Meiosis
The production of offspring by
sexual reproduction includes the fusion of two gametes, each with a complete
haploid set of chromosomes. Gametes are formed from specialized diploid cells.
This specialized kind of cell division that reduces the chromosome number by
half results in the production of haploid daughter cells. This kind of division
is called meiosis.
History
Meiosis was discovered and
described for the first time in sea
urchineggs in 1876 by the German biologist Oscar
Hertwig. It was described again
in 1883, at the level of chromosomes, by the Belgian zoologist Edouard Van Beneden, in Ascaris roundworm eggs. The significance of meiosis for
reproduction and inheritance, however, was described only in 1890 by German
biologist August Weismann, who noted that two cell divisions were necessary to
transform one diploid cell into four haploid cells if the number of chromosomes
had to be maintained. In 1911 the American geneticist Thomas Hunt Morgan detected crossovers in meiosis in the fruit fly Drosophila melanogaster, which helped to establish that genetic traits are
transmitted on chromosomes.
The term meiosis (originally
spelled "maiosis") was introduced to biology by J.B. Farmer and J.E.S. Moore in 1905.
We propose to apply the terms
Maiosis or Maiotic phase to cover the whole series of nuclear changes included
in the two divisions that were designated as Heterotype and Homotype by Flemming. It is derived
from the Greek word μείωσις, meaning 'Lessening'.
Meiosis occurs
Meiosis is actually a type of
cell division that occurs in the testes of males and in the ovaries of females.
Meiosis leads to the formation of cells bearing half the normal number of
chromosomes. The process is also referred to as spermatogenesis in males and
oogenesis in females. Meiosis does not occur in asexual organisms. It occurs
only in organisms that reproduce sexually.
All plants and animals undergo
meiosis. The gametes undergo fertilization to form a zygote or a fertilized
egg.
Process
The preparatory steps that
lead up to meiosis are identical in pattern and name to interphase of the
mitotic cell cycle.
Interphase is divided into
three phases:
·
Growth 1 (G1) phase: In this very active
phase, the cell synthesizes its vast array of proteins, including the enzymes
and structural proteins it will need for growth. In G1, each of the chromosomes
consists of a single linear molecule of DNA.
·
Synthesis (S) phase: The genetic
material is replicated; each of the cell's chromosomes duplicates to become two
identical sister chromatids attached at a centromere. This replication does not
change the ploidy of the cell since the centromere number remains the
same. The identical sister chromatids have not yet condensed into the densely
packaged chromosomes visible with the light microscope. This will take place
during prophase I in meiosis.
·
Growth 2 (G2) phase: G2 phase as seen
before mitosis is not present in meiosis. Meiotic prophase corresponds most
closely to the G2 phase of the mitotic cell cycle.
Interphase is followed by
meiosis I and then meiosis II. Meiosis I separates homologous chromosomes, each
still made up of two sister chromatids, into two daughter cells, thus reducing
the chromosome number by half. During meiosis II, sister chromatids decouple
and the resultant daughter chromosomes are segregated into four daughter cells.
For diploid organisms, the daughter cells resulting from meiosis are haploid
and contain only one copy of each chromosome. In some species, cells enter a
resting phase known as interkinesis between meiosis I and meiosis II.
Meiosis I and II are each
divided into prophase, metaphase, anaphase, and telophase stages, similar in purpose to their analogous
subphases in the mitotic cell cycle. Therefore, meiosis includes the stages of
meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II
(prophase II, metaphase II, anaphase II, telophase II).
Meiosis generates gamete genetic diversity in two ways:
(1)
independent
orientation of homologous chromosome pairs along the metaphase plate during
metaphase I and the subsequent separation of homologs during anaphase I allows
a randomandthe independent distribution of chromosomes to each daughter cell
(and ultimately to gametes).
(2)
Physical
exchange of homologous chromosomal regions by homologous recombination during
prophase I results in new combinations of DNA within chromosomes.During
meiosis, specific genes are more highly transcribed. In addition to strong meiotic stage-specific
expression of mRNA, there are also pervasive translational controls (e.g.
selective usage of preformed mRNA), regulating the ultimatemeiotic
stage-specific protein expression of genesduringmeiosis. Thus, both
transcriptional and translational controls determine the broad restructuring of
meiotic cells needed to carry out meiosis.
Meiosis Stages
Prophase
I
In prophase I of meiosis, the
following events occur:
·
Chromosomes condense and attach to the nuclear envelope.
·
Synapsis
occurs (a pair of homologous
chromosomes lines up closely
together)and a tetrad is formed. Each tetrad is
composed of four chromatids.
·
Genetic
recombination via crossing over
may occur.
- Chromosomes thicken and detach from the nuclear
envelope.
- Similar to mitosis, the centrioles
migrate away from one another and both the nuclear envelope and nucleoli
break down.
Metaphase
I
- Tetrads align at the metaphase plate.
- Note that the centromeres
of homologous chromosomes are oriented toward the opposite cell poles.
Anaphase I
In anaphase I of meiosis, the following events occur:
·
Chromosomes move to the opposite cell poles. Similar to mitosis, microtubules such as the kinetochore
fibers interact to pull the
chromosomes to the cell poles.
·
Unlike in
mitosis, sister
chromatids remain together
after the homologous
chromosomes move to opposite
poles.
At the end of anaphase I of meiosis, the cell enters into telophaseI.Telophase I
In telophase I of meiosis, the following events occur:
·
The spindle
fibers continue to move the homologous
chromosomes to the poles.
·
Once
movement is complete, each pole has a haploid number of chromosomes.
·
In most
cases, cytokinesis (division of the cytoplasm) occurs at the same time as telophase I.
·
At the end
of telophase I and cytokinesis, two daughter cells are produced, each with one-half
the number of chromosomes of the original parent cell.
Prophase II
In prophase II of meiosis, the following events occur:
- The nuclear membrane and nuclei break up while the
spindle
network appears.
- Chromosomes
do not replicate any further in this phase of meiosis.
- The chromosomes begin migrating to the metaphase
II plate (at the cell's equator).
Metaphase II
In metaphase II of meiosis, the following events occur:
·
The chromosomes line up at the metaphase II plate at the cell's
center.
·
The kinetochore
fibers of the sister
chromatids point toward
opposite poles.
At the end of metaphase II of meiosis, the cell enters into anaphase
II.Anaphase II:
In anaphase II of meiosis, the following events occur:
·
Sister
chromatids separate and begin
moving to opposite ends (poles) of the cell. Spindle
fibers not connected to
chromatids lengthen and elongate the cell.
·
Once the
paired sister chromatids separate from one another, each is considered a full chromosome. They are referred to as daughter
chromosomes.
·
In
preparation for the next stage of meiosis, the two cell poles also move further
apart during the course of anaphase II. At the end of anaphase II, each pole
contains a complete compilation of chromosomes.
In telophase II of meiosis, the following events occur:
·
Distinct nuclei form at the opposite poles.
·
Cytokinesis (division of the cytoplasm and the formation of two distinct cells) occurs.
·
At the end
of meiosis II, four daughter
cells are produced. Each cell
has one-half the number of chromosomes as the original parent cell.
Importance of meiosis
The advantage to meiosis is
that the genetic diversity it produces among sexual organisms can help make a
species population more stable by producing a wider variety of traits for the
process of natural selection to act upon. Meiosis relies upon processes that
are similar to those occurring during mitosis during cell division, but
several, such as recombination, occur only in meiosis.