Which sample might represent a sperm cell

This is typically the number of chromosomes found in the single nuclei of sex cells gametes. In the flowering plant life cycle, this is also the chromosome number of the three nuclei within a germinated pollen grain pollen tube and the eight nuclei within an embryo sac.

In plants, the haploid part of the life cycle is called the gametophyte. The term diploid refers to two sets of chromosomes resulting from the union of the sperm and the egg. This is typically the number of chromosomes found in the nuclei of body cells somatic cells of a plant or animal. In plants, the diploid part of the life cycle is called the sporophyte. The following link to life cycle patterns may be helpful in understanding the gametophyte and sporophyte parts of a life cycle.

In the diagrams, everything above the red horizontal line is diploid sporophyte and everything below the red line is haploid gametophyte. The terms sporophyte and gametophyte are used in the life cycles of plants, fungi and algae, but not animals.

They are derived from an egg or sperm that contained more than one set of chromosomes. Gametes that carry more than one set of chromosomes are induced by treating flowers and buds with colchicine. This is especially true of hybrid fruits and vegetables where polyploidy often results in the larger size of vegetables, fruits and flowers.

Polyploid plants may be triploid three sets of chromosomes , tetraploid four sets , pentaploid five sets , hexaploid six sets , and may go all the way to octoploid eight sets. Generally odd polyploids, such as triploid plants, are sterile and cannot produce viable gametes. Gametes are produced by a special type of cell division known as meiosis. During prophase I of meiosis, the matching homologous maternal and paternal chromosomes of the seed and pollen parents must pair up with each other in a process called synapsis.

Triploid plants usually exhibit synaptic failure because there is a third set of chromosomes that has no homologous set to pair up with. Since triploid plants cannot produce viable gametes, they are typically seedless. The commercial advantages of seedless watermelons and bananas are readily apparent.

Even-numbered polyploids such as tetraploid plants are typically fertile because all the maternal and paternal sets of chromosomes have a homologous set of chromosomes to pair up with. Therefore, tetraploid plants can produce seeds. For propagation purposes, fertile tetraploid hybrids are economically desirable. See Plant That Produces Colchicine Sometimes diploid 2n hybrids are sterile and seedless, even though they have two sets of chromosomes.

This is especially true when the seed and pollen parents are two different species interspecific hybrids or different genera bigeneric hybrids. Again, the sterility problem relates to synaptic failure during meiosis. The maternal and paternal sets of chromosomes do not pair up properly because they come from different species and are not truly homologous. A good example of this type of sterility is the diploid 2n rabbage resulting from a cross between a radish Raphanus sativus and a cabbage Brassica oleracea.

The tetraploid 4n rabbage hybrid is fertile because the two sets of radish chromosomes can pair up with each other, and the two sets of cabbage chromosomes can pair up with each other.

Keeping track of the chromosome numbers in plant hybridization can be a little confusing. For example, the diploid number of the rye plant sporophyte is The gametes of rye are haploid and carry only one set of chromosomes D.

It is sterile because the rye D set has no homologous set to pair up with during synapsis. This sterile hybrid seedling is treated with colchicine to produce a plant with twice as many sets of chromosomes i.

Click on the following link for illustrations of vegetative terminology: Botany Vegetative Terminology Flowering Plant Life Cycle: In the flowering plant life cycle, the haploid generation is reduced to a germinated pollen grain containing three nuclei and a 7-celled embryo sac containing eight nuclei.

Diploid microspore mother cells inside the anther undergo meiosis microsporogenesis forming haploid microspores each mother cell dividing into four microspores. The microspores develop into binucleate pollen grains, each containing a tube nucleus and a generative nucleus.

When the pollen grain lands on a receptive stigma it grows into an elongate pollen tube containing a tube nucleus and a generative nucleus, the latter of which divides into two sperm nuclei.

A binucleate angiosperm pollen grain containing a generative nucleus and a tube nucleus. After the pollen grain germinates into a pollen tube, the generative nucleus divides into two sperm nuclei. Because the generative nucleus and sperm nuclei contain cytoplasmic sheaths, they are often referred to as cells in some textbooks.

The tube nucleus controls the growth of the pollen tube as grows down the style and into the ovary of a flower. Eventually it penetrates the micropyle of an ovule and releases its two sperm into the 8-nucleate embryo sac. During double fertilization, one sperm fuses with the egg nucleus to form a zygote.

The other sperm fuses with the two polar nuclei inside the endosperm mother cell to form the endosperm. In corn, this process must occur for each grain that forms. Even more astonishing is the growth of separate pollen tubes down each strand of silk styles.

A diploid megaspore mother cell inside each ovule also undergoes meiosis megasporogenesis and forms four haploid megaspores, three of which abort leaving one functional megaspore. The functional megaspore inside each ovule undergoes nuclear division into a 7-celled, 8-nucleate embryo sac. At one end of the embryo sac are three antipodal cells. At the opposite end is an egg cell flanked by two synergid cells. A large binucleate cell in the center containing two polar nuclei is called the endosperm mother cell.

During pollination, pollen grains land on the stigma where they form pollen tubes that penetrate the style and eventually the ovary of the flower. A separate sperm-bearing pollen tube must reach each ovule in order to fertilize the egg cell inside the embryo sac.

During double fertilization two sperm are introduced into the embryo sac from the long pollen tube. One sperm nucleus fuses with the egg nucleus inside the egg cell to form a diploid 2n zygote which develops into the embryo of the seed. The other sperm nucleus fuses with the two polar nuclei inside the endosperm mother cell to form the tripolid 3n endosperm of the seed.

Microscopic view of the embryo sac megagametophyte of a lily Lilium. Three haploid antipodal cells 1 occur at the upper end of the emryo sac.

A large endosperm mother cell containing two haploid polar nuclei 2 occupies the central portion of the embryo sac. The corms of autumn crocus Colchicum autumnale , a member of the lily family Liliaceae , contain the alkaloid colchicine, a spindle poison causing depolymerization of mitotic spindles into tubulin subunits. This essentially dissolves the spindle and stops the cell from completing its mitotic division.

Because colchicine can stop plant cells from dividing after the chromatids have separated during anaphase of mitosis, it is a powerful inducer of polyploidy. Seeds and meristematic buds can be treated with colchicine, and the cells inside become polyploid with multiple sets of chromosomes more than the diploid number. Polyploidy in plants has some tremendous commercial applications because odd polyploids such as 3n triploids are sterile and seedless. Polyploid plants such as 4n tetraploids typically produce larger flowers and fruits.

In fact, many of the fruits and vegetables sold at supermarkets are polyploid varieties. Colchicine has another medical use for people because it reduces the inflammation and pain of gout. It is also used in cancer chemotherapy to stop tumor cells from dividing, thus causing remission of the cancer. Two additional alkaloids vinblastine and vincristine from the Madagascar periwinkle Catharanthus roseus are also potent spindle poisons.

These alkaloids have proven to be very effective in chemotherapy treatments for leukemia and Hodgkin's disease lymph node and spleen cancer.

Like colchicine, they cause the dissolution depolymerization of protein microtubules which make up the mitotic spindle in dividing cells. This effectively stops the tumor cells from dividing, thus causing remission of the cancer. Before periwinkle alkaloids were used as a treatment there was virtually no hope for patients with Hodgkin's disease. Now there is a 90 percent chance of survival. This is a compelling reason for preserving the diverse flora and fauna in natural ecosystems.

Who knows what cures for dreaded diseases are waiting to be discovered in tropical rain forests or other natural habitats.

Telophase: The chromosomes at each end of the cell begin to organize into separate nuclei, each surrounded by a nuclear membrane. A cleavage furrow or constriction forms in the center of the cell, gradually getting deeper and deeper until the cell is divided into two separate cells.

This cytoplasmic division is referred to as cytokinesis. Cytoplasmic division cytokinesis in a plant cell is accomplished by a partition or cell plate rather than a cleavage furrow. The following illustration shows cell plate formation in an onion root tip cell:. Interphase: Now we are back to interphase again, but now there are two daughter cells.

Each daughter cell is chromosomally identical with the original mother cell. They each have a nucleus that contains a nucleolus and chromatin. The centrioles have divided into four protein bodies and the aster has disappeared. During this phase the chromosomes will replicate and become distinct chromosome doublets as each daughter cell enters prophase.

The five major phases of plant mitosis. Unlike animals cells, plant cells do not have centrioles or asters. During telophase, a partition or cell plate divides the cytoplasm rather than a cleavage furrow. Starfish embryo during the morula stage. It consists of a ball of actively dividing cells superficially resembling the multiple fruit of a mulberry hence, the name morula. At this stage, each cell is unspecialized and can potentially develop into a separate organism.

A human embryo is in the morula stage as it travels down the fallopian tube. At the time of implantation on the uterine wall officially marking the onset of pregnancy , the embryo consists of a hollow sphere or blastocyst blastula consisting of approximately cells roughly the size of a printed period.

Multiple fruit of the black mulberry Morus nigra. Genetic Dominance: Genotype-Phenotype Relationships. Phenotype Variability: Penetrance and Expressivity. Citation: Miko, I.

Nature Education 1 1 Although mitosis and meiosis both involve cell division, they transmit genetic material in very different ways. What happens when either of these processes goes awry? Aa Aa Aa. Gene Transmission in Mitosis. Figure 1. Gene Transmission in Meiosis. Figure 2: Examples of polytene chromosomes. Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.

High-pressure treatment of polytene chromosomes improves structural resolution. Nature Methods 4, All rights reserved. Aberrations That Alter Chromosome Number. Figure 3: Nondisjunction results in daughter cells with unusual chromosome numbers.

Nondisjunction, in which chromosomes fail to separate equally, can occur in meiosis I first row , meiosis II second row , and mitosis third row. These unequal separations can produce daughter cells with unexpected chromosome numbers, called aneuploids. When a haploid gamete does not receive a chromosome during meiosis as a result of nondisjunction, it combines with another gamete to form a monosomic zygote. When a gamete receives a complete homologous chromosome pair as a result of nondisjunction, it combines with another gamete to form a trisomic zygote.

Genetics: A Conceptual Approach , 2nd ed. Figure 4: Jimsonweed seed pod shapes. Trisomy in any of Jimsonweed's 12 chromosomes will cause seed pods to deviate from a wild-type, spherical shape. References and Recommended Reading Belling, J. Genetics: A Conceptual Approach W. Freeman, New York, Article History Close. Share Cancel. Revoke Cancel. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable.

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