Cell division:  mitosis and meiosis

I.  Cell division -- introduction

-  roles for cell division:

• reproduction -- unicellular organisms (mitosis)
• growth and tissue repair in multicellular organisms (mitosis)
• production of gametes, sex cells, for sexual reproduction (meiosis)

-  for an eukaryotic cell to divide a few events required:

• reproductive signal -- signal from outside the cell (but w/in organism) stimulates cell to divide/not divide
  • signals for cell division related to needs of entire organism
  • some cells mitotically active, others not
• replication of DNA within nucleus
• distribution of replicated DNA into two nuclei
• cytoplasmic division 

II.  The cell cycle

-  describes existence of cell -- interplay between periods of growth, periods of preparation for mitosis, periods of mitosis, relative to one another

- has two main phases, interphase and mitosis

A.  Interphase

- a cell spends most of its life in interphase

• G1 phase
• S phase
• Gap 2 phase

1.  G1 phase

• cell is performing its function in organism
• some cells arrested in G1 -- muscle, nerve
• other cells under appropriate stimulation begin preparations for S

2.  S-phase

• DNA replication

3.  G2 phase

• cell is making preparations for mitosis

B.  Mitosis -- M phase of cell cycle

• phase of nuclear division
• sorts genetic information into two cells
• ensures both contain exactly same genetic information

III.  Chromosomes

-  basic unit is a gigantic linear, double stranded molecule of DNA complexed with many proteins

-  after replication (S-phase) each chromosome consists of two chromatids joined at centromere

-  complex of DNA and proteins referred to as chromatin

-  during interphase chromatin is very diffuse:

• cell activity requires that portion of DNA unwound to interact with enzymes

-  prior to and during mitosis chromatin condenses:

• easier to achieve separation of replicated genetic material if DNA arranged in compact units rather than tangled and diffuse

III.  Mitosis

-  distributes exact copies of genetic information to two daughter cells

-  recall that DNA replicated in S-phase

-  also during S-phase the centrosome (microtubule organizing center) duplicates

-  still in interphase -- nuclear membrane visible, chromatin diffuse

A.  Prophase

-  microtubules (MT) emanate from centrosomes (mitotic centers)

• spindle develops

- chromatin coils and compacts 

• chromosomes visible
• each consists of identical, paired chromatids
• chromatids joined at centromere
• kinetochores develop in centromere region, one on each chromatid

B.  Prometaphase -- transition to metaphase

-  nuclear membrane disintegrates into small vesicles

• spindle invades the nuclear region

- microtubules attach to chromatids at kinetochore -- microtubules

• kinetochore of one chromatid attached to MT coming from one pole
• kinetochore of sister chromatid attached to MT coming from other pole

- some microtubules extend from pole to pole and do not attach to chromatids

C.  Metaphase

-  centromere regions connecting paired chromatids become aligned in a plane at cell equator

D.  Anaphase

-  centromere pairs separate

-  new chromosomes -- each containing one of paired chromatids begin to move toward poles

• kinetochores move along microtubules
• microtubules shorten from poles

-  microtubules that extend from pole to pole slide past each other pushing poles of spindle farther apart

•  thus achieve separation of one set of daughter chromosomes from the other

- 10 - 60 minutes to complete journey to poles

E.  Telophase

-  chromosomes stop moving  - reach poles

-  chromatin becomes diffuse

F.  Cytokinesis -- technically not part of mitosis

-  division of the cytoplasm

• initiated by microfilaments of actin and myosin beneath plasma membrane
• proteins interact to form a contraction -- cell pinches in two

Summary: 

• two daughter cells with identical genetic makeup
• thus reproduction by mitosis results in genetic constancy -- clone of offspring genetically identical to parent
  • any variation among offspring due to mutation

IV.  Homologous chromosomes

-  a cell from a specific organism contains a given number of paired chromosomes 

• human -- 23 pairs
• potato -- 24 pairs
• frog -- 13 pairs

-  in a given pair, one chromosome comes from one parent, other chromosome comes from other parent

- members of homologous pair similar in size, appearance, location of centromere

-  a cell that contains two homologs of each chromosome -- diploid cell

-  a cell that contains one homolog of each chromosome -- haploid cell

VI.  Meiosis

-  meiosis and the human life cycle

• two nuclear divisions that reduce the number of chromosomes to haploid number -- preparation for sexual reproduction
• promotes genetic diversity among products

A.  S-phase and late interphase

• DNA replicated
• centrosomes duplicated

B.  Prophase I

• chromatin condenses
• homologs join together and pair up -- synapse
  • chiasmata -- areas of crossing over between chromatids of adjacent homologous chromosomes
  • due to chiasmata exchange of genetic material between homologous chromosomes
• nuclear membrane disintegrates
• spindle in formation

C.  Metaphase I

-  homologous chromosome pairs line up at equatorial plate

• MT from one pole attaches to kinetochore of one chromosome
• MT from opposite pole attaches to kinetochore of homologous chromosome

D.  Anaphase I

-  homologous chromosomes move to opposite poles of cell

• same mechanism as in mitosis
• poles of cell move apart
• each chromosome still comprised of two joined chromatids
• each daughter cell haploid -- contains one set of chromosomes

E.  Telophase

• reformation of nucleus
• chromatin become diffuse
• cytokinesis occurs at some point

F.  Prophase II

• spindle formation
• disintegration of nuclear membrane
• MT attach themselves to kinetochores of sister chromatids

G.  Metaphase II

• chromosomes align themselves at equatorial plate

H.  Anaphase II

- chromatids separate and move towards opposite poles

I.  Telophase II

- chromosomes gather into nuclei, cells divide -- cytokinesis

J.  Summary of meiosis:

1.  formation of haploid cells -- allows for sexual reproduction to occur

2.  leads to genetic diversity:

-  sex cells produced by process different from each other

• synapsis allows maternal chromosome of a pair to interact with paternal one
  • after crossing over two sister chromatids of a given pair not genetically identical.
• which member of a homologous chromosome pair goes to which daughter cell is a matter of chance