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Date Created: 10/26/15
January 12 2010 Each chromosome has many genes often thousands Many eukaryotes including humans do not tolerate having an abnormal number of chromosomesespecially the somatic chromosomes One too many ie three is a trisomygt down syndrome One too few ie one is a monosomygt no known cases none survive to birth all eukaryotic have at least 2 chromosomes 1 pair somatic chromosomes other than sex cells Eukaryotic chromosomes are composed of chromatin a complex of DNA and proteins The long single DNA molecule of each chromosome is highly compacted via coiling around histone complexes 8 protein molecules forming a unit know as a nucleosome Nucleosome compact to form a larger unit known as a solenoid beads on string then compact further to solenoid level Compaction to the solenoid level is characteristic of interphase chromatic called euchromatin Euchromatin DNA is generally available for transcription During mitosis and meiosis the chromatin compacts further via loops and coils of loops Compaction at this level produces chromatin of the heterochromatin form loops of loops level Heterochromatin DNA is generally not available for transcription fig 98 with more compaction chromosomes become short and thick easy to move around Many eukaryotes have pairs of homologous chromosomes therefore two sets diploid 2n One member of each pair was inherited from the mother maternal member and one from the father paternal member linker DNA between histones Humans are diploid and have a 2n number of 46 chromosomes fig 915 is a human karyotype Some have only one set haploid in Some have more than two sets polyploidy 3n 4n etc triploid tetraploid pentaploid Mitosis eukaryotic cell division process used for growth and repair of multicellular species and reproduction of many unicellular species part of the cell cycle Five phases of the cell cycle 31 Gap 1 growth 5 DNA replication synthesis 32 Gap 2 growth and preparations for mitosis 315 and 32 comprise interphase M mitosis events that lead to the separation of the two daughter genomes Ccytokinesis division of the cytoplasm producing the two daughter cells Fig 93 actual duration of cell cycle changes in cell type Duration of the cell cycle Highly variable among species and tissue types and developmental stages of any one species a few minutes to years some cells never complete the cell cycle Cells often enter 30 from 31 in fact most cells of most organisms are in 30 the phase in which they function of m the mitotic phase of the cell cycle in animal cells Possible for some 30 cells to be initiated back into 31 and stages can cont through cell cycle Prophase Chromosomes continue to condense began in 32 Centrioles replicated in 32 of centrosomes separate and develop radial arrays of microtubules asters plants do not have centrioles Microtubules from the centrioles grow out and after reach max condense in metaphase centriolesgt create mitotic spindlereplicated in 32 move apart in prophase Prometaphase in which the nuclear envelope fragments Attach to the kinetochore region of each sister chromatid kinetochore microtu bules attacks proteins that attach microtubules bundles not just one Or remain unattached to the chromosomes polar microtubules nonkinetochore microtubules Metaphase karyotypes are made Mitotic spindle fully formed Chromosomes are aligned on equator of cell metaphase plate equatorial plateplane condensed as much as they will condense Fig 99 just before metaphase not on metaphase plate yet Anaphase Sister chromatids separate and become chromosomes they separate as the result of the enzyme separase hydrolyzing the cohesion protein that holds them together when separate each sister chromatid becomes chromosomes The poles of the mitotic spindle move apart the result of the polar microtubules sliding by each other The kinetochore microtubules shorten at the poles removal of tubulin units and the chromosomes walk down the microtubules using motor proteins tubulin units are removedgt shortening the microtubulin moving the chromosome Telophase Chromosomes reach poles Nuclear envelope forms around each set of chromosomes Chromosomes decondense Cytokinesis actual cell division Usually occurs following telophase but not always Varies among eukaryotes In animals occurs via a cleavage furrow outside in In plants via the formation of a cell plate inside out Summary figs For mitosis and cytokinesis figs 910 and 912 January 7 2010 Succeed in class Read assigned chapters in advance of lecture and recitation Attend lecture take good notes Review notes and read appropriate chapter sections again after lecture Ask questions in recitation Review all topics each week Take advantage of office hours lpHU39IprJNH Begin serious prep for exam at least one week before exam Chapter 9 Chromosomes the cell cycle and cell division Cell division 3 types Binary fission prokaryotes only bacteria Reproduction principle way bacteria increase in numbers Daughter cells are genetically identical barring mutation daughter cells are identical to each other and to parent Mitosis eukaryotes limited to eukaryotes Growth and repair in multicellular species and production of gametes in some not true in humans Reproduction in many unicellular species Daughter cells are genetically identical Daughter cells are identical to each other and parent Meiosis eukaryotes Typically used in production of gametes how we use meiosis Daughter cells are not identical Four events required for cell division via binary fission mitosis or meiosis Reproductive signal initiates division comes from inside or outside cell Replication of DNA Segregation of replicated DNA to the daughter cells Cytokinesis Cell division in prokaryotes binary fission Produces two daughter cells with the same genetic information as the parent cell Events in binary fission The single circular doesn t have an end a loop chromosome is much folded producing a nucleoid region some bacteria have linear but most have a single circular chromosome Replication of the chromosome begins at a single point of origin of replication ori and terminates at the terminus ter The cell elongates by growth replications move away from each other while cell grows The daughter chromosomes separate from each other proteins and cytoskeleton play roles as the cell grows Septation the formation of a barrier separating the daughter cells eventually pinches off the daughter cells when completed the daughter cells separate from each other This occurs as the result of the formation of an FtsZ filament temperature sensitive Z protein ring at midpoint of cell and inward growth of septum fig 92 chromosome not really even as big as red dot indicating ori FtsZ protein is similar to tubulin a major component of microtubules which are used in mitotic and meiotic spindles during cell division in eukaryotes Cell division in eukaryotes is complicated by the presence of more DNA more than one chromosome at least one pair and a nuclear envelope The evolutionary sequence leading from binary fission to mitosis and meiosis is unclear as is the possible evolution from FtsZ Protein to tubulin The details of mitosis differ among different groups of eukaryotes In some protista the nuclear envelope remains intact during mitosis in some the spindle develops within the nucleus in some the chromosomes attach to the inner surface of the nuclear envelope But in most plants and animals the nuclear envelope fragments the spindle develops outside the nucleus and the chromosomes do not attach to the nuclear envelope Eukaryotic chromosomes Discovered and seen to divide in 1882 by Walther Fleming don t need to know He named the cell division process in which this occurs mitosis we will restrict mitosis to the segregation of daughter sets of chromosomes The number of chromosomes differs significantly among eukaryotes ranging from 2 to more than 1000 table 91