Cellular Basis of Reproduction and Inheritance

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					Cellular Basis of Reproduction
        and Inheritance
        Chapter 12 and 13
   Objectives
        Describe  binary fission in bacteria
        Describe the structures that play roles in the mitotic
         phase of the cell cycle: the centrioles, spindle
         microtubules and chromosomes
        Outline the phases of the cell cycle

        Describe the factors that control cell growth and
         how cancer results from a breakdown of this control
        Outline the general progression and overall results
         of meiosis, contrasting them with mitosis
 Explain how meiosis provides possibilities for
 genetic recombination
Introduction
 Life cycle is sequence of life forms from
  one generation to next
 Sexual reproduction involves passing traits
  from two parents to next generation
 Asexual reproduction involves passing traits
  from one parent to next generation
 Cell division is basis of all processes that
  link phases of life cycle
Like beget like (more or less)
   True only for organisms that reproduce
    asexually
     single-celled organisms reproduce asexually by
      dividing in two
        calledbinary fission
        daughter cells receive identical copy of parent’s
         genes
 offspringof multi-cellular organisms not
 genetically identical to parents
    unique  combination of parents traits
    breeders of domestic plants and animals manipulate
     sexual reproduction by selecting offspring that
     exhibit desired traits
   Cells arise from preexisting cells
     cellreproduction called cell division
     two roles
        enables  fertilized egg to develop through various
         stages to adult organism
        ensures continuity from generation to generation
Binary Fission
   Bacterial chromosomes
     genes   carried on single circular DNA molecule
        up   to 500x cell length
     minimal     packaging
                 with few proteins and attached to plasma
        complexed
        membrane at one point
   Binary fission
     prior   to cell division, genome copied
        copies   attached to adjacent parts of membrane
     cellelongation and new plasma membrane
      separates two genomes
     plasma membrane pinches through cell
Eukaryotic Cell Division
   Eukaryotes have large, complex, multiple
    chromosomes
     human   cells contain 50,000-100,000 genes
        organized   into separate, linear chromosomes
     DNA   complexed with proteins
     Just prior to division, chromosomes become
      visible
        remain   visible during division process
 Somatic (body) cells contain 2x chromosomes
 (diploid) compared to sex cells (haploid)
   human   cells:
     • somatic cells-46 chromosomes (2n=46)
     • sex cells-23 chromosomes (n=23)
   Prior to cell division, chromosomes are
    duplicated
     visible chromosomes consist of two identical
      sister chromatids attached at centromere
     sister chromatids are divided among daughter
      cells (now chromosomes)
        each   cell gets identical set of chromosomes
   Cell cycle results in cell multiplication
     most cells in organism divide on regular basis
     dividing cells undergo cycle-sequence of steps
      repeated during each division
   Cell cycle divided into several steps
     interphase   represents 90% or more of cycle
      time
        G1-cell increases in size and increases supply of
         proteins and organelles
        S-DNA synthesis occurs

        G2-cell prepares for division, increases supply of
         proteins necessary for division
 mitotic   (division) phase divided into two steps
    mitosis-nuclear  division
    cytokinesis-cytoplasmic division

    result is two daughter cells with identical
     chromosmes
Mitosis
   While continuum, several established
    dividing points for cell cycle phases
     Interphase: duplication of genetic material, ends
      with visible chromosomes
     Prophase: mitotic spindle forms from MTOC’s;
      ends when chromatin coiled into chromosomes;
      nucleoli and nuclear membrane dissolved
 Metaphase:   spindle formed; chromosomes
  aligned single file with centromeres on
  metaphase plate
 Anaphase: chromosomes separate; migrate to
  spindle poles
 Telophase: reverse of prophase
 Cytokinesis: division of cytoplasm
 movement of chromosomes driven by addition
  or subtraction of protein subunits to kinetichore
  end of spindle microtubules
   Cytokinesis differs in plants and animals
     inanimals, ring of microfilaments contracts
      around periphery of cell
        forms   cleavage furrow that eventually divides
           cytoplasm
 inplants, vesicles containing cell wall material
  collect on spindle equator
    vesicles  fuse from inside out forming cell plate
    cell plate gradually develops into new cell wall
     between new cells
    membranes surrounding vesicles fuse to form new
     parts of plasma membranes
Factors Affecting Cell Division
   Control of cell division important for proper
    growth, development and repair of
    organisms
     growth   factors regulate cell division
        product   of dividing cell
     most plant and animal cells will not divide
      unless in contact with solid surface-anchorage
      dependence
 divisionusually stops when single layer of cells
 formed and cells touch-density-dependent
 inhibition
       to depletion of growth factor proteins in cell
    due
    mass
Growth Factors
   Three major check points in cell cycle
     G1 of interphase
     G2 of interphase
     M phase

   Release of growth factor at each of these
    checkpoints allows cell cycle to continue
Cancer
   Cancer cells not affected by growth factors
    that regulate density-dependent inhibition
     malignant tumor-metastasize
     benign-no metastasis
     named for organ or tissue of origin
     some cancer cells produce factors that keep
      them dividing
 Benign tumor becomes malignant when
 cancerous cells from tumor mass spread to new
 sites and continue to proliferate
   movement   mediated by either blood or lymph
    systems
   Common treatments for cancer
     radiation-disrupts normal processes of cell
      division; cancer cells more susceptible
     chemotherapy-disrupt cell division
Meiosis
   Chromosomes are matched in homologous
    pairs
     share shape, genetic loci; carry genes
      controlling same traits
     each homologue inherited from separate parent
     in humans, 22 pairs are autosomes, remaining
      pair sex chromosomes
        female-two X chromosomes
        male-one X and one Y chromosome
   Gametes have single set of chromosomes
     somatic   cells have two sets of homologues
        diploid   (2n)
     sex   cells have one set of homologues
        haploid(n)
        produced by meiosis

     sexual life cycle involves alternation between
      diploid and haploid
     fusion of haploid gametes at fertilization results
      in diploid zygote
   Meiosis reduces chromosome number from
    diploid to haploid
     occurs  only in diploid cells
     preceded by single duplication of chromosomes
     results in four haploid daughter cells
     consists of two consecutive phases:
        meiosis I-halving of chromosome number
        meiosis II-separation of sister chromatids
   Comparison of mitosis and meiosis
     all   unique events in meiosis occur in meiosis I
        crossing over during prophase I
        separation of homologous pairs during anaphase I

     meiosis    II virtually identical to mitosis
        starting   cells are haploid
            results in two daughter cells with same
     mitosis
      number of chromosomes as parent cells
        can   occur in either diploid or haploid cells
        results in four daughter cells with half
 meiosis
 number of chromosomes as parent cells
    only   occurs in diploid cells
   Independent orientation of chromosomes in
    meiosis and random fertilization lead to
    varied offspring
     during  prophase I each homologue pairs up
      with its “other”
     during anaphase I maternally and paternally
      inherited homologues move to one pole or other
      independently of other pairs
 forn chromosomes, there are 2n different
  combinations of half pairs
    for humans, 223 different combinations
    there are 223x223 combinations possible at
     fertilization (64 billion)
 Homologous chromosomes carry different
  versions of genes
 Crossing over increases genetic variability
             of corresponding segments between
     exchange
     two homologues
        site   of crossing over called chiasma
           between chromatids within tetrads as
     occurs
     homologues pair up during synapsis
 produces new combinations of genes-genetic
  recombination
 can occur several times in variable locations
    variabilitymuch greater than calculated
    two individual parents can never produce identical
     offspring from separate fertilizations

				
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posted:2/8/2012
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