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Purdue BIOL 10200 - Mitosis Notes

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Prokaryotic Cell ReproductionAll species of organisms grow and reproduce via cell reproduction.For bacteria, this is called binary fission.The DNA for bacteria is very simple, but it’s actually about 500 times bigger than the bacteriaitself. So to fit it in, it has to be very compressed and condensed. Once that happens, instead ofbeing put in a nucleus (which, as a prokaryote, the bacteria doesn’t have); it is put in a regioncalled the nucleoid.Whether for eukaryotes or prokaryotes, the compaction of genetic information requires a groupof proteins called “structural maintenance of chromosome” aka SMC.The cohesin and condensin proteins are examples of SMCBinary fission steps:1. It begins with the replication of DNA at the “origin of replication” (discussed in myChapter 14 notes). The origin itself gets duplicated during this process.2. Then it elongates3. Then it divides at the midcell4. The two origins turn until they are at ¼ and ¾ the length of the original cell.5. Then there is decatenation (untanging) of the final replication product6. And finally, the cell’s components are split in half at the midpoint and a septum isproduced to separate the two. This is septationa. Septation involves the formation of a ring composed of many copies of theprotein FtsZ, and that ring is in the midpoint of the cell and it’s where the cellsplits in halfChromosomesChromosomes are in the cells of all eukaryotes. Some have only a single pair, and others haveupwards of 500 pairsHumans have 23 pairs, so 46 totalThe array of chromosomes someone possesses is the karyotypeSometimes, geneticists use the phrases haploid and diploid to refer to one complete set ofchromosomes and the individual chromosomes, respectively. The diploid is always twice thehaploid. EX: humans have a “haploid number” of 23, and a “diploid number” of 46Chromosomes are composed of chromatin.● Chromatin is 40% DNA and 60% protein● And remember, DNA is a nucleic acid, which means it is made up of smallernucleotidesHeterochromatin is not activeEuchromatin is activeDNA makes up small beads called nucleosomes. Nucleosomes basically consist of DNAspooled around a histone octamer (made of 4 main histones: H2A, H2B, H3 and H4.) That inturn makes of chromatinThe protein condensin is involved in condensing the chromosome during replication usingenergy from ATPThe chromosome as we typically know it is only visible for a brief period after it is replicated butwhile the two chromatids are still bound together. (In this state, they are called sisterchromatids). They are bound by the cohesin proteinsEukaryotic Cell Reproduction CycleEukaryotic cell cycle has a few phases:1. G1 (Gap Phase 1): Primary growth phase of the cell, named so because the gapbetween the last cycle’s cytokinesis and the current cycle’s DNA synthesis is filled here.It carries out metabolic activity2. S (Synthesis): Replication of DNA. (The centrosomes/microtubule organizing centerscalled centrioles also duplicate)3. G2 (Gap Phase 2): Preparation for the separation of the newly-replicated DNA. Somemetabolization occurs here to produce proteins needed to split everything. This fills thegap between synthesis and mitosis. Also, microtubules form spindles4. Mitosis: the spindle binds to the chromosomes and pulls them apart, resulting in twodaughter genomes. DNA is segregateda. Prophaseb. Prometaphasec. Metaphased. Anaphasee. Telophase5. Cytokinesis: the cytoskeleton itself splits, creating two daughter cells. Basically,everything that wasn’t DNA is split hereInterphase refers to G1, S, G2Cytokinesis and Mitosis are sometimes called “Mitosis” but we keep them distinctThe time of the cycle varies depending on cell type and organism, and much of that variance isin G1. Much of the time, there’s another phase before G1 called G0 where cells will remaindormant and non-dividing because their dividing proteins are turned off for days or yearsInterphaseThe centromeric region is a visible constriction that holds sister chromatids togetherThe centromere is a sequence of repeated short DNA sequences associated with eachchromatidKinetochore are disk-like proteins associated with each centromere. It is the attachment pointof the microtubulesTelomeres are repeated short sequences of DNA at the end of each chromatid. They arebasically the “feet” of the chromosomes● They help prevent damage to the DNA● They are shortened by aging and diseases, which leads to certain genes not beingexpressed. This is a bad thing● They are replicated by the enzyme telomeraseAfter the S phase, while it might look like one, there are effectively two centromeresMost of the cohesins are replaced with condensins, leaving the sister chromatids attached at thecentromere and loose elsewhereDuring the G2 phase, there is an extensive production to make tubulin, which makesmicrotubules, which makes spindlesFinally, during the G2 phase, microtubule-organizing centers called centrioles form. There aretwo, one for each pole of the cellMitosis and CytokinesisProphase has begun● Chromosomes condense and become visible● Breakdown of the nuclear envelope● The centrioles pull apart, creating an axis of microtubules referred to as “spindle fibers”● Once the centrioles are on opposite sides, they have a bridge called the “spindleapparatus”● Finally, some centrioles go to the plasma membrane to create the aster, a sort ofcushion for when the spindles retract and pull it apart in the next couple of phases. Thisis only in animal cells; plant cells are already quite tough and don’t need an aster● At this point, there are 23 pairs of chromosomes, 46 chromosomes, and 92 chromatidsPrometaphase is next● Spindles attach to chromosomes via the kinetochore● Chromosomes pulled to the centerMetaphase● Chromosomes are aligned at the metaphase plate, the equator of the cell● Sister chromatids are still attached at this point, FYIAnaphase● If checkpoint was passed, simultaneous removal of cohesins from all chromatids andchromatids separate● During Anaphase A: the kinetochores are pulled towards the poles● During Anaphase B: the poles move awayTelophase● Spindles disassemble● Nucleus dissipates, and new nuclear envelopes form around each set of chromatids● Chromosomes decondenseCytokinesis● In the cells of all eukaryotes that lack a cell wall, there is a constricting belt of actinfilaments around the cells called the contractile ring. The diameter of the


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