In
biology,
meiosis ( ) is
a process of reductional division in which the number of
chromosomes per cell is cut in half. In animals,
meiosis always results in the formation of
gametes, while in other organisms it can give rise to
spores. As with
mitosis, before meiosis begins, the DNA in the
original cell is replicated during
S-phase
of the cell cycle. Two cell divisions separate the replicated
chromosomes into four
haploid gametes or
spores.
Meiosis is essential for
sexual
reproduction and therefore occurs in all
eukaryotes (including single-celled organisms)
that reproduce sexually. A few eukaryotes, notably the
Bdelloid rotifers, have
lost the ability to carry out meiosis and have acquired the ability
to reproduce by
parthenogenesis.
Meiosis does not occur in
archaea or
bacteria, which reproduce via asexual
processes such as
binary
fission.
During meiosis, the
genome of a
diploid germ cell, which is
composed of long segments of
DNA packaged into
chromosomes, undergoes DNA replication
followed by two rounds of division, resulting in four
haploid cells. Each of these cells contains one
complete set of
chromosomes, or half of
the genetic content of the original cell. If meiosis produces
gametes, these cells must fuse during
fertilization to create a new diploid cell, or
zygote before any new growth can occur. Thus,
the division mechanism of meiosis is a reciprocal process to the
joining of two genomes that occurs at fertilization. Because the
chromosomes of each parent undergo
homologous recombination during
meiosis, each gamete, and thus each zygote, will have a unique
genetic
blueprint encoded in its DNA. Together, meiosis
and fertilization constitute sexuality in the eukaryotes, and
generate genetically distinct individuals in populations.
In all plants, and in many protists, meiosis results in the
formation of haploid cells that can divide vegetatively without
undergoing fertilization, referred to as spores. In these groups,
gametes are produced by mitosis.
Meiosis uses many of the same biochemical mechanisms employed
during
mitosis to accomplish the
redistribution of chromosomes. There are several features unique to
meiosis, most importantly the pairing and recombination between
homologous chromosomes.
Meiosis comes from the root -meio, meaning less.
History
Meiosis was discovered and described for the first time in
sea urchin egg in
1876, by noted German biologist
Oscar
Hertwig (1849–1922). It was described again in 1883, at the
level of chromosomes, by
Belgian zoologist
Edouard Van Beneden (1846–1910), in
Ascaris worms' eggs.
The significance of
meiosis for reproduction and inheritance, however, was described
only in 1890 by German
biologist
August Weismann (1834–1914), 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 (1866–1945)
observed crossover in
Drosophila
melanogaster meiosis and provided the first genetic
evidence that genes are transmitted on chromosomes.
Evolution
Meiosis is thought to have appeared 1.4 billion years ago. The only
supergroup of
eukaryotes which does not
have meiosis in all organisms is
excavata.
The other five major supergroups,
opisthokonts,
amoebozoa,
rhizaria,
archaeplastida and
chromalveolates all seem to have genes for
meiosis universally present, even if not always functional. Some
excavata species do have meiosis which is consistent with the
hypothesis that this group is an ancient,
paraphyletic grade. An example of eukaryotic
organism in which meiosis does not exist is
euglenoid.
Occurrence of meiosis in eukaryotic life cycles
Gametic life cycle.
Zygotic life cycle.
Sporic life cycle.
Meiosis occurs in eukaryotic life cycles involving
sexual reproduction, comprising of the
constant cyclical process of meiosis and fertilization. This takes
place alongside normal
mitotic cell
division. In multicellular organisms, there is an intermediary step
between the diploid and haploid transition where the organism
grows. The organism will then produce the
germ
cells that continue in the life cycle. The rest of the cells,
called
somatic cells, function within
the organism and will
die with it.
Cycling meiosis and fertilization events produces a series of
transitions back and forth between alternating haploid and diploid
states. The organism phase of the life cycle can occur either
during the diploid state (
gametic or
diploid life
cycle), during the haploid state (
zygotic or
haploid life cycle), or both (
sporic or
haplodiploid life cycle, in which there two distinct
organism phases, one during the haploid state and the other during
the diploid state). In this sense, there are three types of life
cycles that utilize sexual reproduction, differentiated by the
location of the organisms phase(s).
In the
gametic life cycle, of which humans are a part, the
species is diploid, grown from a diploid cell called the
zygote. The organism's diploid germ-line stem cells
undergo meiosis to create haploid gametes (the
spermatozoa for males and
ova
for females), which fertilize to form the zygote. The diploid
zygote undergoes repeated cellular division by
mitosis to grow into the organism. Mitosis is a
related process to meiosis that creates two cells that are
genetically identical to the parent cell. The general principle is
that mitosis creates somatic cells and meiosis creates germ
cells.
In the
zygotic life cycle the species is haploid instead,
spawned by the proliferation and differentiation of a single
haploid cell called the
gamete. Two organisms
of opposing gender contribute their haploid germ cells to form a
diploid zygote. The zygote undergoes meiosis immediately, creating
four haploid cells. These cells undergo
mitosis to create the organism. Many
fungi and many
protozoa are
members of the zygotic life cycle.
Finally, in the
sporic life cycle, the living organism
alternates between haploid and diploid states. Consequently, this
cycle is also known as the
alternation of generations. The
diploid organism's germ-line cells undergo meiosis to produce
gametes. The gametes proliferate by mitosis, growing into a haploid
organism. The haploid organism's germ cells then combine with
another haploid organism's cells, creating the zygote. The zygote
undergoes repeated mitosis and differentiation to become the
diploid organism again. The sporic life cycle can be considered a
fusion of the gametic and zygotic life cycles.
Process
Because meiosis is a "one-way" process, it cannot be said to engage
in a
cell cycle as mitosis does. However,
the preparatory steps that lead up to meiosis are identical in
pattern and name to the interphase of the mitotic cell cycle.
Interphase is divided into three phases:
- Growth 1 phase: This
is a very active period, where the cell synthesizes its vast array
of proteins, including the enzymes and structural proteins it will
need for growth. In G1 stage each of the chromosomes
consists of a single (very long) molecule of DNA. In humans, at
this point cells are 46 chromosomes, 2N, identical
to somatic cells.
- Synthesis phase: The
genetic material is replicated: each of its chromosomes duplicates,
producing 46 chromosomes each made up of two sister chromatids. The
cell is still considered diploid because it still contains the same
number of centromeres. 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 phase:
G2 phase is absent in Meiosis
Interphase is followed by meiosis I and then meiosis II. Meiosis I
consists of separating the pairs of
homologous chromosome, each made up of
two sister chromatids, into two cells. One entire haploid content
of chromosomes is contained in each of the resulting daughter
cells; the first meiotic division therefore reduces the ploidy of
the original cell by a factor of 2.
Meiosis II consists of decoupling each chromosome's sister strands
(
chromatids), and segregating the
individual chromatids into haploid daughter cells. The two cells
resulting from meiosis I divide during meiosis II, creating 4
haploid daughter cells. 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 genetic diversity in two ways: (1) independent
alignment and subsequent separation of homologous chromosome pairs
during the first meiotic division allows a random and independent
selection of each chromosome segregates into each gamete; and (2)
physical exchange of homologous chromosomal regions by homologous
recombination during prophase I results in new combinations of DNA
within chromosomes.
A diagram of the meiotic phases
Meiosis-phases
Meiosis I
Meiosis I separates homologous chromosomes, producing two haploid
cells (
23 chromosomes, N in humans), so meiosis I
is referred to as a
reductional division. A
regular diploid human cell contains 46 chromosomes and is
considered 2N because it contains 23 pairs of homologous
chromosomes. However, after meiosis I, although the cell contains
46 chromatids it is only considered as being N, with 23
chromosomes, because later in anaphase I the sister chromatids will
remain together as the spindle pulls the pair toward the pole of
the new cell. In meiosis II, an
equational
division similar to mitosis will occur whereby the sister
chromatids are finally split, creating a total of 4 haploid cells
(
23 chromosomes, N) per daughter cell from the
first division.
Prophase I
During prophase I, DNA is exchanged between
homologous chromosomes in a process
called
homologous
recombination. This often results in
chromosomal crossover. The new
combinations of DNA created during crossover are a significant
source of
genetic variation, and
may result in beneficial new combinations of
alleles. The paired and replicated chromosomes are
called bivalents or tetrads, which have two chromosomes and four
chromatids, with one chromosome coming
from each parent. At this stage, non-sister chromatids may
cross-over at points called chiasmata (plural; singular
chiasma).
Leptotene
The first stage of prophase I is the
leptotene stage, also
known as
leptonema, from Greek words meaning "thin
threads". During this stage, individual chromosomes begin to
condense into long strands within the nucleus. However the two
sister chromatids are still so tightly bound that they are
indistinguishable from one another.
Zygotene
The
zygotene stage, also known as
zygonema, from
Greek words meaning "paired threads", occurs as the chromosomes
approximately line up with each other into homologous chromosomes.
This is called the bouquet stage because of the way the telomeres
cluster at one end of the nucleus. At this stage, the synapsis
(pairing/coming together) of homologous chromosomes takes
place.
Pachytene
The
pachytene stage, also known as
pachynema,
from Greek words meaning "thick threads", contains the following
chromosomal crossover.
Nonsister chromatids of homologous chromosomes randomly exchange
segments of genetic information over regions of homology.
Sex chromosomes, however, are not wholly
identical, and only exchange information over a small region of
homology. Exchange takes place at sites where
recombination
nodules (the chiasmata) have formed. The exchange of
information between the non-sister chromatids results in a
recombination of information; each chromosome has the complete set
of information it had before, and there are no gaps formed as a
result of the process. Because the chromosomes cannot be
distinguished in the synaptonemal complex, the actual act of
crossing over is not perceivable through the microscope.
Diplotene
During the
diplotene stage, also known as
diplonema, from Greek words meaning "two threads", the
synaptonemal complex degrades
and homologous chromosomes separate from one another a little. The
chromosomes themselves uncoil a bit, allowing some
transcription of DNA. However, the
homologous chromosomes of each bivalent remain tightly bound at
chiasmata, the regions where crossing-over occurred. The chiasmata
remain on the chromosomes until they are severed in Anaphase
I.
In human fetal
oogenesis all developing
oocytes develop to this stage and stop before birth. This suspended
state is referred to as the
dictyotene
stage and remains so until
puberty.
In males, only
spermatogonia (
spermatogenesis) exist until meiosis begins
at puberty.
Diakinesis
Chromosomes condense further during the
diakinesis stage,
from Greek words meaning "moving through". This is the first point
in meiosis where the four parts of the tetrads are actually
visible. Sites of crossing over entangle together, effectively
overlapping, making chiasmata clearly visible. Other than this
observation, the rest of the stage closely resembles
prometaphase of mitosis; the
nucleoli disappear, the
nuclear membrane disintegrates into
vesicles, and the meiotic spindle begins to form.
Synchronous processes
During these stages, two
centrosomes,
containing a pair of
centrioles in animal
cells, migrate to the two poles of the cell. These centrosomes,
which were duplicated during S-phase, function as
microtubule organizing centers nucleating
microtubules, which are essentially cellular ropes and poles. The
microtubules invade the nuclear region after the nuclear envelope
disintegrates, attaching to the chromosomes at the
kinetochore. The kinetochore functions as a
motor, pulling the chromosome along the attached microtubule toward
the originating centriole, like a train on a track. There are four
kinetochores on each tetrad, but the pair of kinetochores on each
sister chromatid fuses and functions as a unit during meiosis
I.
Microtubules that attach to the kinetochores are known as
kinetochore microtubules. Other microtubules will interact
with microtubules from the opposite centriole: these are called
nonkinetochore microtubules or
polar
microtubules. A third type of microtubules, the aster
microtubules, radiates from the centrosome into the cytoplasm or
contacts components of the membrane skeleton.
Metaphase I
Homologous pairs move together along the metaphase plate:As
kinetochore microtubules from both centrioles
attach to their respective kinetochores, the homologous chromosomes
align along an equatorial plane that bisects the spindle, due to
continuous counterbalancing forces exerted on the bivalents by the
microtubules emanating from the two kinetochores of homologous
chromosomes. The physical basis of the independent assortment of
chromosomes is the random orientation of each bivalent along the
metaphase plate, with respect to the orientation of the other
bivalents along the same equatorial line.
Anaphase I
Kinetochore microtubules shorten, severing the
recombination nodules and pulling homologous chromosomes apart.
Since each chromosome has only one functional unit of a pair of
kinetochores, whole chromosomes are pulled toward opposing poles,
forming two haploid sets. Each chromosome still contains a pair of
sister chromatids. Nonkinetochore microtubules lengthen, pushing
the centrioles farther apart. The cell elongates in preparation for
division down the center.
Telophase I
The last meiotic division effectively ends when the chromosomes
arrive at the poles. Each daughter cell now has half the number of
chromosomes but each chromosome consists of a pair of chromatids.
The microtubules that make up the spindle network disappear, and a
new nuclear membrane surrounds each haploid set. The chromosomes
uncoil back into chromatin. Cytokinesis, the pinching of the cell
membrane in animal cells or the formation of the cell wall in plant
cells, occurs, completing the creation of two daughter cells.
Sister chromatids remain attached during telophase I.
Cells may enter a period of rest known as interkinesis or
interphase II. No DNA replication occurs during this stage.
Meiosis II
Meiosis II is the second part of the meiotic process. Much of the
process is similar to mitosis. The end result is production of four
haploid cells (
23 chromosomes, 1N in humans) from
the two haploid cells (
23 chromosomes, 1N * each
of the chromosomes consisting of two sister chromatids) produced in
meiosis I. The four main steps of Meiosis II are: Prophase II,
Metaphase II, Anaphase II, and Telophase II.
Prophase II takes an
inversely proportional time compared to
telophase I. In this prophase we see the disappearance of the
nucleoli and the
nuclear envelope
again as well as the shortening and thickening of the chromatids.
Centrioles move to the polar regions and arrange spindle fibers for
the second meiotic division.
In
metaphase II, the centromeres contain two
kinetochores that attach to spindle fibers from the centrosomes
(centrioles) at each pole. The new equatorial metaphase plate is
rotated by 90 degrees when compared to meiosis I, perpendicular to
the previous plate.
This is followed by
anaphase II, where the
centromeres are cleaved, allowing microtubules attached to the
kinetochores to pull the sister chromatids apart. The sister
chromatids by convention are now called sister chromosomes as they
move toward opposing poles.
The process ends with
telophase II, which is
similar to telophase I, and is marked by uncoiling and lengthening
of the chromosomes and the disappearance of the spindle. Nuclear
envelopes reform and cleavage or cell wall formation eventually
produces a total of four daughter cells, each with a haploid set of
chromosomes. Meiosis is now complete and ends up with four new
daughter cells.
Significance
Meiosis facilitates stable sexual reproduction. Without the halving
of
ploidy, or chromosome count, fertilization
would result in zygotes that have twice the number of chromosomes
as the zygotes from the previous generation. Successive generations
would have an exponential increase in chromosome count. In
organisms that are normally diploid,
polyploidy, the state of having three or more
sets of chromosomes, results in extreme developmental abnormalities
or lethality . Polyploidy is poorly tolerated in most animal
species. Plants, however, regularly produce fertile, viable
polyploids. Polyploidy has been implicated as an important
mechanism in plant speciation.
Most importantly, recombination and independent assortment of
homologous chromosomes allow for a greater diversity of genotypes
in the population. This produces genetic variation in gametes that
promote genetic and phenotypic variation in a population of
offspring.
Nondisjunction
The normal separation of chromosomes in meiosis I or sister
chromatids in meiosis II is termed
disjunction. When the
separation is not normal, it is called
nondisjunction. This results in the production of
gametes which have either too many of too few of a particular
chromosome, and is a common mechanism for
trisomy or
monosomy.
Nondisjunction can occur in the meiosis I or meiosis II, phases of
cellular reproduction, or during
mitosis.
This is a cause of several medical conditions in humans (such as):
Meiosis in mammals
In females, meiosis occurs in cells known as
oogonia (singular: oogonium). Each oogonium that
initiates meiosis will divide twice to form a single
oocyte and two
polar
bodies. However, before these divisions occur, these cells stop
at the diplotene stage of meiosis I and lay dormant within a
protective shell of somatic cells called the
follicle. Follicles begin growth at a
steady pace in a process known as
folliculogenesis, and a small number enter
the
menstrual cycle. Menstruated
oocytes continue meiosis I and arrest at meiosis II until
fertilization. The process of meiosis in females occurs during
oogenesis, and differs from the typical
meiosis in that it features a long period of meiotic arrest known
as the
Dictyate stage and lacks the
assistance of
centrosomes.
In males, meiosis occurs in precursor cells known as spermatogonia
that divide twice to become sperm. These cells continuously divide
without arrest in the
seminiferous
tubules of the
testicles.
Sperm is produced at a steady pace. The process of
meiosis in males occurs during
spermatogenesis.
In female mammals, meiosis begins immediately after primordial germ
cells migrate to the ovary in the embryo, but in the males, meiosis
begins years later at the time of puberty. It is retinoic acid,
derived from the primitive kidney (mesonephros) that stimulates
meiosis in ovarian oogonia. Tissues of the male testis suppress
meiosis by degrading retinoic acid, a stimulator of meiosis. This
is overcome at puberty when cells within seminiferous tubules
called Sertoli cells start making their own retinoic acid.
Sensitivity to retinoic acid is also adjusted by proteins called
nanos and DAZL. Meoisis involves Spermatocytes.
See also
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