Chapter 8 The Cellular Basis of Reproduction and Inheritance PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey Lecture by Edward J. Zalisko 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. Introduction In a healthy body, cell division allows for growth, the replacement of damaged cells, and development from an embryo into an adult. In sexually reproducing organisms, eggs and sperm result from mitosis and meiosis. 1
Figure 8.0_2 Chapter 8: Big Ideas Cell Division and Reproduction The Eukaryotic Cell Cycle and Mitosis Meiosis and Crossing Over Alterations of Chromosome Number and Structure Figure 8.0_3 CELL DIVISION AND REPRODUCTION 2
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 reproduction at the cellular level, requires the duplication of chromosomes, and sorts new sets of chromosomes into the resulting pair of daughter cells. 8.1 Cell division plays many important roles in the lives of organisms Cell division is used 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. 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. 3
THE EUKARYOTIC CELL CYCLE AND MITOSIS 8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division 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. 4
8.3 The large, complex chromosomes of eukaryotes duplicate with each cell division 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. Figure 8.3A Figure 8.3B Chromosomes DNA molecules Sister chromatids Chromosome duplication Centromere Sister chromatids Chromosome distribution to the daughter cells 5
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 separate from each other, now called chromosomes, and sort into separate daughter cells. Figure 8.3B_1 Chromosomes DNA molecules Chromosome duplication Centromere Sister chromatids Chromosome distribution to the daughter cells 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. 6
T 8.4 The cell cycle multiplies cells The cell cycle consists of two stages, characterized as follows: 1. Interphase: duplication of cell contents G 1 growth, increase in cytoplasm S duplication of chromosomes G 2 growth, preparation for division 2. Mitotic phase: division Mitosis division of the nucleus Cytokinesis division of cytoplasm Figure 8.4 I N T E R P H A S E G 1 (first gap) S (DNA synthesis) MI O Cytokinesis Mitosis M G 2 (second gap) T IC PHA SE 8.5 Cell division is a continuum of dynamic changes Mitosis progresses through a series of stages: prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis often overlaps telophase. 7
Figure 8.5_1 INTERPHASE Prophase Centrosomes Early mitotic (with centriole pairs) spindle Centrioles Centrosome Chromatin MITOSIS Prometaphase Fragments of the nuclear envelope Kinetochore Nuclear envelope Plasma membrane Chromosome, Centromere consisting of two sister chromatids Spindle microtubules Figure 8.5_left INTERPHASE Prophase MITOSIS Prometaphase Centrosomes (with centriole pairs) Centrioles Chromatin Early mitotic spindle Centrosome Fragments of the nuclear envelope Kinetochore Nuclear envelope Plasma membrane Chromosome, Centromere consisting of two sister chromatids Spindle microtubules 8.5 Cell division is a continuum of dynamic changes A mitotic spindle is required to divide the chromosomes, composed of microtubules, and produced by centrosomes, structures in the cytoplasm that organize microtubule arrangement and contain a pair of centrioles in animal cells. 8
8.5 Cell division is a continuum of dynamic changes 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. Figure 8.5_2 INTERPHASE 8.5 Cell division is a continuum of dynamic changes Prophase In the cytoplasm microtubules begin to emerge from centrosomes, forming the spindle. In the nucleus chromosomes coil and become compact and nucleoli disappear. 9
Figure 8.5_3 Prophase 8.5 Cell division is a continuum of dynamic changes 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. Figure 8.5_4 Prometaphase 10
Figure 8.5_left INTERPHASE Prophase MITOSIS Prometaphase Centrosomes (with centriole pairs) Centrioles Chromatin Early mitotic spindle Centrosome Fragments of the nuclear envelope Kinetochore Nuclear envelope Plasma membrane Chromosome, Centromere consisting of two sister chromatids Spindle microtubules Figure 8.5_5 Metaphase Metaphase plate MITOSIS Anaphase Telophase and Cytokinesis Cleavage furrow Mitotic spindle Daughter chromosomes Nuclear envelope forming Figure 8.5_right Metaphase MITOSIS Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Mitotic spindle Daughter chromosomes Nuclear envelope forming 11
8.5 Cell division is a continuum of dynamic changes Metaphase The mitotic spindle is fully formed. Chromosomes align at the cell equator. Kinetochores of sister chromatids are facing the opposite poles of the spindle. Figure 8.5_6 Metaphase 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. 12
Figure 8.5_7 Anaphase 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. Figure 8.5_8 Telophase and Cytokinesis 13
Figure 8.5_right Metaphase MITOSIS Anaphase Telophase and Cytokinesis Metaphase plate Cleavage furrow Mitotic spindle Daughter chromosomes Nuclear envelope forming 8.5 Cell division is a continuum of dynamic changes During cytokinesis, the cytoplasm is divided into separate cells. The process of cytokinesis differs in animal and plant cells. 8.6 Cytokinesis differs for plant and animal cells In animal cells, cytokinesis occurs as 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. 14
Figure 8.6A Cytokinesis Cleavage furrow Contracting ring of microfilaments Cleavage furrow Daughter cells 8.6 Cytokinesis differs for plant and animal cells In plant cells, cytokinesis occurs as 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, 3. each cell now possesses a plasma membrane and cell wall. Figure 8.6B Cytokinesis New cell wall Cell wall of the parent cell Daughter nucleus Cell plate forming Cell wall Plasma membrane Vesicles containing cell wall material Cell plate Daughter cells 15
8.7 Anchorage, cell density, and chemical growth factors affect cell division The cells within an organism s body divide and develop at different rates. Cell division is controlled by the presence of essential nutrients, 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. Figure 8.7A Cultured cells suspended in liquid The addition of growth factor Figure 8.7B Anchorage Single layer of cells Removal of cells Restoration of single layer by cell division 16
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 triggers and coordinates key events in the cell cycle. Checkpoints in the cell cycle can stop an event or signal an event to proceed. 8.8 Growth factors signal the cell cycle control system There are three major checkpoints in the cell cycle. 1. G 1 checkpoint allows entry into the S phase or causes the cell to leave the cycle, entering a nondividing G 0 phase. 2. G 2 checkpoint, and 3. M checkpoint. Research on the control of the cell cycle is one of the hottest areas in biology today. Figure 8.8A G 1 checkpoint G 0 G 1 S Control system M G 2 M checkpoint G 2 checkpoint 17
Figure 8.8B Growth factor Plasma membrane EXTRACELLULAR FLUID Relay proteins Receptor protein G 1 checkpoint Signal transduction pathway G 1 M Control system S G 2 CYTOPLASM 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. Cancer cells escape controls on the cell cycle. Cancer cells 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. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors A tumor is an abnormally growing mass of body cells. Benign tumors remain at the original site. Malignant tumors spread to other locations, called metastasis. 18
Figure 8.9 Tumor Glandular tissue Lymph vessels Blood vessel Tumor in another part of the body Growth Invasion Metastasis 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors 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. 8.9 CONNECTION: Growing out of control, cancer cells produce malignant tumors Cancer treatments Localized tumors can be removed surgically and/or treated with concentrated beams of high-energy radiation. Chemotherapy is used for metastatic tumors. 19
8.10 Review: Mitosis provides for growth, cell replacement, and asexual reproduction When the cell cycle operates normally, mitosis produces genetically identical cells for growth, replacement of damaged and lost cells, and asexual reproduction. Figure 8.10A MEIOSIS AND CROSSING OVER 20
8.11 Chromosomes are matched in homologous pairs In humans, somatic cells have 23 pairs of homologous chromosomes and one member of each pair from each parent. 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. 8.11 Chromosomes are matched in homologous pairs Homologous chromosomes are matched in length, centromere position, and 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. Figure 8.11 Locus Centromere Sister chromatids Pair of homologous chromosomes One duplicated chromosome 21
8.12 Gametes have a single set of chromosomes An organism s life cycle is the sequence of stages leading from the adults of one generation to the adults of the next. Humans and many animals and plants are diploid, with body cells that have two sets of chromosomes, one from each parent. 8.12 Gametes have a single set of chromosomes Meiosis is a process that converts diploid nuclei to haploid nuclei. Diploid cells have two homologous sets of chromosomes. Haploid cells have one set of chromosomes. Meiosis occurs in the sex organs, producing gametes sperm and eggs. Fertilization is the union of sperm and egg. The zygote has a diploid chromosome number, one set from each parent. Figure 8.12A Haploid gametes (n = 23) n Egg cell n Sperm cell Meiosis Fertilization Ovary Testis Diploid zygote (2n = 46) 2n Multicellular diploid adults (2n = 46) Mitosis Key Haploid stage (n) Diploid stage (2n) 22
8.12 Gametes have a single set of chromosomes All sexual life cycles include an alternation between a diploid stage and a haploid stage. Producing haploid gametes prevents the chromosome number from doubling in every generation. Figure 8.12B INTERPHASE MEIOSIS I MEIOSIS II Sister chromatids 1 2 3 A pair of homologous chromosomes in a diploid parent cell A pair of duplicated homologous chromosomes 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. Two haploid gametes combine in fertilization to restore the diploid state in the zygote. 23
8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis and mitosis are preceded by the duplication of chromosomes. However, 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. 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. 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. Figure 8.13_left MEIOSIS I: Homologous chromosomes separate INTERPHASE: Chromosomes duplicate Prophase I Metaphase I Anaphase I Centrosomes (with centriole pairs) Centrioles Sites of crossing over Spindle Spindle microtubules attached to a kinetochore Sister chromatids remain attached Tetrad Nuclear envelope Chromatin Sister chromatids Fragments of the nuclear envelope Centromere (with a kinetochore) Metaphase plate Homologous chromosomes separate 24
8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis I Metaphase I Tetrads align at the cell equator. Meiosis I Anaphase I Homologous pairs separate and move toward opposite poles of the cell. 8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis I Telophase I Duplicated chromosomes have reached the poles. A nuclear envelope re-forms around chromosomes in some species. Each nucleus has the haploid number of chromosomes. 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. Meiosis II Prophase II Chromosomes coil and become compact (if uncoiled after telophase I). Nuclear envelope, if re-formed, breaks up again. 25
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 Sister chromatids separate Haploid daughter cells forming Figure 8.13_5 Two lily cells undergo meiosis II 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. 26
8.13 Meiosis reduces the chromosome number from diploid to haploid Meiosis II Telophase II Chromosomes have reached the poles of the cell. A nuclear envelope forms around each set of chromosomes. With cytokinesis, four haploid cells are produced. 8.14 Mitosis and meiosis have important similarities and differences Mitosis and meiosis both begin with diploid parent cells that have chromosomes duplicated during the previous interphase. However the end products differ. Mitosis produces two genetically identical diploid somatic daughter cells. Meiosis produces four genetically unique haploid gametes. Figure 8.14 MITOSIS MEIOSIS I Prophase Parent cell (before chromosome duplication) Site of crossing over Prophase I Duplicated chromosome (two sister chromatids) Chromosome duplication 2n = 4 Chromosome duplication Tetrad formed by synapsis of homologous chromosomes Metaphase Metaphase I Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate 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 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 27
Figure 8.14_1 MITOSIS Prophase Parent cell (before chromosome duplication) Site of crossing over MEIOSIS I Prophase I Chromosome duplication Chromosome duplication Metaphase 2n = 4 Tetrad Metaphase I Chromosomes align at the metaphase plate Tetrads (homologous pairs) align at the metaphase plate Figure 8.14_2 MITOSIS Metaphase Chromosomes align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase 2n 2n Daughter cells of mitosis Figure 8.14_3 MEIOSIS I Metaphase I Tetrads (homologous pairs) align at the metaphase plate Homologous chromosomes separate during anaphase I; sister chromatids remain together Daughter cells of meiosis I MEIOSIS II Anaphase I Telophase I Haploid n = 2 No further chromosomal duplication; sister chromatids separate during anaphase II n n n n Daughter cells of meiosis II 28
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 and random fertilization. 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Independent orientation at metaphase I 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 2 n where n = haploid number of chromosomes. 8.15 Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring Random fertilization The combination of each unique sperm with each unique egg increases genetic variability. 29
Figure 8.15_s1 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Figure 8.15_s2 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Metaphase II Figure 8.15_s3 Possibility A Possibility B Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 30
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 Brown C c White Eye-color genes Black E Meiosis e Pink Tetrad in parent cell (homologous pair of duplicated chromosomes) C C c c E E e e Chromosomes of the four gametes Brown coat (C); black eyes (E) White coat (c); pink eyes (e) 8.17 Crossing over further increases genetic variability 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. Nonsister chromatids join at a chiasma (plural, chiasmata), the site of attachment and crossing over. Corresponding amounts of genetic material are exchanged between maternal and paternal (nonsister) chromatids. 31
Figure 8.17A Chiasma Tetrad Figure 8.17B_1 C c E e Tetrad (pair of homologous chromosomes in synapsis) 1 Breakage of homologous chromatids C E c e 2 Joining of homologous chromatids C c E e Chiasma Figure 8.17B_2 C c E e Chiasma 3 Separation of homologous chromosomes at anaphase I C C c c E e E e 32
Figure 8.17B_3 C C c c E e E e 4 Separation of chromatids at anaphase II and completion of meiosis C C c E e E Parental type of chromosome Recombinant chromosome Recombinant chromosome c e Parental type of chromosome Gametes of four genetic types ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE 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 homologous chromosome pairs, chromosome number, and chromosome structure. 33
Figure 8.18_s5 Centromere Sister chromatids Pair of homologous chromosomes 5 Sex chromosomes 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome Trisomy 21 involves the inheritance of three copies of chromosome 21 and is the most common human chromosome abnormality. 8.19 CONNECTION: An extra copy of chromosome 21 causes Down syndrome Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include: 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. 34
Figure 8.19A Trisomy 21 Figure 8.19B 90 Infants with Down syndrome (per 1,000 births) 80 70 60 50 40 30 20 10 0 20 25 30 35 40 45 50 Age of mother 8.20 Accidents during meiosis can alter chromosome number Nondisjunction is the failure of chromosomes or chromatids to separate normally during meiosis. This can happen during 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. 35
Figure 8.20A_s1 MEIOSIS I Nondisjunction Figure 8.20A_s2 MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Figure 8.20A_s3 MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes Number of chromosomes n + 1 n + 1 n - 1 n - 1 Abnormal gametes 36
Figure 8.20B_s1 MEIOSIS I Normal meiosis I Figure 8.20B_s2 MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction Figure 8.20B_s3 MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n + 1 n - 1 n n Abnormal gametes Normal gametes 37
8.21 CONNECTION: Abnormal numbers of sex chromosomes do not usually affect survival Sex chromosome abnormalities tend to be less severe, perhaps because of the small size of the Y chromosome or X-chromosome inactivation. 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Chromosome breakage can lead to rearrangements that can produce genetic disorders or, if changes occur in somatic cells, cancer. 8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer These rearrangements may include a deletion, the loss of a chromosome segment, a duplication, the repeat of a chromosome segment, an inversion, the reversal of a chromosome segment, or a translocation, the attachment of a segment to a nonhomologous chromosome that can be reciprocal. 38
8.23 CONNECTION: Alterations of chromosome structure can cause birth defects and cancer Chronic myelogenous leukemia (CML) is one of the most common leukemias, 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. Figure 8.23A Deletion Inversion Duplication Reciprocal translocation Homologous chromosomes Nonhomologous chromosomes Figure 8.23B Chromosome 9 Chromosome 22 Reciprocal translocation Activated cancer-causing gene Philadelphia chromosome 39