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UNIT V - DNA & CELL DIVISION Big Campbell – Ch 12, 13, 16 Baby Campbell – Ch 8, 10.1-10.5 Hillis – Chp 7, 9 I. ASEXUAL REPRODUCTION • Purpose Unicellular Organisms Single-celled organisms like bacteria and yeast use asexual reproduction to make new organisms – reproduce . I. ASEXUAL REPRODUCTION • Multicellular Organisms Growth & Development –grow through cell division, rather than unlimited growth because if a cell gets too large, it cannot meet its own needs and maintain homeostasis. Renewal & Repair –used to replace cells that die from normal wear & tear; Ex: skin cells. II. PROKARYOTIC ASEXUAL REPRODUCTION • Binary Fission o Asexual reproduction o Much shorter than euk cell cycle Single chromosome replicates Each copy begins moving to opposite ends of cell Cell elongates When bacterium is 2X original size, cell membrane pinches inward Cell wall deposited 2 identical cells produces Binary Fission, cont. III. EUKARYOTIC CELL DIVISION – THE CELL CYCLE • Can be divided into: Interphase M –Phase or Mitotic Phase III. CELL CYCLE, cont • Interphase Portion of cell cycle in which cell is carrying out normal activities. Approx 90% of normal cell cycle is spent in interphase. DNA found in chromatin form 3 sub-phases G1 -Cell grows, carries out normal cell activities. Organelles are replaced if needed. S or Synthesis - Replication of DNA; occurs in nucleus, also known as “point of no return” G2 - Preparation for mitosis. Centrioles and all organelles are replicated. III. CELL CYCLE, cont • Mitosis Nuclear division Requires all the cells energy, resources Last step is cytokinesis – splitting of the cell III. CELL CYCLE, cont Cell at the END of interphase a. _DNA_ has been replicated – still in chromatin form. b. _Centrioles_ have been replicated in _animal_ cells only. This replication results in 2 pairs of centrioles, composed of _microtubules__. c. The _nuclear envelope_ and _nucleolus are still present. Interphase PROPHASE 1. 2. 3. 4. _Nucleolus_ and _nuclear envelope disassemble. Replicated DNA thickens & condenses Results in _chromosomes_ made up of 2 sister chromatids held together by the _centromere_. _Centrioles_ begin moving to opposite sides of the cell. Microtubules_extend to form spindle fibers from centriole to centriole. Some of these fibers, known as kinetochore microtubules, begin to attach at the kinetochore of each chromatid. (located at the centromere). Other fibers, known as non-kinetochore microtubules span the cell to aid in support and structure. Prophase Late Prophase (Prometaphase) III. CELL CYCLE, cont METAPHASE 1. _Spindle fiber network_ network is fully formed with _centrioles_ at opposite ends. 2. Each _sister chromatid_ is attached by their kinetochore (at the centromere) to a spindle fiber from each pole. 3. Sister chromatids align in _equator (middle)__ of cell. Metaphase Anaphase 1. _Centromeres__ split. 2. Sister _chromatids_ are pulled apart by _kinetochore microtubules_ to opposite ends of the cell. 3. Genetic material is now known as daughter _chromosomes__. 4. Cell is elongating; preparing for two new nuclei. Anaphase Telophase 1. Complete set of _chromosomes_ at each pole of the cell. 2. Spindle fibers _ disassemble. 3. New _nuclear envelope_ forms around nucleus. 4. chromosomes_ uncoil _chromatin_. 5. nucleolus_ reforms _ribosomes_ are produced _protein_ synthesis resumes cell _metabolic activity_ resumes Telophase III. CELL CYCLE, cont • Cytokinesis – IN ANIMAL CELLS Cytokinesis in Plant Cells Telophase and Cytokinesis III. CELL CYCLE, cont IV. CONTROL OF THE CELL CYCLE • Internal Signals o Three major checkpoints in cell cycle G1 G2 M o Regulated by enzymes known as cyclindependent kinases or Cdks Activated when bound to proteins known as cyclins Kinase concentrations fairly constant; cyclin concentrations vary IV. CONTROL OF THE CELL CYCLE, cont • External Signals o Growth Factors Proteins released by certain cells that stimulate other cells to divide. Cells stop dividing when growth factor is depleted. Examples include erythropoetin, interleukin, pdgf IV. CONTROL OF THE CELL CYCLE, cont • External Signals o Density-dependent Inhibition Results from crowded conditions When one cell touches another, cell division stops o Anchorage Dependence Most cells must be in contact with solid surface to divide IV. CONTROL OF THE CELL CYCLE, cont • Cell Cycle Out of Control = CANCER o Cancer cells do not respond to normal cell cycle controls Apoptosis – Programmed cell death o Uncontrolled growth o Deprive normal cells of nutrients IV. CONTROL OF THE CELL CYCLE, cont o Tumor – Mass of abnormal cells Benign – Mass remains at original site Malignant – Mass spreads to other parts of the body Metastasis – Separation of cancer cells from tumor; travel through circulatory system V. MEIOSIS • Somatic Cells o Body cells o Human somatic cells contain 46 chromosomes, 23 from mom, 23 from dad o 2n or diploid o Matched pairs of chromosomes called homologous pairs. Each chromosome making up a homologous pair is known as a homologue. Both carry genes for same traits. The location of a gene on a chromosome is known as a locus. 44 Autosomes 2 Sex chromosomes XX = XY = V. MEIOSIS, cont • Gametes o Egg and sperm cells o Haploid or n o Contain 23 chromosomes o In fertilization, haploid (n) sperm fuses with haploid (n) egg → diploid (2n) zygote V. MEIOSIS, cont • Description of Meiosis o o o o o o Special type of cell division that occurs to produce gametes Occurs in ovaries, testes only Involved specialized cells DNA replicated once, cell divides twice Produces 4 cells with ½ the original chromosome number In humans, V. MEIOSIS, cont V. MEIOSIS, cont V. MEIOSIS, cont • Nondisjunction – Failure of chromosomes to separate properly in meiosis VI. GENETIC VARIATION VI. GENETIC VARIATION, cont • Crossing Over o Further increases genetic variability o Occurs during prophase I when tetrads are forming o Piece of one sister chromatid breaks off & exchanges places with piece of sister chromatid of homologue o Known as chiasma o Occurs very frequently VII. A COMPARISON OF MEIOSIS & MITOSIS VIII. DNA – THE MOLECULE OF INHERITANCE • Chromosome o Single molecule of DNA wrapped in histone proteins. Proteins maintain chromosome structure & control DNA activity o Gene VIII. DNA, cont • Genome o All of an organism’s DNA o Provides working instructions for cell through ______________________ o Must be copied prior to cell division IX. DISCOVERY OF DNA • Early 1900s – Scientists determined genes determined inherited characteristics. Also realized chromosomes were composed of DNA & protein. • Griffith (1928) – Studied 2 strains of bacteria. Determined pathogenicity could be transferred when living non-pathogens were exposed to remains of dead pathogens. • Avery (1944) – Identified “transforming substance” as DNA IX. DISCOVERY OF DNA, cont o Hershey & Chase (1952) Used bacteriophage with labeled phosphorus, sulfur Tested bacterial cells, supernatant following exposure Proved it was the DNA component that was injected into host cell and used to make new virus particles. IX. DISCOVERY OF DNA, cont • Rosalind Franklin (late 1950s) – Produced x-ray crystallography image of DNA; “borrowed” by Watson & Crick Watson & Crick o Realized DNA was a helix composed of 2 nucleotide strands oFranklin suggested backbone of DNA was composed of alternating sugar-phosphate molecules o Watson & Crick determined interior of DNA was made up of paired N-bases o Eventually deduced bases always paired a specific way Chargaff – Chemically proved the same base-pairing rules that Watson & Crick proved structurally X. A CLOSER LOOK AT DNA • Monomers of DNA oNucleotides o Composed of - 5C Sugar (deoxyribose) - phosphate group - nitrogen base Pyrimidines Thymine Cytosine Purines Adenine Guanine X. A CLOSER LOOK AT DNA, cont • Structure of DNA Each strand of nucleotides held together with Double helix 2 nucleotide strands are antiparallel Each strand has a 3’ end (terminus) and a 5’end; named for carbon on deoxyribose X. A CLOSER LOOK AT DNA, cont • Base Pairing XI. DNA REPLICATION • DNA Replication o Prior to cell division, DNA must be replicated o Occurs during _S_ of __interphase_ of mitosis, meiosis o Known as semiconservative model of replication Meselson-Stahl Experiment XI. DNA REPLICATION, cont. • Chromatids Two identical DNA molecules Result of replication Term is only used when identical DNAs are physically attached; described as one chromosome made up of two sister chromatids Centromere – Site where sister chromatids are most closely attached XI. DNA REPLICATION, cont. • Steps of Replication: DNA helicase unwinds the DNA double helix Replication begins at specific points on the DNA molecule known as origins of replication. The Y-shaped region where new strands of DNA are elongating are called replication forks XI. DNA REPLICATION, cont. As DNA is “unzipped”, single-strand binding proteins hold the DNA open A topoisomerase relieves tension creating by unwinding of DNA by making cuts, untwisting, & rejoining the nucleotide strand. DNA polymerase can only add nucleotides to an already-existing strand so an RNA primer is synthesized to get replication going XI. DNA REPLICATION, cont. DNA polymerases add complementary nucleotides to each side of the DNA molecule. DNA polymerase can only add nucleotides to the 3’ end of the growing strand, so the daughter DNA is synthesized 5’ – 3’, which means parental DNA is “read” __ 3’ – 5’__. This means only one side of the DNA (3’ – 5’) molecule can be replicated as a continuous strand. Known as the leading strand. XI. DNA REPLICATION, cont. • Synthesis of lagging strand To synthesize the other new strand of DNA, DNA polymerase must work away from the replication fork. Leads to synthesis of short pieces of DNA known as Okazaki fragments. DNA ligase binds fragments together to form a continuous strand of nucleotides. • Proofreading & Repair DNA Polymerase proofreads nucleotides as they are added XI. DNA REPLICATION, cont. An Overview of Replication XI. DNA REPLICATION, cont. • Telomeres 5’ ends of daughter strands cannot be completed because DNA polymerase can only add nucleotides to the 3’ end Results in shorter and shorter DNA molecules with jagged ends To protect genetic integrity, ends of chromosomes do not contain genes – instead there are nucleotide sequences known as telomeres Contain nucleotide repeat sequences Telomeres shorten each time cell divides - limits the number of times a cell can divide; thought to protect organism from cancer Telomerase – Enzyme produced by stem cells, cancer cells that restores telomere length