Mitosis (Biology 1310) PDF

Biology 1310 - Genes, Cells and Macromolecules - Eukaryotic Cell Division (Mitosis) Prokaryotic cells (like bacteria) have fairly simple cell division - binary fission (Fig. 12.12a) - they don’t have much DNA in their genomes (around 3 million base pairs on average) - s...

Biology 1310 - Genes, Cells and Macromolecules - Eukaryotic Cell Division (Mitosis) Prokaryotic cells (like bacteria) have fairly simple cell division - binary fission (Fig. 12.12a) - they don’t have much DNA in their genomes (around 3 million base pairs on average) - simple division allows proper apportioning of genetic material into daughter cells Eukaryotic cells have considerably more DNA in their genomes (4 billion base pairs in the case of humans) - cell division needs to be more intentional about how genetic material is apportioned into daughter cells Mitosis and meiosis - these are two related, but notably different, concepts that many biology students get confused - both occur at cell division, but in different types of cells - meiosis: specialized division of germ cells - mitosis: every other type of cell (somatic cells) - two daughter cells will be genetically identical to the original cell being divided - basically, copy the genes and give one copy to each daughter cell Diploid organisms (like us) have two copies of every gene (genome) - one from the organism’s mother, one from the father (gametes) Diploid cells are designated as being “2n”, where “n” represents a complete set of one copy of every gene Genes are bundled onto chromosomes - each has many genes, some more than others Humans have 23 different chromosomes (2 x 23 = 46), 2n = 46 - each chromosome contains the same genes in all humans, in virtually the same position along that chromosome - e.g. the superoxide dismutase gene is always on chromo 21 in the middle of the long arm Why chromosomes? - eukaryotic organisms have a lot of DNA - human genome is approx. 4 billion basepairs - bacterial genome is approx. 4 million basepairs - reproduce by binary fission (Fig. 12.12a) - chromosomes keep everything in the eukaryotic genome organized - each chromosome is not too large - consists of a single, long strand of DNA wound twice around “beads” of histone proteins (Fig. 16.23) - nucleosomes are connected like beads along a necklace by DNA - at most stages of the cell cycle, chromosomes exist as a long, thin fibre (chromatin) - when chromosomes replicate, the double-stranded DNA molecules that make up the chromosome are duplicated into two identical copies - the two identical strands of DNA (called sister chromatids) are held together at one spot (centromere) (Fig. 12.4, 12.5) - the centromere is a short sequence of DNA specifically for binding of the two sister chromatids - it is also the attachment site for the kinetochore - each chromosome appears to have a “short arm” and a “long arm”, based on the position of the centromere - soon after, the chromatin strands begin to coil up very tightly (condensation) - this can be seen under the light microscope - it is done in preparation for cell division Mitotic cell cycle (Fig. 12.6) - interphase, which consists of three stages: - G1 (first gap) - S (synthesis) - G2 (second gap) - mitosis - the length of interphase relative to the full cell cycle depends on the physiological state of the cells and tissue - eventually, the cell will need to divide into two Late G2 (Fig. 12.7) - the chromosomes are replicated but not yet condensed - centrosome replicates (animal cells) - microtubules assemble linearly (from the centrosomes in the case of animal cells), radiating out in all directions (aster/spindle) Prophase - spindle elongating from the aster - centrosome moving to opposite poles of cell - chromosomes condensing - nucleoli dissipate Prometaphase - nuclear envelope dissipates - chromosomes attach to termini of both spindles - kinetochore - protein-based - attaches spindle to centromere - chromosomes start to move Metaphase - chromosomes at full condensation - each one lined up along metaphase plate - in humans, there are 46 replicated chromosomes lined up (2 x 23) - 46 pairs of sister chromatids Anaphase - kinetochores pull each chromatid up the spindle fibre (Fig. 12.9) - consists of a variety of proteins, including motor proteins (incl. kinesin and dynein) - cause the spindle fibre to dissolve, effectively “pulling” the chromatid toward the pole - subunits of tubulin protein are released - sister chromatids split - centromeres release from each other - the sister chromatids are now daughter chromosomes - one copy of everything is going to each pole of the cell - the genetic content of each pole is identical Telophase and cytokinesis - chromosomes decondense - nucleolus reforms - nuclear envelope reforms - in animal cells, a cleavage furrow forms (Fig. 12.10) - action of actin and myosin - in plant cells, a cell plate forms - vesicles with materials from the Golgi coalesce, including callose (a variant of cellulose) - fuses with plasma membrane Cell division is controlled by several genes working together - cell-cycle control system (Fig. 12.15) - there are three important checkpoints that determine if a cell can go on to the next stage - G1 checkpoint (restriction point): - involves a series of biochemical reactions and conditions - e.g growth factor binding, and appropriate levels of nutrition - the cell will go into G0 if the correct signals and conditions are not provided (Fig. 12.17) - cancer involves cells inappropriately passing this checkpoint - G2/M checkpoint - is regulated by two types of protein - protein kinases (Fig. 11.10) - especially cyclin-dependant kinases (Cdk’s) - cyclins - one cyclin-Cdk complex is maturation promoting factor (MPF) (Fig. 12.16) - lets cells in G2 go into the mitosis portion of the cell cycle - a specific Cdk is at constant level - a specific cyclin increases gradually through G2 phase - when its level is high, the MPF complex can be made - the Cdk’s can now phosphorylate a variety of proteins - e.g MPF breaks up nuclear membrane, by phosphorylating proteins of the nuclear lamina - it possibly also contributes to chromosomal condensation - during anaphase, then release from Cdk and degrade - levels drop - new cyclin must be made before MPF can be formed once again M checkpoint: allows the cell to go past the M checkpoint and from metaphase into anaphase is. - depends on whether the spindle microtubules are fully connected to the chromosomal kinetochores or not. - If there are any kinetochores not attached to a spindle microtubule, they will inhibit the action of a protein whose role it is to cause the two sister chromatids to release from one another, an enzyme called a separase. - Once all the kinetochores are attached to the spindle, the separase will be activated and the sister chromatids are released. The cell-cycle control system must receive information to determine what it should do - growth factor - e.g. platelet derived growth factor (PDGF) (Fig. 12.18) - density-dependent inhibition (Fig. 12.19) - anchorage dependence - will not divide unless attached to a fixed substratum Several genes must work together to assure proper cell growth and proliferation - one breakdown in the system (mutation) can lead to uncontrolled cell division (cancer) - e.g. doesn’t require a growth factor - often due to environmental pollutants - can occur at many different points - genes that have a normal role in cell division are proto-oncogenes (Fig. 18.23) - if they cause cancer when mutated, they have become oncogenes - transformation: cell becomes cancerous - surface proteins change - can be recognized by immune system and destroyed - tumour: transformed cells evade immune system and proliferate - benign tumour: cells stay within a defined area - malignant tumour: cells invade surrounding tissues, impairing their function - metastasis: cells break off from tumour and enter circulatory system (Fig. 12.20) - can invade tissues around the body -------------------------------------------------- Mastering Biology Study Area For more detail on the specific issues concerning cellular membranes, watch the following animations: - Chapter 12: The Cell Cyle - Chapter 12: Mitosis (Bioflix and Animation) - Chapter 12: Animal Mitosis (time-lapse) - Chapter 12: Spidle formation duringMitosis - Chapter 12: Microtubule Depolymerization - Chapter 12: Cytokinesis - Chapter 12: Cytokinesis in Animal Cell - Chapter 12: Cell Division in Bacteria - Chapter 12: Nuclear Envelope Breakdown and Formation During Mitosis in C. elegans - Chapter 12: Control of the Cell Cycle