BIOL 1P91 - Chapter 16 STUDENT 2024 3 - Eukaryotic Cell Cycle PDF

The Eukaryotic Cell Cycle, Mitosis and Meiosis Chapter 16 1  Chapter 16 Outline  The Eukaryotic Cell Cycle  Mitotic Cell Division  Meiosis  17.2 The Chromosome Theory of Inheritance  Sexual Reproduction  Vari...

The Eukaryotic Cell Cycle, Mitosis and Meiosis Chapter 16 1  Chapter 16 Outline  The Eukaryotic Cell Cycle  Mitotic Cell Division  Meiosis  17.2 The Chromosome Theory of Inheritance  Sexual Reproduction  Variation in Chromosome Structure and Number 2  Cell Division  The adult human body contains between 10 to 50 trillion cells  Except for rare mutations, the DNA sequences of all chromosomes in all cells are the same  Careful reproduction of cells ensures the integrity of the genetic material through two processes:  Mitosis  Meiosis 3  The Eukaryotic Cell Cycle  A cell cycle is a highly regulated series of events that leads to cell division  When cells get ready to divide, the chromosomes become compact enough to be seen with a light microscope  Cytogenetics – field of genetics involving microscopic examination of chromosomes ClickThe 16.1 to edit Master text eukaryotic cell cycle 4 Karyotype: A photographic representation of chromosomes 5  Sets of chromosomes  Eukaryotic chromosomes occur in sets  One set (n) of human chromosomes has 23 different chromosomes (n = 23)  22 autosomes (non sex chromosomes)  Sex chromosomes – X and Y  Most human cells have two sets of chromosomes (= 46 chromosomes total)  Two chromosome sets = diploid or 2n  Gametes(sperm and eggs) have only 1 set of chromosomes = haploid or n 6  Homologs  In diploid species, members of a pair of chromosomes are called homologs or homologous chromosomes  Homologous pairs are nearly identical in size and genetic composition but contain some sequence differences that provide genetic variation  In most cases homologs differ by less than 1%  Sex chromosomes (X & Y) are very different from each other in size and composition 7  The Cell Cycle  G1: first gap  S: synthesis of DNA interphase  G2: second gap  M: mitosis & cytokinesis  A cell may also exit the cell cycle and enter a nondividing phase called G0 8  The cell cycle  During G1 phase, a cell grows and becomes committed to divide  Accumulates molecular changes that promote progression through the cell cycle  In S phase, each chromosome is replicated  Forms a pair of sister chromatids  In G2, a cell synthesizes proteins needed for chromosome sorting and cell division; some growth may occur  In M phase the cell undergoes:  Mitosis, which divides one cell nucleus into two and distributes the duplicated chromosomes so that each daughter cell receives the same complement of chromosomes  Cytokinesis, which divides the cytoplasm into two daughter cells 9  Cell cycle length  The length of the cell cycle varies for different types of cells. Can be:  Several minutes in quickly growing embryos  Several months in slow-growing adult cells  10 to 24 hrs for fast dividing mammalian cells in adults (e.g. skin cells)  For a cell that divides in 24 hours:  G1 – 11 hours  S phase – 8 hours  G2 phase – 4 hours  M phase – 1 hour 10  Control of the Cell Cycle  The cell cycle is highly regulated to ensure cells only divide at the appropriate time  Advancement through the cell cycle is controlled by two types of proteins:  Cyclins  These levels rise and fall during the cell cycle  Cyclin-dependent kinases (cdks)  Cyclins and cdks combine to form an activated cyclin/cdk complex, which phosphorylates & activates other proteins needed to advance the cell cycle 11  Cell cycle checkpoints  Checkpoint proteins control whether a cell advances through the cell cycle  Act as sensors to determine if the cell is in proper condition to divide  Three critical checkpoints:  G1 checkpoint (restriction point) – proteins determine if conditions are favorable for cell division; sense DNA damage  G2 checkpoint – check for DNA damage and ensure that all DNA has been replicated; monitor the levels of proteins needed to advance through M  Metaphase checkpoint – monitor the integrity of the spindle apparatus; checks that all chromosomes are correctly attached to the spindle apparatus 12 A simplified cell cycle 13  Feature Investigation Identification of Cyclins and CDKs  Masui and Markert studied maturation of frog oocytes  Researchers knew frog oocytes were dormant in G2, and that production of progesterone during mating seasons would cause progression to M phase  But how? 14 Masui and Markert Study 15 Masui and Markert Study  Masui and Makert called the factor required to advance the cell cycle “maturation-promoting factor”, or MPF  We now know MPF is a complex of a mitotic cyclin & cyclin dependent kinase (cdk) 16  Mitotic Cell Division  Cell divides to produce two new cells (daughter cells) genetically identical to the original (mother cell)  Involves mitosis (division of one nucleus into two nuclei) followed by cytokinesis (division of one cell into two)  Used for:  Asexual reproduction (e.g. in single-celled yeast or amoeba)  Development and growth of multicellular organisms ClickMitotic 16.2 to edit Master text Cell Division 17  Preparation for cell division  DNA is replicated, producing two identical sister chromatids  Sister chromatids are tightly associated at centromere 18  The spindle apparatus  A structure composed of microtubules, that is responsible for organizing and sorting the chromosomes  Microtubule growth and organization starts at two centrosomes  AKA microtubule-organizing centres (MTOCs)  A single centrosome duplicates during interphase to produce two that define the two poles of the spindle apparatus 19  Recall: Cell cycle  Interphase – (G1, S, G2) Chromosomes are decondensed in the nucleus. DNA replicates in S phase.  Mitosis occurs as a continuum of phases known as:  Prophase  Prometaphase  Metaphase  Anaphase  Telophase  Cytokinesis – division into two daughter cells 20  Phases of Mitosis  Prophase  Chromatids condense into highly compacted structures that are readily visible by light microscopy  Nuclear membrane begins to dissociate into small vesicles; nucleolus no longer visible  Prometaphase  Nuclear envelope completely fragments and spindle apparatus is fully formed  Centrosomes move apart and demarcate the two poles  Sister chromatids become attached to kinetochore microtubules from opposite poles 21  Phases of Mitosis  Metaphase  Characterized by pairs of sister chromatids aligned in a single row along a plane halfway between the poles called the metaphase plate  Anaphase  Connections between sister chromatids are broken & kinetochore microtubules shorten, pulling chromosomes toward the pole to which they are attached  The two poles move away from each other  Telophase  Chromosomes reach their respective poles and decondense  Nuclear membranes re-forms to produce two separate nuclei 22 23 24  Cytokinesis  In most cases, mitosis is quickly followed by cytokinesis  Process is different in animals and plants  Animals – cleavage furrow constricts like a drawstring to separate the cells  Plants – vesicles from the Golgi apparatus form a cell plate, which then forms a cell wall between the two daughter cells  Microtubules are important for the proper positioning of the cleavage plane in animals and the formation of a cell plate in plants  In animals, actin is involved in the formation of the cleavage furrow 25  End Results?  Two daughter cells with the same number of chromosomes as the mother cell  Barring mutations, two daughter cells are genetically identical to each other and to the mother cell from which they were derived Cells when they undergo mitosis: 26 Evolution of eukaryotic mitosis Comparing cell division among different organisms provides insight into how the process progressed throughout evolution 27  Meiosis  Meiosis is the process by which haploid cells are produced from a cell that was originally diploid  For example: A diploid human cell has 46 chromosomes, but after meiosis the gametes (haploid egg or sperm) have 23 chromosomes  Chromosomes must be correctly sorted and distributed to reduce the chromosome number in half  Gametes must receive one chromosome from each of the 23 pairs!  Like mitosis, meiosis begins after a cell has progressed through the G1, S, and G2 phases of the cell cycle, but the DNA is replicated ONCE, followed by TWO rounds of cell division  Meiosis I and meiosis II ClickMeiosis 16.3 to edit Master text 28 Overview of Meiosis 29  Mitosis versus meiosis  Two key events occur at the beginning of meiosis I:  Homologous pairs of chromosomes associate with each other to form a bivalent (or tetrad) in the process of synapsis  Crossing over results in physical exchange between chromosome segments of the bivalent  Increases genetic variation  Connection between chromosomes that have crossed over = chiasma  Number of crossovers is carefully regulated  In humans, avg is >2 per chromosome 30 Crossing Over 31  Meiosis I  Meiosis I separate homologous chromosomes  Prophase I – replicated chromosomes condense, bivalents form and the nuclear membrane starts to fragment  Prometaphase I – nuclear membrane completely broken apart, spindle apparatus is formed, and kinetochore microtubules attach to sister chromatids  Unlike mitosis, pairs of sister chromatids are attached to a single pole  Metaphase I – bivalents are organized along metaphase plate as double row 32 Alignment of chromosomes in metaphase I  Arrangement of chromosomes in the double row at metaphase I is random with regard to maternal and paternal homologs  Because eukaryotic species have many chromosomes per set, there are many possible combinations of maternal and paternal homologs  Possible number of different, random alignments is 2n where n = number of chromosomes per set  For example, humans have 23 chromosomes per set  2n =2 23 = over 8 million possible arrangements of homologs 33  Meiosis I (continued)  Anaphase I - segregation of homologs occurs  Connections between bivalents break, but sister chromatids stay connected together  Each joined pair of chromatids migrates to one pole, while homologous pair moves to the opposite pole  Telophase I - sister chromatids have reached their respective poles and decondense; nuclear membranes reform producing two separate nuclei  Followed by cytokinesis  Meiosis I produces two haploid cells, with no pairs of homologous chromosomes  Reduction division 34 Meiosis I 35  Meiosis II  Separates sister chromatids  DNA is not replicated between meiosis I and meiosis II  Sorting events of meiosis II are similar to those of mitosis but starting point is different  Meiosis II begins with half the number of chromosomes  Otherwise, the steps are the same as mitosis, with the sister chromatids separated in anaphase II 36 Meiosis II 37  Outcome of mitosis vs. meiosis  Mitosis produces two diploid daughter cells that are genetically identical  Meiosis reduces the number of chromosomes in half, producing four haploid daughter cells from one diploid cell  For example, if a cell is 2n = 6 (6 chromosomes in 3 homologous pairs)  Mitosis produces 2 daughter cells with 6 chromosomes  Meiosis produces 4 daughter cells with 3 chromosomes 38 39  Mendel’s laws can be explained by the pairing and segregation of homologous chromosomes during meiosis 40  Chromosome Theory of Inheritance  The chromosome theory of inheritance, proposed by Theodor Boveri & Walter Sutton, showed that the inheritance pattern of traits could be explained by chromosome behaviour during meiosis  A modern view consists of five fundamental principles: 1. Chromosomes contain DNA, which is the genetic material.  Genes are found in the chromosomes 2. Chromosomes are replicated and passed from parent to offspring, and from cell to cell during multicellular development. ClickThe 17.2 to edit Master text Chromosome Theory of Inheritance 41  Chromosome Theory of Inheritance 3. The nucleus of a diploid cell contains two sets of chromosomes, which are found as homologous pairs.  One member of each pair is inherited from the mother and the other from the father. Each set carries a full complement of genes. 4. At meiosis, one member of each chromosome pair segregates into one daughter nucleus and its homologue segregates into the other daughter nucleus.  During the formation of haploid cells, the members of different chromosome pairs segregate independently. 5. Gametes are haploid cells that combine to form a diploid cell during fertilization, with each gamete transmitting one set of chromosomes to the offspring. 42  Chromosomes & Segregation  Locus: Physical location of a gene on a chromosome  Each homologous chromosome carries an allele of the same gene at the same locus 43 Chromosomal basis of allele segregation 44  Chromoso mal basis of independe nt assortment 45  Sexual Reproduction  Process in which two haploid gametes unite in a fertilization event to create a diploid cell called a zygote  After zygote formation, rounds of mitosis create many more diploid cells to form a multicellular organism  Differs from asexual reproduction – offspring formed from a single parent without fusion of gametes ClickSexual 16.4 to edit Reproduction Master text 46  Sexual vs asexual reproduction  Sexual reproduction has some disadvantages:  Two types of gametes must be made in large numbers  Requires specialized body parts  Two sexes must find each other & courtship can use energy  However, sexual reproduction allows for greater genetic variation in offspring, which is a significant advantage at the species level  Scientists hypothesize that sexual reproduction allows for a more rapid adaptation to environmental changes compared to asexual reproduction 47  Life cycles  Sequence of events that produces another generation of organisms  For sexually reproducing organisms, involves an alternation between haploid cells or organisms and diploid cells or organisms  Diploid-dominant species  Includes most species of animals  Multicellular organism is diploid  Produces specialized haploid cells (gametes) in the reproductive organs 48  Life cycles (cont)  Haploid-dominant species  Includes many fungi and some protists  Multicellular organism is haploid  Haploid cells unite to form diploid zygote, which undergoes meiosis immediately to make four haploid spores 49  Life cycles (cont)  Alternation of generations  Includes plants and some algae  Includes diploid multicellular stage (= sporophyte) and haploid multicellular stage (= gametophyte)  Meiosis in sporophyte produces haploid spores, which divide by mitosis to produce the gametophyte  Specialized cells in the gametophyte differentiate into haploid gametes  Two gametes unite to form a diploid zygote that undergoes mitotic cell divisions to produce a sporophyte  Relative size of sporophyte and gametophyte varies among species 50