Week 6 Course Notes
Week 6 Course Notes Biol 2002
U of M
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This 6 page Class Notes was uploaded by Sydney Diekmann on Thursday October 8, 2015. The Class Notes belongs to Biol 2002 at University of Minnesota taught by Dr. Susan Wick, Dr. David Matthes in Fall 2015. Since its upload, it has received 35 views. For similar materials see Foundations of Biology in Biology at University of Minnesota.
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Date Created: 10/08/15
Sections 122 through 124 The Cell Cycle 122 M mitotic or meiotic Phase I two main events 0 division of the nucleus gt replicated chromosomes are divided into two daughter nuclei with identicle chromosomes and genes 0 division of the cytoplasm gt cytokinesiscytoplasmic division I chromosomes are DNA wrapped around globular histone proteins called chromatin 0 at the start of mitosis each chromosome consists of two sister chromatids that are attached to each other at the centromere gt centromere area where proteins called cohesins remain attached during mitosis I during mitosis two sister chromatids separate to form independent daughter chromosomes 0 a copy of each chromosome goes to each daughter cell I 5 mitotic subphases 1 prophase chromosomes condense and spindle apparatus forms gt spindle apparatus structure that produces mechanical force that moves replicated chromosomes during early mitosiss and pulls chromatids apart in late mitosis 2 prometaphase nuclear envelope breaks down and microtubules contact chromosomes at kinetochores gt kinetochores specialized structures where kinetochore microtubules attach to chromosomes to prepare for migration to the middle of the cell for separation 3 metaphase chromosomes migrate to the middle of the cell gt metaphase plate imaginary line between the two spindle poles where chromosomes line up 4 anaphase sister chromatids are separated into daughter chromosomes gt pulled apart to opposite sides of the cell gt daughter chromosomes move to opposite side via the connection of kinetochore proteins to the shortening kinetochore microtubules gt poles of the spindle apparatus are pulled farther apart elongating the cell 5 telophase the nuclear envelope reforms and chromosomes decondense gt when two complete nuclei form chromosomes decondense and cytokinesis begins 123 Cell Cycle Checkpoints I G1 cycle will continue if 0 cell size is adequate o nutrients are sufficient 0 social signals are present 0 DNA is undamaged G2 cycle will continue if 0 chromosomes have replicated successfully 0 DNA is undamaged o activated MPF is present gt MPF M phasepromoting factor Mphase cycle will continue if 0 chromosomes have attached to spindle apparatus 0 chromosomes have property separated o MPF is absent 124 Cancer and Cell Division cancer disease caused by cells that divide in an uncontrollable fashion gt invasion of nearby tissue gt spreads to other sites of the body causes disease because the cells use nutrients and space needed by normal cells which disrupts their overall function cancers occur when cellcycle checkpoints see above have FAILED two primary defects 0 defects that make proteins required for cell growth active when they shouldn t be 0 defects that prevent tumor suppressor genes from shutting down the cell cycle Properties of Cancer Cells malignant tumor invasive cancerous mass of cells benign tumor noninvasive noncancerous mass of cells cells become malignant and cancerous when they detach from the tumor and move to other tissues in the body Loss of CellCycle Control social control 0 in multicellular organisms the passage through the G1 checkpoint also depends on response signals from other cells gt Le individual cells should grow when it is in the best interest of the organism as a whole 0 based on growth factors gt polypeptidessmall proteins that stimulate cell division 0 cells can become cancerous when social controls fail gt Le when cell begins to divide without goahead signal from growth factors Sections 131134 Meiosis I sexual reproduction occurs when a male reproductive cell m and a female reproductive cell egg unite in a process known as fertilization 0 during the formation of gametes reproductive cells there is a distinctive form of cell division called meiosis gt causes each cell to only have 23 chromosomes 0 when sperm and egg combine the resulting cell has 46 chromosomes I meiosis nuclear division that leads to a halving of chromosome number and the production of gametes 131 How Does Meiosis Occur I chromosomes come in distinct shapes and sizes 0 sex chromosomes X and Y chromosomes gt associated with individual s sex XY male and XX female 0 autosomes nonsex chromosomes 0 homologous chromosomeshomologs chromosomes that are the same size and shape gt carry the same genes but may carry different alleles 0 gene section of DNA that influences some hereditary trait in an individual 0 allele different versions of the same gene I chromosomes can come in different numbers and types in an organism o diploid two versions of each type of chromosome are present 0 haploid one type of each chromosome is present gt n haploid number 0 in humans n 23 gt to indicate the number of each type of chromosome a number is placed before n 0 cells can be 2n 2 sets of n chromosomes 0 combination of the number of sets and n is called the cell s my 0 maternal chromosomes come from the mother paternal from the father A Overview of Meiosis I chromosomes are replicated before undergoing meiosis o unreplicated and replicated chromosomes are both considered single chromosomes even though the replicated chromosome contains two sister chromatids gt each chromosome carries a particular set of unique genetic information I meiosis is comprised of two cell divisions meiosis and meiosis II 0 during meiosis the homologs in each chromosome pair separate from each other gt one homolog goes to each daughter cell gt the diploid 2n parent cell produces two haploid n daughter cells 0 during meiosis II the sister chromatids from each chromosome separate gt these cells still have one of each type of chromosome but these daughter chromosomes are no longer replicated o chromosome movements are coordinated by the spindle apparatus that attach to the kinetochores located at the centromere of each chromosome B C meiosis is a reduction division meaning that the number of chromosomes is reduced 0 when two haploid gametes fuse during fertilization a full complement of chromosomes is restored gt resulting cell is diploid and called a zygote 0 each diploid individual receives a haploid chromosome set from mother and a haploid chromosome set from its father Phases of Meiosis begins after chromosomes have been replicated during s phase 0 early prophase gt chromosomes condense spindle apparatus forms nuclear envelope beings to break down synapsis pairing of homologous chromosomes begins 0 pairing of homologous chromosomes results in bivalents 0 late prophase gt chiasmata crossover points can be seen nuclear envelope is broken down there is often multiple chiasmata between nonsister chromatids gt crossing over occurs which is a process of chromosome exchange between two chromatids 0 at a chiasma the nonsister chromatids from each homolog have been physically broken at the same point and attached to each other gt corresponding segments of maternal and paternal chromosomes are exchanged o metaphase gt migration of bivalents to metaphase plate is complete 0 the metaphase plate is not a physical structure just a line used for identification 0 anaphase gt homologs separate and begin moving to opposite poles of the spindle apparatus 0 telophase and cytokinesis gt chromosomes move to opposite poles of the spindle apparatus and the spindle apparatus disassembles Phases of Meiosis two haploid cells gt four daughter cells that contain half the number of chromosomes as parent cell 0 prophase II gt spindle apparatus forms 0 metaphase II gt chromosomes line up at the middle of the spindle apparatus at the metaphase plate 0 anaphase II gt sister chromatids separate and begin moving to opposite poles of the spindle apparatus 0 telophase II and cytokinesis gt chromosomes move to opposite poles of the spindle apparatus and the spindle apparatus disassembles 132 Meiosis Promotes Genetic Variation I due to independent shuffling of maternal and paternal chromosomes and crossing over the chromosomes in one gamete are different from the chromosomes in another 0 changes in chromosomes sets only result from sexual reproduction gt sexual reproduction production of offspring through the production of fusion of gametes 0 based on mitosis gt production of identical daughter cells gt asexual reproduction any mechanism producing offspring that does not involve the production and fusion of gametes I changes in chromosomes produced by meiosis and fertilization are significant because chromosomes contain the cell s hereditary material I aspects of meiosis that create variation among chromosomes 0 separation and distribution of homologous chromosomes 0 crossing over I independent assortment results in a variety of combinations of maternal and paternal chromosomes 0 genetic recombination the appearance of new combinations of alleles gt crossing over results in genetic recombination 133 What Happens When Things Go Wrong in Meiosis A How do mistakes occur I in order for a gamete to get one complete set of chromosomes two steps need to occur 0 the chromosomes in each homologous pair must separate from each other during the first meiotic divisions so that only one homolog ends up in each daughter cell 0 sister chromatids must separate from each other and move to opposite poles of the dividing cell during meiosis II I if both homologs in meiosis or both sister chromatids in meiosis move to the same pole of the parent cell abnormal products will arise o referred to as nondisiunction I meiosis mistakes are common 0 leading cause of spontaneous abortions in humans B Why do mistakes occur I most trisomies and monosomies observed in humans involve the sex chromosomes I errors in meiosis leading to eggs are more common than those leading to sperm I maternal age is an important factor in the occurrence of trisomy 134 Why does Meiosis Exist I asexual reproduction is much more efficient than sexual reproduction because no males are produced A Purifying Selection Hypothesis I sexual individuals are likely to have some offspring that lack the deleterious alleles that are present in a parent 0 gt natural selection against deleterious alleles I reduces the numerical advantage of asexual hypothesis B ChangingEnvironment Hypothesis I if offspring are genetically varied then it is likely that at least some offspring will have combinations of alleles that enable them to be advantageous in changing environments and produce offspring
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