loci (locations on the chromosome). Crossing Over Crossing Over: ○ During Prophase I, homologous chromosomes exchange genetic material at sites called chiasmata, leading to genetic recombination. ○ This process increases genetic variability among offspring. ○ Tetrads are formed to do this process ○ Should not happen between 2 sister chromatids as they are not the same ○ The moment sister chromatids touch they exchange genes Sex Chromosomes Sex Chromosomes: ○ Humans have one pair of sex chromosomes (XX in females, XY in males) that determine an individual's sex. ○ First 22 chromosomes are called autosomes and the last is a sex chromosome Prophase I (continued) Prophase I : ○ The nucleus and nucleolus disappear. ○ Spindle fibers form, and chromosomes continue to condense. ○ Tetrads form and crossing over occurs, which is essential for mixing genetic material between homologous chromosomes. Metaphase I Metaphase I: ○ The shortest phase of meiosis I. ○ Tetrads align at the cell's equator, and independent assortment occurs, where chromosomes separate randomly, contributing to genetic diversity. ○ There are 8 million possible combinations Independent Assortment Independent Assortment: ○ This is a process during Metaphase I where the random alignment of homologous chromosomes leads to different combinations of maternal and paternal chromosomes in the gametes. ○ In humans, this can result in approximately 8 million different combinations of chromosomes. Anaphase I Anaphase I: ○ Homologous chromosomes are separated and pulled to opposite poles of the cell, while sister chromatids remain attached. Telophase I Telophase I: ○ The cell divides into two haploid cells, each with half the number of chromosomes.  ○ Cytokinesis occurs, finalizing the division into two daughter cells. Meiosis II Meiosis II Overview: ○ This division is similar to mitosis, where sister chromatids are separated. ○ Meiosis II leads to the formation of four haploid cells. ○ Meiosis II has no dna replication Stages of Meiosis II Prophase II: ○ Similar to Prophase in mitosis; chromosomes condense, and the spindle apparatus forms. Metaphase II: ○ Chromosomes line up at the equator, but unlike Metaphase I, they are not homologs. Anaphase II: ○ Sister chromatids are finally separated and move to opposite poles. Telophase II: ○ Nuclei reform around the separated chromatids, resulting in four genetically distinct haploid cells, which become gametes. Genetic Variation Genetic Recombination: ○ The result of crossing over, independent assortment, and random fertilization, which increases the genetic diversity within a population. ○ This variation is essential for natural selection and evolutionary processes. Spermatogenesis and Oogenesis Spermatogenesis: ○ The process of sperm cell formation through meiosis, resulting in four sperm cells from one diploid cell. Oogenesis: ○ The process of egg cell formation, typically resulting in one viable egg and polar bodies, which are non-functional and eventually degenerate.  ○ There is only a single egg because of the unequal division of the cytoplasm Karyotype Karyotype: ○ A visual representation of an individual's chromosomes arranged in pairs, used to identify chromosomal abnormalities and determine the sex of an individual. ○ For example, Down syndrome is identified by an extra copy of chromosome 21. Fertilization Fertilization: ○ The fusion of a haploid sperm and egg cell to form a diploid zygote, restoring the chromosome number to 46 in humans. Difference of Meiosis 1 and 2 1. Homologous Chromosomes vs. Sister Chromatids Meiosis I: Homologous chromosomes (each consisting of two sister chromatids) are separated. This is known as a reductional division because the chromosome number is halved. Meiosis II: Sister chromatids are separated. This is similar to mitosis and is called a reduction division because the chromosome number remains the same in order to maintain 46 chromosomes. 2. Chromosome Number Meiosis I: Reduces the chromosome number from diploid (2n) to haploid (n). Meiosis II: The chromosome number remains haploid (n) because sister chromatids separate, not homologous chromosomes. 3. Crossing Over Meiosis I: Crossing over occurs during prophase I, where homologous chromosomes exchange genetic material, leading to genetic diversity. Meiosis II: No crossing over occurs because the homologous chromosomes have already been separated. 4. Alignment at Metaphase Meiosis I: Homologous chromosomes align at the metaphase plate in pairs. Meiosis II: Individual chromosomes (consisting of sister chromatids) align at the metaphase plate. 5. Types of Cells Produced Meiosis I: Produces two haploid cells, each with duplicated chromosomes. Meiosis II: Produces four haploid cells, each with a single set of chromosomes. 6. Duration and Complexity Meiosis I: Generally takes longer because it involves crossing over and homologous chromosome separation. Meiosis II: Usually shorter and simpler, resembling mitotic division. Summary: Meiosis I: Reductional division, homologous chromosomes separate, crossing over occurs. Meiosis II: Equational division, sister chromatids separate, no crossing over.