Learning Objectives Chapter 8 Brief overview of prokaryotic cell replication The three main phases of eukaryotic cell division: Interphase, M phase, C phase Interphase is broken down into three sub-phases (know what is happening during these phases) M phase (mitosis) is broken down into phases (know what happens during mitosis) Cells divide a finite number of times Uncontrolled cell division = cancer Meiosis allows organisms to reproduce sexually while maintaining the same number of chromosomes
Learning Objectives Chapter 9 Meiosis allows organisms to reproduce sexually while maintaining the same number of chromosomes Become familiar with gametes and how they package hereditary information What is reduction division Understand key differences between meiosis I and mitosis Understand key differences between meiosis II and meiosis I Learn the sequence of events that occurs from the beginning of meiosis I to the end of meiosis II What are the mechanisms by which meiosis introduces genetic diversity?
Prokaryotes Have a Simple Cell Cycle Cell division in prokaryotes takes place in two stages, which together make up a simple cell cycle 1. copy the DNA this process is called replication 2. split the cell in two to form daughter cells this process is called binary fission Does this process introduce genetic diversity?
DNA Replication in Prokaryotes the prokaryotic chromosome is a single circle of DNA DNA replication begins with the unzipping of the doublestranded DNA at the origin of replication a new double helix is formed by adding complementary nucleotides to the exposed DNA strands At the end the cell possesses two complete copies of the chromosome
Prokaryotic Cell Division After replication, the cell grows in order to partition the replicated DNA molecules when the cell reaches an appropriate size, the cell splits into two equal halves new plasma membrane and cell wall are added at a point between the partitioned DNA eventually the cell constricts in two to form two daughter cells each daughter cell is a complete, living cell with its own DNA
The Eukaryotic Cell Cycle Eukaryotic cells contain more DNA than prokaryotic cells and the DNA is also packaged differently DNA in eukaryotic cells is linear and packaged into a compact chromosome there is more than one chromosome in a eukaryotic cell
Mitosis vs. Meiosis Eukaryotic cells have two different mechanisms to divide up the DNA mitosis is cell division that occurs in nonreproductive cells these cells are called somatic cells meiosis is cell division that occurs in cells of sexual reproduction these cells are called germ line cells
Eukaryotic Cell Cycle: 3 Phases The eukaryotic cell cycle is divided into distinct phases (each further broken down into subphases) Interphase (G 1,S, and G 2 phases) Mitosis (M phase) Cytokinesis (C phase)
Interphase Interphase is comprised of three phases G 1 phase ( Gap 1 ) the primary growth phase of the cell following division most cells spend the majority of their lifespan in this phase S phase (Synthesis) DNA replication occurs in preparation for cell division G 2 phase ( Gap 2 ) further preparation for cell division, including replication of mitochondria and synthesis of microtubules
M Phase and C Phase Mitosis (M phase) a microtubular apparatus binds to the chromosomes and moves them apart Cytokinesis (C phase) the cytoplasm divides, creating two daughter cells
Essential Biological Process 8A: The Cell Cycle
Animation: How the Cell Cycle Works Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Chromosomes Chromosome number varies among organisms most eukaryotes have between 10 and 50 chromosomes in their somatic cells Chromosomes exist as pairs in somatic cells these pairs are called homologous chromosomes, or homologues homologues contain information about the same traits but the information may vary cells that have two of each type of chromosome are called diploid cells one chromosome of each pair is inherited from the mother and the other is inherited from the father
Chromatids Prior to cell division, each of the homologous chromosomes replicates, forming two identical copies called sister chromatids the sister chromatids are joined together by a structure called a centromere humans have 23 pairs of homologous chromosomes when each chromosome in the pair is replicated, this makes for a total of 92 chromatids
Figure 8.2 The difference between homologous chromosomes and sister chromatids How many chromosomes?
Karyotype A karyotype is an arrangement of chromosomes Figure 8.4 The 46 chromosomes of a human Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homologous pair Chromosomes can be compared based on size, shape, and centromere location The karyotype at right shows the 23 pairs of human chromosomes Andrew S. Bajer
Chromatin Chromosomes are comprised of chromatin, a complex of DNA and protein there is also some RNA associated with chromosomes the DNA in a chromosome is one very long double-stranded fiber that extends unbroken for the length of the chromosome the DNA is coiled in order to allow it to fit into a small space despite being very long Why?
Figure 8.5 Levels of eukaryotic chromosomal organization DNA is coiled around histones
Cell Division - Interphase Interphase sets the stage for cell division chromosomes are first duplicated although not visible, chromosomes begin to wind up tightly in a process called condensation
The cell division that follows interphase is a division of the nuclear contents, known as mitosis mitosis is a continuous process but it is divided, for ease of study, into four distinct stages 1. prophase 2. metaphase 3. anaphase 4. telophase Mitosis
Mitosis: 1 st Phase Prophase in prophase, the condensed chromosomes first become visible with a light microscope the nuclear envelope begins to disintegrate centrosomes (centrioles in animal cells) begin to assemble a network of protein cables called the spindle each cable in the spindle is made of microtubules some of the microtubules attach to the chromosomes when the process is complete, the sister chromatids of a chromosome are attached by microtubules to opposite poles of the cell
Mitosis: 2 nd Phase Metaphase the chromosomes attached to microtubules of the spindle are aligned in the center of the cell the centromeres are aligned along an imaginary plane that divides the cell in half, known as the equatorial plane
Mitosis: 3 rd Phase Anaphase centromeres split sister chromatids separate the microtubules of the spindle are dismantled starting at the poles this pulls the chromatids toward the poles
Mitosis: 4 th Phase Telophase the spindle is dismantled a nuclear envelope forms around the set of chromosomes at each pole the chromosomes begin to decondense the nucleolus reappears
Cytokinesis End of mitosis division of the cytoplasm into halves In animals, cytokinesis occurs by actin filaments contracting and pinching the cell in two this action is evident as a cleavage furrow that appears between the daughter cells In plants, a new cell wall is laid down to divide the two daughter cells the cell wall grows at right angles to the mitotic spindle and is called the cell plate
Essential Biological Process 8B: Cell Division
Animation: Mitosis Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Animation: Mitosis and Cytokinesis Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
What Is Cancer? Cells are programmed undergo only so many times and then die human cells divide about 50 times Cancer is a growth disorder of cells begins when apparently normal cells grow uncontrollably the result is a growing cluster of cells called a tumor malignant tumors are invasive cells from malignant tumors can metastasize, spreading to different areas of the body to form new tumors
Lung Cancer Figure 8.9 Lung cancer cells (300X) Figure 8.10 Portrait of a tumor
Mutation and Cancer Cell division is regulated by proteins called growth factors Cancer is caused by damage to genes the encode growth factors mutation causes damage to genes may result from chemical or environmental exposure, such as UV rays viral exposure may also alter DNA
Cancer Genetics There are two general classes of growth factor genes that are usually involved in cancer proto-oncogenes these genes encode proteins that stimulate cell division mutations to these genes can cause cells to divide excessively when mutated, these genes become oncogenes tumor-suppressor genes these genes normally turn off cell division in healthy cells when mutated, these genes allow uncontrolled cell division
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Inquiry & Analysis Why Do Human Cells Age?
Animation: Telomerase Function Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Sexual Reproduction Gametes are reproductive cells (eggs and sperm) that contain half the complement of chromosomes found in somatic cells (haploid or diploid?) The gametes fuse to form a new cell called a zygote, which contains two complete copies of each chromosome (haploid or diploid?) the fusion of gametes is called fertilization, or syngamy
Meiosis The formation of gametes must involve some mechanism to halve the number of chromosomes found in somatic cells If not the number of chromosomes would double with each fertilization Meiosis is a process of reduction division in forming gametes this ensures a consistent chromosome number across generations
Haploid Gametes Meiosis and fertilization constitute a cycle of sexual reproduction Figure 9.1 Diploid cells carry chromosomes from two parents Somatic cells have two sets of chromosomes making them diploid Gametes have only one set of chromosomes, making them haploid What is the diploid number in humans?
Sexual vs. Asexual Reproduction Some organisms reproduce by mitotic division and do not involve gametes this is called asexual reproduction an example is binary fission in prokaryotes Other organisms are able to reproduce both sexually and asexually For example, strawberry plants flower (sexual reproduction) and send out runners (asexual reproduction) Many fungi have sexual and asexual forms
The Sexual Life Cycle In sexual reproduction, haploid cells or organisms alternate with diploid cells or organisms Figure 9.4 Three types of sexual life cycles
Germ-Line Cells In animals, the cells that will eventually undergo meiosis are reserved early on for the purpose of reproduction these cells are referred to as germ-line cells and are diploid like somatic cells but only the germ-line cells will undergo meiosis to produce haploid gametes
Figure 9.3 The sexual life cycle in animals
Animation: How Meiosis Works Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
The Stages of Meiosis Meiosis involves two divisions, meiosis I and meiosis II DNA is replicated only before meiosis I meiosis I separates the homologous chromosomes meiosis II separates the replicate sister chromatids when meiosis is complete, the result is that one diploid cell has become four haploid cells How many chromosomes are present in a human cell after meiosis I? How many chromatids? After meiosis II?
Meiosis I Meiosis I is traditionally divided into four stages 1. Prophase I homologues pair up and exchange segments 2. Metaphase I the paired homologous chromosomes align on a central plane 3. Anaphase I homologues separate and move to opposite poles 4. Telophase I chromosomes gather at each of the two poles What s the big difference between meiosis I and mitosis?
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Meiosis I: Prophase During prophase I, homologous chromosomes line up together as a pair crossing over occurs between nonsister chromatids of homologous chromosomes the chromatids break in the same place and sections of chromosomes are swapped the result is a hybrid chromosome the pairing is held together by the cohesion between sister chromatids and the crossovers What is the difference between mitosis and meiosis in terms of how chromosomes are held together?
Figure 9.5 Crossing over
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Meiosis I: Metaphase During metaphase I, the orientation of the homologous chromosomes on the metaphase plate is random each possible orientation of which homologue faces which pole results in gametes with different combinations of parental chromosomes this process is called independent assortment In humans, produces over 8 million different chromosome combinations! How is this different from metaphase in mitosis?
Figure 9.6 Independent assortment
Figure Independent assortment increases genetic variability Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Paternal gamete Maternal gamete Diploid offspring Homologous pairs Potential gametes
Meiosis I: Anaphase In anaphase I, the chromosome pairs separate and individual homologues move to each pole
Meiosis I: Telophase In telophase I, the chromosomes gather at their respective poles to form two chromosome clusters How many copies of each chromosome is present after meiosis I?
Meiosis II After meiosis I, a brief interphase occurs where there is no replication of DNA Meiosis II follows and is basically a mitotic division of the products of meiosis I except that the sister chromatids are nonidentical because of crossing over in meiosis I
The Stages of Meiosis II Meiosis II is also divided into four stages 1. Prophase II new spindle forms to attach to chromosome clusters 2. Metaphase II spindle fibers bind to both sides of the centromere and individual chromosomes align along a central plane 3. Anaphase II sister chromatids are pulled to opposite poles Non-disjunction of chromatids at this stage can result in trisomy 4. Telophase II the nuclear envelope is reformed around each of the four sets of daughter chromosomes
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Essential Biological Process 9A: Meiosis I
Essential Biological Process 9A: Meiosis II
Animation: Stages of Meiosis Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
How Meiosis Differs from Mitosis Meiosis has two unique features not found in mitosis synapsis homologous chromosomes pair along their entire lengths and are held together by cohesin proteins; this close association permits crossing over reduction division because meiosis involves two nuclear divisions but only one replication of DNA, the final amount of genetic material passed to the gametes is halved
Animation: Comparison of Meiosis and Mitosis Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Figure 9.8 Unique features of meiosis How do sister chromatids entering meiosis II differ from each other?
Animation: The Function of Cohesin Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Animation: Unique Features of Meiosis Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the Normal or Slide Sorter views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
Figure 9.9 A comparison of meiosis and mitosis