BIOE 120 Week 6 + 7 - The Cellular Basis of Reproduction PDF

The Cellular Basis of Reproduction and Inheritance Chapter 8 Dr. Abdullah A. Alaqel © 2018 Pearson Education Ltd.  Introduction Cell replication is a normal part of maintaining a healthy body....

The Cellular Basis of Reproduction and Inheritance Chapter 8 Dr. Abdullah A. Alaqel © 2018 Pearson Education Ltd.  Introduction Cell replication is a normal part of maintaining a healthy body. Mitosis permit asexual and meiosis permit sexual reproduction. Asexual: offspring arise from a single organism Sexual: offspring requires gametes from both parent. Errors in cell division can lead to cancer, infertility, abortion, or the production of children with serious genetic disorders. © 2018 Pearson Education Ltd.  19.1 Two Types of Cell Division We begin life as a single cell, a zygote, formed by the union of an egg and a sperm. The human life cycle has two types of cell division Meiosis Nuclear division that gives rise to gametes Mitosis Nuclear division that results in identical body cells © 2017 Pearson Education, Inc. © 2017 Pearson Education, Inc.  19.1 Two Types of Cell Division Meiosis Gives rise to gametes that have half the number of chromosomes as the original cell In females Occurs in ovaries Produces eggs In males Occurs in testes Produces sperm © 2017 Pearson Education, Inc.  19.1 Two Types of Cell Division Mitosis Results in identical body cells Occurs during growth and repair © 2017 Pearson Education, Inc.  19.2 Form of Chromosomes Chromosomes are located in the nucleus Structure and function: each is a tightly coiled combination of a DNA molecule and specialized proteins called histones DNA contains genetic information, which directs body development and maintenance Histones help with support and control of gene activity Gene (https://www.youtube.com/watch?v=GbJ asFgJkLg) Specific segment of the DNA Directs synthesis of a protein, which plays a structural or functional role in the cell © 2017 Pearson Education, Inc.  19.2 Form of Chromosomes Somatic cells (soma = body) All cells except eggs and sperm (gametes) Humans have 46 chromosomes Two sets (pairs) of all 23 chromosomes: one set of 23 from each parent Each somatic cell contains two chromosomes with genes for the same traits Called homologous pairs of chromosomes One chromosome of the pair is from the mother One chromosome of the pair is from the father © 2017 Pearson Education, Inc.  19.2 Form of Chromosomes Diploid: a cell with two of each chromosome (2n) Genes also occur in pairs in diploid cells Members of each gene pair are located at the same position on homologous chromosomes © 2017 Pearson Education, Inc.  19.2 Form of Chromosomes Of the 23 pairs of chromosomes Sex chromosomes make up one pair and determine gender; two types: X and Y XX = genetic female XY = genetic male Autosomes make up 22 pairs Determine expression of most of a person’s inherited characteristics © 2017 Pearson Education, Inc.  Principles of Inheritance Genetic information is carried on chromosomes that are in the egg and sperm in equal numbers Homologous pairs of chromosomes The 23 chromosomes received from one parent pair with the 23 chromosomes from the other parent Each member of a homologous pair carries genes for the same traits © 2017 Pearson Education, Inc.  Principles of Inheritance Trait Characteristic Produced by the actions of one or more gene-directed proteins © 2017 Pearson Education, Inc. © 2018 Pearson Education Ltd.  Principles of Inheritance Alleles Different forms of a gene. Produce different versions of the trait they determine. Example: gene for freckles One allele causes freckles to form Other allele does not https://www.youtube.co m/watch?v=nDwyHa4nR zs © 2017 Pearson Education, Inc.  Figure 20.1 In a homologous pair of chromosomes, each member carries genes for the same traits. One member of each pair was inherited from the mother, and the other from the father. A is a segment of DNA located in a specific site on a specific chromosome that gene contains information for producing a particular protein (polypeptide). A pair of alleles. An allele is an alternative version of a gene located on a specific site of a specific chromosome. One allele is inherited from the mother, and the other from the father. © 2017 Pearson Education, Inc.  Cancer cells Cancer cells start as normal cells, but genetic mutations cause them to lose the ability to regulate their division. Cancer cells divide and may spread, invading other tissues, disrupting organ function, and killing the host. Although uncontrolled cell division is harmful, normal cell division is necessary in all forms of life. https://www.youtube.com/watch?v=LEpTTolebqo © 2018 Pearson Education Ltd. Figure 8.0_1 Chapter 8: Big Ideas Cell Division and The Eukaryotic Cell Reproduction Cycle and Mitosis Meiosis and Crossing Alterations of Chromosome Over Number and Structure © 2018 Pearson Education Ltd.  CELL DIVISION AND REPRODUCTION © 2018 Pearson Education Ltd.  8.1 Cell division plays many important roles in the lives of organisms Cell division is at the heart of the reproduction of cells and organisms because cells originate only from preexisting cells. Some organisms reproduce through asexual reproduction, producing offspring that are all genetic copies of the parent and identical to each other. Other organisms reproduce through sexual reproduction, creating a variety of offspring. © 2018 Pearson Education Ltd.  8.2 Prokaryotes reproduce by binary fission Prokaryotic cells reproduce asexually by binary fission, a term that means “dividing in half.” In typical prokaryotes, most genes are carried on one circular DNA molecule that, with associated proteins, constitutes the organism’s chromosome. As the cell replicates its single chromosome: the copies move apart, the plasma membrane pinches inward, and more cell wall is made, which eventually divides the parent cell into two daughter cells. © 2018 Pearson Education Ltd.  8.2 Prokaryotes reproduce by binary fission Checkpoint question Why is binary fission classified as asexual reproduction? © 2018 Pearson Education Ltd. Figure 8.2a_1 Plasma membrane Prokaryotic Cell wall chromosome Duplication of the chromosome 1 and separation of the copies © 2018 Pearson Education Ltd. Figure 8.2a_2 Plasma membrane Prokaryotic Cell wall chromosome Duplication of the chromosome 1 and separation of the copies Continued elongation of the 2 cell and movement of the copies © 2018 Pearson Education Ltd. Figure 8.2a_3 Plasma membrane Prokaryotic Cell wall chromosome Duplication of the chromosome 1 and separation of the copies Continued elongation of the 2 cell and movement of the copies Division into 3 two daughter cells © 2018 Pearson Education Ltd. Figure 8.2b Prokaryotic chromosomes Colorized TEM 32,500× © 2018 Pearson Education Ltd.  THE EUKARYOTIC CELL CYCLE AND MITOSIS © 2018 Pearson Education Ltd.  Mitosis AND Meiosis Mitosis produces two diploid (2n) somatic cells that are genetically identical to each other and the original parent cell. Meiosis produces four haploid (n) gametes that are genetically unique from each other and the original parent (germ) cell. Mitosis involves one cell division, whereas meiosis involves two cell divisions. © 2018 Pearson Education Ltd. Figure 8.14_5 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes Pairs of line up at the homologous metaphase chromosomes plate line up at the metaphase plate Anaphase Anaphase I Telophase Telophase I Homologous chromosomes separate, sister Sister chromatids n=2 chromatids remain attached 2n = 4 separate 2n = 4 during MEIOSIS II anaphase Sister chromatids separate during anaphase II n=2 n=2 n=2 n=2  Mitosis & Meiosis terms: Gamete: A sex cell (in humans: sperm for males, and eggs for females) Meiosis: A two-step process of cell division that is used to make gametes (sex cells) Crossing over: Process in which homologous chromosomes trade parts Interphase: Phase of the cell cycle where the cell grows and makes a copy of its DNA Homologous chromosomes: Set of chromosomes (one from each parent), that are very similar to one another and have the same size/shape. Sister chromatids: Two halves of a duplicated chromosome Diploid (2n): Cell that contains two sets of homologous chromosomes Haploid (n): Cell that contains only a single set of genes © 2018 Pearson Education Ltd.  8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division Before a cell starts dividing, the chromosomes duplicate, producing sister chromatids (containing identical DNA) that are joined together along their lengths by proteins, most closely at a region called the centromere. Cell division involves the separation of sister chromatids and results in two daughter cells, each containing a complete and identical set of chromosomes. © 2018 Pearson Education Ltd. Figure 8.3b Chromosomes Chromosomal DNA molecules Sister chromatids Chromosome duplication Sister chromatids Centromere TEM 38,065× Separation of sister chromatids into two chromo- somes and distribution into two daughter cells Figure 8.3b_1 Chromosomes Chromosomal DNA molecules Chromosome duplication Sister chromatids Centromere Separation of sister chromatids into two chromo- somes and distribution into two daughter cells Figure 8.3b_2 Sister chromatids Centromere TEM 38,065×  8.3 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. Each chromosome contains one long DNA molecule. Individual chromosomes are visible under a light microscope only when the cell is in the process of dividing; otherwise, chromosomes are thin, loosely packed chromatin fibers too small to be seen. © 2018 Pearson Education Ltd.  Cell cycle https://www.youtube.com/watch?v=7NM-UWFHG18 © 2018 Pearson Education Ltd.  Cell cycle G1 phase (first gap phase): During this phase cells that are intended for mitosis, grow and carry out various metabolic activities. S phase (synthesis phase): During this phase, the cell duplicates its DNA. Eukaryotic DNA is coiled around spherical histone proteins to create a rod-shaped structure called the chromosome. During the S phase, each chromosome generates its copy, or sister chromatid. The two sister chromatids fuse together at a point called the centromere, and the complex resembles the shape of the letter "X. © 2018 Pearson Education Ltd.  Cell cycle G2 phase (second gap phase): During this phase the cell continues to grow and generate proteins necessary for mitosis. (G1, S and G2 phases are collectively referred to as "interphase.") © 2018 Pearson Education Ltd.  Cell cycle M phase (mitosis): Mitosis involves the segregation of the sister chromatids. A structure of protein filaments called the mitotic spindle hooks on to the centromere and begins to contract. This pulls the sister chromatids apart, slowly moving them to opposite poles of the cell. By the end of mitosis each pole of the cell has a complete set of chromosomes. The nuclear membrane reforms, and the cell divides in half, creating two identical daughter cells. © 2018 Pearson Education Ltd. Figure 8.4 S G1 (DNA synthesis) (first gap) M G2 (second gap) © 2018 Pearson Education Ltd.  8.5 Cell division is a continuum of dynamic changes Mitosis distributes duplicated chromosomes into two daughter nuclei. After the chromosomes are coiled up, a mitotic spindle made of microtubules moves the chromosomes to the middle of the cell. The sister chromatids then separate and move to opposite poles of the cell, at which point two new nuclei form. © 2018 Pearson Education Ltd. Figure 8.14_5 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes Pairs of line up at the homologous metaphase chromosomes plate line up at the metaphase plate Anaphase Anaphase I Telophase Telophase I Homologous chromosomes separate, sister Sister chromatids n=2 chromatids remain attached 2n = 4 separate 2n = 4 during MEIOSIS II anaphase Sister chromatids separate during anaphase II n=2 n=2 n=2 n=2  Stages of Mitosis Prophase: The duplicated chromosomes are compacted and can be easily visualized as sister chromatids. The mitotic spindle, a network of protein filaments, emerges from structures called centrioles, positioned at either end of the cell. © 2018 Pearson Education Ltd. Figure 8.5_1 MITOSIS INTERPHASE LM 250× Prophase Prometaphase Fragments of Mitotic spindle the nuclear Centrosomes forming envelope Chromatin Centrosome Kinetochore Nuclear Chromosome Centromere Spindle Plasma microtubules envelope membrane consisting of two sister chromatids © 2018 Pearson Education Ltd.  Stages of Mitosis Metaphase: The nuclear membrane dissolves and the mitotic spindle latches on to the sister chromatids at the centromere. The mitotic spindle can now move the chromosomes around in the cell. © 2018 Pearson Education Ltd.  Anaphase: The mitotic spindle contracts and pulls the sister chromatids apart. They begin to move to opposite ends of the cell. Telophase: The chromosomes reach either end of the cell. The nuclear membrane forms again and the cell body splits into two (cytokinesis). At the end of mitosis, one cell produces two genetically identical daughter cells. © 2018 Pearson Education Ltd.  8.6 Cytokinesis differs for plant and animal cells Cytokinesis, in which the cell divides in two, overlaps the end of mitosis. In animals, cytokinesis occurs when a cell constricts, forming a cleavage furrow. In plants, a membranous cell plate forms and then splits the cell in two. © 2018 Pearson Education Ltd. Figure 8.5_6 MITOSIS Metaphase Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Nuclear Mitotic Separated envelope spindle chromosomes forming © 2018 Pearson Education Ltd. 19.4 Mitosis: Creation of Genetically Identical Diploid Body Cells ©©2017 2018Pearson PearsonEducation, EducationInc. Ltd.  Video: Animal Mitosis © 2018 Pearson Education Ltd. Figure 8.6a Cytokinesis Cleavage furrow Contracting ring of microfilaments SEM 113× Daughter cells © 2018 Pearson Education Ltd. Figure 8.6a_2 Cleavage furrow Contracting ring of microfilaments Daughter cells © 2018 Pearson Education Ltd. Figure 8.6b Cytokinesis Cell wall Cell wall of the New parent cell cell wall Daughter LM 1,050× nucleus Cell plate forming Vesicles containing Cell plate Daughter cells cell wall material © 2018 Pearson Education Ltd. Figure 8.6b_1 Cell wall of the parent cell Daughter nucleus LM 1,050× Cell plate forming © 2018 Pearson Education Ltd.  Animation: Cytokinesis © 2018 Pearson Education Ltd.  8.7 The rate of cell division is affected by environmental factors In laboratory cultures, most normal cells divide only when attached to a surface. The cultured cells continue dividing until they touch one another. Most animal cells divide only when stimulated by growth factors, and some do not divide at all. © 2018 Pearson Education Ltd. Figure 8.7a Anchorage Single layer of cells Removal of cells Restoration of single layer by cell division Cancer cells forming clump of overlapping cells © 2018 Pearson Education Ltd. Figure 8.7b Cultured cells suspended in liquid The addition of growth factor Cells divide in Cells fail presence of to divide growth factor © 2018 Pearson Education Ltd.  8.8 Growth factors signal the cell cycle control system A set of proteins within the cell controls the cell cycle. Signals affecting critical checkpoints in the cell cycle determine whether a cell will go through the complete cycle and divide. The binding of growth factors to specific receptors on the plasma membrane is usually necessary for cell division. © 2018 Pearson Education Ltd. Figure 8.8a G1 checkpoint G0 S G1 Control system M G2 M checkpoint G2 checkpoint © 2018 Pearson Education Ltd. Figure 8.8b Plasma membrane Extracellular fluid Growth factor Relay proteins G1 checkpoint Receptor protein Signal S transduction pathway Control G1 system M G2 Cytoplasm © 2018 Pearson Education Ltd.  8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors Cancer cells divide excessively to form masses called tumors. Malignant tumors can invade other tissues. Radiation and chemotherapy are effective as cancer treatments because they interfere with cell division. © 2018 Pearson Education Ltd. © 2018 Pearson Education Ltd. Figure 8.9 Lymph vessels Blood vessel Tumor Tumor in another Glandular part of tissue the body Tumor growth Invasion Metastasis © 2018 Pearson Education Ltd.  MEIOSIS AND CROSSING OVER © 2018 Pearson Education Ltd.  8.11 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, consisting of 23 pairs of homologous chromosomes. The chromosomes of a homologous pair of autosomes carry genes for the same characteristics at the same place, or locus. © 2018 Pearson Education Ltd. Figure 8.11 Pair of homologous duplicated chromosomes Locus Centromere Sister chromatids One duplicated chromosome © 2018 Pearson Education Ltd.  8.12 Gametes have a single set of chromosomes Cells with two sets of homologous chromosomes are diploid. Gametes—eggs and sperm—are haploid cells with a single set of chromosomes. Sexual life cycles involve the alternation of haploid and diploid stages. © 2018 Pearson Education Ltd. Figure 8.12a Haploid gametes (n = 23) Key Haploid stage (n) n Diploid stage (2n) Egg cell n Sperm cell Meiosis Fertilization Ovary Testis 2n Diploid zygote Mitosis and (2n = 46) Multicellular development diploid adults (2n = 46) © 2018 Pearson Education Ltd. Figure 8.12b INTERPHASE MEIOSIS I MEIOSIS II Sister Four haploid cells chromatids Haploid 3 1 2 cells with Sister duplicated chromatids Chromosomes Homologous chromo- separate duplicate chromosomes somes A pair of A pair of separate homologous duplicated chromosomes homologous in a diploid chromosomes parent cell © 2018 Pearson Education Ltd.  8.13 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. The first division, meiosis I, starts with the pairing of homologous chromosomes. In crossing over, homologous chromosomes exchange corresponding segments. https://www.youtube.com/watch?v=kQu6Yfrr6j0 © 2018 Pearson Education Ltd. Figure 8.14_5 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes Pairs of line up at the homologous metaphase chromosomes plate line up at the metaphase plate Anaphase Anaphase I Telophase Telophase I Homologous chromosomes separate, sister Sister chromatids n=2 chromatids remain attached 2n = 4 separate 2n = 4 during MEIOSIS II anaphase Sister chromatids separate during anaphase II n=2 n=2 n=2 n=2  8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis I separates the members of each homologous pair and produces two daughter cells, each with one set of chromosomes. Meiosis II is essentially the same as mitosis: In each of the cells, the sister chromatids of each chromosome separate. The result is a total of four haploid cells. © 2018 Pearson Education Ltd. Figure 8.13_2 MEIOSIS I: Homologous INTERPHASE: chromosomes separate Chromosomes duplicate Prophase I Centrosomes Sites of crossing over Spindle Tetrad Nuclear Chromatin Sister Fragments envelope chromatids of the nuclear envelope © 2018 Pearson Education Ltd. Figure 8.13_3 MEIOSIS I: Homologous chromosomes separate Metaphase I Anaphase I Spindle microtubules Sister chromatids attached to a remain attached kinetochore Centromere Metaphase (with a plate Homologous kinetochore) chromosomes separate © 2018 Pearson Education Ltd. Figure 8.13_4 MEIOSIS II: Sister chromatids separate Telophase II Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II and Cytokinesis Cleavage furrow Sister chromatids Haploid separate daughter cells form © 2018 Pearson Education Ltd. Figure 8.13_5 Telophase I and Cytokinesis Cleavage furrow © 2018 Pearson Education Ltd.  8.14 VISUALIZING THE CONCEPT: Mitosis and meiosis have important similarities and differences Both mitosis and meiosis begin with diploid parent cells that have chromosomes duplicated during the previous interphase. Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes. © 2018 Pearson Education Ltd. Figure 8.14_1 MITOSIS MEIOSIS I Parent cell 2n = 4 © 2018 Pearson Education Ltd. Figure 8.14_2 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids © 2018 Pearson Education Ltd. Figure 8.14_3 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes Pairs of line up at the homologous metaphase chromosomes plate line up at the metaphase plate © 2018 Pearson Education Ltd. Figure 8.14_4 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes Pairs of line up at the homologous metaphase chromosomes plate line up at the metaphase plate Anaphase Anaphase I Telophase Telophase I Homologous chromosomes separate, sister Sister chromatids n=2 chromatids remain attached 2n = 4 separate 2n = 4 during MEIOSIS II anaphase © 2018 Pearson Education Ltd. Figure 8.14_5 MITOSIS MEIOSIS I Parent cell 2n = 4 Chromosome duplication Prophase I Prophase (Occurs once, during S phase Duplicated of preceding interphase) Homologous chromosomes chromosome come together in pairs remains separate Site of crossing over between homologous (nonsister) chromatids Metaphase Metaphase I Chromosomes Pairs of line up at the homologous metaphase chromosomes plate line up at the metaphase plate Anaphase Anaphase I Telophase Telophase I Homologous chromosomes separate, sister Sister chromatids n=2 chromatids remain attached 2n = 4 separate 2n = 4 during MEIOSIS II anaphase Sister chromatids separate during anaphase II n=2 n=2 n=2 n=2 © 2018 Pearson Education Ltd. Figure 8.14_6 MITOSIS MEIOSIS I MEIOSIS II Result: Two genetically identical diploid cells Result: Four genetically unique haploid cells Used for: Growth, tissue repair, asexual reproduction Used for: Sexual reproduction © 2018 Pearson Education Ltd. © 2018 Pearson Education Ltd.  8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Each chromosome of a homologous pair differs at many points from the other member of the pair. Random arrangements of chromosome pairs at metaphase I of meiosis lead to many different combinations of chromosomes in eggs and sperm. Random fertilization of eggs by sperm greatly increases this variation. © 2018 Pearson Education Ltd. Figure 8.15_1 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I © 2018 Pearson Education Ltd. Figure 8.15_2 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Metaphase II © 2018 Pearson Education Ltd. Figure 8.15_3 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 © 2018 Pearson Education Ltd.  Animation: Genetic Variation © 2018 Pearson Education Ltd.  8.16 Homologous chromosomes may carry different versions of genes The differences between homologous chromosomes come from the fact that they can bear different versions of genes at corresponding loci. The position of a gene on a particular chromosome is called the locus (plural = loci). © 2018 Pearson Education Ltd.  8.16 Homologous chromosomes may carry different versions of genes Crossing over is an exchange of corresponding segments between nonsister chromatids of homologous chromosomes. © 2018 Pearson Education Ltd. Figure 8.16 Coat-color Eye-color genes genes Brown Black C E C E Brown coat (C); C E black eyes (E) Meiosis c e c e White coat (c); c e pink eyes (e) White Pink Tetrad in parent cell Chromosomes of (homologous pair of the four gametes duplicated chromosomes) © 2018 Pearson Education Ltd. Figure 8.16b TEM 5,060× Sister Chiasma chromatids Nonsister chromatids Tetrad © 2018 Pearson Education Ltd. Figure 8.16_1 Genetic recombination, which results from crossing over during prophase I of meiosis, increases variation still further. Coat-color Eye-color genes genes Brown C E Black C E Brown coat (C); C E black eyes (E) Meiosis c e c e White coat (c); c e pink eyes (e) White Pink Tetrad in parent cell Chromosomes of (homologous pair of the four gametes duplicated chromosomes) © 2018 Pearson Education Ltd. Figure 8.17_1 MEIOSIS I Paternal Maternal chromosome chromosome Homologous chromosomes (blue) (red) are paired all along their lengths. Centromere Metaphase plate © 2018 Pearson Education Ltd. Figure 8.17_2 MEIOSIS I Paternal Maternal chromosome chromosome Homologous chromosomes (blue) (red) are paired all along their lengths. Centromere Metaphase plate Chiasma Nonsister chromatids form chiasma. Homologous segments cross over, producing hybrid chromosomes. © 2018 Pearson Education Ltd. Figure 8.17_3 MEIOSIS I Paternal Maternal chromosome chromosome Homologous chromosomes (blue) (red) are paired all along their lengths. Centromere Metaphase plate Chiasma Nonsister chromatids form chiasma. Homologous segments cross over, producing hybrid chromosomes. Homologous pairs separate into recombinant chromosomes. © 2018 Pearson Education Ltd. Figure 8.17_4 MEIOSIS I Paternal Maternal chromosome chromosome Homologous chromosomes (blue) (red) are paired all along their lengths. Centromere Metaphase plate Chiasma Nonsister chromatids form chiasma. Homologous segments cross over, producing hybrid chromosomes. Homologous pairs separate into recombinant chromosomes. When the homologous chromosomes separate in anaphase I, half contain a new segment originating from the other member of the homologous pair. © 2018 Pearson Education Ltd. Figure 8.17_5 MEIOSIS I Paternal Maternal chromosome chromosome Homologous chromosomes (blue) (red) are paired all along their lengths. Centromere Metaphase plate Chiasma Nonsister chromatids form chiasma. Homologous segments cross over, producing hybrid chromosomes. Homologous pairs separate into recombinant chromosomes. When the homologous chromosomes separate in anaphase I, half contain a new segment originating from the other member of the homologous pair. MEIOSIS II Sister chromatids separate, each going to a different gamete. © 2018 Pearson Education Ltd. Figure 8.17_6 MEIOSIS I Paternal Maternal chromosome chromosome Homologous chromosomes (blue) (red) are paired all along their lengths. Centromere Metaphase plate Chiasma Nonsister chromatids form chiasma. Homologous segments cross over, producing hybrid chromosomes. Homologous pairs separate into recombinant chromosomes. When the homologous chromosomes separate in anaphase I, half contain a new segment originating from the other member of the homologous pair. MEIOSIS II Sister chromatids separate, each going to a different gamete. Result: two parental Recombinant chromosomes that match the chromosomes originals, and two Parental chromosomes recombinant chromosomes with new gene combinations. © 2018 Pearson Education Ltd.  Animation: Crossing Over © 2018 Pearson Education Ltd.  ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE © 2018 Pearson Education Ltd.  8.18 Accidents during meiosis can alter chromosome number An abnormal chromosome count is the result of nondisjunction, which can result from: the failure of a pair of homologous chromosomes to separate during meiosis I or the failure of sister chromatids to separate during meiosis II. © 2018 Pearson Education Ltd. Figure 8.18_1_1 Meiosis I Nondisjunction © 2018 Pearson Education Ltd. Figure 8.18_1_2 Meiosis I Nondisjunction Meiosis II Normal meiosis II © 2018 Pearson Education Ltd. Figure 8.18_1_3 Meiosis I Nondisjunction Meiosis II Normal meiosis II Gametes Number of n+1 n+1 n−1 n−1 chromosomes Abnormal gametes © 2018 Pearson Education Ltd. Figure 8.18_2_1 Normal meiosis I © 2018 Pearson Education Ltd. Figure 8.18_2_2 Normal meiosis I NONDISJUNCTION DURING MEIOSIS II Nondisjunction © 2018 Pearson Education Ltd. Figure 8.18_2_3 Normal meiosis I NONDISJUNCTION DURING MEIOSIS II Nondisjunction n+1 n−1 n n Abnormal Normal gametes gametes © 2018 Pearson Education Ltd.  8.19 A karyotype is a photographic inventory of an individual’s chromosomes To prepare a karyotype, white blood cells are isolated, stimulated to grow, arrested at metaphase, and photographed under a microscope. The chromosomes are arranged into ordered pairs so that any chromosomal abnormalities can be detected. https://www.youtube.com/watch?v=NG1HEgVbe4w © 2018 Pearson Education Ltd. Figure 8.19_1_3 Packed red Hypotonic and white solution Fixative blood cells Stain White Centrifuge blood Blood culture cells Fluid © 2018 Pearson Education Ltd. Figure 8.19_2 © 2018 Pearson Education Ltd. Figure 8.19_3 Centromere Sister chromatids Pair of homologous chromosomes 3,330× Sex chromosomes © 2018 Pearson Education Ltd.  Checkpoint question What features of the chromosomes shown in the karyotype in Figure 8.19 allow them to be correctly arranged into pairs? © 2018 Pearson Education Ltd.  8.20 CONNECTION: An extra copy of chromosome 21 causes Down syndrome Trisomy 21, the most common chromosome number abnormality, results in a condition called Down syndrome. https://www.youtube.com/watch?v=NDqVlCSM134 https://www.youtube.com/watch?v=bZcGpjyOXt0 © 2018 Pearson Education Ltd. Figure 8.20a 3,350× Trisomy 21 © 2018 Pearson Education Ltd. Figure 8.20a_1 3,350× Trisomy 21 © 2018 Pearson Education Ltd. Figure 8.20b 90 Infants with Down syndrome 80 70 (per 1,000 births) 60 50 40 30 20 10 0 20 25 30 35 40 45 Age of mother Source: Adapted from C. A. Huether et al., Maternal age specific risk rate estimates for Down syndrome among live births in whites and other races from Ohio and Metropolitan Atlanta, 1970–1989, Journal of Medical Genetics 35: 482–90 (1998). © 2018 Pearson Education Ltd.  8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival Nondisjunction of the sex chromosomes during meiosis can result in individuals with a missing or extra X or Y chromosome. In some cases (such as XXY), this leads to syndromes that can affect the health of the individual. In other cases (such as XXX), the body is normal. © 2018 Pearson Education Ltd. Table 8.21 © 2018 Pearson Education Ltd.  8.22 EVOLUTION CONNECTION: New species can arise from errors in cell division Nondisjunction can produce polyploid organisms, organisms with extra sets of chromosomes. Such errors in cell division can be important in the evolution of new species. © 2018 Pearson Education Ltd.  8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements—deletions, duplications, inversions, and translocations—that can produce genetic disorders or, if the changes occur in somatic cells, cancer. © 2018 Pearson Education Ltd. Figure 8.23a Deletion Duplication Homologous chromosomes Inversion Reciprocal translocation Nonhomologous chromosomes © 2018 Pearson Education Ltd. Figure 8.23a_1 Deletion Duplication Homologous chromosomes © 2018 Pearson Education Ltd. Figure 8.23a_2 Inversion Reciprocal translocation Nonhomologous chromosomes © 2018 Pearson Education Ltd. Figure 8.23b Chromosome 9 Chromosome 22 Reciprocal translocation Activated cancer-causing gene © 2018 Pearson Education Ltd.