1. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings All cells come from cells Cellular reproduction is called cell division –Cell division allows an embryo to develop into an adult –It also ensures the continuity of life from one generation to the next 8.2 Cells arise only from preexisting cells

2. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.3A Binary fission of a prokaryotic cell Prokaryotic chromosome Plasma membrane Cell wall Duplication of chromosome and separation of copies Continued growth of the cell and movement of copies Division into two cells

3. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The cell cycle consists of two major phases: –Interphase, where chromosomes duplicate and cell parts are made –The mitotic phase, when cell division occurs 8.5 The cell cycle multiplies cells Figure 8.5

4. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Karyotype

5. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Before a cell starts dividing, the chromosomes are duplicated –This process produces sister chromatids Centromere Sister chromatids Figure 8.4B

6. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Chromosomes contain a very long DNA molecule with thousands of genes –Individual chromosomes are only visible during cell division –They are packaged as chromatin

7. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings When the cell divides, the sister chromatids separate –Two daughter cells are produced –Each has a complete and identical set of chromosomes Centromere Sister chromatids Figure 8.4C Chromosome duplication Chromosome distribution to daughter cells

8. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cell Cycle: Interphase: G1, G0, S, G2 Mitotic Phase: Mitosis / Cytokinesis (Mitosis : PMAT) 8.6 Cell division is a continuum of dynamic changes

9. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Interphase G1 – cell grows / developes G0 – cell does what it normally supposed to do, some cells stay in this phase forever, ex nerve S – DNA Replication G2 – organelles double, enzymes for cell division made

10. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings INTERPHASEPROPHASE Centrosomes (with centriole pairs) Chromatin NucleolusNuclear envelope Plasma membrane Early mitotic spindle Centrosome Chromosome, consisting of two sister chromatids Fragments of nuclear envelope Kinetochore Spindle microtubules Figure 8.6

11. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings METAPHASETELOPHASE AND CYTOKINESIS Metaphase plate SpindleDaughter chromosomes Cleavage furrow Nucleolus forming Nuclear envelope forming ANAPHASE Figure 8.6 (continued)

12. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In animals, cytokinesis occurs by cleavage –This process pinches the cell apart 8.7 Cytokinesis differs for plant and animal cells Figure 8.7A Cleavage furrow Contracting ring of microfilaments Daughter cells

13. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings In plants, a membranous cell plate splits the cell in two Vesicles containing cell wall material Cell plate forming Figure 8.7B Cell plateDaughter cells Wall of parent cell Daughter nucleus Cell wallNew cell wall

14. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cancer cells have abnormal cell cycles –They divide excessively and can form abnormal masses called tumors Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division 8.10 Connection: Growing out of control, cancer cells produce malignant tumors

15. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Homologous pairs – chromosomes from each parent that have same genes but not necessarily same alleles –Human cells have 46, making up 23 pairs of homologous chromosomes MEIOSIS AND CROSSING OVER 8.12 Chromosomes are matched in homologous pairs Chromosomes Centromere Sister chromatids Figure 8.12

16. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Diploid – somatic cells, 2 sets of chromosomes Haploid – gamete cells, 1 set chromosomes 8.13 Gametes have a single set of chromosomes

17. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings At fertilization, a sperm fuses with an egg, forming a diploid zygote –Repeated mitotic divisions lead to the development of a mature adult –The adult makes haploid gametes by meiosis –All of these processes make up the sexual life cycle of organisms

18. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings The human life cycle Figure 8.13 MEIOSISFERTILIZATION Haploid gametes (n = 23) Egg cell Sperm cell Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development

19. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis, like mitosis, is preceded by chromosome duplication –However, in meiosis the cell divides twice to form four daughter cells 8.14 Meiosis reduces the chromosome number from diploid to haploid

20. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Meiosis I - Homologous pairs separate - During Prophase I, tetrads can cross over to swap genetic info - End with 2 Haploid cells with sister chromatids Meiosis II - sister chromatids separate - end with 4 Haploid cells, no sister chromatids

21. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.14, part 1 MEIOSIS I : Homologous chromosomes separate INTERPHASEPROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Nuclear envelope Chromatin Sites of crossing over Spindle Sister chromatids Tetrad Microtubules attached to kinetochore Metaphase plate Centromere (with kinetochore) Sister chromatids remain attached Homologous chromosomes separate

22. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.14, part 2 MEIOSIS II : Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II Cleavage furrow Sister chromatids separate TELOPHASE II AND CYTOKINESIS Haploid daughter cells forming

23. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.18A Tetrad Chaisma Centromere

24. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings How crossing over leads to genetic recombination Figure 8.18B Tetrad (homologous pair of chromosomes in synapsis) Breakage of homologous chromatids Joining of homologous chromatids Chiasma Separation of homologous chromosomes at anaphase I Separation of chromatids at anaphase II and completion of meiosis Parental type of chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types 1 2 3 4 Coat-color genes Eye-color genes

25. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings For both processes, chromosomes replicate only once, during interphase 8.15 Review: A comparison of mitosis and meiosis

26. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.15 MITOSISMEIOSIS PARENT CELL (before chromosome replication) Site of crossing over MEIOSIS I PROPHASE I Tetrad formed by synapsis of homologous chromosomes PROPHASE Duplicated chromosome (two sister chromatids) METAPHASE Chromosome replication 2n = 4 ANAPHASE TELOPHASE Chromosomes align at the metaphase plate Tetrads align at the metaphase plate METAPHASE I ANAPHASE I TELOPHASE I Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I ; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II 2n2n2n2n Daughter cells of mitosis Daughter cells of meiosis II MEIOSIS II Daughter cells of meiosis I Haploid n = 2 nnnn

27. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings GENETIC VARIATION CAN RESULT FROM: - crossing over - homologous pairs rearranging - random fertilization

28. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 8.16 POSSIBILITY 1POSSIBILITY 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1Combination 2Combination 3Combination 4

29. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Preparation of a karyotype Figure 8.19 Blood culture 1 Centrifuge Packed red And white blood cells Fluid 2 Hypotonic solution 3 Fixative White Blood cells Stain 45 Centromere Sister chromatids Pair of homologous chromosomes

30. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings This karyotype shows three number 21 chromosomes An extra copy of chromosome 21 causes Down syndrome 8.20 Connection: An extra copy of chromosome 21 causes Down syndrome Figure 8.20A, B

31. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Abnormal chromosome count is a result of nondisjunction –Either homologous pairs fail to separate during meiosis I 8.21 Accidents during meiosis can alter chromosome number Figure 8.21A Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n – 1 Number of chromosomes

32. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings –Or sister chromatids fail to separate during meiosis II Figure 8.21B Normal meiosis I Nondisjunction in meiosis II Gametes n + 1n – 1nn Number of chromosomes

33. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome Figure 8.21C Egg cell Sperm cell n + 1 n (normal) Zygote 2n + 1

34. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Nondisjunction can also produce gametes with extra or missing sex chromosomes –Unusual numbers of sex chromosomes upset the genetic balance less than an unusual number of autosomes 8.22 Connection: Abnormal numbers of sex chromosomes do not usually affect survival

35. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Table 8.22

36. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings SPERMATOGENESIS –Spermatogenesis: the formation of sperm cells Diploid cells made continuously in seminiferous tubules of testes Differentiated primary spermatocytes Haploid secondary spermatocytes Haploid sperm

37. LE 27-04a Testis Scrotum Diploid cell Differentiation and onset of Meiosis  Primary spermatocyte Secondary spermatocyte Meiosis  (in prophase of Meiosis   (haploid; double chromatids) (haploid; single chromatids) Developing sperm cells Differentiation Sperm cells Epididymis Penis Seminiferous tubule Cross section of seminiferous tubule Center of seminiferous tubule Testis (haploid) nn n n nn nn n n 2n completed Meiosis 

38. LE 27-04b Diploid cell Primary oocyte (arrested in prophase of Meiosis  ) Secondary oocyte In embryo Differentiation and onset of Meiosis  Present at birth Completion of Meiosis  and onset of Meiosis  First polar body 2n n n (arrested at meta- phase of Meiosis  ; released from ovary) Entry of sperm triggers completion of Meiosis  Second polar body n n Ovum (haploid)

39. Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Difference between Oogenesis and Spermatogenesis Oogenesis - starts at birth / primary oocyte arrested in Prophase I - puberty – release secondary oocyte once a month – STOPS after eggs run out - complete Meiosis II if fertilized - 1 egg, polar bodies (unequal divisions) Spermatogenesis - starts at puberty - division continuous - 4 haploid cells