Bio 150 Lecture 8: Cell Cycle, Mitosis, and Meiosis PDF

Cell cycle, Mitosis, Meiosis Ch. 10, CH. 11  Discuss cell cycle Discuss Mitosis Objective Control of the Cell Cycle s Cancer and Cell Cycle Prokaryotic Cell Division Meiosis and sexual reproduction Cell cycle D...

Cell cycle, Mitosis, Meiosis Ch. 10, CH. 11  Discuss cell cycle Discuss Mitosis Objective Control of the Cell Cycle s Cancer and Cell Cycle Prokaryotic Cell Division Meiosis and sexual reproduction Cell cycle Describes stages of a cell’s life from the division of a single parent cell to the production of two genetically identical daughter cells Multicellular organisms use cell division for growth, maintenance and repair of cells and tissues, and for size increase. Sexually reproducing organism (ex Human) begins life as a fertilized egg (zygote). Then, billions of cell divisions produce a multicellular organism Continuity of life is based on cell cycle A sea urchin begins life as a single diploid cell (zygote) that (a) divides through cell division to form two genetically identical daughter cells, visible through SEM. After four rounds of cell division, (b) there are 16 cells, as seen in this SEM image. After many rounds of cell division, the individual develops into a complex, multicellular organism, as seen in this (c) mature sea urchin. Genome of prokaryotes Genome: Cell’s endowment of DNA; genetic information Single, circular chromosome, double- stranded DNA molecule, in nucleoid. No nuclear membrane. Some prokaryotes also have smaller loops of DNA: plasmids, not essential for normal growth. Plasmid can be exchanged between bacteria, sometimes receiving beneficial new genes Ex: Antibiotic resistance: trait often spreads in a bacterial colony through plasmid exchange from resistant donors to recipient Genome of Eukaryotes Several double- Human body/somatic Each species has a stranded linear DNA cells have 46 characteristic number molecules chromosomes= of chromosomes (chromosomes) 23 pairs. Each human somatic Human gametes/sex cell contains two cells (sperm or eggs) homologous sets of have 23 chromosomes (one chromosomes each. set from each parent)- One set of configuration known chromosomes: as diploid (2n.) 1n/haploid. 23 pairs of homologous chromosomes in a female human somatic cell. Condensed chromosomes are viewed within the nucleus (top), removed during mitosis (aka karyokinesis or nuclear division) and spread out on a slide (right), and artificially arranged according to length (left); an arrangement like this is called a karyotype. A method of staining called “chromosome painting” employs fluorescent dyes that highlight chromosomes in different colors.  By fertilization, each gamete (egg/sperm) contributes one set of chromosomes creating a diploid cell (zygote; 2n) having matched pairs of homologous chromosomes Homologous chromosomes have the same genes in the same order, at specific locus. Roughly same length, same centromere Eukaryote position, although may carry different alleles for those genes (versions/variations of the s genes). Genes, functional units of chromosomes, determine specific characteristics by coding for specific proteins Traits: variations of those characteristics. For example, hair color is a characteristic with traits: blonde, brown, or black, and many colors in between Chromosomal structure and compaction in eukaryotes If DNA of 46 chromosomes laid out, it would measure 2m (250,000 x >cell’s diameter) DNA tightly packaged to fit in the nucleus. Also, must be readily accessible for the genes to be expressed. For this reason, strands of DNA condensed into compact chromosomes during certain stages of cell cycle: DNA wrap around a core of eight histone proteins at regular intervals forming nucleosomes (beads on a string, 10 nm) Nucleosomes and linker DNA are coiled into 30-nm chromatin-fiber, further condensing them 3rd level of compaction: a variety of fibrous proteins “pack the chromatin” The complex of DNA and proteins is referred to as chromosomes  Before cell can divide to form two genetically identical daughter cells, DNA must be replicated/copied Cell After replication, chromosomes are composed each of two cycle sister chromatids (each chromatid is a double-stranded DNA molecule) Eukary Identically packed chromatids are bound to each other by cohesin otes proteins Attachment between sister chromatids is the tightest at centromere (highly condensed area)  Cell cycle = interphase + cell division (mitosis) Video: Mitosis (8 min) https://www.youtube.co m/watch?v=f-ldPgEfAHI& t=4s  Cell cycle 1) Interphase: cell grows, (multicellula and DNA duplicated/replicated r organisms) 2) Mitosis: duplicated chromosomes are segregated and distributed into daughter nuclei Cytoplasm divides as well by cytokinesis, resulting in two genetically identical When a cell divides, one of its main daughter cells jobs is to make sure that each of the two new cells gets a full, perfect copy of genetic material Interphase G phase (1st gap), cell active, accumulating 1 building blocks of DNA and associated proteins and energy S Phase (DNA synthesis): DNA in semi- condensed chromatin configuration. Replication occurs, results in identical pairs of DNA molecules—sister chromatids— attached at centromere Centrosome: organelle near nucleus (animal cells), also duplicates two centrosomes; will give rise to the mitotic spindle fibers/microtubules that will orchestrate movement of chromosomes G Phase (2nd Gap): cell grows, replenishes 2 energy stores and makes proteins Each centrosome contains necessary for chromosome manipulation. a pair of rod-like objects, Cytoskeleton is dismantled (resources for mitosis) centrioles, at right angles to each other, absent in plants and fungi Mitosis (karyokinesis) 1- Prophase 2- Prometaphase nuclear envelope breaks chromosomes continue to down condense chromosomes condense (by centrosomes continue to condensin proteins) move towards opposite poles Golgi apparatus and ER, Kinetochores (proteins) fragment and disperse toward appear at centromere and periphery. bind to spindle Microtubules fibers form microtubules from mitotic spindle, extend, Once a mitotic fiber attaches pushing centrosomes apart to a chromosome, chromosome is oriented until kinetochores of sister chromatids face opposite Mitosis 3- 4-Anaphase 5- Metaphase/“change (“upward Telophase (“distance phase” phase”/separation) phase”): Mitotic spindle fully cohesin proteins chromosomes at developed degrade opposite poles begin Centrosomes at sister chromatids to decondense (unr opposite poles separate avel) into chromatin Chromosomes each chromatid, mitotic spindles aligned at now called single depolymerize into metaphase plate, or chromosome, is tubulin monomers (will equatorial plane pulled rapidly form cytoskeletal Sister chromatids still toward the components for tightly attached by centrosome to which daughter cells). cohesin proteins its microtubule is Nuclear envelopes attached form around cell elongated chromosomes  Prometaphase: spindle microtubules from opposite poles attach to each sister chromatid at the kinetochore.  “Cell motion”/division; separation of Cytokinesis cytoplasmic components into 2 daughter cells (2nd part of Animal cells: mitosis) Contractile ring of actin filaments forms inside plasma membrane at former metaphase plate Ring pull the equator inward, forming a fissure: cleavage furrow Furrow deepens, and eventually membrane cleaved in two Plant cells: a new cell wall must form During interphase, Golgi apparatus accumulates enzymes, structural proteins, and glucose prior to breaking into vesicles. During telophase, these vesicles form a phragmoplast (vesicular structure) at metaphase plate They fuse and coalesce from center toward the cell walls forming a cell plate As more vesicles fuse, cell plate enlarges until a new cell wall is built G0 Phase Not all cells adhere to the classic cell-cycle where immediately enter interphase, followed by mitotic phase, and cytokinesis. Cells in G0 are not actively preparing to divide. Rather in a quiescent (inactive) stage, when cells exit cell cycle Some cells enter G0 temporarily due to environmental conditions such as limited availability of nutrients Control and regulation of the cell cycle Regulation by external factors Initiation (or inhibition) of cell division can be triggered by external signals. Cell receives signal, and a series of events allows it to proceed into interphase. Moving forward from initiation point, every parameter required during each phase must be met or the cycle cannot progress Events triggering may be death of nearby cells, or release of hormones, (HGH). Lack of HGH can inhibit cell division, resulting in dwarfism.  Regulation at Internal checkpoints Mistakes in duplication/replication lead to mutations that may be passed to every new cell produced To prevent a compromised cell from continuing to divide, cell cycle monitored by internal controls mechanisms at checkpoints At G1: check reserves, cell size, and DNA integrity (if damage) At G2: check duplication and DNA integrity At M (metaphase): spindle check, whether sister chromatids correctly attached to microtubules at kinetochores  Positive molecular regulators Molecules allow cell cycle to advance to next stage: cyclins and cyclin- dependent kinases (Cdks) Concentration of cyclin proteins regulated by signals As the cell moves to the Direct correlation between cyclin accumulation and next stage, cyclins that the checkpoints. Note the sharp decline of cyclin were active in previous levels following each checkpoint (the transition stage are degraded by between phases of cell cycle) cytoplasmic enzymes  Kinase: enzyme that catalyzes the transfer of a phosphate group from ATP to a special molecule. Cyclin-dependent kinases (Cdks) are kinases that, when activated, can phosphorylate/activate other proteins advancing cell cycle past a checkpoint. To become activated, Cdk must bind to a cyclin protein and then be phosphorylated by another kinase. Target protein activated (phosphorylated) advancing cell division to next stage Negative regulators Molecules that monitor conditions and can stop cycle until specific requirements are met Retinoblastoma protein (Rb), p53, and p21: tumor- suppressor proteins, encoded by tumor suppressor genes E2F transcription factor family, regulates the expression of genes important in DNA replication and mitotic division Cancer Despite levels of control, errors do occur during replication and could be passed on to daughter cells Cancers start when a gene mutation gives rise to a faulty protein that plays a key role in cell reproduction Genes coding for positive cell-cycle regulators are called proto-oncogenes. When mutated, become oncogenes— cause cell to become cancerous, continually dividing. Tumor suppressor genes code for negative regulator proteins, which when activated, prevent uncontrolled division. Cancer: uncontrolled cell growth In an adult organism, normal cell division is balanced by apoptosis (programmed cell death) to maintain a constant cell number in homeostasis. Either an increase in cell division or a decrease in apoptosis leads an increase in the number of cells and tumor formation. P53: A gene that codes for a protein (transcription factor) found inside the nucleus of cells and plays a key role in controlling cell division and cell death Video (2 min): Binary fis sion Prokaryot es: Cell cycle Asexual reproduct ion Meiosis and sexual reproduction  Most multicellular organisms Meiosis and many single-celled organisms reproduce using and sexual reproduction. sexual Involves production by parents of haploid cells reproduc (gametes) and fusion of two haploid cells to form a single, tion genetically recombined diploid cell 2n— a genetically unique organism Meiosis Meiosis: nuclear division Mitosis produces of a diploid cell (2n) to daughter cells whose form four haploid cells nuclei are genetically (n) identical to original (germ cells: parent nucleus spermatogonium and (1 cell 2ntwo cells 2n) oogonium in human undergo meiosis) Meiosis consists of In meiosis, the one round of replication daughter nuclei are (interphase) haploid, different from then two rounds of division resulting in four mother cell, and from nuclei/daughter cells, each each other with half the number of chromosomes as parent cell  Meiosis Meiosis (Updated) – YouTub e This Photo by Unknown Author is licensed under CC BY-SA Interphase- Meiosis G1: cell growth S: DNA replication—sister chromatids, held at centromere G2: preparing for meiosis Meiosis I- separates homologous chromosomes Meiosis II—separates chromatids into individual chromosomes  Prophase 1, prometaphase 1, metaphase 1, anaphase 1 and telophase 1 Prophase I: Synaptonemal complex (lattice of proteins) forms bringing homologous Meiosis chromosomes together tight pairing, synapsis Genes of homologous chromosomes aligned. Cohesin at centromeric regions. Exchange of chromosomal segments (DNA) non-sister chromatids—crossing I over. Chiasma (link area) can be observed visually after the exchange In humans, X and Y sex chromosomes (gonosomes) are not homologous (most their genes differ), but there is a small region of homology. Partial synaptonemal complex develops. In mitosis, homologous chromosomes do not pair up in prophase. Prophase I  Reciprocal exchange of DNA between a paternal and a maternal chromosome. Crossover events can occur almost anywhere along the synapsed chromosomes. Different cells undergoing meiosis produce different recombinant chromatids, with varying combinations of maternal and parental genes. Crossover between non- Multiple crossovers on the chromosome have the same effect, sister chromatids of produce genetically recombined homologous chromosomes. chromosomes. When a recombinant sister chromatid is moved into a gamete cell (by division), it will carry a new combination of maternal and paternal genes. Prophase I Crossovers are first source of genetic variation in nuclei (gametes) produced by meiosis As prophase I progresses, synaptonemal complex begins to break down and chromosomes begin to condense At end of Prophase I, pairs held together only at chiasmata Pairs are called tetrads Prometaphase I then metaphase I Prometaphase I Metaphase 1 Nuclear membrane fully Homologous chromosomes broken (tetrads) at metaphase Chromosomes condensed plate, with kinetochores Attachment of spindle facing opposite poles. fiber microtubules to Each homologous pair is kinetochores oriented randomly at Each tetrad is attached to equator microtubules, with each Remember, homologous homologous chromosome chromosomes not identical; facing a pole different versions (alleles) of Homologous still held same genes together at chiasmata After recombination by crossing over, each gamete (daughter cell) will have a unique genetic makeup  Random, independent assortment. Consider a cell with 2 pairs of chromosomes (2n = 4; n=2). There are two possible homologous chromosome arrangements in metaphase I, which then separated in anaphase I. Total possible number of different gametes is 2n, where n is the number of chromosomes in a set. In example, four possible genetic combinations for gametes. With n = 23 in human cells, there are over eight million possible combinations of paternal and maternal chromosomes.  Anaphase I: microtubules pull homologous chromosomes apart chiasmata broken sister chromatids remain bound at centromere Telophase I: Meiosis 1 separated chromosomes are at opposite poles In some organisms, nuclear membranes form around decondensed continues chromosomes; in others no Cytokinesis: In species of animals and some fungi, cytokinesis separates the cell contents via a cleavage furrow In plants, a cell plate is formed, ultimately lead to formation of cell walls separating daughter cells. Two haploid cells (n) result from meiosis 1. Each cell has one of the homologous chromosomes of each pair.  Crossing-over and independent assortment responsible for genetic Genetic diversity of the gametes diversity That is how siblings are not identical  Meiosis II Sister chromatids will separate in meiosis II, like mitosis, except that the two cells undergoing meiosis 2 have only one set of chromosomes, with two chromatids each Prophase II: Prometaphase II: Metaphase II: Anaphase II: chromosomes nuclear sister chromatids sister chromatids condense again envelopes maximally pulled apart by If nuclear completely condensed align microtubules and envelopes were broken down, at equator move toward formed, they spindle fully opposite poles fragment into formed Non-kinetochore vesicles Each sister microtubules Spindles formed chromatid forms elongate the cell. an individual kinetochore that attaches to microtubules from opposite poles  Chromosomes at opposite poles decondense Nuclear envelopes form around Telophas chromosomes e II and Cytokinesis leads to four unique haploid n cells. cytokines Cells produced are genetically is unique because of recombination of maternal and paternal segments of chromosomes (with their genes) during crossover random assortment of paternal and maternal homologs  Fertilization and meiosis alternate in sexual life cycles Meiosis reduces the chromosome number by half Life cycle Fertilization, joining of two haploid gametes, restores the diploid condition Some organisms have a multicellular diploid stage of sexually that is most obvious and only produce haploid reproductive cells (Animals, including humans) reproduced organisms Other organisms, such as fungi, have a multicellular haploid stage that is dominant Plants and some algae have alternation of generations: have multicellular diploid and haploid life stages apparent to different degrees depending on group. Animals (diploid dominance) Early in development of embryo, specialized diploid cells, germ cells, are produced in gonads (testes and ovaries) Germ cells are capable of mitosis to perpetuate the germ cell line, and of meiosis to produce haploid gametes. Once haploid gametes are formed, cannot divide again. No multicellular haploid life stage Fertilization between two gametes restores diploid state (zygote 2n) Fungi and algae (Haploid dominance) Haploid cells make up the tissues of the dominant multicellular stage (body), form by mitosis During sexual reproduction, specialized haploid cells (formed by mitosis) from two individuals— (+) and (−) mating types—join to form a diploid zygote Zygote undergoes meiosis to form four haploid cells called spores The spores contain a new genetic combination from two parents The spores can remain dormant. In favorable conditions, spores form multicellular haploid structures through many rounds of mitosis Plants (blend of haploid and diploid) Alternation of generations: have both haploid and diploid multicellular organisms as part of life cycle The haploid multicellular plants are called gametophytes (n), because they produce gametes from specialized cells by mitosis. Meiosis not directly involved in the production of gametes in this case (organism making gametes is already haploid) Fertilization between gametes forms a diploid zygote. Zygote will undergo many rounds of mitosis and give rise to a diploid multicellular plant called a sporophyte (2n). Specialized cells of the sporophyte will undergo meiosis and produce haploid spores (1n). Spores will subsequently develop into the gametophytes by mitosis, which make the gametes End of chapter review questions Choose one correct answer An organism’s traits are determined by the specific combination of inherited _____. a. cells. b. genes. c. proteins. d. chromatids. Choose one correct answer Identical copies of chromatin held together by cohesin at the centromere are called _____. a. histones. b. nucleosomes. c. chromatin. d. sister chromatids. Choose one correct answer Which of the following events does not occur during some stages of interphase? a. DNA duplication b. organelle duplication c. increase in cell size d. separation of sister chromatids Choose one correct answer Unpacking of chromosomes and the formation of a new nuclear envelope is a characteristic of which stage of mitosis? a. prometaphase b. metaphase c. anaphase d. telophase Choose one correct answer At which stage of meiosis are sister chromatids separated from each other? a.prophase I b.prophase II c.anaphase I d.anaphase II. Choose one correct answer At metaphase I, homologous chromosomes are connected only at what structures? a.chiasmata b.recombination nodules c.microtubules d.kinetochores Choose one correct answer If a muscle cell of a typical organism has 32 chromosomes, how many chromosomes will be in a gamete of that same organism? a. 8 b. 16 c. 32 d. 64 Choose one correct answer What structure is most important in forming the tetrads? a. centromere b. synaptonemal complex. c. chiasma d. kinetochore Choose one correct answer What is a likely evolutionary advantage of sexual reproduction over asexual reproduction? a. Sexual reproduction involves fewer steps. b. There is a lower chance of using up the resources in a given environment. c. Sexual reproduction results in variation in the offspring.. d. Sexual reproduction is more cost effective. Choose one correct answer Meiosis produces ________ daughter cells. a. two haploid b. two diploid c. four haploid d. four diploid