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Chapter 8 The Cellular Basis of Reproduction and Inheritance Introduction Cancer cells – start out as normal body cells, – undergo genetic mutations, – lose the ability to control the tempo of their own division, and – run amok, causing disease. PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko © 2012 Pearson Education, Inc. Introduction Figure 8.0_2 Chapter 8: Big Ideas In a healthy body, cell division allows for – growth, – the replacement of damaged cells, and – development from an embryo into an adult. Cell Division and Reproduction The Eukaryotic Cell Cycle and Mitosis In sexually reproducing organisms, eggs and sperm result from – mitosis and – meiosis. Meiosis and Crossing Over Alterations of Chromosome Number and Structure © 2012 Pearson Education, Inc. Figure 8.0_3 CELL DIVISION AND REPRODUCTION © 2012 Pearson Education, Inc. 1 8.1 Cell division plays many important roles in the lives of organisms 8.1 Cell division plays many important roles in the lives of organisms Organisms reproduce their own kind, a key characteristic of life. Cell division is used Cell division – is reproduction at the cellular level, – requires the duplication of chromosomes, and – sorts new sets of chromosomes into the resulting pair of daughter cells. © 2012 Pearson Education, Inc. – for reproduction of single-celled organisms, – growth of multicellular organisms from a fertilized egg into an adult, – repair and replacement of cells, and – sperm and egg production. © 2012 Pearson Education, Inc. 8.1 Cell division plays many important roles in the lives of organisms Living organisms reproduce by two methods. – Asexual reproduction – produces offspring that are identical to the original cell or organism and – involves inheritance of all genes from one parent. – Sexual reproduction – produces offspring that are similar to the parents, but show variations in traits and – involves inheritance of unique sets of genes from two parents. © 2012 Pearson Education, Inc. 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division THE EUKARYOTIC CELL CYCLE AND MITOSIS Eukaryotic cells – are more complex and larger than prokaryotic cells, – have more genes, and – store most of their genes on multiple chromosomes within the nucleus. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 2 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division Figure 8.3A Eukaryotic chromosomes are composed of chromatin consisting of – one long DNA molecule and – proteins that help maintain the chromosome structure and control the activity of its genes. To prepare for division, the chromatin becomes – highly compact and – visible with a microscope. © 2012 Pearson Education, Inc. Figure 8.3B Chromosomes DNA molecules Sister chromatids Chromosome duplication Centromere Sister chromatids 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes, resulting in – two copies called sister chromatids – joined together by a narrowed “waist” called the centromere. When a cell divides, the sister chromatids Chromosome distribution to the daughter cells – separate from each other, now called chromosomes, and – sort into separate daughter cells. © 2012 Pearson Education, Inc. Figure 8.3B_1 Chromosomes DNA molecules Chromosome duplication Centromere Sister chromatids 8.4 The cell cycle multiplies cells The cell cycle is an ordered sequence of events that extends – from the time a cell is first formed from a dividing parent cell – until its own division. Chromosome distribution to the daughter cells © 2012 Pearson Education, Inc. 3 Figure 8.4 8.4 The cell cycle multiplies cells INT E RPHASE The cell cycle consists of two stages, characterized as follows: 1. Interphase: duplication of cell contents G1 (first gap) – G1—growth, increase in cytoplasm – S—duplication of chromosomes – Mitosis—division of the nucleus s ito G2 (second gap) M IC OT MIT 2. Mitotic phase: division M si esis okin Cyt – G2—growth, preparation for division S (DNA synthesis) A PH – Cytokinesis—division of cytoplasm SE © 2012 Pearson Education, Inc. Figure 8.5_1 8.5 Cell division is a continuum of dynamic changes MITOSIS INTERPHASE Mitosis progresses through a series of stages: Centrosomes (with centriole pairs) – prophase, Centrioles – prometaphase, Chromatin Prophase Prometaphase Early mitotic spindle Centrosome Fragments of the nuclear envelope Kinetochore – metaphase, – anaphase, and – telophase. Cytokinesis often overlaps telophase. Nuclear envelope Plasma membrane Centromere Chromosome, consisting of two sister chromatids Spindle microtubules © 2012 Pearson Education, Inc. Figure 8.5_left 8.5 Cell division is a continuum of dynamic changes MITOSIS INTERPHASE Prophase Prometaphase A mitotic spindle is – required to divide the chromosomes, – composed of microtubules, and Centrosomes (with centriole pairs) Centrioles Chromatin Early mitotic spindle Centrosome Fragments of the nuclear envelope Kinetochore – produced by centrosomes, structures in the cytoplasm that – organize microtubule arrangement and – contain a pair of centrioles in animal cells. Nuclear envelope Plasma membrane Centromere Chromosome, consisting of two sister chromatids Spindle microtubules © 2012 Pearson Education, Inc. 4 8.5 Cell division is a continuum of dynamic changes Figure 8.5_2 Interphase – The cytoplasmic contents double, – two centrosomes form, – chromosomes duplicate in the nucleus during the S phase, and – nucleoli, sites of ribosome assembly, are visible. INTERPHASE © 2012 Pearson Education, Inc. 8.5 Cell division is a continuum of dynamic changes Figure 8.5_3 Prophase – In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle. – In the nucleus – chromosomes coil and become compact and – nucleoli disappear. Prophase © 2012 Pearson Education, Inc. 8.5 Cell division is a continuum of dynamic changes Figure 8.5_4 Prometaphase – Spindle microtubules reach chromosomes, where they – attach at kinetochores on the centromeres of sister chromatids and – move chromosomes to the center of the cell through associated protein “motors.” – Other microtubules meet those from the opposite poles. – The nuclear envelope disappears. Prometaphase © 2012 Pearson Education, Inc. 5 Figure 8.5_left Figure 8.5_5 MITOSIS INTERPHASE Prophase Prometaphase MITOSIS Anaphase Metaphase Metaphase plate Centrosomes (with centriole pairs) Centrioles Chromatin Early mitotic spindle Cleavage furrow Fragments of the nuclear envelope Centrosome Kinetochore Daughter chromosomes Mitotic spindle Nuclear envelope Telophase and Cytokinesis Plasma membrane Centromere Chromosome, consisting of two sister chromatids Spindle microtubules Figure 8.5_right MITOSIS Anaphase Metaphase Nuclear envelope forming Telophase and Cytokinesis 8.5 Cell division is a continuum of dynamic changes Metaphase – The mitotic spindle is fully formed. – Chromosomes align at the cell equator. Metaphase plate Mitotic spindle Cleavage furrow Daughter chromosomes – Kinetochores of sister chromatids are facing the opposite poles of the spindle. Nuclear envelope forming © 2012 Pearson Education, Inc. Figure 8.5_6 8.5 Cell division is a continuum of dynamic changes Anaphase – Sister chromatids separate at the centromeres. – Daughter chromosomes are moved to opposite poles of the cell as – motor proteins move the chromosomes along the spindle microtubules and – kinetochore microtubules shorten. – The cell elongates due to lengthening of nonkinetochore microtubules. Metaphase © 2012 Pearson Education, Inc. 6 Figure 8.5_7 8.5 Cell division is a continuum of dynamic changes Telophase – The cell continues to elongate. – The nuclear envelope forms around chromosomes at each pole, establishing daughter nuclei. – Chromatin uncoils and nucleoli reappear. – The spindle disappears. Anaphase © 2012 Pearson Education, Inc. Figure 8.5_8 Figure 8.5_right MITOSIS Anaphase Metaphase Metaphase plate Telophase and Cytokinesis Cleavage furrow Telophase and Cytokinesis Mitotic spindle Daughter chromosomes Nuclear envelope forming 8.5 Cell division is a continuum of dynamic changes 8.6 Cytokinesis differs for plant and animal cells During cytokinesis, the cytoplasm is divided into separate cells. In animal cells, cytokinesis occurs as The process of cytokinesis differs in animal and plant cells. © 2012 Pearson Education, Inc. 1. a cleavage furrow forms from a contracting ring of microfilaments, interacting with myosin, and 2. the cleavage furrow deepens to separate the contents into two cells. © 2012 Pearson Education, Inc. 7 Figure 8.6A 8.6 Cytokinesis differs for plant and animal cells Cytokinesis Cleavage furrow In plant cells, cytokinesis occurs as Contracting ring of microfilaments 1. a cell plate forms in the middle, from vesicles containing cell wall material, 2. the cell plate grows outward to reach the edges, dividing the contents into two cells, Daughter cells 3. each cell now possesses a plasma membrane and cell wall. Cleavage furrow © 2012 Pearson Education, Inc. Figure 8.6B 8.7 Anchorage, cell density, and chemical growth factors affect cell division New cell wall Cytokinesis Cell wall of the parent cell Cell wall Cell division is controlled by Plasma membrane – the presence of essential nutrients, Daughter nucleus Cell plate forming The cells within an organism’s body divide and develop at different rates. Vesicles containing cell wall material Cell plate Daughter cells – growth factors, proteins that stimulate division, – density-dependent inhibition, in which crowded cells stop dividing, and – anchorage dependence, the need for cells to be in contact with a solid surface to divide. © 2012 Pearson Education, Inc. Figure 8.7A Figure 8.7B Anchorage Cultured cells suspended in liquid The addition of growth factor Single layer of cells Removal of cells Restoration of single layer by cell division 8 8.8 Growth factors signal the cell cycle control system 8.8 Growth factors signal the cell cycle control system The cell cycle control system is a cycling set of molecules in the cell that There are three major checkpoints in the cell cycle. 1. G1 checkpoint – triggers and – allows entry into the S phase or – coordinates key events in the cell cycle. Checkpoints in the cell cycle can – causes the cell to leave the cycle, entering a nondividing G0 phase. 2. G2 checkpoint, and – stop an event or 3. M checkpoint. – signal an event to proceed. Research on the control of the cell cycle is one of the hottest areas in biology today. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 8.8A Figure 8.8B G1 checkpoint Growth factor G0 EXTRACELLULAR FLUID Plasma membrane Relay proteins G1 S Control system M Receptor protein Signal transduction pathway G2 G1 checkpoint G1 S Control system M G2 M checkpoint CYTOPLASM G2 checkpoint 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors Cancer currently claims the lives of 20% of the people in the United States and other industrialized nations. A tumor is an abnormally growing mass of body cells. Cancer cells escape controls on the cell cycle. Cancer cells – Benign tumors remain at the original site. – Malignant tumors spread to other locations, called metastasis. – divide rapidly, often in the absence of growth factors, – spread to other tissues through the circulatory system, and – grow without being inhibited by other cells. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 9 Figure 8.9 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors Lymph vessels Blood vessel Tumor Tumor in another part of the body Glandular tissue Growth Invasion Cancers are named according to the organ or tissue in which they originate. – Carcinomas arise in external or internal body coverings. – Sarcomas arise in supportive and connective tissue. – Leukemias and lymphomas arise from blood-forming tissues. Metastasis © 2012 Pearson Education, Inc. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors 8.10 Review: Mitosis provides for growth, cell replacement, and asexual reproduction Cancer treatments When the cell cycle operates normally, mitosis produces genetically identical cells for – Localized tumors can be – removed surgically and/or – growth, – treated with concentrated beams of high-energy radiation. – replacement of damaged and lost cells, and – Chemotherapy is used for metastatic tumors. © 2012 Pearson Education, Inc. – asexual reproduction. © 2012 Pearson Education, Inc. Figure 8.10A MEIOSIS AND CROSSING OVER © 2012 Pearson Education, Inc. 10 8.11 Chromosomes are matched in homologous pairs 8.11 Chromosomes are matched in homologous pairs In humans, somatic cells have Homologous chromosomes are matched in – 23 pairs of homologous chromosomes and – length, – one member of each pair from each parent. – centromere position, and The human sex chromosomes X and Y differ in size and genetic composition. The other 22 pairs of chromosomes are autosomes with the same size and genetic composition. © 2012 Pearson Education, Inc. – gene locations. A locus (plural, loci) is the position of a gene. Different versions of a gene may be found at the same locus on maternal and paternal chromosomes. © 2012 Pearson Education, Inc. Figure 8.11 8.12 Gametes have a single set of chromosomes Pair of homologous chromosomes An organism’s life cycle is the sequence of stages leading – from the adults of one generation Locus – to the adults of the next. Centromere Sister chromatids One duplicated chromosome Humans and many animals and plants are diploid, with body cells that have – two sets of chromosomes, – one from each parent. © 2012 Pearson Education, Inc. 8.12 Gametes have a single set of chromosomes Figure 8.12A Haploid gametes (n = 23) n Egg cell n Sperm cell Meiosis is a process that converts diploid nuclei to haploid nuclei. – Diploid cells have two homologous sets of chromosomes. Meiosis Fertilization – Haploid cells have one set of chromosomes. – Meiosis occurs in the sex organs, producing gametes —sperm and eggs. Ovary Testis Diploid zygote (2n = 46) Fertilization is the union of sperm and egg. The zygote has a diploid chromosome number, one set from each parent. 2n Key Multicellular diploid adults (2n = 46) Mitosis Haploid stage (n) Diploid stage (2n) © 2012 Pearson Education, Inc. 11 8.12 Gametes have a single set of chromosomes Figure 8.12B INTERPHASE All sexual life cycles include an alternation between MEIOSIS II Sister chromatids – a diploid stage and – a haploid stage. Producing haploid gametes prevents the chromosome number from doubling in every generation. MEIOSIS I 2 1 A pair of homologous chromosomes in a diploid parent cell 3 A pair of duplicated homologous chromosomes © 2012 Pearson Education, Inc. 8.13 Meiosis reduces the chromosome number from diploid to haploid 8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis is a type of cell division that produces haploid gametes in diploid organisms. Meiosis and mitosis are preceded by the duplication of chromosomes. However, Two haploid gametes combine in fertilization to restore the diploid state in the zygote. – meiosis is followed by two consecutive cell divisions and – mitosis is followed by only one cell division. Because in meiosis, one duplication of chromosomes is followed by two divisions, each of the four daughter cells produced has a haploid set of chromosomes. © 2012 Pearson Education, Inc. 8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis I – Prophase I – events occurring in the nucleus. – Chromosomes coil and become compact. © 2012 Pearson Education, Inc. Figure 8.13_left MEIOSIS I: Homologous chromosomes separate INTERPHASE: Chromosomes duplicate Centrosomes (with centriole pairs) Prophase I Metaphase I Sites of crossing over Spindle microtubules attached to a kinetochore Centrioles Anaphase I Sister chromatids remain attached Spindle – Homologous chromosomes come together as pairs by synapsis. – Each pair, with four chromatids, is called a tetrad. – Nonsister chromatids exchange genetic material by crossing over. Tetrad Nuclear envelope Chromatin Sister chromatids Fragments of the nuclear envelope Centromere (with a kinetochore) Metaphase plate Homologous chromosomes separate © 2012 Pearson Education, Inc. 12 8.13 Meiosis reduces the chromosome number from diploid to haploid 8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis I – Metaphase I – Tetrads align at the cell equator. Meiosis I – Telophase I Meiosis I – Anaphase I – Homologous pairs separate and move toward opposite poles of the cell. – Duplicated chromosomes have reached the poles. – A nuclear envelope re-forms around chromosomes in some species. – Each nucleus has the haploid number of chromosomes. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis II follows meiosis I without chromosome duplication. Each of the two haploid products enters meiosis II. Figure 8.13_right MEIOSIS II: Sister chromatids separate Telophase I and Cytokinesis Prophase II Metaphase II Anaphase II Telophase II and Cytokinesis Cleavage furrow Meiosis II – Prophase II Sister chromatids separate – Chromosomes coil and become compact (if uncoiled after telophase I). Haploid daughter cells forming – Nuclear envelope, if re-formed, breaks up again. © 2012 Pearson Education, Inc. Figure 8.13_5 8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis II – Metaphase II – Duplicated chromosomes align at the cell equator. Meiosis II – Anaphase II – Sister chromatids separate and – chromosomes move toward opposite poles. Two lily cells undergo meiosis II © 2012 Pearson Education, Inc. 13 8.13 Meiosis reduces the chromosome number from diploid to haploid 8.14 Mitosis and meiosis have important similarities and differences Meiosis II – Telophase II Mitosis and meiosis both – Chromosomes have reached the poles of the cell. – begin with diploid parent cells that – A nuclear envelope forms around each set of chromosomes. – have chromosomes duplicated during the previous interphase. However the end products differ. – With cytokinesis, four haploid cells are produced. – Mitosis produces two genetically identical diploid somatic daughter cells. – Meiosis produces four genetically unique haploid gametes. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 8.14 Figure 8.14_1 MEIOSIS I MITOSIS Parent cell (before chromosome duplication) Prophase Duplicated chromosome (two sister chromatids) Chromosome duplication Site of crossing over Prophase I MEIOSIS I MITOSIS Tetrad formed by synapsis of homologous chromosomes Chromosome duplication 2n = 4 Prophase Parent cell (before chromosome duplication) Prophase I Site of crossing over Metaphase I Metaphase Chromosomes align at the metaphase plate Chromosome duplication Tetrads (homologous pairs) align at the metaphase plate Chromosome duplication Tetrad 2n = 4 Anaphase I Telophase I Anaphase Telophase Homologous chromosomes separate during anaphase I; sister chromatids remain together Sister chromatids separate during anaphase Daughter cells of meiosis I Metaphase Metaphase I Chromosomes align at the metaphase plate Haploid n=2 Tetrads (homologous pairs) align at the metaphase plate MEIOSIS II 2n 2n Daughter cells of mitosis No further chromosomal duplication; sister chromatids separate during anaphase II n n n n Daughter cells of meiosis II Figure 8.14_2 Figure 8.14_3 MEIOSIS I MITOSIS Metaphase I Metaphase Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate Anaphase I Telophase I Homologous chromosomes separate during anaphase I; sister chromatids remain together Anaphase Telophase 2n Daughter cells of mitosis Haploid n=2 MEIOSIS II Sister chromatids separate during anaphase 2n Daughter cells of meiosis I No further chromosomal duplication; sister chromatids separate during anaphase II n n n n Daughter cells of meiosis II 14 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Genetic variation in gametes results from Independent orientation at metaphase I – independent orientation at metaphase I and – random fertilization. – Each pair of chromosomes independently aligns at the cell equator. – There is an equal probability of the maternal or paternal chromosome facing a given pole. – The number of combinations for chromosomes packaged into gametes is 2n where n = haploid number of chromosomes. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Figure 8.15_s1 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Random fertilization – The combination of each unique sperm with each unique egg increases genetic variability. © 2012 Pearson Education, Inc. Figure 8.15_s2 Figure 8.15_s3 Possibility A Possibility B Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Two equally probable arrangements of chromosomes at metaphase I Metaphase II Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 15 8.16 Homologous chromosomes may carry different versions of genes Separation of homologous chromosomes during meiosis can lead to genetic differences between gametes. – Homologous chromosomes may have different versions of a gene at the same locus. – One version was inherited from the maternal parent and the other came from the paternal parent. – Since homologues move to opposite poles during anaphase I, gametes will receive either the maternal or paternal version of the gene. Figure 8.16 Coat-color genes Eye-color genes Brown C Black E c White e Pink Meiosis Tetrad in parent cell (homologous pair of duplicated chromosomes) C E C E c e c e Chromosomes of the four gametes Brown coat (C); black eyes (E) White coat (c); pink eyes (e) © 2012 Pearson Education, Inc. 8.17 Crossing over further increases genetic variability Figure 8.17A Genetic recombination is the production of new combinations of genes due to crossing over. Crossing over is an exchange of corresponding segments between separate (nonsister) chromatids on homologous chromosomes. Chiasma – Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over. Tetrad – Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids. © 2012 Pearson Education, Inc. Figure 8.17B_1 C E c e 1 E c e C E c e 3 Joining of homologous chromatids E Chiasma c C Chiasma Breakage of homologous chromatids C 2 Figure 8.17B_2 Tetrad (pair of homologous chromosomes in synapsis) Separation of homologous chromosomes at anaphase I C E C e c E c e e 16 Figure 8.17B_3 C E C c e E c e 4 Separation of chromatids at anaphase II and completion of meiosis C E C e c E c e ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types © 2012 Pearson Education, Inc. 8.18 A karyotype is a photographic inventory of an individual s chromosomes A karyotype is an ordered display of magnified images of an individual’s chromosomes arranged in pairs. Karyotypes – are often produced from dividing cells arrested at metaphase of mitosis and – allow for the observation of Figure 8.18_s5 Centromere Sister chromatids Pair of homologous chromosomes – homologous chromosome pairs, 5 – chromosome number, and Sex chromosomes – chromosome structure. © 2012 Pearson Education, Inc. 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome Trisomy 21 Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include: – involves the inheritance of three copies of chromosome 21 and – is the most common human chromosome abnormality. – mental retardation, – characteristic facial features, – short stature, – heart defects, – susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and – shortened life span. The incidence increases with the age of the mother. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 17 Figure 8.19A Figure 8.19B Trisomy 21 Infants with Down syndrome (per 1,000 births) 90 80 70 60 50 40 30 20 10 0 20 8.20 Accidents during meiosis can alter chromosome number 25 30 35 40 Age of mother 45 50 Figure 8.20A_s1 MEIOSIS I Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during Nondisjunction – meiosis I, if both members of a homologous pair go to one pole or – meiosis II if both sister chromatids go to one pole. Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes. © 2012 Pearson Education, Inc. Figure 8.20A_s2 Figure 8.20A_s3 MEIOSIS I MEIOSIS I Nondisjunction MEIOSIS II Nondisjunction MEIOSIS II Normal meiosis II Normal meiosis II Gametes Number of chromosomes n+1 n+1 n-1 n-1 Abnormal gametes 18 Figure 8.20B_s1 Figure 8.20B_s2 MEIOSIS I MEIOSIS I Normal meiosis I Normal meiosis I MEIOSIS II Nondisjunction Figure 8.20B_s3 8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival MEIOSIS I Normal meiosis I Sex chromosome abnormalities tend to be less severe, perhaps because of MEIOSIS II – the small size of the Y chromosome or – X-chromosome inactivation. Nondisjunction n+1 n-1 Abnormal gametes n n Normal gametes © 2012 Pearson Education, Inc. 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements that can produce These rearrangements may include – a deletion, the loss of a chromosome segment, – genetic disorders or, – a duplication, the repeat of a chromosome segment, – if changes occur in somatic cells, cancer. – an inversion, the reversal of a chromosome segment, or – a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 19 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Figure 8.23A Chronic myelogenous leukemia (CML) – is one of the most common leukemias, Deletion Inversion Duplication Reciprocal translocation – affects cells that give rise to white blood cells (leukocytes), and – results from part of chromosome 22 switching places with a small fragment from a tip of chromosome 9. Homologous chromosomes Nonhomologous chromosomes © 2012 Pearson Education, Inc. Figure 8.23B Chromosome 9 Chromosome 22 Reciprocal translocation Activated cancer-causing gene Philadelphia chromosome 20