Name:_________________________
Meiosis Coloring (10.1)
As studies of sexual reproduction showed that egg and sperm nuclei fuse together in fertilization to form a single
composite nucleus, it became obvious that at some point there must be a mechanism by which the cell reduces
the number of chromosomes by half when such gametes are produced. Otherwise the number of chromosomes
would double with each generation, and cells would soon have to double in size with each generation to have
room for all the chromosomes. In 1890 Hertwig and Boveri discovered that eggs and sperm in animals are
produced by a special kind of cell division that accomplishes that reduction. This type of cell division is called
meiosis (Greek: meioun, "to diminish").
Like mitosis, meiosis begins with the chromosomes in homologous pairs, with each chromosome consisting of
two chromatids. But instead of one division as in mitosis, producing two daughter cells with the same number of
chromosomes as the parent cell, meiosis involves two divisions in succession, resulting in four daughter cells,
each with only one chromosome of each homologous pair. Since there are two divisions, the four phases of
division are gone through twice in succession.
Color titles A through B2. Note that, for simplicity, the centrioles, aster, and spindle apparatus need not
be colored. Begin at the upper right and color the headings Early Prophase I and Late Prophase I and
the associated cells. Read the related text and continue in this manner, coloring the headings and reading the text.
Prophase I is much like the prophase of mitosis: the chromatin coils up to form chromosomes, a spindle is
formed, and the nucleoli and nuclear envelope disappear. However, unlike mitosis, where the chromosomes of
a homologous pair are randomly scattered within the nucleus, in meiosis the chromosomes undergo a process
called synapsis (Greek: "union"), in which each chromosome pairs up with its homologue (the one that is the
same size and shape) to form a compact bundle called a tetrad (Greek: "four," because each consists of four
chromatids). During synapsis, one chromatid of each homologous chromosome crosses over a chromatid of the
opposite homologous chromosome at one or more points. Such a crossover point is called a chiasma (Greek:
"crossing point"; plural, chiasmata). The other two chromatids of that tetrad usually do not cross over.
Chiasmata may persist into metaphase I. This crossing over turns out to have profound consequences for the
process of heredity, as we shall see a few plates from now.
Anaphase I is the same as in mitosis except that the two chromatids making up each chromosome do not
separate; instead, the chromosomes comprising a homologous pair become pulled to opposite ends of the cell,
so that each daughter cell ends up with only one chromosome of each pair, although each
chromosome still consists of two chromatids.
What follows next depends on the species. In some species there is a complete telophase I, including the formation of nuclear envelopes, and an intermediate phase called interkinesis, in which the chromosomes uncoil
completely as if in interphase. In other species, anaphase I proceeds directly into metaphase II. All gradations
between these two extremes are found in nature.
The second meiotic division is even more similar to mitosis than the first. If the chromosomes have uncoiled,
they coil back up in prophase II, and any nuclear envelope that was formed disintegrates again. If the spindle
dissolved, it is re-formed. In metaphase II, the chromosomes attach to the spindle tubules and line up in the
center of the cell. Anaphase II follows, in which the two chromatids of each chromosome are pulled apart by the
spindle tubules to form daughter chromosomes, which are then pulled to opposite ends of the cell. Last, in
telophase II, the spindle dissolves, the nucleoli and nuclear envelopes are formed, and four separate daughter
cells result.
In 1900 Mendel's work was independently rediscovered by three separate scientists and received the attention it
deserved. In 1902 a medical student named Walter Sutton compared this new information on genetics with what
had been discovered in the meantime about chromosomes in cell division and published his hypothesis that
genes must be carried on chromosomes, since they exactly matched the behavior of chromosomes. Both retain
their identity from generation to generation, both occur in pairs in most cells and are single in gametes, and both
are restored to the double number when two cells fuse in fertilization. Sutton also pointed out that the number of
genes must be many times greater than the number of chromosomes and correctly predicted that Mendel's law
of independent assortment would not apply to characteristics whose genes were carried on the same
chromosome pair.