Basis of Inheritance: Meiosis
Mitosis takes a diploid cell and creates a nearly exact
copy. Mitosis has two main functions: (1) it leads to the creation
of all of the somatic (body) cells in humans and other living organisms;
(2) in organisms that undergo asexual reproduction,
diploid parent cells undergo mitosis to create identical
daughter copies of themselves. Mitosis creates a daughter cell with chromosomes
that are identical to the chromosomes in its parent cell.
But humans and most other complex plants and animals each
have a unique set of chromosomes. This diversity of chromosomes
is the result of sexual reproduction, which involves
the contribution of the genetic material from not one, but two parents.
During sexual reproduction the father’s haploid sperm cell and the
mother’s haploid ovum (egg) cell fuse to form a single-celled diploid zygote that
then divides billions of times to form a whole individual.
In order for sexual reproduction to take place, however,
the parents first need to have haploid sperm or
ova, also called sex cells, germ cells,
or gametes. Meiosis is the name for the special type
of cell division that produces gametes.
Process of Meiosis
Unlike the single-cell division of mitosis, meiosis involves
two cellular divisions: meiosis I and meiosis II. Each stage of
meiosis runs through the same five stages as discussed in mitosis.
During the first round of division, two intermediate daughter cells
are produced. By the end of the second round of meiotic division
(meiosis II), the original diploid (2n) cell has
become four haploid (n) daughter cells.
Meiosis I is quite similar to mitosis. However, there
are a number of crucial differences between meiosis I and mitosis,
all of which will be outlined in the discussion of each individual
Just as in mitosis, the cell undergoes DNA replication
during this intermediate phase. After replication, the cell has
a total of 46 chromosomes, each made up of two sister chromatids
joined by a centromere.
The major distinction between mitosis and meiosis occurs
during this phase. In mitotic prophase, the double-stranded chromosomes
line up individually along the spindle. But in meiotic prophase
I, chromosomes line up along the spindle in homologous pairs. Then,
in a process called synapsis, the homologous pairs
actually join together and intertwine, forming a tetrad (two
chromosomes of two chromatids each, or four total chromatids). Often
this intertwining leads the chromatids of homologous chromosomes
to actually exchange corresponding pieces of DNA, a process called crossing-over or
genetic reassortment. Throughout prophase I, sister chromatids behave
as a unit and are identical except for the region where crossover
After prophase I, the meiotic cell enters metaphase I.
During this phase, the nuclear membrane breaks down, allowing microtubules
access to the chromosomes. Still joined at their crossover regions
in tetrads, the homologous pairs of chromosomes, with one maternal and
one paternal chromosome in each pair, align at the center of the
cell via microtubules, as in mitotic metaphase. The pairs align
in random order.
Anaphase I differs slightly from its mitotic counterpart.
In mitotic anaphase, sister chromatids split at their centromeres
and are pulled apart toward opposite poles. In contrast, during
anaphase I, the centromeres do not split: the entire maternal chromosome
of a homologous pair is pulled to one end, and the paternal chromosome
is pulled to the other end.
During telophase I, the chromosomes arrive at separate
poles and decondense. Nuclear membranes re-form around them. The
cell physically divides, as in mitotic cytokinesis.
The Product of Meiosis I
Meiosis I results in two independent cells. One cell contains
the maternal homologous pair, with a small segment of the paternal
chromosome from crossover. The other cell contains the paternal
homologous pair, likewise with a small segment of the maternal chromosome.
Despite the small region of crossover in the chromosomes of each
cell, the maternal sister chromatids are still quite
similar, as are the paternal sister chromatids. Both cells formed
by meiosis I contain a haploid amount of DNA.
The cells produced in meiosis I are different from those
produced in mitosis because both haploid members of the meiotic
pair derive from random assortments of either the maternal or paternal
chromosomes from each homologous pair (with the exception of the small
crossover sections). In mitosis, the cellular division separates
sister chromatids and results in diploid cells containing one maternal
and one paternal copy in each diploid pair.
The cells produced by meiosis I quickly enter meiosis
II. These cells do not undergo DNA replication
before entering meiosis II. The two cells that move from meiosis
I into meiosis II are haploid—each have 23 replicated chromosomes,
rather than the 46 that exist in cells entering both mitosis and
Meiotic division II occurs through the familiar phases
from meiosis I and mitosis. To distinguish the phases, they are
called prophase II, metaphase II, anaphase II, and telophase II.
One important difference between the events of meiosis I and II
is that no further genetic reassortment takes place during prophase
II. As a result, prophase II is much shorter than prophase I. In
fact, all of the phases of meiosis II proceed rapidly.
During meiosis II, chromosomes align at the center
of the cell in metaphase II exactly the way they do in mitotic metaphase.
In anaphase II, the sister chromatids separate, once again in the
same fashion as occurs in mitotic anaphase. The only difference
is that since there was no second round of DNA replication; only
one set of chromosomes exists. When the two cells split at the end
of meioisis II, the result is four haploid cells.
Of the four haploid cells, one cell is composed completely
of a maternal homologue, another of a maternal homologue with a
small segment of paternal DNA from crossover in meiosis I, another
complete paternal homologue, and a final paternal homologue with
a small segment of maternal DNA from crossover in meiosis I. These
four haploid cells are the gametes, the sperm or egg cells, that
fuse together in sexual reproduction to create new individuals.