AP BIOLOGY MITOSIS/MEIOSIS

Transcription

1 AP BIOLOGY MITOSIS/MEIOSIS I. The Cell Cycle A. Eukaryotic Chromosomes 1. DNA in chromosomes of eukaryotic cells is associated with proteins; proteins organize chromosomes. 2. When cell is not undergoing division, DNA in nucleus is a tangled mass of threads called. 3. At cell division, chromatin becomes highly coiled and condensed and now visible as 4. Each species has a characteristic number of chromosomes (2n). a. includes two sets of chromosomes of each type. 1) Found in all of an organism s body (with a few exceptions). 2) Examples include humans (46), crayfish (200), etc. b. contains one of each kind of chromosome. 1) In the life cycle of many animals, only sperm and egg cells have the haploid number. 2) Examples included humans (23), crayfish (100), etc.

2 5. in eukaryotes involves (Karyokinesis) and (division of the cytoplasm). a. (body) cells undergo for development, growth, and repair. 1) This nuclear division leaves the chromosome number constant. 2) A 2n nucleus replicates and divides to provide daughter nuclei that are also 2n. b. A chromosome begins cell division with 1) are two strands of genetically identical chromosomes. 2) At the beginning of cell division, they are attached at a. 3) The is a region of constriction on a chromosome where are attached. B. The Mitotic Spindle 1. C are believed responsible for organizing the spindle. 2. The centrosome is the main microtubule organizing center of the cell. 3. The centrosome has divided before mitosis begins. 4. Each centrosome contains a pair of barrel-shaped organelles called ; plant cells lack centrioles. 5. The spindle contains many fibers, each composed of a bundle of. 6. are made of the protein a. Microtubules assemble when tubulin subunits join, disassemble when tubulin subunits become free, and form interconnected filaments of cytoskeleton. b. Microtubules disassemble as spindle fibers form. C. Mitosis in Amimal Cells 1. Mitosis (karyokinesis) is divided into : prophase, metaphase, anaphase, and telophase. 2. Prophase a. Nuclear division is about to occur because chromatin and chromosomes become visible. b. The nucleolus and the nuclear envelope fragments. c. Already duplicated chromosomes are composed of two sister chromatids held together by a centromere. 1) This configuration in diagrammatic drawings gives accurate chromosome number. 2) Chromosomes have no particular orientation in cell at this time. 3) Specialized protein complexes ( ) develop on each side of centromere for future chromosome orientation.

3 d. Spindle begins to assemble as pairs of centrosomes away from each other. e. Short microtubules radiate out from the pair of centroiles located in each controsome to form starlike 3. Prometaphase a. At this time, the spindle consists of poles, asters, and fibers that are bundles of b. Important event during prometaphase is attachment of chromosomes to the spindle and their movement as they align at the ) of the spindle. c. The kinetochores of sister chromatids capture d. Chromosomes move back and forth until they are aligned at the metaphase plate. 4. Metaphase a. Chromosomes, attached to kinetochore fibers, are aligned at the b. Non-attached spindle fibers, called can reach beyond the metaphase plate and overlap. 5. Anaphase a. Two sister chromatids of each duplicated chromosome separate at centromere. b. Daughter chromosomes, each with a centromere and single chromatid, move to opposite poles. 1) Polar spindle fibers as they slide past each other. 2) Kinetochore spindle fibers disassemble at the kinetochores; this pulls daughter chromosomes to poles. 6. Telophase a. Spindle disappears. b. Chromosomes and return to chromatin; the nuclear envelope reforms and nucleoli reappear. c. Cytokinesis is nearly complete. D. Mitosis in Plant Cells 1. Plant meristematic tissue in tips of roots and shoots of stems retains ability to divide throughout life. 2. Stages are exactly same as in animal cells. 3. Although plant cells have a centrosome and spindle, there are no and asters do not form. E. Cytokinesis in Plant and Animal Cells 1. In Plant Cells a. The rigid cell wall that surrounds plant cells does not permit cytokinesis by furrowing. b. Golgi apparatus produces vesicles that move to the between the daughter nuclei. c. Vesicles fuse forming ; their membranes complete plasma membranes of daughter cells. d. Vesicles also release molecules that signal the formation of plant cell walls. e. Walls are strengthened by the addition of.

4 2. In Animal Cells a. indents the plasma membrane between the two daughter nuclei at a midpoint; progressively divides cytoplasm during cell division. b. Cytoplasmic cleavage begins as draws to a close. c. Cleavage furrow deepens as band of actin filaments constricts between the two daughter cells. d. Narrow bridge exists between daughter cells during telophase; constriction separates cytoplasm. 3. Cell Division in Other Eukaryotic Organisms a. Protists and fungi also undergo mitosis and cytokinesis. b. In fungi and some protists, the nuclear envelope does not fragment but divides and one nucleus goes to each daughter cell. II. Mitosis and Cytokinesis 1. Interphase was considered a "resting state" until DNA replication was detected in the 1950s. 2. Cell cycle involved 4-stage sequence of events. 3. (M=mitosis) is the entire cell division state 4. just prior to DNA replication is when cell grows in size and organelles increase in number. 5. is DNA synthesis period where replication occurs; proteins associated with DNA are also synthesized. 6 occurs just prior to cell division; preparation for mitotic cell division. 7. consists of G1, S, and G2 stages. A. The Cell-Cycle Clock 1. Some cells (e.g., skin cells) divide continuously throughout the life of an organism. 2. and are arrested in the G1 stage; if the nucleus from an arrested cell is placed in cytoplasm of an S-stage cell, it starts to finish the cell cycle. 3. Cardiac muscle cells, are arrested in the ; if fused with a cell undergoing mitosis, it too starts to undergo mitosis. 4. There appear to be stimulatory substances causing a cell to proceed through two critical checkpoints: a. G1 stage S stage b. G2 stage M stage 5. Enzymes known as cyclins and kinases regulate passage of cells through these two checkpoints. a. are enzymes that remove a phosphate group from ATP and add it to another protein; this is a common way for the cell to turn on metabolic pathways. b. proteins activate kinases, which in turn activate enzymes; one destroys cyclin; therefore cyclin levels vary, which gives them their name. M-Kinase combines with M-cyclin and the active complex initiates several mitotic events: 1. chromosome condensation (coiling) 2. nuclear membrane disintegration 3. the synthesis of the spindle apparatus The "S" in S-kinase and S-cyclin refers to DNA synthesis. c. When M-kinase combines with M-cyclin, the kinase phosphorylates a protein causing the cell to move from G2 to the M stage. This causes: 1) the chromosomes to condense, 2) the nuclear envelope to disassemble, and 3) the spindle to form. 4) Then M-cyclin is destroyed. d. Growth Factors 1) Growth factors are molecules that attach to plasma membrane receptors and bring about.

5 2) Ordinarily a cyclin combines with its kinase only when a growth factor is. 3) Cyclin that has gone awry combines with its kinase when growth factor is absent, resulting in a tumor. 4) Tumor suppressor genes usually function to prevent cancer; e.g., tumor suppressor gene causes production of protein that combines with a cyclin kinase complex to stop that kinase from being active; this stops the cell cycle. III. The Cell Cycle and Cancer A. Carcinogenesis is development of cancer. 1. Cancer is a genetic disease requiring a series of mutations toward developing a tumor. 2. A. indicates a failure in controlling cell division, usually due to a faulty gene. 3. Normal gene protein halts the gene cycle when DNA mutates and is in need of repair. 4. Carcinogens are agents that cause cancer and include a. radiation (e.g.,.,.,., etc.) b. organic chemicals (e.g.,., some foods, pesticides, etc.), and c. certain.. 5. A protein mobilizes repair enzymes and stops the cell cycle; only when repaired does a cell cycle begin again. 6. If DNA repairs is not possible, the protein promotes cell death a good thing. B. Apoptosis is programmed. 1. Apoptosis is a sequence of cellular changes involving: a. shattering the nucleus, b. chopping up the chromosomes, and c. packaging the cellular remains into vesicles to the engulfed by macrophages. 2. Apoptosis is caused by cells harboring enzymes called. 3. Cells normally contain caspases by using. 4. Caspases can be released in two ways: a. during development, external signals trigger cells to die, as in the webbing between fingers. b. in adults, cells with severe DNA damage kill themselves. 5. Caspases are activated in two ways. a. Initiators receive a message to activate the executioners which activate the enzymes. b. Initiators, executioners and dismantling enzymes become active when they are clipped and shortened. 6. Apoptosis research may lead to new therapy. a. Tumor cells contain high levels of survivin protein which blocks apoptosis; if we can inactivate survivin, cancer cells would be more vulnerable to radiation and chemicals. b. Excess apoptosis kills off brain cells in Parkinson s disease and stroke; inhibitors of apoptosis could keep the brain cells alive. C. Characteristics of Cancer Cells 1. Cancer cells lack differentiation. a. Unlike normal cells that differentiate into muscle or nerves cells, cancer cells have a general abnormal form. b. Normal cells enter the cell cycle only about.; cancer cells are immortal. 2. Cancer cells have abnormal nuclei. a. The nuclei may have an abnormal number of chromosomes, and be enlarged. b. Some chromosomes may be duplicated or deleted. c. Extra copies of specific genes is more frequent. 3. Cancer cells form tumors. a. Normal cells are anchored and stop dividing when in contact with other cells. b. Cancer cells invade and destroys normal tissue; new growth is called. c. A. tumor is disorganized but encapsulated and does not invade adjacent tissue. 4. Cancer cells undergo angiogenesis and. a. Cancer cells release a. that caused nearby blood vessels to grow and bring more nutrients and oxygen to the tumor. b. Cancer in situ is still in its place of origin and has not spread to other tissues. c.. occurs when metastasis spreads new tumors distant from the primary tumor. 9.4 Prokaryotic Cell Division A. Prokaryotic cells (bacteria).... B. The Prokaryotic Chromosome 1. Prokaryotic chromosome contains DNA and associated proteins, but much less protein than eukaryotic chromosomes. 2. Chromosome consists of. (an irregularly-shaped region, electron-dense, and not enclosed by a membrane). 3. Chromosome, when stretched out, is a circular loop attached to the inside of the plasma membrane; about. times the length of the cell.

6 C. Binary Fission 1.. of prokaryotic cells produces two genetically identical daughter cells by division (fission). 2. Before cell division, DNA is replicated so two chromosomes are attached inside plasma membrane. 3. Following DNA replication, the two chromosomes separate as a cell lengthens and pulls them apart. 4. When cell is approximately.., the plasma membrane grows inward, a new cell wall forms dividing the cell into two approximately equal daughter cells. 5. Generation times of Escherichia coli is.; most bacteria need up to one hour to a day. IV. Halving the Chromosome Number A. Sexual Reproduction 1.. is nuclear division reducing chromosome number from diploid (2n) to haploid (n) number. 2.. is half the diploid number of chromosomes. 3. Requires gamete formation and then fusion of gametes to form a zygote. 4. A. always has a full or diploid (2n) number of chromosomes. 5. If gametes contained same number of chromosomes as body cells, doubling would soon fill cells. B. Homologous Pairs of Chromosomes 1. In a diploid body cell, chromosomes occur as pairs. a. Each set of chromosomes is a.; each member is a homologous chromosome or homologue. b. Homologues look alike; they have same length and centromere position; have similar banding pattern when stained. c. A location on one homologue contains the same types of gene which occurs at the same locus on other homologue. 2. Chromosomes duplicate just before nuclear division. a. Duplication produces two identical parts called sister chromatids, held together at centromere. b. Non-sister chromatids do not share the same centromere. 3. One member of each homologous pair is inherited from either male or female parent; one member of each homologous pair is placed in each sperm or egg. V. Genetic Recombination A. Genetic Recombination 1. Due to genetic recombination, offspring have a.. of genes than their parents. 2. Without recombination, asexual organisms must rely on. to generate variation among offspring; this is sufficient because they have great numbers of offspring. B. Crossing-Over Introduces Variation 1. Crossing-over results in exchange of genetic material between At. homologous chromosomes are held in position by a nucleoprotein lattice (the synaptonemal complex). 3. As the lattice of the synaptonemal complex breaks down at beginning of anaphase I, homologues are temporarily held together by., regions were the non-sister chromatids are attached due to crossing-over.

7 4. The homologues separate and are distributed to separate cells. 5. Due to crossing-over, daughter chromosomes derived from sister chromatids are no longer.. C. Independent Assortment of Homologous Chromosomes 1.. in a cell with only three pairs of chromosomes is eight possible combinations. 2. In humans with 23 pairs of chromosomes, the combinations possible are 8,388,608 possible combinations. D. Fertilization 1. Meiosis increases variation. 2. When gametes fuse at fertilization, chromosomes donated by parents combine. 3. Chromosomally different zygotes from same parents are 70,368,744,000,000 combinations possible without crossing over. 4. If crossing over occurs once, then 4,951,760,200,000,000,000,000,000,000 combinations of genetically different zygotes are possible for one couple. VI. Meiosis Has Phases A. Number of Phases 1. Both meiosis I and meiosis II have four phases: prophase, metaphase, anaphase and telophase. B. Prophase I 1. Nuclear division is about to occur: nucleolus disappears; nuclear envelope fragments; centrosomes migrate away from each other; and spindle fibers assemble. 2. Homologous chromosomes undergo forming bivalents; crossing over may occur at this time in which case sister chromatids are no longer identical. 3. Chromatin condenses and chromosomes become microscopically. C. Metaphase I 1. During prometaphase I, bivalents held together by chiasmata have moved toward the metaphase plate. 2. In metaphase I, there is a fully formed spindle and alignment of the bivalents at the metaphase plate. 3. Kinetochoares are regions just outside centromeres; they attach to spindle fibers call kinetochore spindle fibers. 4. Bivalents independently align themselves at the. of the spindle. 5. Maternal and paternal homologues of each bivalent may be oriented toward either pole. D. Anaphase I 1. The homologues of each bivalent separate and move toward opposite. 2. Each chromosome still has two chromatids. E. Telophase I 1. Only occurs in.. 2. When it occurs, the nuclear envelope reforms and nucleoli reappear. F. Interkinesis 1. This period between. I and. is similar to interphase of mitosis. 2. However, no DNA replication occurs. G. Meiosis II 1. During. the haploid number of chromosomes align at metaphase plate. 2. During., centromeres divide and daughter chromosomes move toward the poles. 3. At the end of telophase II and cytokinesis, there are four haploid cells. 4. Due to crossing-over, each gamete can contain chromosomes with different types of genes. 5. In animals, the haploid cells mature and develop into gametes. 6. In plants, the daughter cells become spores and divide to produce a haploid adult generation. 7. In some fungi and alge, a zygote results from gamete fusion and immediately undergoes meiosis; therefore, the adult is always haploid. VII. Comparison of Meiosis with Mitosis A. How Meiosis I Differs from Mitosis 1. DNA is replicated only once before both mitosis and meiosis; in mitosis there is only. nuclear division; in meiosis there are. nuclear divisions. 2. During prophase I of meiosis, homologous. and undergo crossing-over; this does not occur during mitosis. 3. During metaphase I of meiosis,. homologous chromosomes align at the metaphase plate; in mitosis individual chromosomes align. 4. During anaphase I in meiosis, homologous chromosomes with centromeres intact separate and move to opposite poles; in mitosis at this stage, sister chromatids separate and move to poles. B. How Meiosis II Differs from Mitosis 1. Events of meiosis II are same stages as in mitosis. 2. However, the nuclei contain the haploid number of chromosomes in meiosis. 3. Mitosis produces two daughter cells; meiosis produces four daughter cells.

8 VIII. Viewing the Human Life Cycle A. Life Cycle has Both Meiosis and Mitosis 1.. refers to all reproductive events between one generation and next. 2. In animals, the adult is always diploid [Instructors note: some bees, etc. have haploid male adults]. 3. In animals, it occurs during production of gametes; adult is diploid and gametes are haploid. 4. Mosses are haploid most of their cycle; oak trees are diploid most of their cycle. 5. In fungi and some algae, organisms you see is haploid and produces haploid gametes. 6. In males, meiosis is part of., the production of sperm, and occurs in the testes. 7. In females, meiosis is part of. the production of eggs cells, and occurs in the ovaries. 8. After birth, mitotic cell division is involved in growth and tissue regeneration. B. Spermatogenesis and Oogenesis in Humans 1. Spermatogenesis a. In the testes of males,. with 46 chromosomes divide meiotically to form two. each with 23 duplicated chromosomes. b. Secondary spermatocyes divide to produce four., also with 23 daughter chromosomes. c. Spermatids then differentiate into sperm (spermatozoa). d. Meiotic cell division in males always results in four cells that become sperm. 2. Oogenesis a. In the ovaries of human females,. with 46 chromosomes divide meiotically to form two cells, each with 23 duplicated chromosomes. b. One of the cells, a., receives most cytoplasm; the other cell, a. disintegrates or divides again. c. A secondary oocyte begins meiosis II and then stops at metaphase II. d. At ovulation, the secondary oocyte leaves the ovary and enters an oviduct where it may meet a sperm. e. If a sperm enters secondary oocyte, oocyte is activated to continue meiosis II to completion; result is a mature egg and another polar body, each with 23 daughter chromosomes. f. Polar bodies serve to discard unnecessary chromosomes and retain most of the cytoplasm in the egg. g. The cytoplasm serve as a source of nutrients for the developing embryo.