Fertilization of sperm and egg produces offspring

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1 In sexual reproduction Fertilization of sperm and egg produces offspring In asexual reproduction Offspring are produced by a single parent, without the participation of sperm and egg

2 CONNECTIONS BETWEEN CELL DIVISION AND REPRODUCTION LM Like begets like, more or less Some organisms reproduce asexually And their offspring are genetic copies of the parent and of each other Figure 8.1A

3 Other organisms reproduce sexually Creating a variety of offspring 8 Figure.1B

4 9.2 Cells arise only from preexisting cells Cell division is at the heart of the reproduction of cells and organisms Because cells come only from preexisting cells

5 Colorized TEM 32, Prokaryotes reproduce by binary fission Prokaryotic cells Reproduce asexually by cell division Prokaryotic chromosomes Figure 8.3B

6 As the cell replicates its single chromosome, the copies move apart And the growing membrane then divides the cells Prokaryotic chromosome 1 2 Plasma membrane Cell wall Duplication of chromosome and separation of copies Continued elongation of the cell and movement of copies 3 Division into two daughter cells Figure 8.3A

7 THE EUKARYOTIC CELL CYCLE AND MITOSIS 9.4 The large, complex chromosomes of eukaryotes duplicate with each cell division A eukaryotic cell has many more genes than a prokaryotic cell And they are grouped into multiple chromosomes in the nucleus

8 LM 600 Individual chromosomes contain a very long DNA molecule associated with proteins called Histones. And are visible only when the cell is in the process of dividing If a cell is not undergoing division Chromosomes occur in the form of thin, loosely packed chromatin fibers Figure 8.4A

9 TEM 36,000 Before a cell starts dividing, the chromosomes replicate Producing sister chromatids joined together at the centromere Sister chromatids Centromere Figure 8.4B

10 Cell division involves the separation of sister chromatids And results in two daughter cells, each containing a complete and identical set of chromosomes Centromere Chromosome duplication Sister chromatids Chromosome distribution to daughter cells Figure 8.4C

11 9.5 The cell cycle multiplies cells The cell cycle consists of two major phases INTERPHASE G 1 S (DNA synthesis) G 2 Figure 8.5

12 During interphase Chromosomes duplicate and cell parts are made During the mitotic phase Duplicated chromosomes are evenly distributed into two daughter nuclei

13 9.6 Cell division is a continuum of dynamic changes In mitosis, after the chromosomes coil up A mitotic spindle moves them to the middle of the cell

14 The sister chromatids then separate And move to opposite poles of the cell, where two nuclei form Cytokinesis, in which the cell divides in two Overlaps the end of mitosis

15 LM 250 The stages of cell division Centrosomes (with centriole pairs) INTERPHASE PROPHASE PROMETAPHASE Chromatin Early mitotic spindle Centrosome Fragments of nuclear envelope Kinetochore Nucleolus Figure 8.6 (Part 1) Nuclear envelope Plasma membrane Chromosome, consisting ot two sister chromatids Centromere Spindle microtubules

16 METAPHASE ANAPHASE TELOPHASE AND CYTOKINESIS Metaphase plate Cleavage furrow Nucleolus forming Spindle Daughter chromosomes Nuclear envelope forming Figure 8.6 (Part 2)

17 SEM Cytokinesis differs for plant and animal cells In animals Cytokinesis occurs by a constriction of the cell (cleavage) Cleavage furrow Cleavage furrow Contracting ring of microfilaments Figure 8.7A Daughter cells

18 TEM 7,500 In plants Cell plate forming Wall of parent cell Daughter nucleus A membranous cell plate splits the cell in two Cell wall New cell wall Figure 8.7B Vesicles containing cell wall material Cell plate Daughter cells

19 LM Review of the functions of mitosis: Growth, cell replacement, and asexual reproduction When the cell cycle operates normally, mitotic cell division functions in Growth Figure 8.11A

20 MEIOSIS AND CROSSING OVER 9.12 Chromosomes are matched in homologous pairs The somatic (body) cells of each species Contain a specific number of chromosomes For example human cells have 46 Making up 23 pairs of homologous chromosomes

21 The chromosomes of a homologous pair Carry genes for the same characteristics at the same place, or locus Chromosomes Centromere Figure 8.12 Sister chromatids

22 9.13 Gametes have a single set of chromosomes Cells with two sets of chromosomes Are said to be diploid Gametes, eggs and sperm, are haploid With a single set of chromosomes

23 Sexual life cycles Involve the alternation of haploid and diploid stages Meiosis Haploid gametes (n = 23) n Egg cell n Sperm cell Fertilization Multicellular diploid adults (2n = 46) Diploid zygote (2n = 46) 2n Mitosis and development Figure 8.13

24 9.14 Meiosis reduces the chromosome number from diploid to haploid Meiosis, like mitosis Is preceded by chromosome duplication But in meiosis The cell divides twice to form four daughter cells

25 The first division, meiosis I Starts with synapsis, the pairing of homologous chromosomes In crossing over Homologous chromosomes exchange corresponding segments Meiosis I separates each homologous pair And produce two daughter cells, each with one set of chromosomes

26 Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is a total of four haploid cells

27 The stages of meiosis MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Sites of crossing over Spindle Microtubules attached to kinetochore Metaphase plate Sister chromatids remain attached Nuclear envelope Chromatin Sister chromatids Tetrad Centromere (with kinetochore) Homologous chromosomes separate Figure 8.14 (Part 1)

28 MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS Cleavage furrow PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming Figure 8.14 (Part 2)

29 9.15 Review: A comparison of mitosis and meiosis Mitosis Meiosis Parent cell (before chromosome replication) Meiosis i Prophase Duplicated chromosome (two sister chromatids) Chromosome replication 2n = 4 Chromosome replication Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Metaphase I Anaphase Telophase Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Daughter cells of meiosis I Anaphase I Telophase I Haploid n = 2 Figure n Daughter cells of mitosis 2n No further chromosomal replication; sister chromatids separate during anaphase II n n n n Daughter cells of meiosis II Meiosis ii

30 TEM 2, Crossing over further increases genetic variability Genetic recombination Which results from crossing over during prophase I of meiosis, increases variation still further Tetrad Chiasma Figure 8.18A Centromere

31 ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE 9.19 A karyotype is a photographic inventory of an individual s chromosomes A karyotype Is an ordered arrangement of a cell s chromosomes

32 2,600X Preparation of a karyotype from a blood sample Packed red and white blood cells Hypotonic solution Fixative Stain Blood culture Centrifuge White blood cells 1 A blood Fluid 2 The fluid is discarded, and a hypotonic 3 Another centrifugation step separates the swollen white culture is solution is mixed with the cells. This makes blood cells. The fluid containing the remnants of the red centrifuged to separate the the red blood cells burst. The white blood blood cells is poured off. A fixative (preservative) is mixed blood cells from the culture fluid. cells swell but do not burst, and their with the white blood cells. A drop of the cell suspension chromosomes spread out. is spread on a microscope slide, dried, and stained. Centromere Sister chromosomes Pair of homologous chromosomes Figure The slide is viewed with a microscope equipped with a digital 5 The resulting display is the karyotype. The 46 chromosomes here include camera. A photograph of the chromosomes is entered into a 22 pair of autosomes and 2 sex chromosomes, X and Y. Although difficult to computer, which electronically arranges them by size and shape. discern in the karyotype, each of the chromosomes consists of two sister chromatids lying very close together (see diagram).

33 CONNECTION 9.20 An extra copy of chromosome 21 causes Down syndrome A person may have an abnormal number of chromosomes Which causes problems

34 5,000 Down syndrome is caused by trisomy 21 An extra copy of chromosome 21 Figure 8.20A Figure 8.20B

35 Infants with Down syndrome (per 1,000 births) The chance of having a Down syndrome child Goes up with maternal age Figure 8.20C Age of mother 45 50

36 9.21 Accidents during meiosis can alter chromosome number Abnormal chromosome count is a result of nondisjunction The failure of homologous pairs to separate during meiosis I The failure of sister chromatids to separate during meiosis II