_Meiosis.pptx
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This document provides a detailed overview of meiosis, highlighting its two main phases (meiosis I and II) and various stages such as prophase, metaphase, anaphase, and telophase. It explains key processes such as genetic recombination, the formation of chiasmata, and how meiosis differs across organisms like animals, plants, and fungi. Additionally, it covers the role of enzymes in DNA repair and how various meiotic processes result in haploid gametes necessary for sexual reproduction.
_Meiosis.pptx
- 1. The cell division in Plants and animals 2. Meiosis
- 2. Meiosis- Introduction ● Meiosis a reductional cell division which ensures production of a haploid phase in the life cycle, and fertilization ensures a diploid phase. ● Meiosis take place in two phases-Meiosis I and Meiosis II ● During meiosis, the four chromatids of a pair of replicated homologous chromosomes are distributed among four daughter nuclei. ● Meiosis accomplishes this feat by incorporating two sequential divisions without an intervening round of DNA replication. ● https://youtu.be/kQu6Yfrr6j0
- 3. Stages of Meiosis Meiosis I Meiosis II
- 4. Meiosis I- Prophase I-Leptotene 1. Leptotene: During this stage the chromosomes become compacted and visible in the light microscope. In the electron microscope, however, the chromosomes are revealed to be composed of paired chromatids.
- 5. 2. Zygotene: It is marked by the visible association of homologues with one another. This process of chromosome pairing is called synapsis. chromosome synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. Synaptonemal complex (SC) is a ladder‐like structure with transverse protein filaments connecting the two lateral elements. SC functions primarily as a scaffold to allow interacting chromatids to complete their crossover activities. Meiosis I- Prophase I-Zygotene
- 7. Meiosis I- Prophase I-Pachytene 3.Pachytene: ● During pachytene, the homologues are held closely together along their length by the SC.The major event occur during pachytene is recombination. ● The DNA of sister chromatids is extended into parallel loops. ● A number of electron‐dense bodies about 100 nm in diameter are seen within the center of the SC. These structures have been named recombination nodules because they correspond to the sites where crossing‐over is taking place. ● Recombination nodules contain the enzymatic machinery that facilitates genetic recombination, which is completed by the end of pachytene.
- 8. The synaptonemal complex. ( a ) Electron micrograph of a human pachytene bivalent showing a pair of homologous chromosomes held in a tightly ordered parallel array. K, kinetochore. ( b ) Schematic diagram of the synaptonemal complex and its associated chromosomal fibers. The dense granules (recombination nodules) seen in the center of the SC (indicated by the arrowhead in part a ) contain the enzymatic machinery required to complete genetic recombination, which is thought to begin at a much earlier stage in prophase I. Closely paired loops of DNA from the two sister chromatids of each chromosome are depicted. The loops are likely maintained in a paired configuration by cohesin (not shown). Genetic recombination (crossing‐over) is presumed to occur between the DNA loops from nonsister chromatids, as shown.
- 9. Meiosis I- Prophase I-Diplotene 4. Diplotene: ● The beginning of diplotene, is recognized by the dissolution of the SC, which leaves the chromosomes attached to one another at specific points by X‐shaped structures, termed chiasmata (singular chiasma ). ● Chiasmata are located at sites on the chromosomes where crossing‐over between DNA molecules from the two chromosomes had previously occurred. ● Chiasmata are formed by covalent junctions between a chromatid from one homologue and a nonsister chromatid from the other homologue. These points of attachment provide a striking visual portrayal of the extent of genetic recombination.
- 10. Visible evidence of crossing‐over. ( a , b ) Diplotene bivalents from the grasshopper showing the chiasmata formed between chromatids of each homologous chromosome. The accompanying inset indicates the crossovers that have presumably occurred within the bivalent in a . The chromatids of each diplotene chromosome are closely apposed except at the chiasmata. ( c ) Scanning electron micrograph of a bivalent from the desert locust with three chiasmata (arrows).
- 11. Meiosis I- Prophase I-Diakinesis Diakinesis: ● During the final stage of meiotic prophase I, called diakinesis , the meiotic spindle is assembled and the chromosomes are prepared for separation. ● In those species in which the chromosomes become highly dispersed during diplotene, the chromosomes become recompacted during diakinesis. ● Diakinesis ends with the disappearance of the nucleolus, the breakdown of the nuclear envelope, and the movement of the tetrads to the metaphase plate.
- 12. Meiosis I- Metaphase I ● At metaphase I, the two homologous chromosomes of each bivalent are connected to the spindle fibers from opposite poles. ● In contrast, sister chromatids are connected to microtubules from the same spindle pole, which is made possible by the side‐by‐side arrangement of their kinetochores. ● The orientation of the maternal and paternal chromosomes of each bivalent on the metaphase I plate is random; the maternal member of a particular bivalent has an equal likelihood of facing either pole.
- 13. ● Separation of homologous chromosomes at anaphase I requires the dissolution of the chiasmata that hold the bivalents together. ● The chiasmata are maintained by cohesion between sister chromatids in regions that flank these sites of recombination. ● The chiasmata disappear at the metaphase I– anaphase I transition, as the arms of the chromatids of each bivalent lose cohesion. ● Loss of cohesion between the arms is accomplished by proteolytic cleavage of the cohesin molecules in those regions of the chromosome. Meiosis I- Anaphase I
- 14. ● In contrast, cohesion between the joined centromeres of sister chromatids remains strong, because the cohesin situated there is protected from proteolytic attack. As a result, sister chromatids remain firmly attached to one another as they move together toward a spindle pole during anaphase I. Meiosis I- Telophase I Telophase I of meiosis I produces less dramatic changes than telophase of mitosis. Although chromosomes often undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The nuclear envelope may or may not reform during telophase I. The stage between the two meiotic divisions is called interkinesis and is generally short‐lived.
- 15. Meiosis II ● Prophase II: A much simpler prophase than its predecessor. If the nuclear envelope had reformed in telophase I, it is broken down again. ● Metaphase II ● Anaphase II ● Telophase II: In which the chromosomes are once again enclosed by a nuclear envelope. The products of meiosis are haploid cells with a 1C amount of nuclear DNA.
- 16. The chromosomes become recompacted and line up at the metaphase plate. Unlike metaphase I, the kinetochores of sister chromatids of metaphase II face opposite poles and become attached to opposing sets of chromosomal spindle fibers Metaphase II and Anaphase II Anaphase II begins with the synchronous splitting of the centromeres, which had held the sister chromatids together, allowing them to move toward opposite poles of the cell
- 17. The progression of meiosis in vertebrates ● The progression of meiosis in vertebrate oocytes stops at metaphase II. ● The arrest of meiosis at metaphase II is brought about by factors that inhibit APC Cdc20 activation, thereby preventing cyclin B degradation. ● As long as cyclin B levels remain high within the oocyte, Cdk activity is maintained, and the cells cannot progress to the next meiotic stage. ● Metaphase II arrest is released only when the oocyte (now called an egg) is fertilized. Fertilization leads to a rapid influx of Ca 2+ ions, the activation of APC Cdc20, and the destruction of cyclin B. ● The fertilized egg responds to these changes by completing the second meiotic division.
- 18. Genetic Recombination during Meiosis ● Recombination involves the physical breakage of individual DNA molecules and the ligation of the split ends from one DNA duplex with the split ends of the duplex from the homologous chromosome. ● Recombination is a remarkably precise process that normally occurs without the addition or loss of a single base pair. To occur so faithfully, recombination depends on the complementary base sequences that exist between a single strand from one chromosome and the homologous strand of another chromosome. ● The precision of recombination is further ensured by the involvement of DNA repair enzymes that fill gaps that develop during the exchange process.
- 20. Step I: Two DNA duplexes that are about to recombine become aligned next to one another as the result of some type of homology search in which homologous DNA molecules associate with one another in preparation for recombination. Once they are aligned, an enzyme (Spo11) introduces a double‐stranded break into one of the duplexes. Step II:The gap is subsequently widened.Resection may occur by the action of a 5 ′ → 3 ′ exonuclease or by an alternate mechanism. Regardless, the broken strands possess exposed single‐stranded tails, each bearing a 3 ′ OH terminus. Step III: One of the single‐stranded tails leaves its own duplex and invades the DNA molecule of a non-sister chromatid, hydrogen bonding with the complementary strand in the neighboring duplex. Step IV: The RecA recombinase polymerizes along a length of single‐stranded DNA forming a nucleoprotein filament. RecA enables the single‐stranded DNA to search for and invade an homologous double helix. Eukaryotic cells have homologues of RecA (e.g., Rad51) that are thought to catalyze strand invasion. Strand invasion activates a DNA repair activity that fills the gaps.
- 21. Step IV and V: As a result of the reciprocal exchange of DNA strands, the two duplexes are covalently linked to one another to form a joint molecule (or heteroduplex ) that contains a pair of DNA crossovers, or Holliday junctions , that flank the region of strand exchange. Step V: Recombination intermediate need not be a static structure because the point of linkage may move in one direction or another (an event known as branch migration ) by breaking the hydrogen bonds holding the original pairs of strands and reforming hydrogen bonds between strands of the newly joined duplexes. Step VI: To resolve the interconnected DNA molecules of the Holliday junctions and restore the DNA back to two separate duplexes, another round of DNA cleavage must occur. Depending on the particular DNA strands that are cleaved and ligated, two alternate products can be generated. In one case, the two duplexes contain only short stretches of genetic exchange, which represents a noncrossover. Step VII:In the alternate pathway of breakage and ligation, the duplex of one DNA molecule is covalently joined to the duplex of the homologous molecule, creating a site of genetic recombination (i.e., a crossover).
- 22. Types of meiosis in different organisms Gametic or terminal meiosis Zygotic or initial meiosis Sporic or intermediate meiosis
- 23. Gametic or terminal meiosis In this group, which includes all multicellular animals and many protists, the meiotic divisions are closely linked to the formation of the gametes. Gametic mitosis in male vertebrates: ● In male vertebrates, for example, meiosis occurs just prior to the differentiation of the spermatozoa. Spermatogonia that are committed to undergo meiosis become primary spermatocytes, which then undergo the two divisions of meiosis to produce four relatively undifferentiated spermatids. ● Each spermatid undergoes a complex differentiation to become the highly specialized sperm cell ( spermatozoon ).
- 24. Gametic mitosis in female vertebrates: ● In female vertebrates, oogonia become primary oocytes, which then enter a greatly extended meiotic prophase. ● During this prophase, the primary oocyte grows and becomes filled with yolk and other materials. It is only after differentiation of the oocyte is complete (i.e., the oocyte has reached essentially the same state as when it is fertilized) that the meiotic divisions occur. ● Vertebrate eggs are typically fertilized at a stage before the completion of meiosis (usually at metaphase II). Meiosis is completed after fertilization, while the sperm resides in the egg cytoplasm.
- 25. Zygotic or initial meiosis ● In this group, which includes only protists and fungi, the meiotic divisions occur just after fertilization to produce haploid spores. ● The spores divide by mitosis to produce a haploid adult generation. ● Consequently, the diploid stage of the life cycle is restricted to a brief period after fertilization when the individual is still a zygote.
- 26. Sporic or intermediate meiosis ● In this group, which includes plants and some algae, the meiotic divisions take place at a stage unrelated to either gamete formation or fertilization . ● If we begin the life cycle with the union of a male gamete (the pollen grain) and a female gamete (the egg), the diploid zygote undergoes mitosis and develops into a diploid sporophyte . ● At some stage in the development of the sporophyte, sporogenesis (which includes meiosis) occurs, producing spores that germinate directly into a haploid gametophyte. ● The gametophyte can be either an independent stage or, as in the case of seed plants, a tiny structure retained within the ovules. ● In either case, the gametes are produced from the haploid gametophyte by mitosis .
- 27. A comparison of three major groups of organisms based on the stage within the life cycle at which meiosis occurs and the duration of the haploid phase.