Chapter 3

CHAPTER 3: CELL DIVISION 3.1 Principles of Cell Division -

First part of cell theory: all living things are made up of one or more cells, all cells formed from pre existing cells by cell division 1) single cell (parent)  two cells (daughters) : mitosis (cell division: daughter cell  – same # chromosomes) – chromosomes) – equitable distribution of nuclear contents/ cytoplasm, organelles in cytokinesis. 2) to prepare: cell replicates each DNA molecule (found in chromosomes. Humans = 46). 3) separation – separation  – ensure genetically identical + future cell divisions. All body cells – cells  – same genetic information (from same f ertilized cell).

3.2 The Cell Cycle Interphase

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Mitosis Prophase

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Cell cycle  – sequence of events  – continuous process: stage between nuclear divisions: interphase  – rapid growth in mass, replication of chromosomes, doubling of cytoplasmic components, another period of growth, preparation for further divisions. Cells grow, make structural proteins to repair damaged parts, transport nutrients to where they are needed, eliminate wastes, prepare for mitosis by building proteins (involved in construction of enzymes for chemical reactions) E.g. reactions that control synthesis of DNA + replication of genetic information in chromosomes During interphase: genetic material = chromatid (all DNA molecules + proteins in nucleus (individual chromosome: 1 DNA molecule + proteins)  – long, thin strands – strands  – tangled, fibrous mass Each chromosome duplicates Original + duplicate attached by centromere structure = sister chromatids = considered 1 chromosome Chromosomes shorten and thicken (visible in microscope) Animal cells: 1) small body in cytoplasm separates, move to opposite poles 2) tiny structures centrioles (small protein bodies) provide attachment for spindle fibres (protein structures) guide wires for attachment and movement of chromosomes. Both = spindle apparatus 3) Centromere  joining two chromatids = helps anchor chromosomes to spindle fibres. 4) Nuclear membrane: dissolves to allow separation of chromosomes + ce ll organelles.

Metaphase

Anaphase

Telophase

Cytokinesis

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Plants: no centrioles Chromosomes (sister chromatids)  center of cell (equatorial plate) = midway between poles Chromosomes = dark, thick filamentous structures attached to spindle fibres Difficult to count #  most are entangled, chromatids become intertwined Centromeres divide, sister chromatids move to opposite poles Same number and type of chromosomes Occasionally: segments will break apart, reattach Chromosomes reach opposite poles, begin to lengthen Spindle fibres dissolve Nuclear membrane forms around each mass of chromatin Cytoplasm divides Distinct from nuclear division Animal cell: furrow develops, pinching off cell in two parts Plant cell: separation accomplished by c ell plate forming between chromatin masses  cell wall

3.3 A Cell Clock  -

Research on cultured cells: internal memory of # of divisions. Biological clock regulates # of cell divisions available Age, specialization – specialization  – stop dividing Skin cells / cells in digestive tract  – reproduce more often than muscle cells, nerve cells, s ecretory cells 2 types divide continuously: sperm-producing cells spermatocytes, cells of cancerous tumor Once spermatocyte  sperm cell  loses ability to divide further Cancer cells – cells – accelerated rate – rate  – genes cannot regulate proliferation and direct specialization. E.g. Leukemia: reduced ability (white blood cells) to fight infections, because they need time to specialize More specialized, less able  – mitosis. Fertilized egg cell  – differentiation begins after – after  – same time, biological clock Note: goals of biotechnology industry: create rapidly dividing cells. E.g. agriculture industry: plant group (chemical: gibberellin)/ medicine: generatin g tissues for skin grafts or new organ parts’

3.4 Cloning -

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‘asexual reproduction’: identical offspring from a single cell/tissues: or near identical. Some – accidental changes in genes. No variation of traits (e.g. male + female cells) Plant Clones 1958: Fredrick Stewart: cloned carrot. Most orchids: clones = production of plants with predictable characteristics Groundwork for genetic engineering Trick in cloning: delaying specialization/differentiation. When they do, they use only segments of DNA (identical to parents’). Animal Clones Blastula: embryonic stage – stage  – ball of cells produced by cell division following fertilization of an egg Robert Briggs + Thomas King: nuclear transplants in frogs Common grass frog – frog  – extracted nucleus from unfertilized egg cell. Fine glass, micropipette  cytoplasm, sucked out nucleus. Cell now: enucleated Nucleus of cell from frog embryo  – blastula stage – stage  – removed, inserted  enucleated cell Began to divide  adult frog, characteristics of transplanted nucleus Different results – results  – nucleus taken at later stages Nucleus capable of bringing cell to adult: totipotent

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Not all  nucleus from later stage, gastrula stage  – did not progress far. Difference: specialization = mechanism in cell turns off some of the genes that allow cell division. Another experiment: nuclei from gut cells (fully specialized) of African clawed toad tadpoles  egg cells, nuclei destroyed by ultraviolet radiation. Some  adult clones Cloning from Adult Cells Adult mammalian cell nuclei  adult mammal: no cell divi sion = genes controlling cell division switched off. Until Recently: cloning only by splitting off c ells from developing embryo  divide  cells aggregate/specialize Must be taken before eight-cell stage of development = totipotent. After: specialization Dr. Ian Wilmut (Rosalind Institute in Scotland)  – extracted nucleus from udder cell of adult Finn Dorsett sheep  enucleated egg cell of adult Finn Dorsett sheep. Developed in petri dish until early embryo stage  womb of Scottish Blackface sheep. Offspring, Dolly  genetic clone of Finn Dorsett adult Finn Dorsett genetic information: from frozen mammary tissue. Dr. Wilmut’s team concluded: previous failures: 1) c ells too actively replicating DNA and dividing 2 ) w rong stage of cell cycle 3) wrong set of genes had been turned off a s cell specialized. Scottish team: starved cells  – few days – days – before extracting nucleus. After: acted  – unspecialized  transferred to enucleated egg cell. Possible: starving  – slows down metabolism + turn genes back on that are shut off  Benefits: 1) animals with organs for organ transplants 2) experimenting effectiveness of drugs 3) agriculture: strongest livestock can be cloned.

(see liver tissue transplants + biotechnology in agriculture page 98-99)

3.5 Cancer

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Broad group of diseases: uncontrolled, unregulated, accelerated growth of cells After specialization: division only for damaged cells. Cell destruction/replacement balance = healthy. Cells communicate information: e.g. callus: accelerated division  – replaces damaged cells, increases cell numbers of protective outer layers of skin  – shielding delicate nerve + blood vessels = inner lay ers. Cancer cells – cells – reproduce in isolation (unlike usual). Artificial culture  once every 24 hours (1 billion in a month). In body: slower. Even embryo cells can’t divide that quickly. Slow cancer: cancer cells  – messages from adjacent cells regulating rate of reproduction Can reproduce without directions. Cancer cells do not adhere to other ca ncer cells, or normal cells. Can dislodge from one area, move: metastasis: difficult to locate and control. Another difference: no maturing/ specialization specialization (unlike 100 different types of cells in human body, uni que shapes + functions) = another threat: can’t carry out functions of normal cells. cells.

3.6 Meiosis -

Process: sex cells/gametes form in testes and ovaries/ a ka Reduction Division: prevents redoubling Mitosis: same # chromosomes  – parent + daughter Meiosis: Daughter cells have half # - all gametes = same. #: haploid chromosome number/ # chromosomes in all cells: diploid number/

n.

2n.

Offspring carry genetic info from each of parents (23 each). Paired chromosome: homologous chromosomes: similar shape, size, gene arrangement  – deal w/ same traits – traits  – interact during meiosis. How= characteristics

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Stages of Meiosis: 2 nuclear divisions = 4 n cells. Second phase: separation of 2 chromatids. Meiosis I: 1) Prophase 1: nuclear membrane dissolves, centriole splits  – opposite poles, spindle fibres form. Sypnapsis: 1) form homologous pairs: each chromosome: homologe, composed of sister chromatids. Whole: tetrad: 4 chromosomes. 2) crossing over: chromosomes intertwine, sometimes break and exchange segments: exchange of genetic material.

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2) Metaphase 1: homologous chromosomes attach to spindle fibres, line  equatorial plate 3) Anaphase 1: 1) segregation: homologous chromosomes  opposite poles. 2) reduction division: 1 member of each homologous pair  new cell.

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4) Telophase 1: new membrane forms. Chromosomes in nuclei: not identical. Meiosis II: Pairs of chromatids separate  opposite poles (no replication prior) 1) Prophase II: beginning of 2

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division: 1) nuclear membrane dissolves 2) spindle fibres form

2) Metaphase II: chromosomes  equatorial plate 3) Anaphase II: individual chromatids  pole. Nuclear membrane forms 4) Telophase II: 2

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nuclear division  2

division of cytoplasm: 4 haploid daughter cells.

3.7 Comparing Mitosis and Meiosis -

Mitosis: multicelled, single-celled, eukaryotic, nonreproductive cells. Interphase: diploid chromosome number. Before mitotic division: DNA replication Reproductive cells: diploid chromosome number in interphase. Before meiosis 1, DNA replication. None between meiosis I and II. Difference: end results. Mitosis: clones of original. Meiosis: four cells different from each other and parent. Combined with fertilization = variation in traits observed in species that reproduce sexually. Occurs through: 1) crossing over during prophase 1 exchanges genes. 2) metaphase 1: paternal and maternal randomly assorted. 3) fertilization: different combinations of chromosomes + genes  gametes unite

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3.8 Reproduction and Cell Division

Plants

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Animals

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Gametogenesis: formation of sex cells during meiosis Flowering plants: pollen (mail sex cells: n) + structures (egg cells: n )= zygote (male + female gamete)  mitosis  either spores or seeds  new generation Plants, eg. Pine trees, roses: zygote  mitosis  spore producing bodies (sporophytes) = n cells  – protective wall (resistance to drought, unfavorable environmental conditions  germination  gamete-producing bodies, gametophytes. Alternation between sporeproducing and gamete-producing bodies = generalized plant life cycle Animals: meiosis (testes (sperm cells) + ovaries (egg cells)  two haploid gametes  diploid zygote  mitosis  specialization and become gamete-producing cells  meiosis  haploid cells

Differences Female Male female gametes  one of daughter cells, ootid  – Sperm cells:  movement: streamlined, no receives most of the cytoplasm. Others  – polar excess weight. bodies  – die, nutrients absorbed by organism = 1 ovum (egg cell). Egg cell: use nutrients + organelles  future cell divisions

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Baby females: 2 million oocytes (immature cells)  ovaries. Most absorbed into body, remaining: 300 000  – 400  500 released in reproductive years. Primary oocytes  – in meiosis 1, suspended st  – prophase 1 until  puberty. 1 menstrual cycle, meiosis resumes 1 at a time.

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1 billion sperm cells every day: many more: diploid spermatocytes (gives rise to sperm cells)  – capable – capable  – mitotic division before meiosis.

Menopause: end: age 40-55. Oocytes: nonfunctional, unresponsive, no longer released Sex Chromosomes member. - 1906: meiosis in testes cell of fruit fly: 1 chromosome: hook shaped, no homologous member. - In females: 2 rod shaped chromosomes of identical length - Male: 1 rod shaped, and smaller hook shaped chromosome = sex chromosomes. Not: autosomes (not involved in sex determination) - Rod shaped: X, hook shaped: Y. Females: two homologous X. Males: one X + one Y. Abel to synapse for part of their lunch, function as homologous chromosomes  meiosis.

3.9 Abnormal Meiosis: Nondisjunction Nondisjunction Nondisjunction: Nondisjunction: Meiosis  2 homologous chromosomes  same pole = 1 daughter cell missing 1, other 1 extra = won’t function properly. Abnormal chromosome # - abnormal separation: either meiosis 1 (all gametes abnormal in number), meiosis II (half gametes abnormal in number) nondisjunction  gametes w/ 22 + 24 chromosomes. Gamete with 24  – both chromosomes - Trisomy: nondisjunction from one of the homologous pairs  joins w/ normal gamete of 23 from opposite sex = zygote containing 47. Zygote = 3 chromosomes in place of normal pair. - Monosomy: sex cell 22 chromosomes + normal gamete = zygote w/ 45 = only 1 of chromosomes rather than homologous pair. - Begin division  each cell  greater or less than 46 Nondisjunction Nondisjunction Disorders - Karyotype chart: picture of chromosomes arranged in pairs. Obtained  small sample of tissue + solution that stimulates mitotic division. Different solution: stops at metaphase (most condensed form). Placed onto slide, stained  distinctive bands appear. Photograph taken, enlarged, each cut out and paired w/ homologue. Similar  – size, length, centromere location, banding pattern. All aligned in decreasing size order. Sex chromosomes  last. Down 95% of cases extra Common traits: round, full 1 in 600 babies.  Syndrome chromosome in number 21: face / enlarged, creased Generally associated w/ trisomic disorder. Syndrome  tongue / short height / mental retardation, group of disorders that occur large forehead. though some  wide together. range mental abilities. Risk increases with age of mother: forties  1 in 40 chance. 25% greater than woman in

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Turner Syndrome

Klinefelter syndrome

: monosomic disorder: female  – single X chromosome. In egg cell  – both homologous X chromosomes  same pole in meiosis I. Egg w/ no X + normal with 1 X = zygote w/ 45 chromosomes. nondisjunction in sperm or egg. Two x chromosomes, 1 Y chromosome.

Appear female, undeveloped sexually / short / thick, widened necks.

Male at birth  high levels of female sex hormones at sexual maturity. Sterile.

twenties. 1 in every 3000 babies. Most fetuses are th miscarried before 20 week – week  – pregnancy.

1 in every 500 male babies.