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Transcript
Reproduction Mitosis & Meiosis Cell Division • allows for repair & replacement of worn out cells • basis of reproduction in every organism • unicellular organisms – cell division reproduces entirely new organisms • allows organisms to develop from one fertilized egg cell into multicellular organism of 60 trillion cells DNA • body must have way to ensure that each time a cell divides information is maintained & directly copied • information is found in chromosomes – can only be seen during cell division • remainder of the time exists as mass of very long fibers -chromatin • each chromosome = one long DNA molecule containing thousands of genes Chromosomes • genes are found on chromosomes in the nucleus • number-specific to a species • human cells except ovum & sperm have 46 chromosomes • dog cells have 78 Chromosomes • during cell division genetic material makes an exact duplicate of itself • resulting in a chromosome containing two identical copies or sister chromatids • joined by a centromere • when cell divides chromatids separate • one goes to one daughter cell • other to another daughter cell • resulttwo cells with identical genetic material Cell Cycle • ordered sequence of events that begins when cell is formed & continues until cell divides • two broad stages • interphase – growing stage • mitotic phase – cell division stage Interphase • 90% of cycle • normal functions are performed • cell prepares for cell division • everything in cytoplasm is doubled • cell increases in size • chromosomes duplicate Substages of Interphase •G1 phase •S phase •G2 phase G1 Phase • mitochondria, cytoskeletal elements, ER, ribosomes, Golgi membranes & cytosol are made in quantities for two cells • continues until G2 stage • centrioles begin to replicate • may last hours, days, weeks, or months S Phase • 6-8 hours • chromosom es duplicate –DNA replicates G2 Phase • 2-5 hours • last minute protein synthesis • completion of centriole replication • each chromosome consists of 2 identical sister chromatids linked by centromere • • • • • Mitotic Phase M phase cell divides produces two identical daughters cell divided into two stages Mitosis – nuclear division – duplicated DNA is separated into 2 nuclei – sister chromatids separate at centromere – one goes into each of two daughter cells • Cytokinesis – cytoplasm divides into two cells Cytokinesis • cytoplasm division • animal cells-cleavage • first sign-appearance of cleavage furrow • microfilaments surround cell • pulled tight to divide cytoplasm • plant cells • cell plate or cleavage plate forms inside cell & grows outward • eventually new piece of wall divides cell into two Stages of Mitosis • mitosis is continuous • divided into four main stages • Prophase • Metaphase • Anaphase • Telophase Interphase • mitosis begin after interphase Prophase • begins when chromosomes coil tightly. • become visible as individual structures • there are 2 copies of each chromosome • each termed a sister chromatid • connected by centromere • as chromosomes appearnucleoli disappear Prometaphase • nuclear envelope disappears • spindle fibers form among chromosomes • kinetochore of each chromatid attaches to spindle fiber • centrioles begin to move to opposite poles due to spindle fibers Metaphase • mitotic spindle fully formed • chromosomes line up on metaphase plate Anaphase • begins when centromere of each chromosome comes apart separating sister chromatids • kinetochores move daughter chromosomes to opposite poles of cell • ends when complete collection of chromosomes has reached poles of cell • • • • • • • • • Telophase nuclear membrane forms nuclei enlarge chromosomes uncoil chomatin filaments form while nucleoli reappear mitosis is completed cells prepare to return to interphase in order to make two complete cells cytoplasm must divide Cytokinesis usually takes place at same time as telophase Identify the Stages of Mitosis Meiosis Reduction Division Chromosomes • every nucleus in every somatic cell carries genetic blueprint • 46 chromosomes • each paired with a like chromosome • 23 pairs • 23 chromosomes came from our mothers • 23 from our fathers Homologous Chromosomes • pairs of chromosomes are homologous • carry same genes • genes code for a particular trait • come in several forms or alleles • genes may be alike – Homozygous • genes may be unlike – Heterozygous Diploid & Haploid • • • • • • • • • • cells containing 23 pairs of chromosomes are diploid abbreviated-2n 2n = 46 all cells in human are diploid with exception of gametes – sperm & egg cells have haploid number – half number in diploid cell 23 chromosomes n = 23 during fertilization gametes fuse producing diploid zygote which develops into a diploid organism haploid gametes keep chromosome number from doubling in each generation gametes are made by a special type of cell division-meiosis or reduction division Meiosis • basis of sexual reproduction • reduction division • cells produced contain half number of chromosomes as typical body cell • one diploid cell4 haploid cells-4 sperm or 1 egg & 3 polar bodies • occurs in stages • many resemble stages of mitosis • preceded by replication of chromosomes • followed by two successive nuclear divisions: meiosis I (reduction) & meiosis II (division) Phases of Meiosis I • • • • • • interphase prophase I metaphase I anaphase I telophase I cytokinesis Interphase • chromosomes duplicate • end of stage chromosomes composed of two attached, identical sister chromatids • centrosomes have duplicated Prophase I • chromatin coils up so individual chromosomes become visible • homologous chromosomes-each composed of two chromatids pair up • form tetrad • composed of 2 chromatids forming thick, 4-strand structure • spindle starts to form between them Crossing Over • during prophase I synapsis forms (chiasmata) • crossing over • chromatids break • become reattached to different homologous chromosomes – rearranges genetic information • important to producing variability Metaphase I • tetrads line up on metaphase plate • sister chromatids still attached by centromeres • spindle fibers are attached to kinetochores at centromere region of each homologous chromosome pair Anaphase I • tetrads separate • drawn to opposite poles by spindle fibers • centromeres remain intact so each pole has two chromosomes attached to centromere • only tetrad has separated Telophase I • chromosomes arrive at poles of cell • each in duplicate form • cytokinesis usually takes place at same time Meiosis II • essentially same as mitosis Prophase II & Metaphase II • Prophase II • nuclear envelope (if formed) dissolves • spindle fibers form moving chromosomes to middle of cell • Metaphase II • spindles move chromosomes to metaphase plate with kinetochores of sister chromatids of each chromosome pointing to opposite poles Anaphase II & Telophase II • anaphase II • centromeres of sister chromatids separate • move toward opposite poles of cell • telophase II • nuclear envelopes form at the poles • cytokinesis • occurs at same time Genetic Variation • like begets like • truer of asexual than sexual reproduction • in sexually reproducing species like does not exactly beget like • none of you look exactly like your parents • none of your siblings look exactly like you – unless you are an identical twin • each offspring inherits a unique combination of genes from parents producing unique combinations of traits • genetic variability is due to two factors Genetic Variation • • • • • • • • • • • half your chromosomes came from your father half came from your mother giving you 46 when you produce sperm or egg cells with a haploid number of chromosomes some gametes got your mom’s chromosome & some your dad’s metaphase I-each homologous pair of chromosomes aligns on metaphase plate orientation of homologous pair to poles is random there are 4 possible gametes that could form this is true if an organism has only two pair of chromosomes humans have 23 pairs an independent orientation at metaphase 1 for humans with 23 pairs of chromosomes this works out to 8 million possible chromosome combinations Genetic Variation • also due to crossing over • during prophase I-synapsis of chromosomes occurs • genetic information is exchanged between pairs of homologous chromosomes • results in new genetic combinations • offspring inherit gene combinations totally different from those inherited from previous generations Genetic Variation • Fertilization • contributes to genetic variability • any egg may be fertilized by any sperm • one egg represents one of eight million possibilities being fertilized • sperm represents one of eight million possibilities • resulting zygote has any one of 64 trillion possible combinations Chromosomal Abnormalities • mistakes can occur in meiosis • detected using diagnostic tool • karyotype Aneuploidy • aberrant number of chromosomes • usual cause-non disjunction • Monosomy – one chromosome lags behind another – left out of newly formed cell nucleus resulting in one daughter cell with a normal chromosome number and one with a deficiency – not compatible with life • polysomy – too many chromosomes • autosomal polysomay may result in viable fetus but is nearly always associated with severely disability • Advanced maternal age, radiation, viruses & chemicals • implicated in chromosomal abnormalities Trisomy 21-Down Syndrome • • • • • • • • • • • • • • • extra 21st chromosome most common chromosomal disorder leading cause of mental challenges occurs in 1/700 live births first described-1866 mental retardation protruding tongues low set ears poor muscle tone short stature epicanthal folds flat face often congenital heart deformities increased susceptibility to respiratory infections and leukemia ¾ of fetuses with syndrome are still born or miscarried Sex Chromosome Disorders • unusual number of sex chromosomes • typically has less debilitating symptoms than extra autosomal chromosomes • may be because Y chromosome carries few genes Klinefelter Syndrome • extra X chromosome-XXY • abnormal sexual development • not diagnosed until puberty • secondary sex characteristics do not develop • boy lacks testosterone leading to infertility • child is tall with long arms & legs • female hair distribution, breast enlargement and high pitched voice • testosterone therapy reduces feminine characteristics Turner Syndrome • only one X chromosome • monosomy disorder – only one not fatal in humans • females have characteristic appearance-often short • web of skin between neck & shoulders • ovaries do not developsterility • may be poor development of secondary sex characteristics • estrogen can alleviate symptoms