Meiosis

Meiosis is the biological cell Chromosome Halve the number of divisions. Cell division and chromosome copy only once, continuous cell division two times, this is a special way to halve the number of chromosomes of split. Meiosis is not only the guarantee mechanism of species stable chromosome number, but also the species to adapt to the evolving mechanism of environmental change. The results of meiosis is mature germ cell In less than half the chromosome number of primordial germ cells. Meiosis (Meiosis) range is sexual reproduction Biological; period is to mature germ cells from primordial germ cell development.
Meiosis
Meiosis refers to the sexual reproduction of the individual in the formation of a special division occurs during germ cell, different from Mitosis and Amitosis Meiosis occurs only in. life cycle It is a stage of sexual reproduction of the organism Meiocyte Mature, form gamete A special division in the process of. Be fertilized When the male and female gametes, restore parental chromosome number, chromosome number of the species in order to maintain a constant.

Meiosis Effect

In the process of meiosis and non homologous chromosomes Sister chromatid Between the exchange ( Gene recombination So, the gametes) inheritance Diversification, increased the adaptability of offspring on the environment, so it is not only to guarantee the mechanism of meiotic chromosome stability number of species, and species to adapt to the evolving mechanism of environmental change. Meiosis is not only to maintain a stable species genetic material transfer means; in the process of meiosis, the chromosomes of non sister chromatids crossing over (Association), non homologous chromosomes. Free combination as well as Four split In the non crossover segments of sister chromosomes, increased the gene mutation types, enhanced group Genetic diversity And provide more raw materials for natural selection......

Meiosis The main classification

Summary
Meiosis ( Meiosis The process is a copy of DNA), and continuous cell division two times form haploid Sperm and eggs (Figure 13-12), the fertilization Role again diploid (or polyploid ) process, chromosome and non sister chromatid exchange between non homologous meiotic process,. gamete The genetic diversity, increased offspring adaptability, so meiosis is not only the guarantee mechanism of biological stable chromosome number, but also the species to adapt to evolving mechanism of environmental change.
Meiosis can be divided into three main types:
Gametic meiosis
Gametic meiosis (gametic meiosis), also called Terminal meiosis (terminal meiosis), which is characterized by meiosis and gametogenesis together. In the male vertebrate In a Spermatogonial cells For Primary spermatocyte After meiosis, the process is: the primary spermatocyte division produces 2 copy Secondary spermatocyte 2, secondary spermatocyte again split, during DNA replication, the formation of a total of 4 sperm cells. After a series of spermatogenic cells Metamorphosis Mature, sperm . In female vertebrates, a Oocyte After replication of meiosis to form 1 first Polar body (small, the first polar body is usually not split into two of the second polar body) and 1 secondary oocytes (larger), The secondary oocyte Divided into an egg cell (larger) and a The second polar body (smaller), a total of the formation of an egg cell and two polar bodies (the last one egg cell, two polar body degradation).
Spore meiosis
Spore meiosis (sporic meiosis), also called intermediate meiosis (intermediate (meiosis). Intermediary meiosis In plants and some algae). Its characteristic is the meiosis and Gametogenesis There is no direct relationship between the formation of haploid meiosis is Gametophyte (microspore and megaspore). After two times of microspore mitosis contains a Vegetative nucleus And two Male gamete (sperm) mature pollen (male gametophyte), megaspore undergoes three mitosis embryo sac (female gametophyte), containing an egg nucleus, two Polar nuclei , 3 Antipodal cells And two Help cell .
Zygotic meiosis
Zygotic meiosis (zygotic meiosis), also called the initial meiosis (initial meiosis), found only in fungi and certain Prokaryote Meiosis occurs in. Zygote formation After the formation of haploid spores, spore by mitosis to produce new haploid offspring. In addition also has some biological somatic meiosis (somatic meiosis) phenomenon, such as mosquito larvae in the gut, some by Endomitoses The formation of polyploid Cells (up to 32X), in the pupal stage through meiosis reduces Chromosome ploidy The number of cells increased. By meiosis Close connection The two division of. usually Meiosis I Separation is Homologous chromosomes , heterotypic division (heterotypic Division) or meiosis (reductional division). Meiosis II The separation of sister chromosomes, similar to mitosis, so called homeotypic division (homotypic or division) Equal division (equational division). And mitosis in order to describe the convenience of meiosis is divided into several main stages and several sub periods.

Meiosis Splitting process

Note: meiosis can be divided into two stages, interval and Split phase The interval is divided into G1, S, and G2. Mitosis is divided into meiosis stage (minus one), second division (meiosis period minus two). In the range of knowledge in high school, a reduction of the 1980s and the early reduction of two can be regarded as the same time, we called it the end of a reduction. (minus one end and minus two during a short interval before it can be ignored)
 The process of meiosis The process of meiosis
The 1 cell division before the interval, DNA and chromosome replication, but the same number of chromosomes, two sister chromatids of each chromosome contains replication, the number of DNA to two times the original cell.
2 minus one early Homologous chromosomes The Federation Form. Four split (or "four body"), appeared spindle, nucleolus The nuclear membrane disappeared. Homologous nonsister chromatid interchange may occur.
3 minus one homologous chromosome metaphase. Centromere In the symmetrical arrangement Equatorial plate Both ends。 (and animal cell mitosis is roughly the same as that of animal cell mitotic kinetochores are arranged on the equatorial plate)
4 minus one later, the separation of homologous chromosomes, chromosome free combination, move to the cell poles.
5 minus one end cell One divides into two., formed the secondary spermatocyte or the formation of the secondary oocyte and The first polar body .
 In several meiotic division rules change curve In several meiotic division rules change curve
6 minus two early secondary spermatocytes (secondary Oocyte ) in the chromosome gathered again again Spindle .
7 minus two metaphase chromosome centromere in Equatorial plate On.
8 minus two chromosome centromere separation, chromosome shift The poles .
9 by the end of two cell One divides into two., secondary Spermatocyte formation Sperm cells The secondary oocyte formation. Egg cell And second Polar body .

Meiosis The specific process

The changes of meiosis project overview
Comparison project
Spermatogonial cells
Primary spermatocyte
Secondary spermatocyte
Sperm cells
interval
Early stage
Metaphase
Later stage
Late
Early stage
Metaphase
Later stage
Late
Chromosome variation
2N
2N
2N
2N
2N
N
N
N
2N
2N-N
N
DNA molecular changes
2A
2A-4A
4A
4A
4A
2A
2A
2A
2A
2A-A
A
Chromatid change
Zero
0-4n
4N
4N
4N
2n
2n
2n
Zero
Zero
Zero

The changes of meiosis project overview
Comparison project
Spermatogonial cells
Primary spermatocyte
Secondary spermatocyte
Sperm cells
interval
Early stage
Metaphase
Later stage
Late
Early stage
Metaphase
Later stage
Late
Chromosome variation
2N
2N
2N
2N
2N
N
N
N
2N
2N-N
N
DNA molecular changes
2A
2A-4A
4A
4A
4A
2A
2A
2A
2A
2A-A
A
Chromatid change
Zero
0-4n
4N
4N
4N
2n
2n
2n
Zero
Zero
Zero

Meiosis The first meiosis

interval
Can be divided into three stages: G1 stage, S stage and G2 stage. According to the research of modern cell biology, cell division were divided into three stages: the first stage of the gap, called G1; synthesis phase, called phase S; second gap period, called G2. The G1 and G2 phase is mainly related to protein synthesis and RNA, S to complete DNA replication.
G1
The characteristics of G1 phase: G1 period is from the last cell proliferation After the completion of the starting cycle. G1 is a growth period. The main RNA and protein biosynthesis in this period, and prepare for the next stage of S phase DNA synthesis. As the synthesis of various enzymes associated with DNA replication, mitochondria and ribosomes are increased, endoplasmic reticulum expansion in the update, from the endoplasmic reticulum Golgi body and lysosome were also increased. Animal cells 2 centrioles also separated from each other and start copying. That is to say S reserves of material and energy.
S
The characteristics of S phase from G1 phase to S phase is the key of cell proliferation. The main features of S is the synthesis of DNA, DNA molecular replication is carried out in this period. As long as the synthesis of DNA at first, carry on cell proliferation activities will be split into two sub cells, until.
G2
The characteristics of G2 phase: phase G2 also called "cell division period of preparation" because it is mainly behind the division phase ( M Prepare). In G2 phase, synthesis termination DNA, but also RNA and protein synthesis, but synthesis gradually decreased. Especially Tubulin The synthesis for mitotic phase (M phase) assembly of spindle microtubules provide raw materials. A complete copy of the 2 particles in the G2 particle phase center center.
Early stage
According to the chromosome morphology can be divided into 5 stages:
"Leptotene" (agglutination leptotene stage)
Slender, linear chromosomes in nucleus, nucleus and nucleolus Increase in size. Each chromosome contains two Sister chromatid .
"Zygotene" (zygotene pairing stage)
Within the cell Homologous chromosomes The 22 sides are closely matched, called this phenomenon The Federation . Because the pairing of a pair of homologous chromosomes in 4 Chromatid , known as the four split (or" Four of the body ").
"Pachytene" (recombinant pachytene stage)
Chromosome continuous thicker, at the same time, four points in the body Non sister chromatid Between the DNA fragment exchange, which leads to the parents swap genes, produce Gene recombination But, each chromatid still have identical genes.
"Diplotene" (synthetic diplotene stage)
Occurrence of chromatid crossing began to separate. The cross is often more than occurred in a site, therefore, V, X, present on chromosomes 8, O and other shapes.
"Diakinesis" (diakinesis recondensation stage)
A close and close to the state of aggregation around nuclear chromosome. Later, the nuclear membrane and nucleolus disappeared, finally formed Spindle .
Metaphase
Each pair of Homologous chromosomes Both moved to the center of the cell Equatorial plate The centromere, arranged in pairs on both sides of the equatorial plate and spindle formation in the cytoplasm.
 In the late period of meiosis In the late period of meiosis
Later stage
from Spindle The traction, the paired Homologous chromosomes Their separation, were moved to The poles .
Late
Arrive at the poles of the non homologous chromosomes and gathered to reproduce the nuclear envelope, nucleolus Then, the cell divides into two daughter cells. The chromosome number of the two sub cells, only half of the original. The new generation cells followed by a split second.
 Telophase Telophase
Be careful:
Is the first time in the 1 chromosome replication Interkinesis For, once the copy is complete, is called spermatogonia Primary spermatocyte .
2 a primary spermatocyte after the first meiotic division into two secondary spermatocytes, a Primary oocytes After the first meiosis to become a The secondary oocyte And the first polar body.
The first division of meiosis is to achieve the purpose of 3 Homologous chromosomes The separated (halving of chromosome number of each pair of sister chromatids with centromere binding, only a single chromosome number is two times the number of chromosomes). Halve the number of DNA molecules. (relative to the copy of it)
All sexual reproduction in plants and animals, from primordial germ cells (such as animal spermatogonia or oogonia) to mature germ cells (sperm or egg cell) in the process, must undergo meiosis. Meiosis is the cell division for two times, and in the process of splitting chromosome copy only once cell division mode. The results of meiosis, chromosome number in cells was reduced by half than the original.
The combined forming process of animal sperm and egg cells, the basic process about meiosis.
The process of sperm formation of sperm is formed in testis and testis in animal. The students have spermatogonia, each containing spermatogonia and somatic cells within the same chromosome number. A part of spermatogonia increased slightly, chromosome replication. At this time, spermatogonia and primary spermatocytes became. Primary spermatocyte after two consecutive cell divisions became mature sperm.
The process of spermiogenesis (Graphic)
The first division started soon, primary spermatocytes in 22 paired homologous chromosomes (two chromosomes paired shapes and sizes are generally the same, a father, a mother from the party, called the homologous chromosomes). 22 pairs of homologous chromosomes, called the federation. Then under the optical microscope can see clearly that each chromosome contains two sister chromatids, but these two sister chromatids are connected by a wire with a. At this time, each of a pair of homologous chromosomes contain four chromatids, this is called the four split.
Subsequently, all four points are arranged in the center of the cell, the kinetochore attached to the spindle on. Soon, as attached to the spindle with the centromere continuously shortening, four split into two (i.e. the synapsis of homologous chromosomes from each other), the traction by attachment of spindle cells, respectively to the poles. Then, the cell split, a primary spermatocyte divides into two secondary spermatocytes. So, half the number of chromosomes of secondary spermatocytes in only the original cell - primary spermatocytes (each chromosome still contains two sister chromatids, the centromere is still a). This is the first division of sperm formation process, in the course of cell division, chromosome number was reduced by half
Meiosis prophase of meiosis metaphase of meiosis I first anaphase telophase
It should be emphasized that: synapsis of homologous chromosomes separated, chromosome has a certain independence; two chromosomes each move to which one is random, that is to say, between different pairs of chromosome is free combination. The formation process of sperm and egg cells, has such characteristics.
The first division, followed by the second division. At this time, each chromosome two secondary spermatocytes in the centromere is split into two, two sister chromatids are separated completely, each with a centromere. In this way, the two sister chromatid has two chromosomes. The affects of spindle, the two chromosomes respectively to the cell poles. Then, the cell split, two secondary spermatocyte divides into four cells, four cells that contain only the number of sperm cells, reduced by half
Sperm cells after deformation, the formation of sperm. The sperm head contains a nucleus, a long tail, so the sperm can swim.
The formation process of egg egg, basically the same as the sperm formation process, the specific circumstances as shown below.
The formation process of egg cell
The egg cell is formed in the animal in the ovary. Ovary having ovogonium. Each oogonium and somatic cells contain the same number of chromosomes. Some of the egg cell volume increases, chromosome replication. At this time, the oogonia become primary oocytes. The first division started soon, the primary oocytes in homologous chromosomes undergo synapsis. The four split followed. Then, the completion of the first division. Split into two cells, ranging in size, large is called the secondary oocyte, called small polar bodies, they all have halved the number of chromosomes.
 Cell mitosis Cell mitosis
The secondary oocyte after a split, forming a large cell, this is the egg cell, and also formed a small cell, which is called the second polar body, but all the polar body after degradation. The egg cell contains only half of the decrease in the number of chromosomes, that is to say, half of the primary oocyte number of chromosomes in eggs.
Fertilization of sperm and egg cells are combined into a zygote process called fertilization. Fertilization is the head of the sperm into the egg cell, the tail on the outside. After the sperm head penetrates into the egg cell, its nucleus and egg nuclei together, so in zygotes from sperm chromosome and from the oocyte to chromosome and join together, half of them from sperm (father), half from the egg cell (the mother). In this way, the zygote returned to the number of chromosomes in somatic cells. Therefore, for the sexual reproduction of life, a constant number of sex chromosomes in meiosis and fertilization for each species before and after generation of somatic cells, the biological heredity and variation, are very important.

Meiosis The second division of meiosis

The second meiotic division and meiosis immediately, may also be a short pause. No chromosomal replication. Each chromosome centromere division, sister chromatids separate, respectively, move to the cell poles, sometimes accompanied by cell deformation.
Early stage
Similar to the first pre meiotic chromosome, first is scattered in the cell. Then again gathered, Nuclear membrane , nucleolus Disappear again, spindle form again.
Metaphase
Chromosome Centromere The cells are aligned to the central Equatorial plate On. Note that no longer exists Homologous chromosomes The.
Later stage
Each chromosome centromere separation, two Sister chromatid Will be separated, two chromosomes. In traction spindles, the two chromosomes were moved to the cell poles.
Late
To reproduce the nuclear membrane and nucleolus and chromosomes to opposite poles, respectively into two sub cells. The two daughter cells with chromosome number Primary spermatocyte Less than half. At this point, the second split ends.
Note: the 1 meiotic split second to centromere division, realize chromatid separation. Results is the same number of chromosomes, halve the number of DNA molecules.
2 of the two secondary spermatocyte meiosis after second split into four sperm cells, sperm cells must undergo a series of complex morphological changes to become sperm. The result is a spermatogonia undergo meiosis and a series of morphological development and eventually become the four sperm.
3 a secondary oocyte meiosis after second split into an egg cell and a second polar body. Soon, the first and the second polar body will disappear. The result is a oogonia after meiosis eventually become an egg cell.

Meiosis Genetic significance

First, to ensure the stability of sexual reproduction of individual organisms and the chromosome number of generations. Through meiosis leads to sex cells (gametes) the chromosome number in half, from somatic cells of 2n (n is a Chromosome set In the number of chromosomes) chromosomes into male and female gametes n chromosomes, after sexual gametes, zygote chromosome number and return to the parental 2n level, the sexual reproduction of the offspring chromosome number of parents always keep the inherent guarantee genetic material The relatively stable.
Two, in the process of sexual reproduction creation provides the material basis of genetic variation:
By 1 Non homologous chromosomes The combination of the non random; Homologous chromosomes To enter the free combination of gametes, formation of gametes can produce a variety of genetic combinations, with male and female gametes can occur after individual variation varied, the species will survive and evolve, provide abundant material for natural selection.

Meiosis Biological significance

Meiosis is the basis of genetics . Specific performance in:
1 in the meiosis process, because of the separation of homologous chromosomes, respectively in different Sub cell Therefore, in the sub cell having only one chromosome of each pair of homologous chromosomes. meiosis Homologous chromosomes The separation is the cytological basis of gene separation law.
2 homologous chromosome synapsis, non sister chromatid exchange occurs fragment may symmetric between monomers on the position is between paternal and maternal chromosome genetic material The exchange. This exchange can make chromosome linkage with gene recombination, which is on the chromosome Gene linkage The cellular basis and exchange.
Because of meiosis, so that each species of generations are able to maintain the diploid number of chromosomes. In the process of meiosis chromosome re combination of homologous chromosome exchange occurred, the genetic basis of gamete diversification, the offspring of the changes in environmental conditions have greater adaptability.
1 to ensure the stability of sexual reproduction of individual organisms and generations through meiotic chromosome number of cells (gametes) led to the halving of chromosome number, 2n chromosomes from somatic cells into gametes n chromosomes after sexual gametes, zygote chromosome number and return to the parental 2n level to sexual the reproductive offspring always maintain the inherent parental chromosome number. To ensure the relative stability of genetic material.
The 2 is in the process of sexual reproduction creation provides the material basis of genetic variation:
By 1 Non homologous chromosomes The combination of non homologous chromosomes; to freely enter the formation of gametes, provide abundant material for natural selection.
Through the exchange of 2 non sister chromatid fragments in pachytene meiosis stage, due to the non sister chromatid exchange may occur on the corresponding fragment, the homologous genetic material on chromosome recombination, genetic variation is different from the parent form.

Meiosis Mitotic comparison

1 meiosis continuous cell division two times and mitosis cells only divide a;
Results of the 2 meiotic chromosome number is halved, and the mitotic chromosome number is unchanged;
3 after meiosis, a cell into four containing different combinations of genetic material of cells (four points in consideration of non sister chromatid exchange fragment) or 22 identical daughter cells (not consider monomer fragment exchange). And after mitosis, a cell formed only two genetically identical cells;
4 the process of meiosis has its unique homologous chromosome pairing and non homologous sister chromosome monomer of the local exchange, and no mitosis
The 5 part is the occurrence of meiosis in animal testis or ovary primordial germ cells (the male animal occurred in the testis, higher plants in anther and ovule), mitosis occurs as somatic cells (i.e. when the primordial germ cells of the original cells proliferation belongs to mitosis).
6 primary oocyte cytoplasmic division asymmetric division, and the first polar body and the second experience gradually disappear, and mitosis does not produce the phenomenon.
How to distinguish between mitosis and meiosis
The number of chromosome 1 see: odd words must be at the second division (but not necessarily reduce the II chromosome number is odd), or may be mitosis or meiosis i;
2 see non homologous chromosomes: if not necessarily second meiotic division. (left) if Homologous chromosomes The third step, see;
3 see homologous chromosome behavior change: if there is a federation of four points, and the centromere is located in the equatorial plate on both sides, the separation of homologous chromosomes and other phenomena should be the first meiosis. In the absence of such behavior may be mitosis.

Meiosis Be careful

1 Chromatid When the DNA number is equal to 2 times the number of original DNA
 Comparison between mitosis and meiosis Comparison between mitosis and meiosis
2 chromatids, DNA is equal to the number of original DNA number
The chromosome number is equal to 3 Centromere number

Meiosis Meiotic formula

Primary cell preparations,
At the beginning of the first mother cell association.
Row plate after homologous points,
This staining was not paired.
A mother like and have the same wire,
Then the row plate split in.
Say goodbye to a sister,
Again the matter non constriction.
A complex cell staining two crack,
Halve the number of homologous.
Divide the egg quality differences,
Back to welcome the edge solution.