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Prokaryotic Cell Reproduction All 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 bacteria itself So to fit it in it has to be very compressed and condensed Once that happens instead of being put in a nucleus which as a prokaryote the bacteria doesn t have it is put in a region called the nucleoid Whether for eukaryotes or prokaryotes the compaction of genetic information requires a group of proteins called structural maintenance of chromosome aka SMC The cohesin and condensin proteins are examples of SMC Binary fission steps 1 It begins with the replication of DNA at the origin of replication discussed in my Chapter 14 notes The origin itself gets duplicated during this process 2 Then it elongates 3 Then it divides at the midcell 4 The two origins turn until they are at and the length of the original cell 5 Then there is decatenation untanging of the final replication product 6 And finally the cell s components are split in half at the midpoint and a septum is produced to separate the two This is septation a Septation involves the formation of a ring composed of many copies of the protein FtsZ and that ring is in the midpoint of the cell and it s where the cell splits in half Chromosomes Chromosomes are in the cells of all eukaryotes Some have only a single pair and others have upwards of 500 pairs Humans have 23 pairs so 46 total The array of chromosomes someone possesses is the karyotype Sometimes geneticists use the phrases haploid and diploid to refer to one complete set of chromosomes and the individual chromosomes respectively The diploid is always twice the haploid EX humans have a haploid number of 23 and a diploid number of 46 Chromosomes 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 smaller nucleotides Heterochromatin is not active Euchromatin is active DNA makes up small beads called nucleosomes Nucleosomes basically consist of DNA spooled around a histone octamer made of 4 main histones H2A H2B H3 and H4 That in turn makes of chromatin The protein condensin is involved in condensing the chromosome during replication using energy from ATP The chromosome as we typically know it is only visible for a brief period after it is replicated but while the two chromatids are still bound together In this state they are called sister chromatids They are bound by the cohesin proteins Eukaryotic Cell Reproduction Cycle Eukaryotic cell cycle has a few phases 1 G1 Gap Phase 1 Primary growth phase of the cell named so because the gap between the last cycle s cytokinesis and the current cycle s DNA synthesis is filled here It carries out metabolic activity 2 S Synthesis Replication of DNA The centrosomes microtubule organizing centers called centrioles also duplicate 3 G2 Gap Phase 2 Preparation for the separation of the newly replicated DNA Some metabolization occurs here to produce proteins needed to split everything This fills the gap between synthesis and mitosis Also microtubules form spindles 4 Mitosis the spindle binds to the chromosomes and pulls them apart resulting in two daughter genomes DNA is segregated a Prophase b Prometaphase c Metaphase d Anaphase e Telophase 5 Cytokinesis the cytoskeleton itself splits creating two daughter cells Basically everything that wasn t DNA is split here Interphase refers to G1 S G2 Cytokinesis and Mitosis are sometimes called Mitosis but we keep them distinct The time of the cycle varies depending on cell type and organism and much of that variance is in G1 Much of the time there s another phase before G1 called G0 where cells will remain dormant and non dividing because their dividing proteins are turned off for days or years Interphase The centromeric region is a visible constriction that holds sister chromatids together The centromere is a sequence of repeated short DNA sequences associated with each chromatid Kinetochore are disk like proteins associated with each centromere It is the attachment point of the microtubules Telomeres are repeated short sequences of DNA at the end of each chromatid They are basically 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 being expressed This is a bad thing They are replicated by the enzyme telomerase After the S phase while it might look like one there are effectively two centromeres Most of the cohesins are replaced with condensins leaving the sister chromatids attached at the centromere and loose elsewhere During the G2 phase there is an extensive production to make tubulin which makes microtubules which makes spindles Finally during the G2 phase microtubule organizing centers called centrioles form There are two one for each pole of the cell Mitosis and Cytokinesis Prophase 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 spindle apparatus Finally some centrioles go to the plasma membrane to create the aster a sort of cushion for when the spindles retract and pull it apart in the next couple of phases This is 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 chromatids Prometaphase is next Spindles attach to chromosomes via the kinetochore Chromosomes pulled to the center Chromosomes are aligned at the metaphase plate the equator of the cell Sister chromatids are still attached at this point FYI If checkpoint was passed simultaneous removal of cohesins from all chromatids and chromatids separate During Anaphase A the kinetochores are pulled towards the poles During Anaphase B the poles move away Spindles disassemble Nucleus dissipates and new nuclear envelopes form around each set of chromatids Chromosomes decondense Metaphase Anaphase Telophase Cytokinesis In the cells of all eukaryotes that lack a cell wall there is a constricting belt of actin filaments around the cells called the contractile ring The diameter of the belt decreases and the cell is eventually pinched off to create a cleavage furrow eventually separating the cells In plant cells it s different The cell wall is so hard it can t

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Purdue BIOL 10200 - Mitosis Notes

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