Fertilization is the unification of the male and female gametes, the sperm and the egg, to form a zygote.

Remember pollination is not fertilization. Pollination is the process of growing the 2 sperms in the pollen grain and transferring them from the anther to the stigma in angiosperms so they can begin the process of fertilization.

Pollen Tube Formation

  1. Pollen sticks to the stigma. The pollen grain itself never goes down into the style. 
     
  2. A chemical stimulus and moisture on the stigma of the pistil induces the correct type of pollen tube nucleus to commence growing. 
  3. Fertilization can only occur once the pollen tube penetrates the stigma and grows down through the style into the ovary.
     
  4. The tube nucleus is a rapid growing cell which requires a continuous source of glucose which style provides.
     
  5. The ovules emit a chemical signal: probably a Ca+2, compound to guide the pollen tube’s growth to the ovary.
     
  6. Only one pollen tube will enter each ovule. 
  7. The two slower sperm will swim down the tube to the ovary while the generative sperm will fertilize the egg cell. The haploid sperm and haploid egg combine to form a diploid  zygote, while the other sperm and the two haploid  polar nuclei of the large central cell of the megagametophyte form a triploid  nucleus (some plants may form polyploid nuclei). The large cell of the megagametophyte will then develop into the endosperm , the nutrient rich tissue which will provide all the nourishment for the developing embryo to grow. The ovary, surrounding the ovules, develops into the fruit, which protects the seeds.

The Sexual Cycle

Each of the gametes (egg and sperm) involved in sexual reproduction carries a single set of chromosomes. This is known as the haploid condition and is symbolized with the letter “n” which carries the same number of chromosomes and type of characteristics of that species.

When a sperm fuses with an egg in fertilization the new zygote nucleus receives two complete sets of chromosomes, one set from the egg and one from the sperm. This is known as the diploid state and is symbolized with the letter “2n”.

To carry out sexual reproduction, a diploid organism must produce haploid gametes from its diploid cells. This is accomplished by meiosis, a sequence of nuclear divisions that reduces the chromosome number from diploid (2n) to haploid (n). Meiosis always involves two successive divisions, normally yielding from the original diploid nucleus a set of four haploid nuclei or cells, called a tetrad.

Meiotic Division:

This occurs in two stages, meiosis I and meiosis II. The dividing of the cells once at each stage before meiosis begins, during S phase of the cell cycle. The DNA of each chromosome is replicated, so that each chromosome has two sister chromatids; a diploid organism now has a tetraploid DNA amount in the cell.

The first stage of meiosis begins with a cell that has (if it is from a diploid organism) two copies of each type of chromosome, one from each of the mother and father known as homologous chromosomes, each of which has two sister chromatids. The homologous chromosomes pair up and may exchange genetic material with each other in a process known ascrossing over. Each pair then separates as two cells are formed, each with one chromosome (two chromatids) from every homologous pair. The chromatids composing a chromosome may differ from one another if crossing over occurred. The chromosomes present in each of the two cells will be complementary subsets from the original set, some originally from the mother and some originally from the father.

In the second stage, each chromosome splits into two; each half, each sister chromatid, is separated into two new cells, which are haploid. (Note: The instant the “sister chromatids” are seperated, they are called daughter chromosomes, not chromatids.) This occurs in both of the cells formed in meiosis I. Therefore from each original cell, four genetically distinct haploid cells are produced. These cells can mature into gametes.

Meiosis I – the 1st cell division.

* DNA is duplicated before this division begins.

* Parent and daughter cells are diploid (2n).

* Prophase I: This is the longest phase of meiosis. During prophase I, DNA is exchanged between homologous chromosomes in a process called homologous recombination. This often results in chromosomes crossing over. The new combinations of DNA created during crossover are a significant source of, and may result in beneficial new combinations of alleles. The paired and replicated chromosomes are called bivalents or tetrads, which have two chromosomes and four, with one chromosome coming from each parent. The process of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids may cross over at points called chiasmata (singular chiasma).

* Leptotene: The first stage of prophase I is known as the leptotene stage orleptonema and is derived from the Greek words meaning “thin threads”.In the leptonema stage of prophase I, individual chromosomes; each consisting of two sister chromatids, change from the diffuse state they exist in during the cell’s period of growth and gene expression, and condense into visible strands within the nucleus. The two sister chromatids are still tightly bound that they are indistinguishable from one another. During leptotene, lateral elements of the  synaptonemal complex  assemble. Leptotene is of very short duration and progressive condensation and coiling; like springs, of chromosome fibers takes place.

* Metaphase I:The homologous pairs move together along the metaphase plate. As kinetochore microtubules from both centrioles attach to their respective kinetochores. Kinetechores are the protein structure on chromatids where the spindle fibers attach during cell division to pull the sister chromatids apart. The homologous chromosomes align along an equatorial plane that bisects the spindle. Due to the continuous counter balancing of forces exerted on the bivalents; by the microtubules emanating from the two kinetochores of homologous chromosomes results in the physical basis of the independent assortment of chromosomes is the random orientation of each bivalent along the metaphase plate, with respect to the orientation of the other bivalents along the same equatorial line.

* Anaphase I: Kinetochore (bipolar spindles) microtubules shorten, severing the recombination nodules and pulling homologous chromosomes apart. Since each chromosome has only one functional unit of a pair of kinetochores, whole chromosomes are pulled toward opposing poles, forming two haploid sets. Each chromosome still contains a pair of sister chromatids. During this time disjunction occurs, which is one of the processes leading to genetic diversity as each chromosome can end up in either of the daughter cells. Nonkinetochore microtubules lengthen, pushing the centrioles farther apart. The cell elongates in preparation for division down the center.

* Telophase I: The first meiotic division effectively ends when the chromosomes 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. Sister chromatids remain attached during telophase I.

Cells may enter a period of rest known as interkinesis  or interphase II. No DNA replication occurs during this stage.

Meiosis – the 2nd cell division.

* DNA is not duplicated before this division begins.

* The parent cell is diploid (2n), daughter cell is haploid (n).

Mitosis 

is the process, in the cell cycle, by which the chromosomes in the cell nucleus are separated into two identical sets of chromosomes, each in its own nucleus. In general, mitosis is followed immediately by cytokinesis, which divides the cytoplasm organelles cell membrane, and later karyokinesis, which divides the nucleus, dividing the cell into two new cells containing roughly equal shares of these cellular components. Mitosis and cytokinesis together define the mitotic (M) phase of the cell cycle —the division of the mother cell into two daughter cells, genetically identical to each other and to their parent cell.

Angiosperm Egg Cells Form In Flower Ovules

Angiosperm Egg Cell Formation

  1. The mother cell divides by meiosis to produce 4 haploid megaspores. 
     
  2. Three of these 4 megaspores disintegrate. 
     
  3. The fourth megaspore undergoes 3 cycles of mitosis to produce the female gametophyte, consisting of 7 cells with a total of 8 haploid nuclei. The central cell contains 2 polar nuclei. 
     
  4. Three cells are found at each end of the large central cell of the ovule. The middle cell located near the micropyle and between the 2 synergids is the egg cell.


Double fertilization is needed for seed formation

  1. One of the 2 generative sperm; of the pollen grain, will fertilize the egg, resulting in a 2n zygote.
     
  2. The other generative sperm; of the pollen grain, fertilizes the 2 polar nuclei with each haploid, resulting in a triploid (3n) endosperm, a nutritive tissue for the embryo. This triploid endosperm contains the bulk of the food stores of cereal grains.
  1. The seed coat that forms around the endoplasm is not a product of fertilization. 
     
  2. There fore most seeds consist of the seed husk or skin of some type, the nutritive endosperm (cotyledon), and the embryo.

Apomixis – apo is Greek Latin for away from and mixis to mingle. A few plants like the common Dandelion are able to bypass the sexual processes normally involved in seed formation and produce their seeds without fertilization. The embryo may develop from a diploid cell of the ovule rather than from a zygote. From the genetic point of view, apomixes seeds are equivalent to being vegetatively produced and are clones of the mother plant.

Remember:

1. A Zygote is formed when sperm and egg join.

2. In Angiosperms, the pathway for sperm to reach the egg is through the pollen tube.

3. Meiosis is the process that reduces the chromosome number from diploid to haploid.

4. The first division of gametes occurs through two cell divisions and only one set of DNA.

5.Three of the four megaspores produced by the Angiosperm mother cell disintergrate prior to maturity.

6. Double fertilization is needed for seed formation.

7. The pollen grain itself does not go into the style.

8. The tube nucleus grows rapidly requiring a continual source of glucose from the style.

9. Ovules guide the pollen tube’s growth by producing Double Positive Calcium ions. 

10. When a parent cell divides, the new cells are called daughter cells.

11. The endosperm of a seed contains three sets of chromosomes.

12. A seed has three parts, the coat ,husk or skin or other outer layer, the endosperm or cotyledon/s and the embryo.

Further Comments from Members:

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