Meiosis, the first meiosis and the second meiosis; Sexual reproductive organs, ovaries, testes and testes; Primordial germ cells, spermatogonia, oogonia, primary spermatocytes (eggs), secondary spermatocytes (eggs), spermatocytes (eggs), sperm and polar bodies; Synapses and tetrads; Chromosomes, homologous chromosomes, sister chromatids; fertilize the soil or land

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meiosis

Only sexually reproducing organisms have primordial germ cells that undergo meiosis. Organs with primordial germ cells (gonads) are called gonads, and females are ovaries and males are testes. Meiosis is a mode of division in which chromosomes are copied only once, but cells divide twice in succession. The result of division is that the chromosome number of daughter cells is reduced by half compared with that of original cells (or somatic cells). To understand the process of meiosis, we should pay attention to the following points: first, the replication time of chromosomes has been completed long before gonadal cells develop into sexual mother cells, that is, before homologous chromosomes unite; Secondly, synapsis occurs in the early stage of chromosome shortening and thickening, and the process of synapsis cannot be seen under the optical microscope, so the meiosis diagram in the textbook shows that two chromatids are not drawn on one chromosome in the synapsis diagram; Thirdly, the purpose of the first meiosis is to separate homologous chromosomes from each other to halve the number of chromosomes, to make non-homologous chromosomes freely combine when homologous chromosomes are separated from each other, and to cross-exchange chromatids between homologous chromosomes, which is the cytological basis of the three genetic laws; 4. The main feature of the second division of meiosis is centromere division, which separates chromatids from each other, so the result of division is that the number of chromosomes remains unchanged, but the number of DNA molecules is reduced by half; Fifth, the division of secondary mother cells in the secondary division process is similar to the mitosis process, but unlike the mitosis process, there are generally no homologous chromosomes.

As for the comparison between meiosis and mitosis, the emphasis is on the comparison between the second division process and mitosis process of meiosis. ① Comparison between metaphase of mitosis and metaphase of meiosis: Homologous chromosomes exist from beginning to end during mitosis, but not during meiosis. There are two grounds for distinguishing homologous chromosomes in senior high school biology stage: one is the size of chromosomes, which are generally similar or identical in shape and size; The second is the position of the centromere, which includes both the terminal centromere and the intermediate centromere. The centromere position of homologous chromosomes should be the same. The image characteristics of metaphase mitosis are: the number of chromosomes is generally even, the chromosomes are identical in pairs, each chromosome has two chromatids, and centromeres are arranged in the center of the equatorial plate. The image feature of the second metaphase meiosis is that there are odd and even chromosomes, but no two identical chromosomes can be found, that is, the size is different or the centromere position is different, which means that there are no homologous chromosomes, but each chromosome has two chromatids, as shown in the figure. ② Comparison of mitotic images between late mitosis and late meiosis, as shown in the figure. The similarity between late mitosis and late meiosis is centromere division, that is, two chromatids of each chromosome are separated from each other to form two chromosomes, which move to the two poles of the cell under the traction of spinning. But the difference is that there should be homologous chromosomes in cells at the late mitosis stage, while there are generally no homologous chromosomes in cells at the late meiosis stage. A set of chromosomes moving in the same direction should be analyzed in reading pictures. As shown in the figure, in a group of chromosomes moving upward, two large chromosomes are the same and can be regarded as a pair of homologous chromosomes. The two small chromosomes are the same and can be regarded as another pair of homologous chromosomes. A group of chromosomes moving upward in the picture are different in size and shape, so there are no homologous chromosomes. Therefore, Karmaputra is regarded as the anaphase of mitosis, and Karmaputra is regarded as the second anaphase of meiosis.

The similarities and differences of meiosis between spermatogonia and oogonia are that the process of chromosome behavior and number change is the same, but the distribution of cytoplasm is different. During meiosis, the cytoplasm of spermatogonia divides equally twice in succession. During the meiosis of oogonia, the cytoplasm of two consecutive divisions is not equal. During the first division, the cytoplasm is mainly distributed in the secondary oocyte, and during the second division, the cytoplasm is mainly distributed in the egg cell, and there is almost no cytoplasm in the polar body.

Sperm cells are formed by meiosis of spermatogonia. Sperm cells must be deformed to form sperm. In the process of deformation, a lot of cytoplasm is lost, leaving only the nucleus and a tail composed of protein. There is no deformation process in the formation of egg cells, and the egg cells are large in size, and a large amount of nutrients are stored in the cytoplasm to prepare for the development of fertilized eggs.

The changes of chromosome number and DNA content during meiosis can be represented by two curves as shown in the figure.

The process of fertilization refers to the fusion of sperm nucleus and egg nucleus, so half of the genetic material in the fertilized egg nucleus comes from the father and half from the mother, but all the genetic material in the fertilized egg cytoplasm comes from the mother. Sperm and egg cells with half chromosomes are fertilized to form zygotes. When the chromosome number returns to the original state, the genetic material of parents is integrated into offspring individuals, which not only ensures the stability of the chromosome number of parents and offspring of the same organism, but also has important significance for heredity, so that offspring can obtain the genetic material of parents, which is conducive to the survival and evolution of organisms and is also of great significance for biological variation. As shown in the figure.

Several problems that should be paid attention to in the study of meiosis;

Question 1: In the whole process of the second meiosis, the characteristics of chromosome changes are completely consistent with those of mitosis, which is an ordinary mitosis.

Discrimination: An important feature of mitosis is that two sister chromatids on a chromosome separate and enter different daughter cells. At this time, the result of the second meiosis is the same as that of mitosis. But the daughter cells produced by mitosis are somatic cells with pairs of homologous chromosomes. After the first meiosis, homologous chromosomes enter different daughter cells, so there are no homologous chromosome pairs in the secondary spermatocytes (oocytes). Therefore, the second mitosis does not have paired homologous chromosomes and daughter cells, so the second meiosis is by no means an ordinary mitosis, but only a special mitosis.

Question 2: Germ cells must be produced by meiosis.

Discrimination: Germ cells refer to cells that reproduce the next generation, such as sperm, egg cells and spores. When sexually reproducing organisms produce germ cells, most of them undergo meiosis; However, there are exceptions. For example, the formation of sperm and egg cells in higher plants is mitosis, and spores produced by asexual organisms such as Rhizopus and Penicillium are also a kind of germ cells, but they are called asexual germ cells, which are produced by mitosis and can directly form new individuals without two two combination. It can be seen that biological germ cells are produced by meiosis and mitosis.

Question 3: Chromosomes in somatic cells produced by mitosis all exist in the form of homologous chromosome pairs, while chromosomes in germ cells produced by meiosis all exist in the form of non-homologous chromosomes.

Discrimination: Generally speaking, chromosomes in somatic cells produced by mitosis exist in the form of homologous chromosome pairs, while chromosomes in germ cells produced by meiosis exist in the form of non-homologous chromosomes. But there are exceptions, such as bees and ants. Their male individuals (drones and ants) are haploid organisms produced by unfertilized eggs through parthenogenesis, and their cells have no homologous chromosomes, so their somatic cells produced by mitosis also have no homologous chromosomes. In polyploid organisms, there are many homologous chromosomes. For example, potato is autotetraploid, and somatic cells have four homologous chromosomes. There are also homologous chromosomes in the germ cells formed by meiosis.