MITOSIS AND MEIOSIS


     Verification of Mendel's theory of inheritance was
provided by cytologists in the early years of the 20th
century.  Microscopic analysis of reproductive cells
revealed the dynamics of gamete formation which provided the
cellular mechanism behind Mendel's two laws.  Gametes are
formed by the process of meiosis which we will now explore
and later relate directly to Mendels laws.  To assist your
understanding of the details of meiosis, I will contrast
this form of cell division with another (mitosis).


               Reproduction and Cell Division

     Although we usually think of reproduction as a process
involving adult organisms, it is best understood as a
process of cell division.  There are two different types of
cell division:  mitosis which occurs in all cells of a
multicellular organism and meiosis which is restricted to
reproductive cells.  Asexual reproduction, which results in
offspring genetically identical to the parent, is the result
of mitosis; sexual reproduction produces offspring which are
genetically variable due to the process of meiosis.

     In the nucleus of the cell are molecules of DNA which
contain the genes or hereditary material.  The DNA is
complexed with protein to form chromosomes.  Each cell in a
multicellular organism contains the same number of
chromosomes and the same number of genes as all of the other
cells in the organism and this number is characteristic of
the entire species to which that organism belongs.  The
number of different chromosomes, i.e., those containing
different sets of genes, is called the haploid number,
designated by the letter "n".  In our species, Homo sapiens,
n = 23 which means that each cell in our body contains 23
different or nonhomologous chromosomes.  In many species,
including ourselves, each chromosome is represented twice so
that the total number of chromosomes in each cell is 2n (the
diploid number).  The two representatives of the same
chromosome are called homologous chromosomes and homologous
chromosomes contain the same genes (loci).  Other species
may have more than two sets of chromosomes per cell and this
phenomenon is called polyploidy.  Species with three
homologous chromosomes are called triploids (3n), those with
four homologues are tetraploids (4n), etc.  We will confine
our discussion to diploid species which have only two
homologous chromosomes.
                 The process of mitosis

      The cell cycle describes the events which occur in the
life history of a eukaryotic cell and consists of two major
phases: a long interphase followed by a relatively short
division phase.  The product of division is two daughter
cells which then enter the interphase of their life cycle.
The interphase appears to be a period of no activity since
the choromosomes are not visible, but each of the three
parts or stages of interphase are characterized by cell
activity.  The G1 stage is the first stage of interphase and
is a period of growth.  Next is the S stage during which the
DNA is replicated.  The chromosomes are not visible because
they unwind from their coiled appearance during the division
cycle and when spread out they are metabolically active both
in RNA synthesis during the G1 or growth phase and in DNA
synthesis during the S stage.  After the DNA has been
replicated, the cell enters the last stage of interphase
during which the cell prepares for division.  This last
stage is called the G2 stage and it differs form the G1
stage in that the nucleus now contains twice the amount of
DNA it had in the G1 stage.

     The division part of the cell cycle has four distinct
phases: prophase, metaphase, anaphase and telophase.  During
prophase the chromosomes coil and so become visible again.
Since the DNA has already replicated, each chromosome
consists of two DNA molecules in the form of two sister
chromatids held together by a structure called the
centromere.  These chromosomes then line up in random order
in the center of the nucleus (metaphase).  The sister
chromatids then separate and migrate to separate halves of
the cell (anaphase).  The cell then divides into two equal
halves, each of which has the same number of chromosomes and
DNA molecules as the original parent cell when it was in its
G1 stage.  This final stage of the division cycle wherein
the cytoplasm divides to produce two genetically identical
daughter cells is called the telophase.

     Note that when the sister chromatids separate, they are
called chromosomes; so the term chromosome can refer to a
nucleoprotein complex with one DNA molecule, or to a
duplicated complex in the form of sister chromatids (each
chromatid consisting of one DNA molecule complexed with
protein).

                 The process of meiosis

     The process of cell division wherein gametes (haploid
sex cells) are produced from a diploid reproductive cell is
called meiosis.  Meiosis only occurs in reproductive cells,
i.e., in oocytes located in the female ovary which give rise
to eggs (ova) and in spermatocytes located in the male
testis which give rise to sperm or spermatozoa.  This
process is more complicated than mitosis because it involves
two division cycles which give rise to four gametes rather
than the two daughter cells produced by mitosis.  (In
spermatogenesis or the production of sperm, the cytoplasm
divides equally so that four equal-sized spermatozoa are
produced; in oogenesis the cytoplasm divides unequally so
that most of the cytoplasm ends up in one egg - the other
three products are called polar bodies because they consist
of inviable haploid nuclei with very little cytoplasm.)

     The differences between meiosis and mitosis can be
summarized as follows:

          l.  Meiosis involves two consecutive division
cycles so that four haploid gametes are
produced; mitosis involves a single division
cycle which gives rise to only two diploid
daughter cells.

          2.  The interphase, including DNA replication, is
the same for the first cycle of meiosis as was
described for mitosis, but unlike the
equational division of mitosis the first
division in meiosis is a reductional division
in which the two daughter cells are haploid,
not diploid.  This reductional division occurs
because before metaphase the homologous
chromosomes pair up in a process called
synapsis so that at metaphase the homologous
pairs line up rather than the individual
chromosomes as happens in mitosis.
Consequently, during anaphase of the first
meiotic division, the homologous chromosomes
separate rather than the sister chromatids of
individual chromosomes.  The key event that
must be understood is synapsis; if you
understand synapsis you understand meiosis!

          3.  The interphase between the first and second
divisions of meiosis is short and does not
involve the replication of DNA.  Since DNA is
replicated during the S phase of the
interphase before the first division, each
chromosome has already been duplicated and is
in the form of sister chromatids.  Note that
each of the two daughter cells resulting from
the first or reductional division of meiosis
has the same number of DNA molecules as the
parent reproductive cell but only half the
number of chromosomes.  Read the last sentence
again and be sure you understand it!  The
reason the cell has only half the number of
chromosomes is because the homologous pairs
separated and went to different daughter
cells.  The reason the number of DNA molecules
is the same is because each chromosome is in
the form of two sister chromatids.  The second
meiotic division is an equational division
like mitosis and like mitosis results in the
separation of the sister chromatids.  The
gametes produced after the second division of
meiosis have half the number of chromosomes
and half the number of DNA molecules as the
original parent reproductive cell.

     In asexual reproduction offspring are produced by
mitosis and so they are genetically identical to the
parents, but in sexual reproduction the offspring is the
product of gamete fusion (syngamy) from two genetically
different parents.  Each parent contributes one haploid
gamete and so syngamy restores the diploid number of
chromosomes.  Consequently, the diploid offspring differs
genetically from both of its parents.  Although syngamy may
involve the fusion of gametes of equal size (isogametes),
for the most part sexual reproduction is characterized by
the fusion of one large gamete (egg) and one small gamete
(sperm).  Individuals which produce many, small gametes are
called males and those producing few, large gametes are
females.  The fusion of an egg and a sperm (fertilization)
produces a diploid, one-celled organism called a zygote.
The zygote then divides by mitosis to produce a
multicellular organism.

       Meiosis, Mendel's Laws and Gametic Variation

     By producing variable gametes, the process of meiosis
is responsible for the tremendous amount of genetically
controlled individual variation due to sexual reproduction.
Although Mendel was unaware of the mechanics of cell
division, the process of meiosis explains why his laws
operate - the topic of our next discussion.