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Reproduction

Reproduction. Mitosis & Meiosis. Cell Division. basis of reproduction in every organism unicellular organisms cell division reproduces entirely new organisms allows organisms to develop from one fertilized egg cell into a multicellular organism of 60 trillion cells

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Reproduction

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  1. Reproduction Mitosis & Meiosis

  2. Cell Division • basis of reproduction in every organism • unicellular organisms • cell division reproduces entirely new organisms • allows organisms to develop from one fertilized egg cell into a multicellular organism of 60 trillion cells • allows for repair & replacement of worn out cells

  3. DNA • body must have a way to ensure that each time a cell divides information is maintained & directly copied • information is found in chromosomes • can only be seen clearly during cell division • remainder of the time exists as mass of very long fibers -chromatin • composed of DNA & proteins • each chromosome = one long DNA molecule containing thousands of genes • genes carry specific information

  4. Chromosomes • genes are found on chromosomes in the nucleus • number of chromosomes is specific to a species • human cells except ovum & sperm have 46 chromosomes • dog cells have 78

  5. 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 • resulttwo cells with identical genetic material

  6. 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

  7. Interphase • 90% of cycle • cell metabolizes & performs its job • normal functions are performed • cell prepares for cell division • everything in cytoplasm is doubled • cell increases in size • chromosomes duplicate

  8. Substages of Interphase • G1 phase • S phase • G2 phase

  9. 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

  10. S Phase • 6-8 hours • chromosomes duplicate • DNA replicates

  11. G2 Phase • 2-5 hours • last minute protein synthesis • completion of centriole replication • each chromosome consists of 2 identical sister chromatids linked by a centromere

  12. Mitotic Phase • M phase • cell divides • produces two identical daughters cell • can be 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

  13. Cytokinesis • cytoplasm division • animal cells-cleavage • first sign-appearance of cleavage furrow • microfilaments surround cell • pulled tight to divide cytoplasm • plant cells • cell plate forms inside cell & grows outward • eventually new piece of wall divides cell into two

  14. Stages of Mitosis • mitosis is continuous • divided into four main stages • Prophase • Metaphase • Anaphase • Telophase

  15. Interphase • mitosis begin after interphase

  16. 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 appearnucleoli disappear

  17. 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

  18. Metaphase • mitotic spindle fully formed • chromosomes line up on metaphase plate

  19. Anaphase • begins when centromereof each chromosome come apartseparating sister chromatids • kinetochores move daughter chromosomes to opposite poles of cell • ends when complete collection of chromosomes has reached poles of cell

  20. 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

  21. Identify the Stages of Mitosis

  22. Meiosis Reduction Division

  23. 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

  24. 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

  25. Diploid & Haploid • cells containing 23 pairs of chromosomes are diploid • abbreviated-2n • 2n = 46 • all cells in human body 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

  26. Meiosis • basis of sexual reproduction • reduction division • cells produced contain half number of chromosomes as typical body cell • one diploid cell4 haploid cells • 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)

  27. Phases of Meiosis I • interphase • prophase I • metaphase I • anaphase I • telophase I • cytokinesis

  28. Interphase • chromosomes duplicate • end of stage chromosomes are composed of two attached, identical sister chromatids • centrosomes have duplicated

  29. Prophase I • chromatin coils up so individual chromosomes become visible • homologous chromosomes-each composed of two chromatids pair up • forms a tetrad • composed of 2 chromatids forming thick, 4-strand structure • spindle starts to form between them

  30. Crossing Over • during prophase I synapsis forms • crossing over • chromatids break • become reattached to different homologous chromosomes • rearranges genetic information • important to producing variability

  31. 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

  32. Anaphase I • tetrads separate • drawn to oppositepoles by spindle fibers • centromeres remain intact so each pole has two chromosomes attached to centromere • only tetrad has separated

  33. Telophase I • chromosomes arrive at poles of cell • each in duplicate form • cytokinesis usually takes place at same time

  34. Meiosis II • essentially same as mitosis

  35. 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

  36. 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

  37. 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

  38. 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 • total neach has umber of combinations of chromosomes equals 2n • for humans with 23 pairs of chromosomes this works out to 8 million possible chromosome combinations

  39. Genetic Variation • also due to crossing over • during prophase I-synapsis of chromosomes occurs • genetic information is exchanged between pairs of homologous chromosomes • segment of one chromatid changes places with same segment of its homologous chromosome • results in new genetic combinations • offspring inherit gene combinations totally different from those inherited from previous generations

  40. 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

  41. Chromosomal Abnormalities • mistakes can occur in meiosis • detected using a diagnostic tool known as a karyotype

  42. Aneuploidy • aberrant number of chromosomes • usual cause-non disjunction of paired homologous chromosomes • anaphase lag • one chromosome lags behind another and is left out of newly formed cell nucleus resulting in one daughter cell with a normal chromosome number and one with a deficiency of one called monosomy • not compatible with life • polysomy • too many chromosomes • autosomal polysomay may result in a viable fetus but is nearly always associated with severely disability • extra or missing sex chromosomes • more common & less debilitating • radiation, viruses & chemicals • all been implicated in chromosomal abnormalities • advanced maternal age is known to increase risk of chromosome number abnormality Down Syndrome

  43. 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

  44. 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

  45. Klinefelter Syndrome • extra X chromosome-XXY • 1/2000 live births • abnormal sexual development • not diagnosed until puberty • secondary sex characteristics do not develop • boy lacks testosterone leading to infertility • physically child is tall with long arms & legs • has female hair distribution, breast enlargement and a high pitched voice • testosterone therapy reduces feminine characteristics

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