lecture 5 (4)

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  • The ability of organisms to reproduce distinguishes living things from nonliving matter

  • Cell division is the basis of the continuity of life

  • Unicellular organisms reproduce by division of one cell

  • Multicellular organisms depend on cell division for development, growth, and repair

  • Cell division is part of the cell cycle

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  • There are three types of cell division: binary fission, mitotic cell division, and meiotic cell division

  • Binary fission occurs in prokaryotes and produces two exact copies of the original cell

  • Mitotic cell division occurs in somatic cells of eukaryotes and produces two exact copies of the parent cell

  • Meiotic cell division occurs in sex cells of eukaryotes and involves two successive stages of division

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  • The cell cycle consists of the mitotic phase and interphase

  • Interphase makes up about 90% of the cell cycle and can be divided into G1, S, and G2 phases

  • Chromosomes are duplicated during the S phase

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  • G1 phase prepares the cell for division and leads to the S phase

  • S phase involves DNA synthesis and replication of genetic material

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  • G2 phase assembles the necessary materials for mitosis and cytokinesis

  • M phase includes nuclear division (mitosis) and cell division (cytokinesis)

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  • Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell

  • Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase

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  • Prophase is the first stage of mitosis

  • Chromosomes appear as long thin threads and consist of two chromatids

  • Centromeres and kinetochores are present at the centromere region

  • Chromatids are the threads of chromosomes

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  • Prophase: chromatids become thicker and shorter

  • Spindle fibers begin to form at the cell poles

  • Nuclear membranes and nucleoli disappear

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  • Metaphase: nuclear membranes and nucleoli disappear

  • Chromosomes are arranged at the cell equator

  • Chromosomes reach their maximum thickness and minimum length

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  • Anaphase: centromeres split, separating sister chromatids

  • Spindle fibers contract, causing migration of chromatids to opposite cell poles

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  • Telophase: daughter chromosomes are separated into two groups

  • Daughter chromosomes increase in length and decrease in thickness

  • Nuclear envelope reappears around each daughter chromosome group

  • Daughter nuclei are formed at the two poles of the cell

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  • Cytoplasmic division (cytokinesis) begins in late anaphase and is completed in telophase

  • Animal cells undergo midline constriction to separate into two daughter cells

  • Plant cells form a cell plate between the two daughter nuclei

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  • Asexual reproduction produces genetically identical offspring through mitosis

  • Meiosis produces gametes in animals

  • Gametes fuse to form a diploid zygote that develops into a multicellular organism

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  • Meiosis reduces the number of chromosome sets from diploid to haploid

  • Meiosis consists of two sets of cell divisions: meiosis I and meiosis II

  • Four daughter cells are produced in meiosis, each with half as many chromosomes as the parent cell

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  • Meiosis I and meiosis II are the main divisions in meiosis

  • Each division has multiple stages: prophase, metaphase, anaphase, and telophase

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  • Meiosis I: prophase I has stages of leptonema, zygonema, pachynema, deplonema, and diakinesis

  • Leptonema: chromosomes appear as long and thin threads

  • Zygonema: homologous chromosomes pair up

  • Pachynema: crossing over occurs between non-sister chromatids

  • Deplonema: homologous chromosomes repel each other

  • Diakinesis: nuclear envelope and nucleolus disappear, chromosomes become thicker and shorter

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  • Prophase I: pachynema stage involves crossing over and the formation of chiasmata

  • Chiasmata are points of crossing over between non-sister chromatids

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  • Prophase I: diakinesis stage involves the disappearance of the nuclear envelope and nucleolus

  • Chromosomes become thicker and shorter

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  • Metaphase I: chromosomes reach their maximum thickness and shortness

  • Homologous chromosome pairs are arranged at the cell equator

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  • First anaphase (Anaphase-I)

    • Every chromosome migrates to one of the two cell poles.

    • Centromeres do not divide.

    • Each chromosome still consists of two chromatids.

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  • First telophase (Telophase-I)

    • The nuclear envelope is reformed around each chromosome group at the two cell poles.

    • Nucleoli reappear.

    • Two haploid daughter nuclei are formed.

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  • Meiosis II occurs in four phases:

    • Prophase II

    • Metaphase II

    • Anaphase II

    • Telophase II and cytokinesis

  • Meiosis II is very similar to mitosis.

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  • Second Meiosis division (Meiosis-I): Second prophase (Prophase-II)

    • The nucleus contains one chromosome set (haploid set).

    • Chromosomes become shorter and thicker.

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  • Second metaphase (Metaphase-II)

    • Nuclear envelope and nucleoli disappeared.

    • Spindle fibers appear at the two cell poles.

    • The chromosomes arranged on the equatorial plane of the cell.

    • Centromere of every chromosome divides.

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  • Second anaphase (Anaphase-II)

    • Chromatids of each chromosome migrate to one of the two cell poles.

    • Sister chromatids.

    • Haploid daughter cells separate forming.

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  • Second telophase (telophase-II)

    • Chromatids reached cell poles.

    • The nuclear envelope and nucleoli reappear at each pole.

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  • Prophase II

  • Metaphase II

  • Anaphase II

  • Telophase II and Cytokinesis

  • Sister chromatids separate

  • Haploid daughter cells forming

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  • Comparison of mitosis and meiosis in diploid cells

  • MITOSIS

  • MEIOSIS

  • MEIOSIS I

  • Prophase I

  • Chiasma

  • Chromosome replication

  • Homologous chromosome pair

  • Chromosome replication

  • 2n = 6 Parent cell

  • Prophase

  • Replicated chromosome

  • Metaphase

  • Metaphase I

  • Anaphase I

  • Telophase I

  • Haploid n = 3 Daughter cells of meiosis I

  • MEIOSIS II

  • Daughter cells of meiosis II

  • n n n n 2n 2n Daughter cells of mitosis

  • Anaphase

  • Telophase

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  • Comparison of mitosis and meiosis

  • Meiotic division

  • Number of divisions

  • Consists of one division

  • Consists of two divisions; first and second meiotic divisions.

  • Main function

  • Keeps the chromosome number

  • Reduce the chromosome from one generation to number. generation.

  • Types of the resulted cells

  • Produces somatic cells

  • Produces gamete cells.

  • Comparison of Mitosis and Meiosis

  • Prophase stage

  • No crossing over occurs.

  • Consists of 5 substages.

  • Crossing occurs during the synapsis.

  • Metaphase

  • Chromosomes arranged on the equatorial plane.

  • Homologous chromosomes pairs arranged opposite each other on the equatorial plane.

  • Anaphase

  • Centromeres are split.

  • Centromeres do not split.

  • Each Chromatids migrate towards cell chromosome migrates to a pole poles.

  • Opposite to the pole to which its homologous chromosome migr