Encyclopedia
In
biology,
meiosis is the process that allows one diploid cell to divide in a special way to generate haploid cells in
eukaryotes. The word "meiosis" comes from the Greek
meioun, meaning "to make smaller," since it results in a reduction in chromosome number.
Meiosis is essential for
sexual reproduction. It therefore occurs in most
eukaryotes, including single-celled organisms. A few
eukaryotes, notably the Bdelloid
rotifers, have lost the ability to carry out meiosis and acquired the ability to reproduce by
parthenogenesis. Meiosis does not occur in
archaea or prokaryotes, which reproduce by asexual cell division processes.
During meiosis, the genome of a diploid germ cell, which is composed of long segments of
DNA called
chromosomes, undergoes DNA replication followed by two rounds of division, resulting in haploid cells called gametes. Each gamete contains one complete set of
chromosomes, or half of the genetic content of the original cell. These resultant haploid cells can fuse with other haploid cells of the opposite gender or mating type during
fertilization to create a new diploid cell, or zygote. 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
genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic
blueprint encoded in its DNA. In other words, meiosis is the process that produces genetic variation.
Biochemically, meiosis uses some of the same 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, which enable them to separate from each other.
History
Meiosis was discovered and described for the first time in
sea urchin eggs in 1876, by noted German biologist Oscar Hertwig . It was described again in 1883, at the level of chromosomes, by
Belgian zoologist Edouard Van Beneden , in Ascaris worms' 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 observed
crossover in
Drosophila melanogaster meiosis and provided the first true genetic interpretation of meiosis.
Occurrence of meiosis in eukaryotic life cycles
Meiosis occurs in all 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.
The organism phase of the life cycle can occur between the haploid to diploid transition or the diploid to haploid transition. Some species are diploid, grown from a diploid cell called the zygote. Others are haploid instead, spawned by the proliferation and differentiation of a single haploid cell called the gamete. Humans, for example, are diploid creatures. Human stem cells undergo meiosis to create haploid gametes, which are
sperm cells for males or ova for females. These gametes then fertilize in the uterus of the female, producing a diploid zygote. The zygote undergoes progressive stages of mitosis and differentiation to create an
embryo, the early stage of human life.
There are three types of life cycles that utilise sexual reproduction, differentiated by the location of the organisms stage.
In the
gametic life cycle, of which humans are a part, the living organism is diploid in nature. Here, we will generalize the example of human reproduction stated previously. The organism's diploid germ-line stem cells undergo meiosis to create haploid gametes, 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 living organism is haploid. 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, and indeed its diagram supports this conclusion.
Process
Because meiosis is a "one-way" process, it cannot be said to engage in a
cell cycle that 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:
- Gap 1 phase: Characterized by increasing cell size from accelerated manufacture of organelles, proteins, and other cellular matter.
- Synthesis phase: The genetic material is replicated.
- Gap 2 phase: The cell continues to grow.
It is immediately followed by meiosis I, which divides one diploid cell into two haploid cells by the separation of homologous chromosomes, and meiosis II, which divides two haploid cells into four haploid cells by the separation of sister chromatids. Meiosis I and II are both divided into
prophase,
metaphase,
anaphase, and
telophase subphases, similar in purpose to their analogous subphases in the mitotic cell cycle. Therefore, meiosis encompasses the interphase , meiosis I , and meiosis II .
Meiosis I
Prophase I
In the
prophase stage, the cell's genetic material, which is normally in a loosely arranged pile known as
chromatin, condenses into visible threadlike structures. Along the thread, centromeres are visible as small beads of tightly coiled chromatin.
The first stage of Prophase I is the
leptotene stage, during which individual chromosomes begin to condense into long strands within the nucleus.
The
zygotene stage then occurs as the chromosomes approximately line up with each other into homologous chromosomes. The combined homologous chromosomes are said to be
bivalent. They may also be referred to as a
tetrad, a reference to the four sister chromatids. During this stage, one percent of DNA that wasn't replicated during S phase is replicated. The significance of this cleanup act is unclear.
The
pachytene stage heralds crossing over. Nonsister chromatids of homologous chromosomes randomly exchange segments of genetic information. Because the chromosomes cannot be distinguished in the synaptonemal complex, the actual act of crossing over is not perceivable through the microscope.
During the
diplotene stage, the synaptonemal complex degrades. Homologous chromosomes fall apart and begin to repel each other. The chromosomes themselves uncoil a bit, allowing some transcription of DNA. They are held together by virtue of
recombination nodules, betraying the sites of previous crossing over, the chiasmata.
Chromosomes recondense during the
diakinesis stage. Sites of crossing over entangle together, effectively overlapping, making chismata clearly visible. In general, every chromosome will have crossed over at least once. The nucleoli disappears and the nuclear membrane disintegrates into vesicles.
During these stages,
centrioles are migrating to the two poles of the cell. These centrioles, which were duplicated during interphase, function as microtubule coordinating centers. Centrioles sprout microtubules, essentially cellular ropes and poles, during crossing over. They invade the nuclear membrane after it 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 two kinetochores on each tetrad, one for each centrosome. Prophase I is the longest phase in meiosis.
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.
Metaphase I
As kinetochore microtubules from both centrioles attach to their respective kinetochores, the homologous chromosomes align equidistant above and below an imaginary equatorial plane, due to continuous counterbalancing forces exerted by the two kinetochores of the bivalent. Because of independent assortment, the orientation of the bivalent along the plane is random. Maternal or paternal homologues may point to either pole.
Anaphase I
Kinetochore microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, 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 further apart. The cell elongates in preparation for division down the middle.
Telophase I
The first meiotic division effectively ends when the centromeres 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.
Cells enter a period of rest known as interkinesis or interphase II. No DNA replication occurs during this stage. Note that many plants skip telophase I and interphase II, going immediately into prophase II.
Meiosis 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 are arranged by spindle fibres. The new equatorial plane is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plane.
In
metaphase II, the centromeres contain two kinetochores, organizing fibers from the centrosomes on each side. This is followed by
anaphase II, where the centromeres are cleaved, allowing the kinetochores to pull the sister chromatids apart. The sister chromatids by convention are now called sister chromosomes, and they are pulled toward opposing poles.
The process ends with
telophase II, which is similar to telophase I, marked by uncoiling, lengthening, and disappearance of the chromosomes occur as the disappearance of the microtubules. Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of four daughter cells, each with an haploid set of chromosomes. Meiosis is complete.
Significance of meiosis
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 than the zygotes from the previous generation. Successive generations would have an exponential increase in chromosome count, resulting in an unwieldy genome that would cripple the reproductive fitness of the species.
Polyploidy, the state of having three or more sets of chromosomes, also results in developmental abnormalities or lethality.
Most importantly, however, meiosis produces genetic variety in gametes that propagate to offspring. Recombination and independent assortment allow for a greater diversity of genotypes in the population. A system of creating diversity allows a species to maintain stability under environmental changes.
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 more or less of the usual amount of genetic material, 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, including:
- Down Syndrome - trisomy of chromosome 21
- Patau Syndrome - trisomy of chromosome 13
- Edward Syndrome - trisomy of chromosome 18
- Klinefelter Syndrome - an extra X chromosome in males
- Turner Syndrome - only one X chromosome present in females
- XYY Syndrome - an extra Y chromosome in males
- Triple X Syndrome - an extra X chromosome in females
Meiosis in humans
In females, meiosis occurs in precursor cells known as oogonia that divide twice into oocytes. These stem 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 is called oogenesis.
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 is called
spermatogenesis.
External links
See also
