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Journey Through The Cell
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1. A prokaryotic genome is often a single long DNA molecule; eukaryotic genomes usually consist of several such molecules. A typical human cell has 3 meters of DNA. Before a cell can divide all of said DNA, it must be coiled and then apportioned to give each daughter cell a complete genome. 2. The replication and distribution of so much DNA is done because the DNA molecules are packaged into chromosomes. Every eukaryotic species has a characteristic number of chromosomes in each cell nucleus. Reproductive cells, or gametes—sperm cells and egg cells—have half as many chromosomes as somatic cells (all body cells save gametes), or 23 chromosomes in humans. 3. In each generation of humans, meiosis reduces the chromosome number from 46 to 23. Fertilization joins gametes and doubles the chromosome number to 46 again and mitosis conserves that number in every somatic cell of the new individual. The role of meiosis in reproduction and inheritance will be discussed later on. 4. Cell cycle phases. Mitotic phase- includes both mitosis and cytokensis; usually the shortest part of the cell cycle. Interphase- often account for about 90% of the cycle. During interphase, the cell grows and copies chromosomes. Interphase divides into subphases, the cell grows by producing proteins and cytoplasmic organelles. *Chromosomes are duplicated only during the S-phase (S stands for DNA synthesis) *Cell grows (g) *Copies chromosomes(s) *Completes preparation for cell division (G2) *Divides (M) 5. Process of Mitosis Prophase- changes in both the nucleus and the cytoplasm. In the nucleus, the chromatin fibers become more tightly coiled, condensing into discrete chromosomes. *The nucleoli disappear. *Each duplicated chromosome appears as two identical chromatids joined together. *In cytoplasm, the mitotic spindle begins to form; made of microtubules radiating from two centrosomes. *Centrosomes move from each other, apparently propelled along the surface of the nucleus by the lengthening bundles of microtubules between them. Prometaphase- nuclear envelope fragments. Microtubules of the spindle can invade the nucleus and interact with the chromosomes, which are further condensed. *Bundles of microtubules extend from each pole toward the middle of the cell. *2 chromatids of a chromosome now have a specialized structure called a kinetochore, located at the centomere region. Microtubules attach to the kinetochores. *Interaction causes the chromosomes to begin jerk movements. Non-kinetochore microtubules interact with those from the opposite pole of the cell. Metaphase- the chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle’s two poles.
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