Foundations of Biology II William Coffman Spring 2012 Chapter 11 The Cell Cycle Cell Division Processes 1 Binary Fission a Limited to bacteria prokaryotes prokaryote before nucleus b Reproduction c Daughter cells are genetically identical clones 2 Mitosis 3 Meiosis a Eukaryotes only b Growth and repair c Reproduction d Daughter cells are genetically identical to parent cell in many unicellular species in multicellular species and production of gametes in some a Limited to eukaryotes b Typically used in production of gametes c Daughter cells are not genetically identical to parent cell or to each other 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 o Each daughter gets correct DNA component Cytokinesis o Actual division of cells o Not necessary not part of mitosis many cells undergo mitosis without cytokinesis BINARY FISSION Events of binary fission region Produces two daughter cells with the same genetic information as the parent cell o The singular circular chromosome is typically much folded producing a nucleoid o Replication of the chromosome begins at a single point of origin of replication ori and terminates at the terminus ter o Cell elongates by growth o The daughter chromosomes attach to the inner surface of the plasma membrane and separate as the cell growth proceeds o Septation the formation of a barrier separating the daughter cells eventually pinches off the daughter cells This occurs as the result of the formation of a barrier FtsZ protein ring at the midpoint of a cell and inward growth of septum septation occurs as the contractile protein ring contracts FtsZ Filament temperature sensitive mutant Z provides new plasma membrane FtsZ protein is similar to tubulin a major component of microtubules that are used in mitotic and meiotic spindles during cell division in eukaryotes Foundations of Biology II William Coffman Spring 2012 Mitosis and Meiosis Cell division in eukaryotes is complicated by the presence of more DNA than one chromosome at least one homologous pair and a nuclear envelope o Evolutionary sequence leading from binary fission to mitosis and meiosis is unclear as it is possible and probable that evolution from FtsZ protein to tubulin could have occurred not known for sure The details of mitosis differ among different groups of eukaryotes In some Protista eukaryotes very diverse some multicellular the nuclear envelope remains intact during mitosis in some the spindle develops within the nucleus in some 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 envelope Eukaryotic Chromosomes Discovers and seen divide in 1882 by Walter Fleming He named the cell division process by which this occurs mitosis mitosis restricted to the segregation of daughter sets of chromosomes The number of chromosomes differs significantly among eukaryotes ranging from two to more than 1000 in some plants and fungi Each chromosome has many genes often thousands on a DNA molecule o Many eukaryotes including humans do not tolerate having an abnormal number of chromosomes especially somatic chromosomes One too many 3 trisomy One too few 1 monosomy Humans cannot survive without two of each somatic chromosome miscarriage Approximately 50 of miscarriages are due to trisomy or monosomy Eukaryotic chromosomes are composed of chromatin a complex of DNA and proteins o The long single DNA molecule of each chromosome is highly compacted via coiling around histone complexes 8 protein molecules forming a unit known as a nucleosome 4 types of histones contains 2 molecules of each kind 5th kind doesn t count is used to combine DNA to the nucleosome o Nucleosomes compact to form a larger unit known as a solenoid Compaction to the solenoid level is characteristic of interphase chromatin called euchromatin Euchromatin DNA is generally available for transcription o During mitosis and meiosis the chromatin compacts further via loops and coils of loops solenoid compacts coils on itself Compaction at this level produces chromatin of the heterochromatic form Heterochromatic DNA is generally not available for transcription genes are too compacted fig 11 2 Foundations of Biology II William Coffman Spring 2012 Heterochromatic DNA is shorter and thicker than euchromatic DNA EXCEPTION one X chromosome of every female is heterochromatic during interphase while the other is looped and coiled Many eukaryotes have pairs of homologous chromosomes therefore two sets diploid 2n males have 22 homologous pairs females have 23 o One member of each pair was inherited from the mother maternal member and one from the father paternal member o Humans are diploid and have a 2n number of 46 chromosomes Some have only one set haploid n o Many do not reproduce sexually Some have more than two sets polyploidy 3n 4n 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 Mitosis Interphase G1 gap 1 growth S DNA replication G2 gap 2 growth and preparation for mitosis M mitosis events that lead to the separation of the two daughter genomes C cytokinesis division of the cytoplasm producing the two daughter cells OPTIONAL Fig 11 3 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 o Cells often enter G0 from G1 in fact most cells of most organisms are in G0 the phase in which the function as muscle cells liver cells etc Stages of Mitosis in ANIMAL cells PROPHASE o Chromosomes continue to condense began in G2 o Centrioles replicated in G2 of centrosomes separate and develop radial arrays of microtubules asters First animal without centrosome discovered flatworm o Microtubules from the centrioles grow out and after PROMETAPHASE METAPHASE o Nuclear envelope fragments o Attach chromosomes polar microtubules slide past each other pullers to right and left kinetochore microtubules OR remain unattached to Foundations of Biology II William Coffman Spring 2012 o Mitotic spindle fully formed o Chromosomes are aligned on equator of cell metaphase plate o Chromosomes are
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