Mitosis Meiosis.pdf

The nuclear envelope fragments. 🧬 L8 ⌋ Mitosis Microtubules emerging from the centrosomes at the...

The nuclear envelope fragments. 🧬 L8 ⌋ Mitosis Microtubules emerging from the centrosomes at the poles of the mitotic spindle extend to the nuclear ⌈ region. Each sister chromatid has a protein called kinetochore within the centromere. Mitosis Some of the spindle microtubules attach to the Mitosis is a fundamental process in cell biology that kinetochore. allows eukaryotic cells to divide and create two identical daughter cells from a single parent cell. Proteins associated with spindle microtubules move This process is essential for growth, tissue repair, and the chromosomes toward the center of the cell. asexual reproduction in many organisms. Mitosis ensures that each daughter cell receives an identical set of chromosomes and genetic Metaphase information as the parent cell. 4 Stages of Mitosis ○ PMAT Prophase Metaphase Anaphase Telophase Prophase Centrosomes are now at opposite poles of the cell. The chromosomes have all arrived at the metaphase plate, a plane that is equidistant between the spindle's two poles. The chromosomes' centromeres lie at the metaphase plate. For each chromosome, the kinetochores of the sister chromatids are attached to the kinetochore microtubules coming from opposite poles. The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope. Anaphase The nucleoli disappear. Each duplicated chromosome appears as two identical sister chromatids joined at their centromeres and, in some species, all along their arms by cohesins (sister chromatid cohesion). The mitotic spindle (named for its shape) begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters (“stars”). The centrosomes move away from each other, propelled partly by the lengthening microtubules between them.  In anaphase, the cohesins are cleaved by an Cytokinesis in Animal Cells In animal cells, cytokinesis involves the division of the enzyme called separase. cell's cytoplasm and organelles to create two Sister chromatids separate and move along the separate daughter cells. kinetochore microtubules toward opposite ends of the cell. Cleavage Furrow Formation The microtubules shorten by depolymerizing at During the late anaphase of mitosis, a structure their kinetochore ends. known as the cleavage furrow forms. Nonkinetochore microtubules from opposite poles The cleavage furrow is a contractile ring composed overlap and push against each other, elongating of actin filaments and associated proteins. the cell. The cleavage furrow contracts like a drawstring, gradually pinching the cell membrane inward. At the end of anaphase, duplicate groups of This constriction process is similar to pulling a string chromosomes have arrived at opposite ends of the tighter around the middle of a balloon. elongated cell. Cytokinesis begins during anaphase or telophase, and the spindle eventually disassembles. Telophase and Cytokinesis Two daughter nuclei form in the cell. Nuclear Cytokinesis in Plant Cells envelopes arise from the fragments of the parent Cytokinesis in plant cells differs from animal cells cell’s nuclear envelope and other portions of the because plant cells have rigid cell walls surrounding their cell membranes. endomembrane system. Nucleoli reappear. The chromosomes become less condensed. Vesicle Formation Any remaining spindle microtubules are During late telophase of mitosis (or meiosis), small vesicles called "cell plate vesicles" are produced by depolymerized. the Golgi apparatus. In animal cells, cytokinesis occurs by a process These vesicles contain cell wall materials such as known as cleavage, forming a cleavage furrow. cellulose. In plant cells, a cell plate forms during cytokinesis. Cell Plate Formation The cell plate vesicles move to the center of the Cytokinesis dividing cell, aligning along the future plane of Cytokinesis is the final stage of cell division, and it division, which corresponds to the location of the differs in its execution between animal cells and metaphase plate. plant cells due to their structural and functional The cell plate vesicles fuse together, gradually differences: enlarging and forming a structure called the "cell plate."  The cell plate effectively divides the cell into two compartments, similar to how the cleavage furrow loci). functions in animal cells. Cell Wall Synthesis Enzymes and other molecules are deposited in the cell plate, and cellulose synthesis occurs, resulting in the formation of a new cell wall between the two daughter cells. Homologous Chromosomes Sexually reproducing organisms inherit their genetic information from both parents, resulting in two sets of chromosomes—one from the mother and one from the father. These pairs of chromosomes, with one originating from the maternal side and the other from the paternal side, are referred to as homologous 🧬 L9 ⌋ Meiosis chromosomes. ⌈ Inheritance of Genes Offspring resemble their parents more than they do unrelated individuals. Heredity Is the transmission of traits from one generation to the next. Variation is demonstrated by the differences in appearance that offspring show from parents and siblings. Characteristics of Homologous Chromosomes Offspring acquire genes from parents by inheriting chromosomes. They exhibit identical At corresponding loci In a literal sense, children do not inherit particular structural attributes, such positions, they carry the physical traits from their parents. as similar sizes, banding same genes. While the It is genes that are actually inherited. patterns, and centromere genes are identical in terms ○ Genes are the units of heredity and are positions. of function, they may made up of segments of DNA. possess different alleles, ○ Genes are passed to the next generation contributing to genetic via reproductive cells called gametes variation within the (sperm and eggs). organism. ○ A gene’s specific position along a chromosome is called its locus (plural Behavior of Chromosome Sets in the Human Life Cycle Fertilization is the process in which gametes, specifically the sperm and the egg, unite to form a fertilized egg known as a zygote. This zygote possesses one set of chromosomes contributed by each parent. As the zygote develops, it undergoes mitosis to generate somatic cells, eventually maturing into an adult organism. During sexual maturity, the ovaries and testes produce haploid gametes. Gametes are the only haploid cells in animals. They are produced by meiosis and undergo no further cell division before fertilization. Gametes fuse to form a diploid zygote that divides by mitosis to develop into a multicellular organism. Meiosis I Meiosis is a type of cell division that occurs in sexually reproducing organisms to produce gametes, which are haploid cells (containing half the usual number of chromosomes), such as sperm and egg cells in animals or pollen and ovules in plants. Unlike mitosis, which results in the formation of two genetically identical diploid cells, meiosis leads to the formation of four genetically different haploid cells. Meiosis involves two sequential divisions: ○ meiosis I ○ meiosis II Each of these divisions consists of specific stages, which include ○ 4 Stages of Meiosis PMAT Prophase Metaphase  Anaphase ○ Anaphase I Telophase ○ Telophase I and Cytokinesis Prophase I Chromatin Condensation At the beginning of prophase I, the chromatin in the cell's nucleus condenses, becoming more tightly coiled and visible under a microscope. This condensation is the first step in the formation of visible chromosomes. Nuclear Envelope The nuclear envelope, which surrounds the Breakdown nucleus, starts to disintegrate during prophase I. This breakdown allows the contents of the nucleus to interact with the cytoplasm and facilitates the movement of chromosomes during meiosis. Spindle Fiber Formation As prophase I progresses, the microtubules of the centrioles begin to extend from the centrosomes located at opposite poles of the cell. These spindle fibers will eventually attach to the centromeres of the chromosomes. Synapsis Synapsis is the process of homologous chromosomes coming together and forming tetrads (also called bivalents). These tetrads align next to each other. A zipper-like structure called the synaptonemal complex holds the homologs together tightly. During synapsis, DNA Like mitosis, meiosis is preceded by the replication of breaks are repaired, joining chromosomes DNA from one non-sister chromatid to the Meiosis takes place in two consecutive cell divisions, corresponding segment of called meiosis I and meiosis II another. The two cell divisions result in four daughter cells, Crossing Over Homologous rather than the two daughter cells in mitosis chromosomes within tetrads may undergo a Each daughter cell has only half as many process called crossing chromosomes as the parent cell over. This involves the exchange of genetic material between chromatids of homologous Meiosis I: Reduction Division chromosomes, creating Meiosis I is the first of two sequential divisions that new combinations of traits occur during the process of meiosis. It is often on the chromatids and referred to as the "reduction division" because it contributing to genetic reduces the chromosome number by half, resulting diversity among the in cells that are haploid (containing half the normal resulting gametes. number of chromosomes). Division in meiosis I Chiasmata Are the points where chromatids of homologous occurs in four phases: chromosomes cross over ○ Prophase I and recombine (singular: ○ Metaphase I chiasma). These visible structures are a physical manifestation of the genetic exchange taking place. Metaphase I In metaphase I, pairs of homologous chromosomes line up at the metaphase plate, with one chromosome facing each pole. Microtubules from one pole are attached to the kinetochore of one chromosome of each pair, while microtubules from the other pole are attached to the kinetochore of the other chromosome. Anaphase I  In anaphase I, pairs of homologous chromosomes separate. In the beginning of telophase I, each half of the cell One chromosome of each pair moves toward has a haploid set of chromosomes; each opposite poles, guided by the spindle apparatus. chromosome still consists of two sister chromatids. Sister chromatids remain attached at the Cytokinesis usually occurs simultaneously, forming centromere and move as one unit toward the pole. two haploid daughter cells. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms. Telophase I and Cytokinesis I Meiosis II Meiosis II follows meiosis I and is similar in some aspects to mitosis. However, meiosis II involves the division of haploid cells produced in meiosis I into four haploid daughter cells, each with a unique combination of genetic material. No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated. Division in meiosis II also occurs in four phases: ○ Prophase II ○ Metaphase II ○ Anaphase II ○ Telophase II and Cytokinesis II Prophase II  In prophase II, a spindle apparatus forms. In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate. Metaphase II In anaphase II, the sister chromatids separate. The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles. Telophase II and Cytokinesis In metaphase II, the sister chromatids are arranged at the metaphase plate. Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical. The kinetochores of sister chromatids attach to microtubules extending from opposite poles. Anaphase II In telophase II, the chromosomes arrive at opposite poles. The nuclear membrane forms, and the  chromosomes begin decondensing. Cytokinesis separates the cytoplasm, resulting in the formation of four haploid daughter cells. 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.