cell division

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are not properly aligned. G₂ checkpoint Mitosis checkpoint Mitosis will occur if DNA has replicated properly Apoptosis will occur if DNA is damaged and cannot be repaired. *S Stage Following G1, the cell enters the S stage, when DNA replication occurs. Beginning of S stage, each chromosome is composed of 1 DNA double helix. Following DNA replication, each chromosome is composed of 2 identical DNA double helices. →Each double helix is called a chromatid. → 2 identical chromatids are called sister chromatids. ● ● ● * Mitosis •Somatic Cells: Body cell ; excludes cells that undergo meiosis and become sperm or eggs. The nuclei of human somatic cells contain 46 chromosomes made up of 2 sets of 23, each Set is inherited from each parent. Gametes: Haploid sex cell ; e.g., egg or sperm. Gametes contain half the chromosomes (one set of 23). ● → The sister chromatids remain attached until they are separated during mitosis. • Typical human cell contains 2m of DNA, about 250,000 times greater than the cell 's diameter. Eukaryotic chromosomes are made up of chromatin, a complex of Fibrils of DNA associated proteins, mainly histones. → Histones: Family of basic proteins that associate with DNA & help them to be packaged into chromatin. • 5 stages of mitosis: Prophase 2 Prometaphase 3 Metaphase 4Anaphase Telophase *End of G2 (Early Prophase) Centrosome: Central - microtubule - organizing center of cells. In human cells, it ● contains 2 centrioles. • The centrosome duplicates into two (results in 4 centrioles). • Chromosomes, duplicated in S phase, cannot be seen individually cuz they didn't condense yet. * Prophase • At this point, parental chromosomes are already duplicated composed of 2 sister chromatids held together at a centromere. •The nucleolus disappears the nuclear envelope fragments. The spindle begins to assemble. • An array of microtubules radiates toward the plasma membrane from the centrosomes. → These structures are called radial arrays or asters. * Prometaphase • Microtubules of the spindle invade the nuclear area interact with the chromosomes, which have become even more condensed. →Microtubules extend from each centrosome toward the middle of the cell. Centrosomes (with centriole pairs) cytoplasm array (aster) Nucleolus Each of the 2 chromatids of a chromosome now has a kinetochore, a specialized protein structure located at the centromere. Some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules". Early mitotic Aster spindle 4 radial Chromosome, consisting of two sister chromatids Fragments of nuclear envelope Nuclear Plasma envelope membrane Kinetochore chromosomes microtubules never huse Chromatin (duplicated) (3) Centromere 4 chromosome Nonkinetochore microtubules 8 kineto- chore Kinetochore microtubule These jerk the chromosomes back & forth. Nonkinetochore microtubules interact with those from the opposite pole of the spindle (without physically touching). * Metaphase •Longest phase of mitosis (~20 min.) • Centrosomes are now at opposite ends of the cell. •The chromosomes convene to the metaphase plate, an imaginary plane that is equidistant between the spindle's 2 poles. The chromosome's centromeres lie on the metaphase plate. *Anaphase •Shortest stage of mitosis. • 2 sister chromatids separate at the centromere. Daughter chromosomes, each with a centromere single chromatid composed of a single double helix, move toward opposite poles. → Because these microtubules are attached at the centromere region, the chromosomes move centromere first (~1 μm /min). chromosomes should lie Metaphase plate horizontally Spindle Centrosome at one spindle pole Daughter chromosomes non kinetochore mt are still elongated • The cell elongates as the nonkinetochore microtubules lengthen. →Because of this, by the end of anaphase, the cell reaches its longest diameter. *Telophase • 2 daughter nuclei begin to form in the cell. • Nuclear envelopes arise from the fragments of the parent cell's nuclear envelope other portions of the endomembrane system. Chromosomes become less condensed. * Cytokinesis Is the division of the cytoplasm. Cleavage furrow Nucleolus forming → It begins in anaphase telophase, but it doesn't reach completion until the following interphase begins. •In animal cells, a cleavage furrow is formed. This is an indentation of the membrane between the 2 daughter nuclei. →The cleavage furrow deepens when a band of actin Filaments form a contractile ring. • In plants, the rigid cell wall doesn't allow the Formation of a cleavage furrow. Instead, the golgi produces vesicles which form the cell plate. Nuclear envelope forming