What is the relationship between fertilization and meiosis? – Kgb Answers
Discuss the relationship between sex and reproduction, and compare sexual and asexual Recognize the essential elements of the process of meiosis. Sexual reproduction requires fertilization, the union of two cells from two individual . are modified, and a new connection is made between the non-sister chromatids. . Review the process of meiosis, observing how chromosomes align and. Meiosis, the process of cells splitting, plays roles of great importance in sexual reproduction. It helps randomly select which chromosomes carry.
These homologous chromosomes line up gene-for-gene down their entire length, allowing crossingover to occur. This is an important step in creating genetic variation. In metaphase I, the 23 pairs of homologous chromosomes line up along the equator of the cell. During mitosis, 46 individual chromosomes line up during metaphase.
Some chromosomes inherited from the father are facing one side of the cell, and some are facing the other side. During anaphase I the spindle fibers shorten, and the homologous chromosome pairs are separated from each other. One chromosome from each pair moves toward one pole, with the other moving toward the other pole, resulting in a cell with 23 chromosomes at one pole and the other 23 at the other pole. The sister chromatids remain attached at the centromere.
The spindle fiber disassembles and the nucleus reforms. This is quickly followed by cytokinesis and the formation of two haploid cells, each with a unique combination of chromosomes, some from the father and the rest from the mother.
Meiosis II is essentially the same as mitosis, separating the sister chromatids from each other. Once again the nucleus breaks down, and the spindle begins to reform as the centrioles move to opposite sides of the cell.
The spindle fibers align the 23 chromosomes, each made out of two sister chromatids, along the equator of the cell. The sister chromatids are separated and move to opposite poles of the cell. As the chromatids separate, each is known as a chromosome. Anaphase II results in a cell with 23 chromosomes at each end of the cell; each chromosome contains half as much genetic material as at the start of anaphase II. The nucleus reforms and the spindle fibers break down.
Each cell undergoes cytokinesis, producing four haploid cells, each with a unique combination of genes and chromosomes. Meiosis is a process in which a diploid cell divides itself into four haploid cells. Meiosis and Genetic Variation Sexual reproduction results in infinite possibilities of genetic variation.
This occurs through a number of mechanisms, including crossing-over, the independent assortment of chromosomes during anaphase I, and random fertilization. Crossing-over occurs during prophase I.
Crossing-over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Recall during prophase I, homologous chromosomes line up in pairs, gene-for-gene down their entire length, forming a configuration with four chromatids, known as a tetrad. At this point, the chromatids are very close to each other and some material from two chromatids switch chromosomes, that is, the material breaks off and reattaches at the same position on the homologous chromosome Figure 5.
This exchange of genetic material can happen many times within the same pair of homologous chromosomes, creating unique combinations of genes. This process is also known as recombination. A maternal strand of DNA is shown in red. Paternal strand of DNA is shown in blue. Crossing over produces two chromosomes that have not previously existed. The process of recombination involves the breakage and rejoining of parental chromosomes M, F.
As mentioned above, in humans there are over 8 million configurations in which the chromosomes can line up during metaphase I. It is the specific processes of meiosis, resulting in four unique haploid cells, that results in these many combinations.
Meiosis and Fertilization - The Cell - NCBI Bookshelf
Figure 6 compares mitosis and meiosis. This independent assortment, in which the chromosome inherited from either the father or mother can sort into any gamete, produces the potential for tremendous genetic variation. Together with random fertilization, more possibilities for genetic variation exist between any two people than individuals alive today.
Sexual reproduction is the random fertilization of a gamete from the female using a gamete from the male. A sperm cell, with over 8 million chromosome combinations, fertilizes an egg cell, which also has over 8 million chromosome combinations. That is over 64 trillion unique combinations, not counting the unique combinations produced by crossing-over. In other words, each human couple could produce a child with over 64 trillion unique chromosome combinations. Mitosis produces two diploid daughter cells, genetically identical to the parent cell.
Meiosis produces four haploid daughter cells, each genetically unique. Gametogenesis At the end of meiosis, haploid cells are produced. These cells need to further develop into mature gametes capable of fertilization, a process called gametogenesis Figure 7. Gametogenesis differs between the sexes. In the male, the production of mature sperm cells, or spermatogenesis, results in four haploid gametes, whereas, in the female, the production of a mature egg cell, oogenesis, results in just one mature gamete.
Analogies in the process of maturation of the ovum and the development of the spermatids. Four haploid spermatids form during meiosis from the primary spermatocyte, whereas only 1 mature ovum, or egg forms during meiosis from the primary oocyte. Three polar bodies may form during oogenesis.
These polar bodies will not form mature gametes. During spermatogenesis, primary spermatocytes go through the first cell division of meiosis to produce secondary spermatocytes. These are haploid cells. Secondary spermatocytes then quickly complete the meiotic division to become spermatids, which are also haploid cells. The four haploid cells produced from meiosis develop a flagellum tail and compact head piece to become mature sperm cells, capable of swimming and fertilizing an egg.
The compact head, which has lost most of its cytoplasm, is key in the formation of a streamlined shape. The middle piece of the sperm, connecting the head to the tail, contains many mitochondria, providing energy to the cell.
The sperm cell essentially contributes only DNA to the zygote. On the other hand, the egg provides the other half of the DNA, but also organelles, building blocks for compounds such as proteins and nucleic acids, and other necessary materials. The egg, being much larger than a sperm cell, contains almost all of the cytoplasm a developing embryo will have during its first few days of life.
What is the relationship between fertilization and meiosis?
Therefore, oogenesis is a much more complicated process than spermatogenesis. Oogenesis begins before birth and is not completed until after fertilization. Oogenesis begins when an oogonia singular, oogoniumwhich are the immature eggs that form in the ovaries before birth, with the diploid number of chromosomes undergoes mitosis to form primary oocytes, also with the diploid number.
It proceeds as a primary oocyte undergoes the first cell division of meiosis to form secondary oocytes with the haploid number of chromosomes. A secondary oocyte undergoes the second meiotic cell division to form a haploid ovum if it is fertilized by a sperm. The one egg cell that results from meiosis contains most of the cytoplasm, nutrients, and organelles. This unequal distribution of materials produces one large cell, and one cell with little more than DNA.
This other cell, known as a polar body, eventually breaks down. The larger cell undergoes meiosis II, once again producing a large cell and a polar body. Mitotic division in the spore leading to the formation of a microgametophyte or pollen. Only the metaphase is shown here. The chromosomes lay in the equatorial plane of the cell. Nearly ripe pollen grain: Ripe pollen grain in which the texture of the outer cell wall, the exine, can be recognized. The grainy dark purple structure in the middle of the pollen grain is the vegetative nucleus.
Diagram in 3 parts: Ripe pollen grain consisting of the vegetative cell VC and therein the smaller generative cell GC. After landing on the stigma St the pollen grain germinates and forms a pollen tube. In the pollen tube the generative cell divides into two sperm cells SC. The pollen tube grows to the embryo sac ES and delivers the two sperm cells that are involved in double fertilization.
At the male side, in the loculi of the anthers, a microspore mothercell produces four microspores that are initially joined in a tetrad see photograph here below.
From each -haploid- microspore a microsporophyt develops by mitosis. In angiosperms the microsporophyte correspond to the pollen grains.
- Sexual life cycles
In each pollen grain goes to the first mitotic division to form a vegetative and a generative cell. The generative cell leads generates through a second mitosis two gametes sperm cells. More images and information on pollen development can be found elsewehre on this site. In the pistil, meiosis occurs in the ovaria. Also here asymmetrical divisions during meiosis leads to accumulation of the cytoplasm in only one of the meiotic products: The other cells disappear.
The large cell forms an embryo sac, the actual macrogametophyt. Within the ovules three further mitotic divisions usually occur, giving in many species rise to three antipodes, two polar nuclei, one egg cells and two synergids.
Find numerous photographs of all stages of the microsporogenesis and microgametogenesis from meiosis tot pollen formationand of the megasporogenesis and megagametogenesis from meiosis to egg and fertilization or syngamy in Lilium on the very complete webpages at the site of the International Association of Sexual Plant Reproduction Research IASPRR. See also instructive and complete article by Ross E. Koning about "Pollen and Embryo Sac". Ross Koning; download pdf version, with permission; KB. The much smaller generative cell, or her mitotic products, the two -haploid- sperm cells, are enclosed inside the vegetative compartment and migrate forward along with the growing tube tip.
The pollen tube penetrates the stigma and continues to grow through the style to an ovule inside the ovarium. Each ovule contains one embryo sac ES ,in fact a small female gametophyte, bearing a. When the pollen tube reaches the embryo sac it bursts open and releases the two sperm cells, the actual sex cells.
Then double fertilization occurs. One sperm cell fertilizes the egg cell so that a diploid zygote arises, from which an embryo develops.
The other sperm cell fertilizes the large central cell in the middle of the embryo sac and give rises through series of divisions to the endosperm, which function as a storage organ of nutrient for the seed. Sometimes the storage food is transfered from the endosperm to the cotyledons. The ripe dry ovule with the mature embryo and the endosperm is the seed. Fruits are the ripened ovary with inside one e. Pollen tube growth through the pistil in tomato Pollen tubes in a pollinated pistil of tomato Solanum Lycopersicum cv money maker.
The course of pollen tubes could be visualized by treatment with the fluorescing marker aniline blue that specifically binds to callose, a special cell wall component which is abundant in pollen tubes. Pollen tubes growing through the stigma St and the style Sty. Detail of pollen tubes arrows in the style. The bright spots are callose plugs which separate the living and growing part of the pollen tube in the tip and the remaining degenerating part. Fluorescing pollen tubes can be seen in the lower part of the style Sty and the underlying ovarium Ova containing ovules Ovu.
Detail of three ovules Ovu. The two upper ovules have been penetrated by pollen tubes arrow growing toward the embryo sac ES, corresponding with the dark ovale shape on this photograph.
In each pollen tube two sperm cells are present which are involved in double fertilization Embryosac and mature embryo in plants A. Transverse section through an ovule of Lily Lilium sp.
In the center an embryo sac ES can be seen surrounded by its two protective layers the integumenten I1 and I2. Together they form the ovule.