Meiosis Lecture Notes PDF

1.Compare and contrast mitosis and meiosis ◦Summarize the steps of mitosis and meiosis Mitosis: Purpose: Growth, repair, and asexual reproduction. Outcome: Produces two genetically identical daughter cells with the same number of chromosomes as the parent cell (diploid). Cell T...

1.Compare and contrast mitosis and meiosis ◦Summarize the steps of mitosis and meiosis Mitosis: Purpose: Growth, repair, and asexual reproduction. Outcome: Produces two genetically identical daughter cells with the same number of chromosomes as the parent cell (diploid). Cell Type: Somatic (body) cells. Number of Divisions: One. Meiosis: Purpose: introduce genetic variation into a population through Sexual reproduction. Outcome: Produces four genetically distinct daughter cells with half the number of chromosomes as the parent cell (haploid). Cell Type: Germ cells (sperm and eggs). Number of Divisions: Two. ◦List the sequential phase of meiosis and describe the major events of each phase Sequential Phases of Meiosis and Major Events Meiosis I: 1. Prophase I: ○ Homologous chromosomes pair up and exchange genetic material (crossing over). ○ The nuclear envelope breaks down, and the spindle forms. 2. Metaphase I: ○ Homologous chromosomes align at the metaphase plate. ○ Spindle fibers attach to the chromosomes. ○ Seahorse will NOT be activated to separate sister chromatids 3. Anaphase I: ○ Homologous chromosomes (not sister chromatids) are pulled to opposite poles. 4. Telophase I: ○ Chromosomes arrive at the poles, chromosomes decondense. ○ Cytokinesis follows, resulting in two haploid daughter cells. Meiosis II: 1. Prophase II: ○ Chromosomes condense again in each haploid daughter cell. ○ The spindle reforms. 2. Metaphase II:  ○ Chromosomes align at the metaphase plate. ○ Spindle fibers attach to the centromeres. 3. Anaphase II: ○ Sister chromatids are pulled apart to opposite poles. 4. Telophase II: ○ Chromatids reach the poles and begin to decondense. ○ The nuclear envelope reforms, and cytokinesis follows. 5. Outcome: Four genetically distinct haploid daughter cells are formed. Subphases of Prophase I in Meiosis Prophase I is a critical stage in meiosis where homologous chromosomes pair up and exchange genetic material. It is divided into five subphases: 1. Leptotene: ○ Chromosomes begin to condense and become visible under a microscope. Each chromosome consists of two sister chromatids, but they are still closely associated with each other, appearing as thin threads. 2. Zygotene: ○ Homologous chromosomes begin to pair up in a process called synapsis. This pairing is facilitated by the formation of a protein structure known as the synaptonemal complex. The chromosomes align along their entire lengths, forming pairs called tetrads. 3. Pachytene: ○ Chromosomes condense further, and crossing over occurs. During crossing over, non-sister chromatids of homologous chromosomes exchange genetic material at specific sites called chiasmata. This genetic recombination increases genetic diversity. 4. Diplotene: ○ The synaptonemal complex begins to disassemble, and homologous chromosomes start to separate slightly. However, they remain connected at the chiasmata, where crossing over occurred. The chromosomes become more visible, and the chiasmata can be seen as points where the homologous chromosomes are still attached. 5. Diakinesis: ○ Chromosomes condense to their maximum level, becoming shorter and thicker. The nuclear envelope begins to break down, and the spindle apparatus starts to form. The chiasmata move towards the ends of the chromosomes (terminalization), preparing the chromosomes for segregation. Prophase I is crucial for ensuring genetic diversity through recombination and for preparing homologous chromosomes for accurate segregation during meiosis I. ◦List and explain the differences between meiosis and mitosis Differences Between Meiosis and Mitosis 1. Number of Divisions: ○ Mitosis involves one division ○ Meiosis involves two.  2. Number of Daughter Cells: ○ Mitosis produces two daughter cells ○ Meiosis produces four 3. Genetic Variation: ○ Mitosis is poor at introducing genetic variation (2 identical daughter cells) ○ Meiosis results in genetically diverse cells due to crossing over and independent assortment. 4. Chromosome Number: ○ Mitosis maintains the chromosome number (diploid) ○ Meiosis reduces it by half (haploid) 5. Purpose: ○ Mitosis is for growth, repair, and asexual reproduction ○ Meiosis is for sexual reproduction, drives evolution 6. Occurrence of Synapsis: ○ Synapsis (pairing of homologous chromosomes) and crossing over occur only in meiosis ○ Not in mitosis. 7. Type of Cells Involved: ○ Mitosis occurs in somatic cells ○ Meiosis occurs in germ cells. Similarities Between Meiosis and Mitosis 1. Beginning Stage a. Meiosis I begins the same way Mitosis Begins i. Cell passes through S phase and thus contains duplicated genetic material ◦Identify the chromosome content expected following conclusion of mitosis and meiosis Chromosome Content Following Mitosis and Meiosis Mitosis: ○ Chromosome Content: After mitosis, each daughter cell has the same number of chromosomes as the original parent cell, which is diploid (2n). For humans, this means each daughter cell has 46 chromosomes (23 pairs). Meiosis: ○ Chromosome Content: After meiosis, each daughter cell has half the number of chromosomes as the original parent cell, which is haploid (n). For humans, this means each gamete (sperm or egg) has 23 chromosomes. ◦Summarize the processes involved that generate chromosome content changes in mitosis vs meiosis (this will be revisited in the next block) Processes Involved in Chromosome Content Changes Mitosis: ○ Process Overview: Mitosis involves one round of cell division following DNA replication, ensuring that each daughter cell receives an identical set of chromosomes. ○ Key Steps Influencing Chromosome Content: Replication: During the S phase of interphase, each chromosome is duplicated, resulting in two sister chromatids. Separation: During anaphase, sister chromatids are separated and pulled to opposite poles of the cell, ensuring each daughter cell receives a full set of chromosomes. Meiosis: ○ Process Overview: Meiosis involves two rounds of cell division (meiosis I and meiosis II) following a single round of DNA replication, reducing the chromosome number by half. ○ Key Steps Influencing Chromosome Content: Replication: Similar to mitosis, DNA replication occurs during the S phase, resulting in chromosomes composed of two sister chromatids. Reductional Division (Meiosis I): Homologous chromosomes are separated during anaphase I, reducing the chromosome number by half (from 2n to n). Equational Division (Meiosis II): Similar to mitosis, sister chromatids are separated during anaphase II, but since meiosis I already reduced the chromosome number, the resulting cells remain haploid. 2.Explain Interkinesis and how it differs from the final steps of meiosis or mitosis ​Interkinesis: A Detailed Explanation Interkinesis is an intermediate phase that occurs between the two meiotic divisions—meiosis I and meiosis II. Unlike interphase in mitosis, interkinesis is typically shorter and lacks DNA replication. Its primary function is to prepare the cell for the second round of meiotic division. Key Features of Interkinesis: 1. Absence of DNA Replication: ○ During interkinesis, the cell does not undergo DNA synthesis (S phase). This is crucial because the chromosomes have already been  replicated before meiosis I, and further replication would prevent the reduction of chromosome number that is characteristic of meiosis. 2. Nuclear Envelope Reformation: ○ In some species, a partial or complete reformation of the nuclear envelope occurs around the chromosomes during interkinesis. This reformation is usually temporary and may be absent in some organisms. 3. Chromosome Decondensation: ○ Chromosomes may partially decondense during interkinesis, though they typically remain more condensed than they would be during interphase of mitosis. 4. Preparation for Meiosis II: ○ The cell prepares for meiosis II, which resembles a mitotic division but starts with haploid cells. Cell membrane splits cell into two, and meiotic spindle breaks down so that a new spindle can form in preparation for the second division. How Interkinesis Differs from the Final Steps of Mitosis or Meiosis 1. Comparison with Cytokinesis (Final Step of Mitosis): ○ Cytokinesis i. is the process of cytoplasmic division that occurs at the end of mitosis, resulting in two genetically identical daughter cells, each with a full set of chromosomes (diploid, 2n). ○ Interkinesis i. in contrast, does not involve cytokinesis leading to daughter cells with half the chromosome number (haploid, n). Instead, it is a pause between two nuclear divisions in meiosis, without significant changes to the chromosome number or cytoplasm. 2. Comparison with Telophase II and Cytokinesis (Final Step of Meiosis): ○ At the end of meiosis II, i. telophase II is followed by cytokinesis, resulting in four genetically distinct haploid daughter cells, each with half the chromosome number of the original diploid cell. ○ Interkinesis occurs after meiosis I and before meiosis II, i. Unlike the final steps of meiosis II, it does not result in the formation of new cells. Instead, it is a brief interlude that separates the two meiotic divisions without altering the chromosome number. 3. Chromosome State: ○ After mitosis, chromosomes return to a decondensed state as the cell enters interphase, ready for the next cell cycle. ○ In contrast, during interkinesis, chromosomes may only partially decondense, retaining a state of readiness for the next division. 4. Cellular Division Process: ○ In mitosis, cytokinesis is a definitive step that divides the cell into two. ○ In meiosis, interkinesis is not a division step but a preparatory phase that precedes the second meiotic division. Biological Significance of Interkinesis Interkinesis is significant because it allows the cell to reorganize and prepare for the second meiotic division without undergoing DNA replication. This is critical for maintaining the reduction in chromosome number, which is the hallmark of meiosis. The phase ensures that the genetic material is properly prepared and aligned for the subsequent division, which will produce haploid gametes ready for fertilization. 3.Assess consequences for errors in major events of each phase of mitosis and meiosis ◦Determine the stage at which a cell would stall based on the error described Consequences of Errors in Mitosis and Meiosis 2. Errors in Meiosis a. Prophase I Errors: Error Type: Failure in homologous chromosome pairing or crossing over. Consequence: Non-disjunction, leading to gametes with abnormal chromosome numbers, which can cause conditions like Down syndrome (trisomy 21) or miscarriages. Stalling Point: The cell might stall during pachytene of prophase I if the synaptonemal complex formation or crossing over is defective, as these are crucial for ensuring proper homologous recombination and segregation. b. Metaphase I Errors: Error Type: Incorrect alignment of homologous chromosome pairs at the metaphase plate. Consequence: Non-disjunction, resulting in gametes with an extra chromosome or a missing chromosome. Stalling Point: The cell would stall at the metaphase I checkpoint if bivalents are not properly aligned or if spindle fibers do not correctly attach to kinetochores. c. Anaphase I Errors: Error Type: Failure to separate homologous chromosomes. Consequence: Gametes with duplicated or missing chromosomes, leading to aneuploid offspring. Stalling Point: If the spindle apparatus fails to pull homologs apart, the cell may either stall indefinitely, triggering apoptosis, or incorrectly proceed, leading to gametes with abnormal chromosome numbers. d. Telophase I/Interkinesis Errors: Error Type: Failure in nuclear envelope reformation or cytokinesis, resulting in cells with improper chromosomal content. Consequence: Cells entering meiosis II with incorrect chromosome numbers, leading to defective gametes. Stalling Point: The cell might proceed with meiosis II despite errors, but the resulting gametes will be nonviable or lead to developmental defects. e. Prophase II to Telophase II Errors: Error Type: Similar to those in mitosis—errors in spindle formation, chromatid separation, or cytokinesis. Consequence: Production of gametes with missing or extra chromatids, leading to genetic disorders or nonviable zygotes. Stalling Point: Similar checkpoints as in mitosis, where errors in chromatid separation during metaphase II or anaphase II would cause stalling or result in defective gametes. Summary of Potential Stalling Points: 1. G2/M Checkpoint: Errors in chromosome condensation or spindle formation (Prophase of Mitosis/Meiosis I). 2. Metaphase Checkpoint: Errors in chromosome alignment or spindle attachment (Metaphase of Mitosis, Metaphase I, and Metaphase II). 3. Post-mitotic/meiotic Checkpoints: Errors after anaphase leading to defective cytokinesis or chromosomal content. Conclusion Errors during the different phases of mitosis and meiosis can lead to severe consequences, including aneuploidy, genetic disorders, and cancer. The cell has evolved various checkpoints to detect and correct these errors, stalling the cell cycle at critical points to ensure proper division. If these errors are not corrected, they can lead to cell death or the propagation of mutations that contribute to disease. 4.Explain the process of oogenesis and assess the connections with secondary sexual development ◦List the steps involved in oogenesis including the “pauses” in meiosis Oogenesis: The Process and Its Connection with Secondary Sexual Development Oogenesis is the process by which female gametes, or ova (eggs), are produced in the ovaries. This process is complex and involves several stages, with specific pauses during meiosis that are critical for proper development. Oogenesis is intricately connected to secondary sexual development, which occurs during puberty. Steps Involved in Oogenesis 1. Fetal Development (Before Birth): ○ Primordial Germ Cells: These cells migrate to the developing ovaries during early fetal development. ○ Oogonia Formation: Primordial germ cells differentiate into oogonia, which are diploid cells (2n) that undergo mitotic divisions to increase in number. ○ Primary Oocytes Formation: Oogonia begin meiosis I but halt at prophase I. At this stage, they are referred to as primary oocytes. Each primary oocyte is surrounded by a layer of granulosa cells, forming a primordial follicle. ○ First Pause: The primary oocytes remain arrested in prophase I, specifically in the diplotene stage, until puberty. This arrest can last for many years, from fetal life until the female reaches reproductive maturity. 2. Puberty to Menopause: ○ Completion of Meiosis I: Just before ovulation, one primary oocyte completes meiosis I to form a secondary oocyte (haploid, n) and a polar body. The polar body typically degenerates. ○ Second Pause: The secondary oocyte immediately enters meiosis II but halts at metaphase II. This second arrest occurs until fertilization. ○ Ovulation: The secondary oocyte is released from the ovary during ovulation and enters the fallopian tube. ○ Fertilization: If the secondary oocyte is fertilized by a sperm, meiosis II is completed, resulting in the formation of a mature ovum and a second polar body. The ovum then fuses with the sperm nucleus, forming a diploid zygote. Pauses in Meiosis During Oogenesis 1. First Pause: Prophase I (Diplotene stage) of meiosis I during fetal development. Primary oocytes remain arrested here until puberty.  2. Second Pause: Metaphase II of meiosis II after ovulation. The secondary oocyte remains arrested here until fertilization. Connection with Secondary Sexual Development Secondary sexual development refers to the physical changes that occur during puberty, driven by hormonal changes, particularly the increase in estrogen levels in females. This development is closely linked to the stages of oogenesis. Onset of Puberty: ○ At the onset of puberty, the hypothalamus begins to release gonadotropin-releasing hormone (GnRH), which stimulates the pituitary gland to secrete FSH and luteinizing hormone (LH). ○ FSH stimulates the growth and maturation of the ovarian follicles, initiating the resumption of oogenesis from the first pause in prophase I. ○ As the follicles mature, they produce increasing amounts of estrogen, which is crucial for the development of secondary sexual characteristics, including breast development, widening of the hips, and the onset of the menstrual cycle. Follicular Phase and Secondary Sexual Characteristics: ○ The maturation of follicles and the production of estrogen during each menstrual cycle support the maintenance and further development of secondary sexual characteristics. ○ The cyclical nature of oogenesis and follicular development continues throughout the reproductive years, with each menstrual cycle marking the potential for ovulation and fertilization. ◦Identify the gametogenesis stages that correlate with secondary sexual development in humans Gametogenesis Stages Correlated with Secondary Sexual Development Fetal Development: The formation of primary oocytes occurs, but no secondary sexual development is observed at this stage. Puberty: The re-initiation of oogenesis, with the first completion of meiosis I, directly correlates with the onset of secondary sexual development. The production of estrogen by the maturing follicles triggers the development of secondary sexual characteristics. Reproductive Years: The continuous cycle of follicular development, ovulation, and possible fertilization (completion of meiosis II) occurs in  tandem with the maintenance of secondary sexual characteristics driven by hormonal fluctuations. ◦Determine the expected outcomes for errors or changes in gametogenesis based on meiotic stages 1. Errors During Meiosis I Stage Involved: Prophase I, Metaphase I, Anaphase I, or Telophase I. Types of Errors: ○ Non-disjunction: Failure of homologous chromosomes to separate during Anaphase I. ○ Crossing Over Errors: Abnormalities in the recombination process during Prophase I, such as unequal crossing over or failure to crossover. Expected Outcomes: ○ Aneuploidy: This can result in gametes with an abnormal number of chromosomes (e.g., n+1 or n-1). If fertilization occurs, this can lead to disorders such as: Trisomy: The presence of an extra chromosome in the zygote. For example, Trisomy 21 causes Down syndrome. Monosomy: The absence of a chromosome in the zygote, which is often lethal. An example is Turner syndrome (45,X), where an individual has only one X chromosome. ○ Unbalanced Gametes: If crossing over errors occur, the resulting gametes may contain duplicated or deleted segments of chromosomes, potentially leading to genetic disorders or nonviable embryos. ○ Miscarriage or Infertility: Severe aneuploidies or structural abnormalities typically result in spontaneous abortion or infertility. 2. Errors During Meiosis II Stage Involved: Metaphase II, Anaphase II, or Telophase II. Types of Errors: ○ Non-disjunction: Failure of sister chromatids to separate during Anaphase II. ○ Spindle Attachment Errors: Improper attachment of spindle fibers to chromatids. Expected Outcomes:  ○ Aneuploidy in Gametes: Similar to Meiosis I errors, but the error affects the separation of sister chromatids rather than homologous chromosomes. This leads to: Trisomy or Monosomy: As with errors in Meiosis I, fertilization of these abnormal gametes can result in trisomy or monosomy conditions, depending on which chromosome is involved. ○ Genetic Mosaicism: If an error occurs during meiosis II in an early embryonic cell, some cells will be normal, and others will have an abnormal number of chromosomes, leading to a mosaic pattern. This can result in a less severe form of a genetic disorder or milder clinical symptoms. 3. Errors During Oogenesis vs. Spermatogenesis Oogenesis: The long pauses in meiosis (e.g., arrest in prophase I) increase the risk of errors, particularly non-disjunction, as the primary oocytes age. This is why the risk of chromosomal abnormalities such as Down syndrome increases with maternal age. Spermatogenesis: Errors are generally less common due to continuous sperm production throughout a male’s life. However, errors in meiosis II can still occur, leading to similar outcomes as described above. 4. Spindle Assembly and Checkpoint Errors Spindle Assembly Checkpoint Failure: If the spindle assembly checkpoint fails to detect improper alignment of chromosomes or spindle attachment, the cell may proceed with meiosis, resulting in aneuploid gametes. Outcome: The fertilized zygote may have severe chromosomal abnormalities, leading to developmental disorders, miscarriage, or conditions like polyploidy (where the zygote has more than two sets of chromosomes). 5. Errors During Crossing Over (Prophase I) Error Type: Incomplete or abnormal recombination. Expected Outcomes: ○ Reduced Genetic Diversity: Insufficient recombination can lead to reduced genetic variation in gametes, potentially reducing the adaptability of the species. ○ Chromosomal Rearrangements: Abnormal crossing over can result in structural chromosomal aberrations such as translocations,  inversions, or duplications, leading to various genetic disorders or nonviable gametes. Summary of Outcomes Aneuploidy: Resulting from errors in chromosome separation during meiosis I or II, leading to conditions like trisomy or monosomy. Chromosomal Abnormalities: Due to faulty recombination or spindle attachment, resulting in structural changes that can cause genetic disorders or developmental abnormalities. Miscarriage or Infertility: Often the result of severe chromosomal abnormalities that lead to nonviable embryos. Genetic Disorders: Errors that allow abnormal gametes to contribute to a zygote can result in conditions such as Down syndrome, Turner syndrome, or Klinefelter syndrome. 5.Summarize the early stages of embryogenesis and identify the expected differences in cellular developmental potential associated with each stage (this will be revisited in the signaling lecture) ◦Define: totipotent, pluripotent, growth, development, differentiation, determination, specialization, fertilization, and organogenesis Early Stages of Embryogenesis and Cellular Developmental Potential Embryogenesis refers to the process by which a fertilized egg develops into a fully formed embryo. The early stages of this process are critical, as they establish the foundation for all future development. Each stage of embryogenesis is characterized by differences in the developmental potential of cells, which influences their ability to give rise to various cell types and tissues. Stages of Early Embryogenesis 1. Fertilization: ○ Definition: The process by which a sperm cell fuses with an oocyte (egg) to form a zygote, the first cell of a new organism. ○ Developmental Potential: The zygote is totipotent, meaning it has the potential to develop into any cell type, including the extraembryonic tissues (e.g., placenta) and the embryo itself. 2. Cleavage:  ○ Description: The zygote undergoes rapid mitotic divisions without growth, leading to an increase in cell number without an increase in overall size. ○ Developmental Potential: The cells, called blastomeres, remain totipotent during the early stages of cleavage. 3. Morula: ○ Description: By the 16-32 cell stage, the embryo is called a morula. The cells begin to compact, forming a solid ball. ○ Developmental Potential: The cells in the morula are still totipotent, but as they start to differentiate into an inner and outer cell mass, their potency begins to shift. 4. Blastocyst Formation: ○ Description: The morula develops into a blastocyst, characterized by the formation of a fluid-filled cavity (blastocoel) and differentiation into two distinct cell types: the trophoblast (outer cell layer) and the inner cell mass (ICM). ○ Developmental Potential: The trophoblast cells become specialized in forming the placenta and supporting tissues, losing their totipotency. The inner cell mass becomes pluripotent, meaning it can give rise to any cell type in the body but cannot form extraembryonic tissues like the placenta. 5. Gastrulation: ○ Description: The blastocyst undergoes gastrulation, a process that forms the three primary germ layers: ectoderm, mesoderm, and endoderm. ○ Developmental Potential: Cells become more specialized as they differentiate into specific germ layers, further restricting their developmental potential. 6. Organogenesis: ○ Description: The germ layers differentiate into specific tissues and organs. ○ Developmental Potential: Cells become determined and specialized to perform specific functions within the body. Definitions of Key Terms Totipotent: The ability of a cell to develop into any cell type, including both embryonic and extraembryonic tissues. Example: the zygote and early blastomeres. Pluripotent: The ability of a cell to develop into any cell type within the body but not into extraembryonic tissues. Example: cells of the inner cell mass of the blastocyst. Growth: The increase in size and number of cells, contributing to the overall size of the organism. Development: The process by which an organism grows and develops, involving cell division, differentiation, and morphogenesis. Differentiation: The process by which unspecialized cells become specialized in structure and function. Determination: The commitment of a cell to a specific fate, often occurring before differentiation.  Specialization: The final stage of differentiation, where a cell adopts a specific function within the organism. Fertilization: The union of a sperm and an oocyte, leading to the formation of a zygote. Organogenesis: The process by which the three germ layers develop into the internal organs of the organism. ◦Explain the differences in the morula and blastocyst in terms of potency and roles for cells during development Differences Between Morula and Blastocyst in Terms of Potency and Roles Morula: ○ Potency: Cells in the morula are totipotent, meaning they have the potential to develop into any cell type, including both the embryo and extraembryonic structures. ○ Roles: The morula serves as a transitional stage between the zygote and the blastocyst. Its main function is to increase the number of cells and initiate the first steps of differentiation. Blastocyst: ○ Potency: The blastocyst contains two cell types with different potencies: Trophoblast (outer cell layer): These cells have lost totipotency and are specialized to form extraembryonic tissues, such as the placenta. Inner Cell Mass (ICM): These cells are pluripotent, meaning they can give rise to any cell type in the body but cannot form extraembryonic tissues. ○ Roles: The blastocyst is crucial for implantation into the uterine wall and for the establishment of the embryo proper. The trophoblast initiates the formation of the placenta, while the ICM gives rise to the entire embryo How does meiosis prophase I differ from meiosis prophase II? (LO1) - Chromosomes condense and align differently in meiosis I (multiple stages that are designed to help ensure homologous chromosomes align and exchange information) How does meiosis prophase I differ from mitotic prophase? (LO1) - How does interkinesis during meiosis differ from cytokinesis of mitosis or the end of meiosis? (LO2) - Cytokinesis is a complete conclusion of mitosis At which stage of early embryogenesis would you expect to see differential gene expression and differences in downstream potential between 2 cells in the absence of mutation? (LO5) - AFTER asymmetrical cell divisions enabling different responses OR different level of exposures to morphogens/factors - Essentially, we wouldn't expect to see major differences until the cells themselves begin to change or are exposed to differences in external stimuli (once specialization occurs, blastocyst)