WEB TUTORIAL 2.3
Meiosis
OVERVIEW
Meiosis takes place in reproductive (germ) cells and results in the production of gametes
or spores with a haploid number of chromosomes. This tutorial examines the steps
involved in meiosis and illustrates how genetic variation can result from this process.
TEXTBOOK REFERENCES
Meiosis Reduces the Chromosome Number from Diploid to Haploid in Germ Cells and
Spores (pp. 27-28)
Interphase and Meiosis
During the interphase before meiosis, the cell duplicates its chromosomes. Although the
chromosomes are dispersed and not yet distinctly visible at this stage, we show the chromosomes as distinct bodies for the sake of clarity.
Meiosis consists of two nuclear divisions designated meiosis I and meiosis II. Meiosis I can
be further subdivided into prophase I, metaphase I, anaphase I, and telophase I. Meiosis
II can be divided into prophase II, metaphase II, anaphase II, and telophase II.
Prophase I
The first stage of meiosis I is prophase I, which is further subdivided into leptonema,
zygonema, pachynema, diplonema, and diakinesis.
During leptonema, the chromatin begins to condense and the chromosomes become visible. For the example shown here with n = 2, there are 2n or 4 chromosomes, half paternal
(blue) and half maternal (red). In leptonema, a process called homology search begins that
will eventually lead to alignment of homologous pairs of chromosomes.
During zygonema, the chromosomes continue to shorten and thicken. Also, homologous
chromosomes enter into an initial alignment with each other, called rough pairing because
the homologs are still separated by a significant distance. The paired homologs are called
bivalents.
Also during zygonema, centrioles begin moving to polar positions and a spindle system
forms.
During pachynema, the chromosomes continue to coil and shorten, and the two members
of a bivalent become more closely paired. This close pairing is synapsis. During this stage,
the fact that the chromosomes are doubled first becomes visible. Each bivalent contains
four chromatids.
Each member of a duplicated chromosome is a sister chromatid. Chromatids from homologous chromosomes are nonsister chromatids. The four-chromatid structure is called a
tetrad; each tetrad contains two pairs of sister chromatids.
During diplonema, areas of contact called chiasmata appear between nonsister chromatids. Chiasmata are thought to represent points of chromosome exchange, or crossing
over. The exchanges occur in pachynema, but chiasmata are visible when the homologs
begin to separate slightly in diplonema. Crossing over leads to new combinations of genetic material and is a significant source of genetic diversity.
Also in diplonema, the centrioles move to the poles, a spindle system forms, and the
nuclear membrane breaks down.
Diakinesis is the final stage of prophase I. In diakinesis, the chromosomes separate further,
but nonsister chromatids remain associated loosely through the chiasmata.
In this final stage of prophase I, the nuclear envelope breaks down, spindle fibers similar
to those that we saw in mitosis penetrate the nuclear region, and the centromeres of each
tetrad become attached to the spindle fibers and begin moving the tetrads to the equatorial plane.
Metaphase I
With prophase I complete, steps similar to those observed in mitosis now occur. In
metaphase I, the chromosomes have become maximally shortened and thickened, and the
interaction of the tetrads with the spindle fibers moves them to the equatorial plane of the
cell.
During meiosis I, a single centromere holds each pair of sister chromatids together. As
metaphase I gives way to anaphase I, each tetrad is split, with one pair of sister chromatids
(a dyad) pulled to one pole and the other pair pulled to the opposite pole.
Anaphase I
The separation of chromosomes to opposite poles is called disjunction. At the completion
of anaphase I, a haploid number (n) of dyads arrives at each pole.
The alignment of each tetrad at the metaphase plate prior to anaphase I is random.
Therefore, when the tetrads split and move to the poles during anaphase I, four possible
combinations of paternal and maternal chromosomes can result. This is the genetic basis
for the principle of independent assortment.
Telophase I
In many organisms, a nuclear membrane begins to form around the dyads in telophase I,
and the cells enter a short intermeiotic period before beginning meiosis II. In other organisms, the cells go directly from anaphase I into the second meiotic division. In some cases
a short period of interphase may occur, but the chromosomes do not replicate.
Prophase II
In prophase II, each dyad consists of a pair of sister chromatids attached by a common centromere. In this second meiotic division (which is similar to mitosis), each gamete or spore
formed will receive exactly one chromatid from an original tetrad.
Prophase II begins with the breakdown of the nuclear membrane (if one re-formed in
telophase I), the movement of centrioles to opposite poles, the growth of spindle fibers and
their attachment to the centromeres, and the movement of chromosomes toward the equatorial plane.
Metaphase II
In metaphase II, the dyads have been moved to the equatorial plane by their interaction
with the spindle fibers.
Anaphase II
By anaphase II, the centromeres have divided and the sister chromatids from each dyad
are pulled to opposite poles.
Now it is clear that following cytokinesis, one member of each homologous chromosome
pair will end up in each new cell. Each such chromosome is called a monad.
Telophase II
After cytokinesis in telophase II, four haploid gametes result, with each having a haploid
number (n) of chromosomes. Notice that the haploid state results from a doubling of the
chromosome pair, followed by two divisions.
If any crossing over occurred in prophase I, the monads may be a combination of both
maternal and paternal genetic information (represented here by red and blue segments).
Thus, offspring will receive a mixture of genetic information originally present in the offspring's grandparents.
CONCLUSION
Although meiosis has many similarities to mitosis, meiosis results in the conversion of a
cell with a diploid number (2n) of chromosomes into a gamete or spore with a haploid
number (n) of chromosomes. This conversion occurs when a diploid cell undergoes the
two nuclear divisions that constitute meiosis, ultimately producing four gametes that each
contain one member of each homologous pair of chromosomes. Meiosis results in extensive genetic variation by virtue of chromosome exchange between maternal and paternal
chromatids during crossing over and their random segregation during the first meiotic
division. During sexual reproduction, these gametes combine to reconstitute the full
diploid complement of the parental cells.
YOU
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SHOULD NOW BE ABLE TO
Describe the events that occur during prophase I of meiosis.
Explain the role of chiasmata in the generation of genetic diversity.
Explain how the separation of chromosomes during metaphase I illustrates the
principle of independent assortment.
Compare and contrast the events of meiosis I and meiosis II.
KEY TERMS
anaphase
cell cycle
cell plate
centriole
centromere
cytokinesis
G0 phase
G1 phase
G2 phase
interphase
kinetochore
metaphase
metaphase plate
prometaphase
prophase
S phase
spindle fibers
telophase