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Lecture 7 Mitosis & Meiosis. Cell Division  Essential for body growth and tissue repair  Interphase  G 1 phase  Primary cell growth phase  S phase.

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Presentation on theme: "Lecture 7 Mitosis & Meiosis. Cell Division  Essential for body growth and tissue repair  Interphase  G 1 phase  Primary cell growth phase  S phase."— Presentation transcript:

1 Lecture 7 Mitosis & Meiosis

2 Cell Division  Essential for body growth and tissue repair  Interphase  G 1 phase  Primary cell growth phase  S phase  DNA replication  G 2 phase  Microtubule synthesis  Mitosis  Nuclear division  Cytokinesis  Cytoplasmic division

3 Chromosomes: Carriers of Inherited Traits  Chromosomes were first observed by the German embryologist Walther Fleming in 1882  Chromosomes exist in somatic cells as pairs  Homologous chromosomes or homologues  The number of chromosomes varies enormously from species to species  The Australian ant Myrmecia spp. has only 1 pair  Some ferns have more than 500 pairs

4 Human Chromosomes  Humans have 46 chromosomes  This display is termed a karyotype  The 23 pairs of homologous chromosomes can be organized by size

5 Basic Terminology  Diploid cells have two copies of each chromosome  Replicated chromosomes consist of two sister chromatids  These are held together at the centromere

6 Chromosomes  Chromosomes are composed of chromatin  Complex of DNA (~ 40%) and proteins (~ 60%)  A typical human chromosome contains about 140 million nucleotides in its DNA  This is equivalent to  About 5 cm in stretched length  2,000 printed books of 1,000 pages each!  In the cell, however, the DNA is coiled

7 Chromosomes: Packaged DNA  The DNA helix is wrapped around positively-charged proteins, called histones  200 nucleotides of DNA coil around a core of eight histones, forming a nucleosome  The nucleosomes coil into solenoids  Solenoids are then organized into looped domains  The looped domains appear to form rosettes on scaffolds

8 Mitosis  The phases of mitosis are: Prophase XmXm XfXf XmXm XfXf XmXm XfXf Condense Metaphase XmXm XfXf XmXm XfXf XmXm XfXf Line Up Anaphase ImIm IfIf ImIm IfIf ImIm IfIf IfIf ImIm IfIf ImIm IfIf ImIm Separate Telophase ImIm IfIf ImIm IfIf ImIm IfIf IfIf ImIm IfIf ImIm IfIf ImIm Divide  Cytokinesis  Cleavage furrow formed in late anaphase by contractile ring  Cytoplasm is pinched into two parts after mitosis ends

9 Early Prophase Early prophase Early mitotic spindle Pair of centrioles Centromere Aster Chromosome, consisting of two sister chromatids Figure 3.32.2 Prophase PLAY  Asters are seen as chromatin condenses into chromosomes

10 Late prophase Fragments of nuclear envelope Polar microtubules Kinetochore Kinetochore microtubule Spindle pole Late Prophase Figure 3.32.2 Prometaphase PLAY  Nucleoli disappear  Centriole pairs separate and the mitotic spindle is formed

11 Metaphase Metaphase plate Spindle Metaphase Figure 3.32.4  Chromosomes cluster at the middle of the cell with their centromeres aligned at the exact center, or equator, of the cell  This arrangement of chromosomes along a plane midway between the poles is called the metaphase plate Metaphase PLAY

12 Anaphase Figure 3.32.5 Daughter chromosomes Anaphase  Centromeres of the chromosomes split  Motor proteins in kinetochores pull chromosomes toward poles Anaphase PLAY

13 Telophase and Cytokinesis Telophase and cytokinesis Nucleolus forming Contractile ring at cleavage furrow Nuclear envelope forming  New sets of chromosomes extend into chromatin  New nuclear membrane is formed from the rough ER  Nucleoli reappear  Generally cytokinesis completes cell division Telophase PLAY

14 Controlling the Cell Cycle 1.Cell growth is assessed at the G 1 checkpoint 2.DNA replication is assessed at the G 2 checkpoint 3.Mitosis is assessed at the M checkpoint G 0 is an extended rest period  The eukaryotic cell cycle is controlled by feedback at three checkpoints Control of the cell cycle PLAY

15  During fetal development, many cells are programmed to die Cell Death  Human cells appear to be programmed to undergo only so many cell divisions  About 50 in cell cultures  Only cancer cells can divide endlessly Programmed cell death Fingers and toes form from these paddle-like hands and feet

16 What is Cancer?  Cancer is unrestrained cell growth and division  The result is a cluster of cells termed a tumor  Benign tumors  Encapsulated and noninvasive  Malignant tumors  Not encapsulated and invasive  Leave the tumor and spread throughout the body  Can undergo metastasis

17 What is Cancer?  Most cancers result from mutations in growth-regulating genes  There are two general classes of these genes  1. Proto-oncogenes  Encode proteins that stimulate cell division  If mutated, they become oncogenes  2. Tumor-suppressor genes  Encode proteins that inhibit cell division  Cancer can be caused by chemicals, radiation or even some viruses

18 Cancer, cell division, and p53 protein How tumor suppression genes work PLAY

19 Receiving the signal to divide Passing the signal via a relay switch Amplifying the signal Releasing the “brake” Checking that everything is ready Stepping on the gas Stopping tumor growth New molecular therapies for cancer  Potential cancer therapies are being developed to target seven different stages in the cancer process  Stages 1-6  Prevent the start of cancer within cells  Focus on the decision-making process to divide  Stage 7  Act outside cancer cells  Prevents tumors from growing and spreading

20  Diversity is what allows gene populations to survive changes in their environment  The greater the diversity, the more likely some individuals will have the traits to survive a major change  Genetic diversity is the raw material that fuels evolution  No genetic process generates diversity more quickly than sexual reproduction Importance of Generating Diversity

21 Contains two sets of chromosomes Two Types of Reproduction  Sexual reproduction  Maximizes diversity  Involves the alternation of meiosis and fertilization  Meiosis halves the number of chromosomes from each parent cell  Fertilization restores the number of chromosomes  Asexual reproduction  Creates a faithful copy  Does not involve fertilization Contain one set of chromosomes

22 The Sexual Life Cycle  The life cycles of all sexually-reproducing organisms follows the same basic pattern  Haploid cells or organisms alternate with diploid cells or organisms  There are three basic types of sexual life cycles Spend most of life in haploid form Spend most of life in diploid form Spend half of life in haploid form and half in diploid form

23 The Sexual Life Cycle in Humans

24 Discovery of Meiosis  Meiosis was first observed by the Belgian cytologist Pierre- Joseph van Beneden in 1887  Gametes (eggs and sperm) contain half the complement of chromosomes found in other cells  The fusion of gametes is called fertilization or syngamy  It creates the zygote, which contains two copies of each chromosome

25 Mitosis Meiosis Prophase XmXm XfXf XmXm XfXf XmXm XfXf Condense Metaphase XmXm XfXf XmXm XfXf XmXm XfXf Line Up Anaphase ImIm IfIf ImIm IfIf ImIm IfIf IfIf ImIm IfIf ImIm IfIf ImIm Separate XfXf XmXm XmXm XmXm XfXf XfXf IfIf IfIf ImIm IfIf IfIf ImIm ImIm ImIm ImIm ImIm IfIf IfIf XmXm XfXf XmXm XfXf XmXm XfXf ImIm ImIm IfIf ImIm IfIf ImIm ImIm ImIm IfIf IfIf IfIf IfIf XfXf XmXm XmXm XmXm XfXf XfXf XmXmfXfX mXmX XfXf XmXm fXfX Meiosis I mXmX XfXf XmXmfXfX XmXm fXfX Meiosis II XfXf XmXm XmXm XmXm XfXf XfXf Telophase ImIm IfIf ImIm IfIf ImIm IfIf IfIf ImIm IfIf ImIm IfIf ImIm Divide Unique features of meiosis PLAY

26 Meiotic Cell Division: Meiosis I Figure 27.7  Tetrads line up at the spindle equator during metaphase I  In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to opposite ends of the cell  At the end of meiosis I each daughter cell has:  Two copies of either a maternal or paternal chromosome  A 2n amount of DNA and haploid number of chromosomes  In telophase I:  The nuclear membranes re-form around the chromosomal masses  The spindle breaks down  The chromatin reappears, forming two daughter cells Prophase IProphase I, Metaphase I, Anaphase I, Telophase IMetaphase IAnaphase ITelophase I PLAY

27 Meiotic Cell Division: Meiosis II  Mirrors mitosis except that chromosomes are not replicated before it begins  Meiosis accomplishes two tasks:  It reduces the chromosome number by half (2n to n)  It introduces genetic variability Meiosis II PLAY

28 Differences Between Mitosis & Meiosis Purpose Where it occursBody cellsSpecial reproductive organs Number of divisions after one chromatin replication event OneTwo Number of daughter cells TwoFour Homologous chromosomes in daughter cells Two (diploid)One (haploid) Makeup of daughter cells vs. parent cell Same as parent cellDifferent from parent cell Mitosis Meiosis Individually Homologous pairs in Meiosis I, Individually in Meiosis II How chromosomes Line up at metaphase Differences between meiosis &. mitosis PLAY Create new cells (faithful copies) Create new individuals (diversity)

29 Evolutionary Consequences of Sex  Sexual reproduction increases genetic diversity through three key mechanisms 1.Independent assortment 2.Crossing over 3.Random fertilization

30  In humans, a gamete receives one homologue of each of the 23 chromosomes  Humans have 23 pairs of chromosomes  2 23 combinations in an egg or sperm  8,388,608 possible kinds of gametes Independent assortment Three chromosome pairs 2 3 combinations

31  DNA exchanges between maternal and paternal chromatid pairs Crossing over  This adds even more recombination to independent assortment that occurs later

32  The zygote is formed by the union of two independently- produced gametes  Therefore, the possible combinations in an offspring  8,388,608 X 8,388,608 =  70,368,744,177,664  More than 70 trillion!  And this number does not count crossing-over Random fertilization

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