MITOSIS & MEIOSIS
The process of DEVELOPMENT that was examined earlier is
dependent on the processes of mitosis and meiosis that we will
explore now. Specifically, egg and sperm production occur via meiosis,
and the cell divisions that transform the fertilized egg into a morula,
blastula, gastrula, etc. occur via mitosis.
Preliminary: the Cell Cycle
A newly-created human cell, produced via the fertilization of an
egg or as a result of mitotic cell division, contains 46 chromosomes. At
this point, each chromosome contains a single piece of DNA complexed
with a vast number of protein molecules. The cell is said to be in the G1
phase of the cell cycle. After a period of "housekeeping" and/or growth,
the cell enters the S phase of the cycle. During this period, each
chromosome is replicated ; that is, an exact duplicate copy of each
piece of DNA is made, and newly-synthesized protein molecules
complex with the replicated DNA. The duplicate copies of the original
chromosomes remain attached to the originals via structures known as
centromeres. Thus, at the end of the S phase, a cell contains 46
replicated chromosomes, 46 centromeres, and 92 chromatids (originals
+ duplicates). The G2 phase of the cycle follows; it is another period of
"housekeeping" and/or growth. Together the G1, S, & G2 phases
constitute interphase, the time in which a cell is not engaged in either
of the cell division processes of mitosis or meiosis. The chromosomes
are not distinctly visible structures during interphase, so none of the
processes described above can actually be seen in the microscope. All
you will see is a uniformly-stained nucleus, with perhaps a nucleolus,
with a clearly defined nuclear envelope. (The cells that you scraped
from your mouth in the first lab were in interphase.)
When a cell completes the G2 phase, it has the option to proceed
into either mitosis or meiosis. In mitosis, the cell will divide to produce
two cells identical to each other and identical to the original cell; the two
daughter cells will be in the G1 phase, and the cycle can begin again.
In meiosis, the cell can divide twice to produce four cells; the products
are not identical to one another nor identical to the original cell ; they do
not re-enter the cell cycle, but develop into gametes (eggs or sperm).
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The gametes contain half the original chromosome number; e.g.,
human sperm contain 23 chromosomes.
Cells have one other option. A cell in the G1 phase might
"decide" to take on a specialized, differentiated function in the body. It
might, for example, become a neuron. Once this decision is made, the
cell exits the cycle to a non-dividing state called G0. A cell in G0 almost
never re-enters the cell cycle; neurons, for example, never divide.
The cell cycle is summarized below.
Mitosis
The purpose of mitosis, as stated above, is to produce two
daughter cells identical to each other and to the original cell in terms of
chromosome content. Mitosis, like meiosis, begins at the end of the G2
phase. In the first phase of mitosis, prophase, the nuclear envelope
disperses, and the nucleoli disappear. More importantly, the diffusely
distributed chromosomes begin to coil up into distinct structures which
become visible in the light microscope. The mitotic spindle, a network
of microtubules, assembles and prepares to guide the chromosomes to
opposite poles of the cell. Once the chromosomes have condensed,
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they move to the equator of the cell (equidistant from the two poles); this
is metaphase. At metaphase, the chromosomes are most easily
counted, and they can be seen to consist of two chromatids attached
at a centromere. In the next phase, anaphase, the sister chromatids
separate from one another as the centromeres split, and the
chromosomes (formerly chromatids) move toward the poles. Finally, in
telophase, the chromosomes begin to disperse, the nucleoli reappear,
and the nuclear envelopes reassemble. The two daughter nuclei are
partitioned into two cells as cytoplasmic division (cytokinesis) ensues.
Note that the daughter cells must be identical to one another because
identical sister chromatids moved to the opposite poles during
anaphase.
Mitosis can be observed easily wherever cells are dividing
rapidly. Since mitosis is virtually identical in all higher organisms, it can
be studied in experimental organisms, and the knowledge acquired is
applicable to humans. You will observe the stages of mitosis in onion
root tips and whitefish blastulae; the stages in humans are
indistinguishable from these.
Where would you look for rapidly dividing human cells if you
wanted to study mitosis in human cells directly?
Interphase: The interphase nucleus is round or oval in shape and
possesses a distinct nuclear envelope and nucleolus. The
chromosomes are dispersed.
Prophase: The prophase nucleus contains condensing chromosomes;
the nuclear envelope is disintegrating, as are the nucleoli.
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Interphase (onion root tip)
Prophase (onion root tip)
Metaphase: Chromosomes are aligned along the equator in preparation
for the splitting of the centromeres; spindle fibers are attached to
chromosomes.
Metaphase (onion root tip)
Metaphase (whitefish blastula)
Anaphase: Sister chromatids separate as distinct chromosomes and
move along spindle fibers toward poles.
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Anaphase (onion root tip)
Anaphase (whitefish blastula)
Telophase: Chromosomes at poles begin to disperse to interphase
condition, and the nuclear envelope and nucleoli reassemble.
Telophase (onion root tip)
Meiosis
The purpose of meiosis is to produce gametes with half the
number of chromosomes of the original cell . Meiosis is essential for
sexual reproduction in higher organisms - if gametes were produced by
mitosis, every generation would see a doubling of chromosome number
on the union of egg and sperm. Meiosis insures the constancy of
chromosome number from generation to generation. In addition,
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meiosis allows for the generation of new chromosome combinations,
generating novel organisms- you don't look exactly like your brothers
and/or sisters because of meiosis.
Consider a hypothetical organism formed by the union of egg and
sperm each carrying two chromosomes (as shown below). One
chromosome in each gamete is metacentric, with the centromere near
the middle, and the other is acrocentric, with the centromere near the
end. The two metacentric chromosomes are homologous, meaning
they are identical in shape and size and carry similar (though not
necessarily identical) information. As an example, one might carry a
blue eye gene, the other a brown eye gene. Similarly, the two
acrocentric chromosomes are homologous, with one coming from the
mother and the other from the father . Before either mitosis or meiosis
can occur, the cell must proceed through the S phase of the cell cycle
so that the chromosomes can be replicated. At the start of meiosis. in
prophase I, the chromosomes condense, becoming visible, then in
metaphase I, they align along the equator with the homologous
chromosomes in side-by-side pairs. By contrast, in mitotic metaphase,
the chromosomes lie unpaired, in random order, along the equator.
Refer to the comparison below.
Unfertilized
Egg
Sperm
Fertilized Egg
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Meiotic
Metaphase
Mitotic
Metaphase
Before proceeding further: What's the difference between
"homologous chromosomes" and "sister chromatids"?
Homologous chromosomes :
Sister chromatids :
In meiotic anaphase I, the homologous chromosomes separate
(segregate) and move to opposite poles, as shown below. By contrast,
in mitotic anaphase, sister chromatids separate and move to opposite
poles.
Meiotic Anaphase
Mitotic Anaphase
In meiotic telophase I, the chromosomes return briefly to the
dispersed state, just as in mitotic telophase. The cells then goes
through another division without chromosome replication, passing
through prophase II, metaphase II, anaphase II, and telophase II,
stages identical to mitotic division stages. These stages are illustrated
below.
Meiotic Telophase I
Mitotic Telophase
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Prophase II
Metaphase II
Anaphase II
Telophase II
Notice that the gametes which result from the two rounds of
meiotic division have half the number of chromosomes of the original
cell and that each gamete contains one chromosome of each type.
Now that you have been through descriptions of both mitosis and
meiosis, it's time to find out whether you really understand the two
processes. You have been provided with red and blue beads with which
to build chromosome models. Make a sperm containing three blue
chromosomes, one a small metacentric, another a larger metacentric,
and the third a large acrocentric. Then make an egg containing red
chromosomes homologous to those in the sperm. Fertilize the egg.
Replicate the chromosomes, attaching the sister chromatids using the
yellow centromeres. Align the chromosomes as in mitotic metaphase,
randomly along the equator (Make poles using pieces of chalk.). Split
the centromeres and move the chromosomes as in mitotic anaphase,
then divide the cell into two cells as in telophase.
What's the result? Are the two daughter cells identical to each
other and identical to the original cell?
Now try meiosis. Using the same fertilized egg, replicate the chromosomes and attach the sister chromatids via yellow centromeres.
Align the chromosomes as side-by-side homologous pairs as in meiotic
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metaphase I. Move homologous chromosomes away from one another
toward the poles without splitting any centromeres, then divide the cell
into two. In each cell, align the chromosomes randomly along the
equator, then split the centromeres and move the chromosomes (former
sister chromatids) toward the poles. Divide each cell into two.
What's the result? Does each gamete have half the number of
chromosomes found in the original cell? Does each gamete have
one representative of each chromosome type?
How does meiosis permit the generation of new chromosome
combinations? Try this: repeat the meiosis model as above, aligning all
of the red chromosomes to the left of center and the blue chromosomes
to the right of center at meiotic metaphase I. What kinds of gametes
result? (Compare with the original blue paternal and red maternal
gametes.) Repeat again, but switch one red chromosome from the left
to the right and place the homologous blue chromosome on the left at
meiotic metaphase I. What kinds of gametes result this time? Do you
think a cell knows which chromosomes are "red" or blue" as it aligns
them for meiosis? How many different kinds of gametes could be
produced in the model above?
Internet Resources
Animations illustrating mitosis and meiosis can be found at
http://www.biology.arizona.edu/cell_bio/cell_bio.html.
The National Center for Biotechnology Information has everything
you want to know (and much more!) about chromosomes and genes at
http://www.ncbi.nlm.nih.gov/genome/guide/human/.
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