Cell Reproduction


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Cell Reproduction, Mitosis, Meiosis, Human Life Cycle

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Cell Reproduction

  1. 1. Cell Reproduction Introduction •Two types of Cell Reproduction –Cell Division/Reproduction involving Mitosis –Cell Division/Reproduction involving Meiosis Mitotic Reproduction: Asexual; produces 2 identical daughter cells from original mother cell. Meiotic Reproduction: Produces 4 non-i ni lg mee ”rm o emoh r d ta “a ts f e c o n te cell; reduces the number of chromosomes in the cell by ½; leads to sexual reproduction 1-1
  2. 2. Chromosome Number Terminology Chromatin-tangled mass of threadlike DNA in nondividing cell –Chromosomes-condensed rod-shaped DNA molecules formed during division –Diploid (2N) number-characteristic chromosome number in common somatic eukaryotic cells; chromosomes exist in pairs due to presence of two complete sets of chromosomes –one maternal set + one paternal set –Haploid (N) number- half the diploid number; only one set of mixed maternal and paternal chromosomes; found only in gametes (Technically, all prokaryotes are haploid) 1-2
  3. 3. Cell Type Terminology •Somatic Cells: All non-sex cells; normal body cells (skin, muscle, liver, root, leaf, stem, etc.) –Theoretically always Diploid •Gametes: Sex cells –sperm, egg (ovum), sexual spores –Theoretically always Haploid 1-3
  4. 4. DNA/Chromosome Terminology Monads –Unreplicated/unduplicated chromosomes i.e. 46 in humans (23 pairs = 1 set of 23 maternal + 1 set of 23 paternal) Dyads –Replicated chromosomes i.e. Each consists of two identical DNA molecules c ld“ ae sister chromatids” h haeh l l wi r e c d tg te a a“ o eh r t centromere” . 1-4
  5. 5. DNA/Chromosome Terminology (cont.) •Homologous Chromosomes (Homologues): –2 chromosomes (a pair = 1 maternal + 1 paternal) that carry genes for the same traits –Genes of the gene pairs may or may not be identical • not identical, are known as alleles If 1-5
  6. 6. 5.1 Cell increase and decrease • Increase and decrease of cell numbers –Cell division increases number of somatic cells •Mitosis/Karyokinesis-division of nucleus of cell •Cytokinesis-division of cytoplasm •Occurs throughout life; growth, development, repair –Apoptosis- programmed cell death decreases cell number •Occurs throughout life also •Prevents abnormal cells from proliferating 1-6
  7. 7. C ln ra ea dd ce s , o t . e i e s n e ra e c n’ lc d • The cell cycle – Set of events that occur between the time a cell divides and the time the resulting daughter cells divide – 2 major parts: Interphase and Cell Division – 5 Phases: Interphase, Prophase, Metaphase, Anaphase, Telophase • Stages of interphase –longest phase of the cycle – Normal cell functions occur as well as preparation for division – G1 stage-normal metabolic activities; cell is actively growing/maturing; organelles double in number; cell accumulates materials needed for division – S stage-DNA replication – G2 stage-synthesis of proteins needed for division; mitochondria and chloroplasts replicate 1-7
  8. 8. The cell cycle •Fig 5.1 a 1-8
  9. 9. C ln ra ea dd ce s , o t . e i e s n e ra e c n’ lc d •Mitotic stage –Follows interphase –Includes prophase, metaphase and anaphase •Control of cell cycle –The protein cyclin must be present for stages to progress –G2 checkpoint-stops cycle if DNA is not done replicating or is damaged –M checkpoint-stops if chromosomes not aligned –G1 checkpoint-protein p53 stops cycle if DNA damaged 1-9
  10. 10. Control of the cell cycle •Fig. 5.1 b 1-10
  11. 11. C ln ra ea dd ce s , o t . e i e s n e ra e c n’ lc d •Apoptosis –Progressive series of events resulting in cell destruction –Cells rounds up, and loses contact with surrounding cells –Nucleus breaks up and cell undergoes fragmentation –Mediated by 2 sets of enzymes called caspases –One set initiates the events –The other set activates enzymes that digest the cell 1-11
  12. 12. Apoptosis •Fig 5.2 1-12
  13. 13. Maintaining the chromosome n mb r o t . u e c n’ d •Overview of cell division involving mitosis –Nuclear division in which chromosome number stays constant –DNA replication produces duplicated chromosomes –Each duplicated chromosome is composed of 2 sister chromatids held together by a centromere –Sister chromatids are genetically identical –During mitosis, the centromere divides and each chromatid becomes a daughter chromosome 1-13
  14. 14. Mitosis overview •Fig 5.3 1-14
  15. 15. Animal Cell Mitosis •Mitosis in detail - Animal cells –Prophase-nuclear membrane disappears, centrosomes migrate, spindle fibers produced by centrioles appear; chromosomes become visible scattered –Metaphase-chromosomes line up at equator, associated with spindle fibers –Anaphase-centromeres divide, sister chromatids separate and migrate to opposite poles, cytokinesis begins –Telophase-nuclear membranes form, spindle disappears, cytokinesis occurs 1-15
  16. 16. Late interphase •Fig 5.4 1-16
  17. 17. Phases of animal cell mitosis •Fig 5.5 1-17
  18. 18. Animal Cell Mitosis (cont.) •Cytokinesis in animal cells •Cleavage furrow forms between daughter nuclei •Contractile ring contracts deepening the furrow •Continues until separation is complete 1-18
  19. 19. Animal cell cytokinesis •Fig 5.8 1-19
  20. 20. Plant Cell Mitosis •Mitosis in plant cells –Occurs in meristematic tissues –Same phases and events as animal cells except: •Spindle fibers are produced by the golgi bodies •Cytokinesis i b “ el lt fr t n s y c lpae omai ” o –Plant cells do not have centrioles or asters •Cytokinesis in plant cells –Flattened, small disk appears between daughter cells –Golgi apparatus produces vesicles which move to d k fr a“ Plate” i ;oms Cell s –Release molecules which build new cell walls –Vesicle membranes complete plasma membranes 1-20
  21. 21. Phases of plant cell mitosis •Fig 5.6 1-21
  22. 22. Plant cell cytokinesis •Fig 5.7 1-22
  23. 23. Prokaryotic Cell Division •Cell division in prokaryotes - Binary Fission –Prokaryotes have a single DNA molecule and no nucleus –Therefore, do NOT do mitosis –Chromosomal replication occurs before division –Cell begins to elongate to twice its length –Cell membrane grows inward until division is complete 1-23
  24. 24. 5.3 Reducing the chromosome number •Overview of cell division involving Meiosis – 2 divisions (Meiosis I and Meiosis II) producing 4 haploid daughter cells – Cells are diploid at beginning of meiosis – Pairs of chromosomes are called homologues – Meiosis I (Reductional Division) •Homologues line up side by side at equator - they synapse •When pairs separate, each daughter cell receives one member of the pair •Cells will be haploid (chromosome number is reduced by ½) 1-24
  25. 25. Overview of meiosis •Fig 5.9 1-25
  26. 26. Reducing the chromosome number c n’ o t. d •O ev wo me s , o t . v ri f i i c n’ e os d –Meiosis II (Equational Division –just like mitosis) • replication of DNA occurs in this division No •Centromeres divide and sister chromatids migrate to opposite poles to become individual chromosomes •Each of the four daughter cells produced has the haploid chromosome number and each chromosome is composed of one 1-26 chromatid
  27. 27. Reducing the chromosome number c n’ o t. d •Meiosis in detail –Meiosis I- genetic recombination occurs in 2 ways •Crossing over-exchange of segments of DNA between homologues •Independent assortment of chromosome pairs 1-27
  28. 28. Independent alignment •Fig 5.11 1-28
  29. 29. Synapsis and crossing over •Fig 5.10 1-29
  30. 30. R d c gtec rmo o n mb r o t . e u i h ho s me u e c n’ n d • Phases of meiosis I –Prophase I •Synapsis occurs, nuclear membrane breaks down •Homologues line up side by side and crossing over occurs –Metaphase I •Homologous pairs line up at equator such that maternal or paternal member may be oriented toward either pole –Anaphase I •Homologous chromosomes (each still consisting of 2 chromatids) undergo independent assortment into daughter cells –Telophase I •Cytokinesis produces 2 daughter cells which are haploid 1-30
  31. 31. Meiosis I in animal cells •Fig 5.12 1-31
  32. 32. Reducing the chromosome number c n’ o t. d • Interkinesis-period between meiosis I and meiosis II • Phases of meiosis II –Prophase II- • Cells have 1 member of each homologous pair –Metaphase II • Chromosomes line up at the equator –Anaphase II • Centromeres divide and daughter chromosomes migrate –Telophase II • Nuclei form, cytokinesis 1-32
  33. 33. Meiosis II in animal cells •Fig 5.13 1-33
  34. 34. R d c gtec rmo o n mb r o t . e u i h ho s me u e c n’ n d • Nondisjunction-causes various syndromes which result from abnormal chromosome numbers – Failure of homologous chromosomes to separate during anaphase – Failure of sister chromatids to separate during anaphase II – Ex: Down syndrome results from nondisjunction of chromosome 21 • Genetic recombination – Promotes genetic variability – Independent alignment of paired chromosomes during metaphase I – Crossing over in prophase I – Both assure that gametes will contain different combinations of chromosomes – When fertilization occurs, the resulting offspring will genetically unique 1-34
  35. 35. 5.4 Comparison of meiosis and mitosis •In comparison of meiosis to mitosis note that: –DNA replication occurs only once prior to both –Meiosis requires 2 divisions, mitosis only 1 –Meiosis produces 4 daughter cells, mitosis produces 2 –Daughter cells from meiosis are haploid, those from mitosis are diploid –Daughter cells from meiosis are genetically variable, while those from mitosis are genetically identical 1-35
  36. 36. Comparison of meiosis and mitosis c n’ o t. d •Fig 5.14 1-36
  37. 37. Comparison of mitosis and meiosis cnd o ’. t •Table 5.1 •Table 5.2 1-37
  38. 38. 5.5 The human life cycle •The human life cycle –Requires both mitosis and meiosis –In females meiosis is part of the process of oogenesis –In males meiosis is part of spermatogenesis –At fertilization, the resulting zygote divides by mitosis for the processes of growth and development –Mitosis is used for repair throughout life 1-38
  39. 39. Life cycle of humans •Fig 5.15 1-39
  40. 40. T eh ma l cc , o t . h u n i yl c n’ fe e d •Spermatogenesis –Begins at puberty and continues throughout life –Occurs in seminiferous tubules of testes –Primary spermatocytes (2n) divide in meiosis I to form 2 secondary spermatocytes (1n) –Secondary spermatocytes divide in meiosis II to produce 4 sperm 1-40
  41. 41. T eh ma l cc , o t . h u n i yl c n’ fe e d •Oogenesis –Occurs in the ovaries –Primary oocyte (2n) divides in meiosis I to produce 1 secondary oocyte (1n) and 1 polar body • Division is unequal as secondary oocyte receives most of the cell contents and half the chromosomes • Polar body functions only to receive half of the chromosomes –Secondary oocyte begins meiosis II but stops at metaphase II; polar body may also divide –At puberty, after ovulation secondary oocyte is activated if fertilized to complete division –Meiosis II produces 1 ovum and 1 polar body 1-41
  42. 42. T eh ma l cc , o t . h u n i yl c n’ fe e d • o e e i c n’ O g n s ,o t. s d –Products of oogenesis are 1 large ovum and up to 3 small polar bodies –Ovum receives nearly all cytoplasm and organelles and half the chromosomes –Polar body gets the remaining half of the chromosomes –Allows ovum to have all the cellular “ c i r”t e d fr mby n ma h ey i e s o e ro i n n c development 1-42
  43. 43. Spermatogenesis and oogenesis 1-43 •Fig 5.16
  44. 44. T eh ma l cc , o t . h u n i yl c n’ fe e d •Summary –Spermatogenesis and oogenesis both utilize meiosis –Spermatogenesis begins at puberty and continues throughout life –Spermatogenesis produces 4 sperm per primary spermatocyte •Results in production of many sperm –Oogenesis results in 1 oocyte and up to 3 polar bodies per primary oocyte •Divisions are unequal, ovum receives most cell contents –Oogenesis begins prior to birth, stops until puberty, then resumes in a cyclic pattern –Cyclic release of oocytes continues until menopause when the process stops 1-44