BIOL1017 CELL BIOLOGY Department of Life Sciences, Faculty of Science and Technology, the University of the West Indies Mona Campus JAMAICA Mitosis LECTURE 16 Dr. Machel Emanuel § DNA is a very long polymer, up to several centimeters in length. During the cell cycle DNA tend to be packed into a compact structure. § A eukaryotic chromosome consist of one or two linear, double stranded DNA molecules bound with many proteins, is refer to as chromatin. § After replication during the S phase, two double stranded DNA molecules: the sister chromatids. § Throughout G2 phase the sister chromatids are held together along most of their length by a protein complex called cohesion complexes. § At mitosis most of the cohesion is removed, except in a region called centromere, where the chromatids remain held together. § At the end of G2 and the beginning of mitosis, a second group of proteins called condensins coats the DNA molecules, making them compact. § The level of packing achieved is largely due to proteins called histones, which are positively charged. § These DNA-histone interactions, result in the formation of bead like units called nucleosomes § During inter phase the DNA is accessible to proteins involved in replication and transcription. § Once a mitotic chromosome is formed its compact nature makes it inaccessible to replication and transcription factors. § Mitosis during the M phase of the cell cycle leads to the production of two nuclei that are genetically identical to each other and to the nucleus of the cell that entered the cell cycle in G1. § Mitosis ensures the accurate segregation of each copy of a eukaryotic cell’s multiple chromosomes into daughter nuclei. § Mitosis is a continuous process in which each event flows smoothly into the next. The centrosomes determine the plane of division § The spindle is a dynamic microtubule structure that moves sister chromatids apart during mitosis. § Before the spindle is formed, its orientation is determined by the centrosome, an organelle in the cytoplasm. § During S phase the centrosome duplicates, and at the beginning of prophase the two centrosomes separate from one another, moving to opposite ends of the nuclear envelope. § During anaphase the chromosomes move towards the poles . § The cells of plants and fungi lack centrosomes, but distinct microtubule organizing centers at each end of the cell play the same role. § The positions of the centrosomes determine the plane where the animal cell will divide. The spindle begins to form during prophase § The appearance of the nucleus changes as the cell enters prophase. § At this stage, most of the cohesion that held the two products of DNA replication together since the S phase is removed, so individual chromatids become visible. § Late in prophase, specialized structures called kinetochores develop in the centromere region, one on each chromatid. Structures assist in chromosome movement § Each of the two centrosomes, now at opposite sides of the nucleus, serves as a mitotic center, or pole towards which chromosomes will move. § During prophase and prometaphase, microtubules form between the poles and the chromosome to make up the spindle. The spindle serves as a structure to which the chromosomes attach as a framework keeping poles apart. § The microtubules are initially unstable, constantly forming and falling apart until they contact the kinetochores to become more stable. There are two groups of microtubules in the spindle: 1. Polar microtubules from the framework of the spindle and run from one pole to the other 2. Kinetochore microtubules, which form later, attached to the kinetochores on the chromosomes. This ensures the two chromatids will eventually move to opposite poles. § The movement of chromatids achieves the main goal of mitosis as the segregation of genetic material must occur before the cell can divide. Chromosome separation and movement highly organized § During metaphase, the chromosomes line up at the equatorial position of the cell. § Two key processes of anaphase to consider: 1. Separation of the chromatids 2. The mechanism of the actual movement towards the poles Chromatid separation § The separation of chromatids occurs at the beginning of anaphase. § It is controlled by a M phase cyclin CDK, which activates another protein complex called the anaphase-promoting complex. § A cell cycle check point often called the spindle assembly check point occurs at the end of metaphase to inhibit the anaphase-promoting complex if one of the chromosomes are not attached. § Sister chromatids share centromere/ Daughter chromosomes have own centromere. Chromosome movement Three mechanisms operate to move the chromosomes to the opposite poles: 1. The kinetochores contain molecular motor proteins which use energy from ATP hydrolysis to do the work of moving the chromosomes along the microtubules. 2. The kinetochore microtubules, shorten, drawing the chromosome towards the poles. 3. The centrosomes move apart aiding in separation. § The last stage of mitosis is telophase, when the spindle disappears and the nuclear envelope forms around each set of daughter chromosomes § Finally the cytoplasm divides to form two daughter cells during cytokinesis. Cytokinesis divides the cytoplasm § In animal cells cytokinesis begins to make a groove of the cell membrane. § A contractile ring composed of microfilaments of actin and associated myosin, form a ring on the cytoplasmic surface of the cell membrane. § These two proteins interact to produce a contraction, pinching the cell in two. § The microfilaments assemble rapidly from actin monomers that are present in the interphase cytoskeleton. § In plant cells as the spindle breaks down after mitosis, membranous vesicles derived from the Golgi apparatus appear along the plane of cell division, roughly midway between two daughter nuclei. § The vesicles are propelled along the microtubules by the moto protein kinesin and fuse to form a new cell membrane. § At the same time they contribute their contents to the cell plate, which is beginning of a new cell wall.
