a different approach to study of Life and cells:
GENETICS & DEVELOPMENT... how cells pass information across generations..
is based upon Replication of DNA and Cell Division : Key Concepts* the cellular basis of Inheritability... the mechanisms of Cell Reproduction in gametic cells...   egg cells & sperm cells cells reproduce identically, yet allow gene variation (i.e., new traits can appear) "All living cells arise from pre-existing cells" GENETICS asks.... HOW? What are mechanisms at cellular & molecular levels ? What is the physical basis of inheritability?

DEVELOPMENT looks.... at mechanisms of the LIFE CYCLES of cells & organisms 1. cellular mechanisms of reproduction in organisms: mitosis & meiosis             2. growth of organism..... zygote* to adult (embryology) cell differentiation - how one cell becomes different from another differential gene activity - genes are active at different times totipotency & cloning - exact genetic copies of cells 











METHODS of CELL REPRODUCTION, which include...

 Asexual Cell Division (ex:  bacteria & archaea & yeast)...

     Fission*  - [figure] = 2 equal halves (cyanobacteria  &  protozoans)

     Budding* - outgrowths detach = new organism (unequal split)  (hydra)    

  Eukaryotic (higher) organism cell division...    more commonly referred to by
                        by the terms designating their nuclear divisions, including:


     Mitosis* - asexual =  identical genetic copies [cytokinesis
                              genetically equal somatic cells 
c7 fig 1.5 lung cell
                                                                                     amoeba, sand dollar, bone marrow cell divisions
     Meiosis* - sexual = cycle produces sperm & egg cells with    
                               1/2 chromosome number &
new gene combos










-  in Eukaryotic Asexual Reproduction is via a Cell Cycle...
                        results in copying & equal duplication of parental cell's  DNA  
                        and the equal division of chromosomes into two daughter cells,
                        which are genetically identical.
                        the rates vary =   epithelial cells
  1x/day    -    liver cells  1x/yr.

Mitosis, is a part of the whole Life of a Cell... known as the "CELL CYCLE"...   
Cell Cycle has 3 Stages, which is often depicted as a circle 360o --> the cell cycle*                
     Interphase     - period between successive cytokenetic divisions of a cell
                                3 parts...    G1 = before   DNA synthesis = S,    &   G2 - period after S.

MITOSIS*     - graphic stages image  &  flourescent stained stages of nuclear division
     Cytokinesis* - physical division of cell into two parts: [fig 12.10*]     animals/plants  
                            - what happens to the organelles during cytokinesis

                     What amount of times is each of the phases of Cell Cycle ?  
                      Fibroblast cell cultures*   +     different cycle times*








       skip = 
S-phase: pulse chase experiment* 






         BW-stages of Mitosis
- in color*           animation of mitosis*view@home     
          the nuclear division phase of asexually dividing cells
  Prophase -                        
      chromatin condenses into chromosomes - 

                                (23703 bytes)

    Prometaphase -
      chromosome MT's attach to kinetochores
             fig 12.7*
      each homolog has 2 chromatids

prometaphase.jpg (22842 bytes)

 Metaphase -            
      chromosomes align at equator                  
            fig 12.7*
      homologs align independently of each other

metaphase.jpg (20290 bytes)

 Anaphase -
      MT are attached to kinetochore
      and the chromatids are pulled & pushed apart

                                (25058 bytes)

 Telophase - at opposite poles of cells as cytokinesis starts    
      ex:   animal cells & confocal pics  

                                (19976 bytes)

  Recognition Exercices:     onion root tip cells*       a practice ID*
 next              another mitosis animation  &   a review of mitotic stages  








Naming and Numbering of the chromosome -  (to protect the innocent)                                         

 Structure*  [nucleosome]    of   chromosomes*
  Genes occur in chromatin of nucleus, 
        which condense into  CHROMOSOMES
        (colored bodies) visible only during
        aninmation of how DNA packaged?*

        bacteria have about 3,000 genes                   &       1  chromosome (circular DNA molecule only)
        humans have some   22000+ genes              &     46  chromosomes

        Humans have 46 chromosomes:
               or  22 homologous autosomal pairs
     and  one pair of sex chromosomes (X & Y).

                 22 maternal chromosomes + one XX pair
22 paternal chromosomes  + one XY pair











Cell Division and the Cell Cycle
      2001 Nobel prize for cell cycle regulators

   Regulation is by
"Growth Factors
*" - proteins or steroid hormones that promote cell
            division, as epidermal growth factor (EGF) or platelet derived growth factor (PDGF)...

   MPF - is Mitotic Promoting Factor...  a complex* of two proteins  CDK  &  CYCLIN
                   MPF is a kinase enzyme, one that switches on/off target cell cycle
                    proteins by phosphorylating them (adds a phosphate group to them)...

[inactive cell cycle proteins---ATP ---> ADP---> active-protein-P
   MPF promotes entrance into mitosis from the G2 phase by phosphorylating multiple
            proteins during mitosis including one that leads to destruction of cyclin itself.

cdk - (cyclin dependent kinase) is a cell division control protein that is active 
                          only when bound to cyclin;  

   cyclin - a protein whose amount varies cyclically* throughout interphase;
  when in high concentration cyclin binds to cdk activating the MPF...  
                                 [cyclin + cdk = MPF]...  favors the initiation of Mitosis

   Growth Factors proteins regulate at critical...   Cell cycle checkpoints* 











   "MEIOSIS"...  is nuclear division phase of sexually dividing GAMETES
   Compare physical differences
* between nuclear divisions of MEIOSIS & MITOSIS

   thus the distinct differences are:
                                                     Key Concepts*
meiosis  =  homologous chromosome pair in prophase-1
            mitosis   =  homologs do not pair at all

            meiosis  =  4 progeny cells  [1 = 2 = 4]... 
thus  2  cell divisions
            mitosis   =  2 daughter cells only...           
thus  1  cell division

            meiosis  =  one-half # of chromosomes                     (haploid)
            mitosis   =  same # of chromosomes as parent cell   (diploid)

            meiosis  =  new combinations of genes not parents in parents 
                                      & chromosomes sort randomly of each other

            mitosis  =  daughter cells are genetically identical

                              Overview of Meiosis Animation*










Stages of the Sexual Cell Division Life Cycle have same names as asexual division:

              Interphase,              MEIOSIS (nuclear division),            Cytokinesis
                   but,  there are 2 Divisions    Meiosis I       and      Meiosis II
                                                                   1 cell   =   2 cell   =   4 cells 
   Meiosis I...                                                  
             Prophase I     =  chromosomes condense...   what's new ?
* - homologs PAIR together ---> tetrad
* - (animation*) - exchange occurs at a chiasma
             Metaphase I   =  chromosomes align at equator                                 
             Anaphase I    =  chromosomes migrate toward poles
             Telophase I    =  chromosome at poles - cell domains separate

                           Meiosis I separates homologs of homologous pair


   Meiosis II...   is like mitosis [ P-M-A-T, but without an S phase]            
Meiosis II  separates chromatids of one homolog of a homologous pair.
                          Stages of Meiosis I & II
* (fig 13.10) of Mitosis/Meiosis 
             An evolutionary advantage is that Meiosis provides for lots of Genetic Variation 

                          Independent Assortment
* is random alignment of homologous pairs
                          Crossing Over
- [figure*] is exchange of chromosome material
                            Meiosis results in significant Genetic Variation
Summary of Meiosis animation*view@home  

    next page                                                  





Summary of MEIOSIS
... sexual cell division   
1.  Nuclear division phase of sexual cell reproduction cycle
2.  Two successive divisions, results in 4 daughter cells...
               Meiosis 1  & 
Meiosis 2
3.  Reduction/division occurs...    from  diploid  ---->  haploid
             daughter cells ½ number of parental cell chromosomes
4.  Stages have same nomenclature as Mitosis
               prophase, metaphase, anaphase, telophase = M1 & M
5.  Often only one S phase, where DNA is duplicated
               often may be no interphase between M1 & M
6.  Homologous pairs separate in Meiosis 1
               Chromatids separate in Meiosis 2 (mitotic-like)
7.  Random Assortment occurs......  homologs align 
               at equitorial plates independent of each other
8.  Crossing over... may occur in Prophase I...
               synapsis = close pairing homologs allows exchange
               chiasma = point exchange of sister chromatids
                                                   a table of differences








Evolutionary consequences of sexual cell division...
new gene/chromosome combos
     that did not exist in either parent alone,
     which will become the stuff of evolution
dancing baby  
Why sex may have evolved?

Evolutionarily, sex may not be advantageous. Asexually dividing  organisms pass
100% of their genome to offspring, but sexual reproduction passes only half (50%) the genome of an individual. However, it does provide genetically variability.

Evolutionary biologists tracked a multicellular eukaryote (monogonont
rotifers-  microscopic, aquatic invertebrates) which can reproduce both sexually and asexually) for nearly 100 generations and found that the species was more likely to switch from asexual to sexual reproduction, if it encountered varying physical settings, suggesting sex may help species adapt to diverse environments.

Thus sex seems more likely to evolve in
heterogenous environments
, because it more rapidly creates new gene combos that may have an advantage.

               back      next lecture                                    Key Concepts*

                      copyright c2023    last update - April 2023        
                        Charles Mallery,    Biology 150, Department of Biology,   U. of Miami,  Coral Gables, FL 33124








  Sexual Cell Reproduction   (Meiosis - nuclear division phase) 

      Only specialized sex cells [GAMETES] can undergo meiosis...  

             Meiosis  --->  produces  cells with
half chrm # = 23 giving human sperm & egg
             Fertilization   (sperm + egg)  --->  development of an organism...

      where does meiosis (nuclear division) occur during the sexual cell division cycle ?
            cell generations are defined by cell DNA content (chromosome numbers present)...

Diploid generation life cycle (alternates with haploid generation)*---> (chrm # = 46)
Alternation of Generations in Plants
*      &       Human life Cycle*     

fig 13.7* 

                        Duplication of DNA during Meiosis and Mitosis*  

               etritium labelling experiments*