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BIO1022: Cell division

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CELL DIVISION Cellular organisation of the genetic material  Genome – a cell’s genetic information.  A prokaryotic genome is often a long DNA molecule.  An eukaryotic genome usually has a number of DNA molecules.  Before the cell can divide to form genetically identical daughter cells, all of this DNA must be copied and then the two copies separated so that each daughter cell ends up with a complete genome.  The replication and distribution of so much DNA is okay because the DNA molecules are packaged into chromosomes.  Each eukaryotic species has a specific number of chromosomes in each cell nucleus.  Gametes – sperm and eggs.  Eukaryotic chromosomes are made of chromatin.  Chromatin – a complex of DNA and associated protein molecules.  The associated proteins maintain the structure of the chromosome and help control the activity of the genes. Distribution of chromosomes during eukaryotic cell division  Before DNA replication, each chromosome is in the form of a long, thin chromatin fiber.  After DNA replication, the chromosomes denses: each chromatin fiber becomes densely coiled and folded, making the chromosomes much shorter and thick.  Each duplicated chromosomes has two sister chromatids.  Centromere – a specialised region where the two chromatids are most closely attached.  Later in the cell division process, the two sister chromatids of each duplicated chromosome separate and move into two new nuclei, one forming at each end of the cell.  Once the sister chromatids separate, they are considered individual chromosomes.  Mitosis – division of the nucleus.  Cytokinesis – division of the cytoplasm. Phases of the cell cycle  Mitotic (M) phase includes both mitosis and cytokinesis is the shortest part of the cell cycle.  Mitotic cell division alternates with a much longer stage called Interphase.  During interphase, the cell grows and copies its chromosomes in preparation for cell division.  Interphase can be divided into sub-phases: -G1 phase (first gap) - S phase (synthesis) - G2 phase (second gap)  During the three sub-phases, the cell grows by producing proteins and cytoplasmic organelles (ER, mitochondria).  Chromosomes are only duplicated in the S phase.  A cell grows (G1), continues to grow as it copies its chromosomes (S), grows more as it completes preparations for cell division (G2), and divides (M).  Mitosis is broken down into five stages: - Prophase - Prometaphase - Metaphase - Anaphase - Telophase  Overlapping with the latter stages of mitosis, cytokinesis completes the mitotic phase. The mitotic spindle: a closer look  The mitotic spindle begins to form in the cytoplasm during prophase.  The mitotic spindle is made of microtubules and associated proteins.  While the mitotic spindle assembles, the other microtubules of the cytoskeleton partially dissemble.  During interphase in animal cells, the single centrosome replicates forming two centrosomes, which remain together near the nuclear.  The two centrosomes move apart during prophase and prometaphase of mitosis as spindle microtubules grow out from them.  By the end of prometaphase, the two centrosomes, one at each pole of the spindle, are at opposite ends of the cell.  Kinetochore – a structure of proteins associated with specific sections of chromosomal DNA at the centromere.  Each of the two sister chromatids of a replicated chromosome.  During prometaphase, some of the spindle microtubules attach to the kinetochores.  At metaphase, the centromeres of all the duplicated chromosomes are on a plane midway between the spindle’s two poles.  Anaphase starts when the cohesins holding together the sister chromatids of each chromosome are cleaved by enzymes.  In a dividing animal cell, these microtubules are responsible for elongating the whole cell during anaphase.  Nonkinetochore microtubules from opposite poles overlap each
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