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CAS BI 108 (112)
Chapter 11.3

Chapter 11.3 Notes

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Department
Biology
Course
CAS BI 108
Professor
Francis Monette
Semester
Spring

Description
11.3: What Happens during Mitosis? Simple answer: segregation of replicated DNA. Prior to mitosis, eukaryotic DNAis packed into very compact chromosomes • Chromosome: one or two long, linear double stranded DNAmolecules bound with many proteins (DNA/Proteins = chromatin) • Before S phase each chromosome contains only one double-stranded DNAmolecule. After it replicates during S phase there are two double stranded DNAmolecules: the sister chromatids • Sister chromatids are held together by protein complex called cohesin.  Cohesin is removed at the end of mitosis. But some remains at the centromere where the chromatids remain held together • Condesins coat the DNAmolecules and make them more compact after G2 • Packing is achieved by histones, which are positively charged (amino acids) that bind to negatively phosphate groups on DNA  DNA-Histone interactions and histone-histone interactions result in nucleosomes  During this phase of the cell cycle, DNAis accessible for proteins involved in replication and transcription  However, once a mitotic chromosome is formed, its compactness makes it inaccessible to replication and transcription factors Overview: Mitosis segregates copies of genetic information A single nucleus gives rise to two nuclei that are genetically identical to each other and the parent nucleus. Prophase: Chromatin condenses Separate chromatids visible Prometaphase: Nuclear envelope breaks down Chromosomes (two chromatids) attach to spindle apparatus Metaphase: Chromosomes line up at the midline Anaphase: Chromatids separate and move away from each other toward opposite poles The centrosomes determine the plane of cell division SpindleApparatus: (mitotic spindle) moves sister chromatids apart. Made of microtubules. Its orientation is determined by the centrosome The centrosome consists of a pair of centrioles, each one is a hollow tube formed by microtubule triplets. These centrioles are at right angles to each other. During S phase the centrosome doubles. Then, during prophase they move to opposite ends of the nuclear envelope. This identifies the “poles” toward which chromosomes move during anaphase. Note: plants and fungi lack centrosomes. The events of the centrosomes contribute to division in this way: determine the plane at which the cell will divide. i.e. the spatial relationship between the two new cells. The spindle begins to form during prophase At this point, most of the cohesin that has held the replicated DNAtogether is removed, so individual chromosomes become visible.  However, there is a small amount of cohesion at the centromere. Later in prophase, kinetochores develop on the centromere, one on each chromosome. Each of the two
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