LO20-真核细胞分裂.docx

X Hello and Welcome! In this online session,you wil learn about Cell Division of Eukaryotes. Page 1/53 School of Biological Sciences Recognise the difference between eukaryotic chromosomal organisation compared to the prokaryotic one; Distinguish between haploid and diploid states; Distinguis...

X Hello and Welcome! In this online session,you wil learn about Cell Division of Eukaryotes. Page 1/53 School of Biological Sciences Recognise the difference between eukaryotic chromosomal organisation compared to the prokaryotic one; Distinguish between haploid and diploid states; Distinguish between chromosomes and chromatids; Recognise the processes of mitosis and meiosis; Describe the concept of reduction division in meiosis; and Identify the differences between mitosis and meiosis. NANYANG TECHNOLOGICAL UNIVERSITY School of Biological Sciences NANYANG School of Biological Sciences TECHNOLOGICAL UNIVERSITY Diploid Chromosome Numbers of Some Eukaryotes Type of Organism Scientific Name (Common Name) Chromosome Number Fungi Plants Saccharomyces cerevisae(yeast) Pisum sativum (garden pea) Solanium tuberosum(potato) Ophioglossum vulgatum (Southern adder's tongue fern) 32 14 48 1,320 Animals Drosophila melanogaster (fruit fly) Homo sapiens(human) Carassius auratus(goldfish) 8 46 94 Page 4/53 School of Biological Sciences Page 5/53 Homologous chromosomes Centromere Prior to cell division,each of the homologous chromosomes replicates, forming two identical copies called sister chromatids. The sister chromatids are joined together by a structure called a centromere. Humans have twenty-three pairs of homologous chromosomes. When each chromosome in the pair is replicated,this makes for a total of ninety-two chromatids. Homologous chromosomes Homologous chromosomes Sister Chromatids School of Biological Sciences Mitosis Meiosis A cell division mechanism that occurs primarily in non-reproductive cells. A cell division mechanism that occurs in cells that participate in sexual reproduction, which you will learn more about in the next section: Asexual and Sexual Reproduction. These cells are called somatic cells. These cells are called germ line cells. The eukaryotic cell cycle is divided into distinct phases: Interphase Prophase Metaphase 3 Interphase (G,S,G,phases) Cytokinesis (C phase) Cytokinesis Telophase Anaphase Interphase (G1,S,G2 phases) G,Phase The primary growth phase of the cell following division. Most cells spend the majority of their lifespan in this phase. S Phase DNA replication occurs in preparation for cell division. G,Phase Further preparation occurs,e.g. replication of centrioles condensation of chromosomes The eukaryotic cell cycle is divided into distinct phases: Interphase Prophase Metaphase 3 Interphase (G,5,G,phases) Cytokinesis (C phase) Cytokinesis Telophase Anaphase Mitosis (M phase) The step that follows interphase is a division of the nuclear contents, a process known as mitosis. Prophase 2 Metaphase During mitosis,a division of the 3 nuclear contents occurs. Mitosis is a continuous process whereby a micro-tubular apparatus binds to the chromosomes and moves them apart. For ease of study,it is divided into four distinct stages: Prophase; Metaphase; Anaphase;and Telophase. Mitosis (M phase) 5 Anaphase Telophase Prophase (G,S,G₂phases) The chromosomes condense. The spindle (attached to the chromosomes)forms. The nuclear envelope breaks down. 2 Centrioles Page 14/53 Metaphase The chromosomes line up on the central plane of the cell. 6 Metaphase Spindle fiber Centromere and kinetochore (G,5,G₂phases) Cytokinesis (C phase) 5 Metaphase Anaphase The spindle fibres disappear. 5 Telophase The eukaryotic cell cycle is divided into distinct phases: Interphase Prophase Metaphase 3 Interphase (G,S,G,phases) Cytokinesis (C phase) Cytokinesis Telophase Anaphase Cytokinesis (C phase) It occurs at the end of mitosis. yhtplvlit roughly daughter cells In animals,cytokinesis occurs by actin filaments contracting and pinching the cell in two. In plants,a new cell wall is laid down to divide the two daughter cells. Page 19/53 Animal Cell Cleavage furrow Plant Cell Cell Wall Nuclei Vesicle containing membrane components fusing to form cell plate Haploid sperm Haploid egg School of Biological Sciences This reduction division resulting in the formation of gametes is achieved through the process of meiosis. Meiosis and fertilisation are two key events that are the hallmark of sexual reproduction. Meiosisl DNA is replicated only before Meiosis l. Meiosis I separates the homologous chromosomes. Meiosis ll separates the replicated sister chromatids. When meiosis is complete,the result is that one diploid cell has become four haploid cells. Meiosis l Traditionally divided into four stages: Prophase I Metaphase l Anaphase l Telophase l Homologous Microtubule spindle Homologous One set of paired chromosomes chromosomes apparatus attaches pairs of chromo- arrives at each pole,and nuclear further condense to chromosomes. somes separate and and pair. Homologous pairs move to opposite Crossing over occurs. align along spindle poles. Spindle fibres form. equator. Prophase l Page 28/53 The homologues chromosomes pair up. Crossing over occurs between non-sister chromatids of homologous chromosomes. Metaphase l Page 29/53 The paired homologous chromosomes align on a central plane of the cell. > Anaphase l Page 30/53 and move to opposite poles of the cell. Telophase l The chromosomes gather at each of the two poles to form two chromosome clusters Prophase II Chromosomes recondense. Telophase Il Chromatids arrive at each pole,and cell divisions begins, Cell division is complete.Each cell ends Prophase Il Page 33/53 New spindle forms to attach to chromosome clusters. Metaphase I Spindle fibres bind to both sides of the centromere. Individual chromosomes align along a central plane. Anaphase I The sister chromatids are pulled apart at the centromere. The chromatids move to opposite poles. Telophase II The nuclear envelope is reformed around each of the four sets of daughter chromosomes. X Lastly in Telophase ll,the nuclear envelope is reformed around each of the four sets of daughier chromosomes. Synapsis The homologous chromosomes pair along their entire lengths. together by cohesin proteins. This close association permits crossing over between sister and non-sister chromatids. ① Overview Sister chromatids of one duplicated chromosome Sister chromatids of chromosome's homologue Synapsis Page 37/53 Reduction Division ① Overview The final amount of genetic material passed to the gametes is halved. It is due to two nuclear divisions but only one replication of DNA Meiosis l Diploid germ-line cell Chromosome duplication Haploid gametes Meiosis I School of Biological Sciences Meiosis Il Meiosis Mitosis Homologous chromosomes pair up Homologous chromosomes do not normally pair up Crossing over No crossing over Two cell divisions One cell division Four daughter cells Two daughter cells Daughter cells haploid (n) Daughter cells diploid (2n) School of Biological Sciences Take Home Messages In this section on Cell division Eukaryotes,you need to take home the following messages: 1.Chromosomes are organised as linear chromosomes in eukaryotic cells,and the number varies greatly among the organisms. 2.Cells which have only one set of chromosomes are referred to as"haploid".Cells which have two sets of chromosomes are referred to as"diploid". 3.Chromosomes in diploid organisms exist as homologous pairs in somatic cells. 4.Eukaryotic cells undergo two different mechanisms to divide up the DNA into daughter cells:mitosis and meiosis. Page 42/53 School of Biological Sciences In this section on Cell division Eukaryotes,you need to take home the following messages. 5. Mitosis is a part of the cell cycle and consists of four phases: Prophase;Metaphase;Anaphase;and Telophase. 6.The two daughter cells generated by mitosis has a chromosome collection identical to that of the parent. 7. Meiosis is responsible for reduction division that generates haploid gametes.It is made up of Meiosis I and Meiosis ll,each consisting of four phases,similarly to mitosis. 8.Meiosis has features not found in mitosis -synapsis of homologous chromosomes,and reduction division resulting in four daughter cells.