Transcript chromosomes
Ch. 8 Cell Reproduction Why do cells make new cells? 1. Growth 2. Maintenance – cells wear out 3. Repair - injury 4. Regeneration – lost body part 5. Reproduce asexually 8.1 Some organisms reproduce asexually • Their offspring are genetic copies of the parent and of each other Figure 8.1A – All organisms can reproduce sexually • Creating variety in the offspring Figure 8.1B 8.3 Prokaryotes reproduce by binary fission • One chromosome • Chromosome copies itself • Cell lengthens, copies move apart • Cell membrane divides cell in two Eukaryotic nuclei contain many chromosomes Each chromosome contains thousands of genes - must be organized before cell can dividing Prokaryotic chromosomes LM 340 Colorized TEM 32,500 One-celled protists also do fission, but must do mitosis first to divide chromosomes evenly 8.5 The cell cycle - two phases Interphase – most of cell life - non-dividing cell INTERPHASE G1 S (DNA synthesis) G2 Mitotic (M) phase – dividing cell Figure 8.5 Cell Cycle INTERPHASE G1 growth, normal life functions S “synthesis” – DNA replicates G2 final growth; prepares to divide MITOTIC PHASE Mitosis – chromosomes condense, organize and divide - each new cell gets one copy of every chromosome Cytokinesis – cytoplasm divides Some cells divide often embryo Bone marrow stem cells Plants and simple organisms Skin cells Digestive lining cells Some divide rarely or not at all Nerve cells Muscle cells Cartilage or tendon cells BEFORE a cell divides DNA condensation •DNA is ALREADY COPIED (replicated) •coils, packs, condenses Forms dense “CHROMOSOMES” •keeps copies organized and intact until cell splits CHROMOSOMES CONDENSE BEFORE CELL DIVISION DNA DNA wraps around histones These group to form chromatin Groups of histones form nucleosomes This coils and wraps until it all fits into the nucleus. One chromosome (copied and packed for cell division) SEM of human chromosomes Sister chromatids identical copies Centromere holds chromatids together Prokaryotes have a single, circular chromosome, no histones; no nucleus Double-stranded (replicated) chromosome Eukaryotic chromosomes Nucleus in non-dividing cell Early mitosis Chromosome is in CHROMATIN form CHROMATIN begins to condense before a cell can divide Ch.10 Chromosomes of eukaryotes duplicate in cell division Before cell divides - DNA replicates a. Helicase “unzips” molecule b. Original strands are templates c. Free nucleotides in nucleus d. Polymerase – base-pairing rules two identical molecules Semi-conservative replication • Original strands serve as templates • New molecules have one original strand and one new strand PHASES OF MITOSIS PROPHASE – cell organizes, prepares •chromatin •nuclear membrane, nucleolus •spindle and asters •centrioles PROMETAPHASE – chromosomes condensed •move toward middle •centrioles to opposite poles •spindle METAPHASE – chromosomes in middle •Centromeres and spindle fibers LM 250 – The stages of cell division INTERPHASE PROPHASE Centrosomes (with centriole pairs) Chromatin Early mitotic spindle PROMETAPHASE Centrosome Fragments of nuclear envelope Kinetochore Nucleolus Nuclear envelope Figure 8.6 (Part 1) Chromosome, consisting Plasma membrane ot two sister chromatids Centromere Spindle microtubules ANAPHASE -chromosomes separate • spindle fibers pull • single copies to opposite poles of cell TELOPHASE – return to normal • chromosomes relax/uncoil into chromatin • nuclear membranes form; nucleoli appear • spindle fibers disappear CYTOKINESIS - division of cytoplasm • identical daughter cells Cleavage furrow ANAPHASE METAPHASE Cleavage furrow Metaphase plate Spindle Figure 8.6 (Part 2) TELOPHASE AND CYTOKINESIS Daughter chromosomes Nuclear envelope forming Nucleolus forming Cell division differs for plant and animal cells 8.7 Animal cytokinesis • Microfilaments SEM 140 • Cleavage furrow Cleavage furrow • Pinch cytoplasm in two Cleavage furrow Contracting ring of microfilaments Daughter cells Figure 8.7A Cell plate forming Wall of parent cell Daughter nucleus In plants - no centrioles or asters TEM 7,500 (has spindle) - cell plate - new cell wall grows Cell wall New cell wall along sides of cell plate Vesicles containing cell wall material Figure 8.7B Cell plate Daughter cells Plant cell mitosis Control of Cell Division Chemical signals tell a cell when to divide Some made by cell; some from cell environment Normal rate: growth, repair - asexual reproduction in some organisms Slow rate: some cell types divide rarely (liver) - aging slow healing, lose some cells Fast rate: some cell types (skin, digestive lining) - embryo, growing zones (bones, root tips) Uncontrolled cancer Growth factors • Proteins secreted by cells, can start or stop cell division • When cells crowded, growth factors used up stop dividing After forming a single layer, cells have stopped dividing. Providing an additional supply of growth factors stimulates further cell division. Figure 8.8B Cyclins - proteins produced by cell at constant rate - when reach high level, tell cell to divide Programmed Cell Death apoptosis 1. Cells damaged too much to repair – self-destruct 2. Embryonic development – lose unneeded cells 8.10 Cancer – uncontrolled cell division • Cancer cells ignore cycle controls – Repeated and rapid cell divisions; immortal • tumor – mass of non-functioning cells – Benign – not cancer; do not spread to nearby tissue – May get large enough to stop tissue function • Malignant tumor – cancer – Will invade and destroy neighboring tissue • Metastatic tumor – spreads to other body parts – Starts new cancers Cumulative DNA damage Cells begin to divide abnormally Continue dividing, invade healthy tissue (malignant) Spread through blood or lymph stream, can start new tumors (metastatic) Cells look abnormal – in structure; size of nucleus :: cytoplasm • If not treated early, cancer will spread • kills by destroying organ function Lymph vessels Tumor Blood vessel Glandular tissue A tumor grows from a single cancer cell. Figure 8.10 Cancer cells invade neighboring tissue. Cancer cells spread through lymph and blood vessels to other parts of the body. BE CAREFUL when sunning • Wear sunscreen, sunglasses • Avoid brightest part of the day • Don’t use tanning beds The most common cancer - skin Melanoma -deadliest Melanoma Basal cell Squamous Cell Cancer treatment Radiation – high-energy, carefully aimed at tumor Chemotherapy – drugs specific for tumor types • interfere with cell division • some normal cells destroyed, too – Skin (lose hair); digestive lining (nausea) Surgery – remove tumor and nearby cells Boost immune system – healthy diet, reduce stress Asexual Reproduction New organism from one parent – offspring identical genetically Advantages: 1) no mate – good for solitary, sessile organisms or when population density is low 2) continues a successful gene makeup 3) usually fast, large numbers Disadvantage: no genetic variation Asexual Reproduction One-celled organisms, some simple multicelled vegetative propagation in plants LM 10 Hydra with bud Yeast, sponges can also bud Regeneration – replace lost body part starfish can replace an arm Lizards can replace a tail flatworms can regrow a body Stem cells and differentiation Differentiate – cells specialize as embryo develops Stem cell – unspecialized cell - can differentiate to form many kinds of cells - depends on chemical signals from nearby cells Multipotent or totipotent Stem cells for cloning tissues Meiosis for Sexual Reproduction Sexual Reproduction - two parents a. Offspring are genetic mix of both parents b. Have a NEW combination of genes Advantage – genetic variation in offspring a. Some may have traits that favor survival b. Can pass these traits on to offspring c. Darwin’s theory - “ survival of the fittest” d. Variation in individuals allows a species to evolve Sexual Reproduction in bacteria and protists CONJUGATION a. Recipient cell gets new genes b. Bacteria and protists Complex organisms – make special cells a. gametes – sperm and egg b. Gametes combine in fertilization - make a zygote new organism Chromosome Number: Diploid and Haploid Homologous chromosomes a. matched chromosome pairs b. one member of pair from each parent c. carry genes for the same traits d. 22 autosome pairs; one pair sex chromosomes X, Y Gene for one trait Cells with paired chromosomes are diploid a. 2n (n = number) b. Human: 2n = 46 (23 pairs) b. Somatic (body) cells are diploid 2 sets of chromosomes - 2 of every gene Locus – location of gene on a chromosome Fruit fly 2n = 8 Meiosis is “Reduction Division” gametes – sperm and egg Haploid (n) - one set of chromosomes - one member from each homologous pair human: n = 23 Meiosis Reduces the Chromosome Number 2n parent cell DNA replicates in interphase First division – pairs separate Second division – sister chromatids separate 4 haploid daughter cells Homologous pairs separate in MEIOSIS TWO cell divisions Diploid cell - Has pairs (2n=2) - Daughter cells have ½ parent chromosome number Meiosis I - Pairs separate (n = 1) Meiosis II - copies separate (n = 1) Haploid cells - (n = 1) Crossing over – only in meiosis a. In Prophase I b. Homologous chromatids trade pieces c. Increases genetic variation Meiosis I: homologous pairs separate - makes two daughter cells, but sister chromatids are still attached MEIOSIS I: Homologous chromosomes separate INTERPHASE Centrosomes (with centriole pairs) Nuclear envelope PROPHASE I METAPHASE I Sites of crossing over Spindle Chromatin 2n parent cell Sister chromatids synapsis Tetrad ANAPHASE I Microtubules Metaphase attached to plate kinetochore Centromere (with kinetochore) pairs line up Sister chromatids remain attached Homologous chromosomes separate pairs separate disjunction Meiosis II: sister chromatids separate 4 haploid cells MEIOSIS II: Sister chromatids separate TELOPHASE I AND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II AND CYTOKINESIS Cleavage furrow Sister chromatids separate 2n n two daughter cells one chromosome set each two copies (sisters) chromatids separate disjunction Haploid daughter cells forming 4 daughters one set single copies 8.15 Review: Comparing mitosis and meiosis Mitosis Meiosis 2n copies Parent cell (before chromosome replication) Prophase I Prophase Duplicated chromosome (two sister chromatids) 2n Meiosis i Tetrad formed by synapsis of homologous chromosomes Chromosome replication Chromosome replication 2n = 4 Metaphase Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together Metaphase I 2n copies 2n copies copies 2n single 2n Daughter cells of mitosis 2n No further chromosomal replication; sister chromatids separate during anaphase II Anaphase I Telophase I Haploid n=2 Daughter cells of meiosis I n n n Daughter cells of meiosis II copies n single Meiosis ii n n Causes of genetic variation 1. Homologous pairs have different genes • same traits, but may be different forms 2. Crossing over – homologs trade pieces before separating new gene combinations 3. Pairs position in Metaphase I - independent • n pairs 2n possible combinations 4. Random fertilization of eggs by sperm • Any egg or sperm is equally likely to be used 5. Gene or chromosome mutation - Error in replication or cell division 8.16 Chromosomes line up randomly in meiosis – Many different gene combinations in haploid gametes Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4 Figure 8.16 Making sperm and egg Sperm: 2n parent cell 4 haploid sperm Ovum: 2n parent cell 1 haploid egg + haploid polar bodies Ovum needs all the cytoplasm Ovum and polar body (0.1mm) Sperm needs only DNA - and flagellum - and mitochondria for power - and acrosome to penetrate ovum When meiosis goes wrong Nondisjunction - do not separate correctly In mitosis defective nucleus, cell usually dies In meiosis defective gamete wrong number of chromosomes in zygote 8.21 Accidents during meiosis can alter chromosome number Nondisjunction in meiosis I Normal meiosis I Normal meiosis II Nondisjunctio n in meiosis II Gametes Gametes n1 n1 n1 n 1 n 1 n 1 n Number of chromosomes Number of chromosomes Nondisjunction in meiosis I All gametes abnormal Nondisjunction in meiosis II Some gametes normal n Wrong chromosome number in zygote wrong number in every cell in organism • Fertilization after nondisjunction trisomy in zygote Trisomy = 3 Egg cell n+1 Zygote 2n + 1 Sperm cell n (normal) Abnormal chromosome number = aneuploidy KARYOTYPE picture of a person’s chromosomes Photographed during mitosis - sorted into homologous pairs - largest-to-smallest - sex chromosomes last Abnormalities visible: - missing or extra - pieces broken or moved - pieces added or lost autosomes sex chrom. Trisomy 21 Normal male karyotype Normal female karyotype Down Syndrome Trisomy chromosome # 21 8.22 Abnormal number of sex chromosomes usually do not affect survival in humans • Nondisjunction of large chromosomes is usually lethal • Down Syndrome - # 21 is very small, carries few genes • In sex chromosomes, leads to varying degrees of malfunction, but usually not lethal Turner Syndrome XO Characteristic facial features Web of skin Constriction of aorta Poor breast development Under developed ovaries Figure 8.22B Turner Syndrome Klinefelter Syndrome XXY Klinefelter Syndrome Abnormalities of Sex Chromosomes in Humans 8.23 Other chromosome changes can cause birth defects and cancer Chromosomes break – pieces lost or rearranged - in somatic cells increases cancer risk - in gametes genetic disorders Deletion Translocation Translocation Duplication Homologous chromosomes Inversion Activates a cancer-causing gene