Meiosis in Pollen Mother Cells

TITLE: MEIOSIS IN POLLEN MOTHER CELLS

OBJECTIVE: To observe meiosis in Lillium using pollen squash method.

INTRODUCTION:

Meiosis is the process in cell division in sexually reproducing organisms that reduces the number of chromosomes from diploid to haploid (2nn). Meiosis involves two consecutive divisions of the nucleus and leads to the production of reproductive cells (gametes) in animals and to the formation of spores in plants, fungi, and most algae (the spores grow into organisms that produce gametes by mitosis). This enables organisms to reproduce sexually as gametes (sperm and eggs are haploid and thus the fusion of two gametes doubles the number of chromosomes. During the first division, meiosis begins when the chromosomes, which have already duplicated, condense along the center of the nucleus, and pairs of homologous chromosomes undergo crossing over, whereby some of their genetic material is exchanged. The pairs of chromosomes then separate and move to opposite ends of the cell, and the cell itself divides into two cells. In the second stage, each of these two cells also divides into two cells. Meiosis thus produces four cells, each of which contains half the number of chromosomes as the original cell. Some or all of the four cells may become functional gametes or spores.

Lillium anthers were chosen in this experiment because they have nice meiotic figures as they produce the male gametophyte, that is, the chromosomes are more easily observed than in many other species.

MATERIALS:

Lillium sp, forceps, white tile, hot plate, aceto-orcein, microscope slides, 1.0M HCL, coverslip, and microscope.

METHOD:

Fine forceps were used to remove anthers from flower buds.
The anthers were place in a watch glass containing 2cm3 of aceto-orcein and a few drops of 1.0M HCL were added.
The watch was held with forceps or a test tube holder and was warmed gently over a low Bunsen flame until the liquid just begins to steam.
It was left for 10minutes to take up the stain, then the anthers were placed on a microscope slide.
Three or ...

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MATERIALS:

Lillium sp, forceps, white tile, hot plate, aceto-orcein, microscope slides, 1.0M HCL, coverslip, and microscope.

METHOD:

Fine forceps were used to remove anthers from flower buds.
The anthers were place in a watch glass containing 2cm3 of aceto-orcein and a few drops of 1.0M HCL were added.
The watch was held with forceps or a test tube holder and was warmed gently over a low Bunsen flame until the liquid just begins to steam.
It was left for 10minutes to take up the stain, then the anthers were placed on a microscope slide.
Three or four drops of fresh stain were added and anthers were broken up using the glass rod.
A coverslip was applied carefully.
The slide was placed between several sheets of blotting paper. It was then squashed firmly, avoiding any sideways movement.
The slide is observed under the microscope.
The cells with visible chromosomes, which should be stained darkly was watched.

RESULTS:

MEIOSIS I

MEIOSIS II

DISCUSSION:

Lillium anthers were chosen in this experiment because they have nice meiotic figures as they produce the male gametophyte, that is, the chromosomes are more easily observed than in many other species.
Meiosis I (description of process in detail):

Prophase I

The chromosomes become visible as single threads.
Homologous chromosomes pair up along their length (synapsis).
More condensation makes them appear shorter and fatter and it is possible to distinguish the chromatids of each chromosome of a pair.
In some regions the members of a pair repel each other but they remain attracted to each other.
At points of attraction, non sister chromatids break and rejoin. This process is known as crossing over, resulting in the formation of chiasmata.
The nucleoli disappear, the centrioles migrate to opposite poles of the cell and the nuclear envelope breaks down.

Metaphase I

Pairs of homologous chromosomes (bivalent) become attached to the spindle.
The centromere attach to individual spindle fibres and the bivalents are arranged across the equator of the cell
The centomere of each homologous pair repel each other, the sister chromatids remain associated.
The order of the chromosomes on the equator is entirely random.

Anaphase I

Separation of homologous chromosomes
The centromeres are pulled by the spindle fibres to opposite poles.

Telophase I

Chromosomes begin to uncoil and a nuclear envelope forms around each group.
If there is no interphase, then prophase II does not occur. However if the nuclear envelopes and nucleoli were reformed in the telophase I, then these will disappear and two new spindles will form.

Meiosis II (description of process in detail):

Prophase II

Nuclear membrane reformed in the telophase I, then they are broken down in prophase II.

Metaphase II

Each chromosome becomes attached to a spindle fibre by its centrometre and the chromosomes are aligned along the equator of the cell.

Anaphase II

Centromeres divide and separation of the sister chromatids to opposite poles is achieved, pulled by the spindle fibres.

Telophase II

The chromatids reach their respective poles and become less distinct.
Daughter chromosomes become dispersed and less visible.
Nucleoli disappear.
Nuclear membrane form around the groups of chromosomes.
The spindle disappears, leaving four cells known as a tetrad.

Cytokinesis follows and four haploid daughter cells are produced, each of which is genetically different from the others.
There are a few significant materials and apparatus used in this experiment. Below are the materials/apparatus and their function:

1 mole of HCL – used to soften the cells. The acid will chemically break down the middle lamellae which will allow the cells to spread easily.
Acetic orcein – in order to get a clearer picture of the cell division process, acetic orcein is used to stain the nucleus and chromosomes.
Warm plate – heating helps stain the chromosomes darker. The slide should be heated by putting it on the plate for 10 seconds.

One of the steps in the experiment includes the squashing of anthers on the microscope slide. Anthers must be squashed to form a single layer of cells as double layers might resort in distorted and unclear image of cells.
The squashed anthers must first be examined under the microscope at low power to ensure the cells are spread into a single layer. When the cells are successfully squashed, examine under high power.
Some students fail in observing the stages of meiosis of a pollen grain. This happens because some of the pollen grains has already matured and no longer undergo meiosis. There is also another reason for this, some stages of pollen grain cannot be observed because it is not squashed evenly, to form a single layered cell.
The importance of meiosis in plants:

Meiosis halves the number of chromosomes in the gametes, so that when two haploid gametes fuse, the diploid number is restored.
Genetic variation is introduced in the offspring. There are three causes of variation:

Crossing over – crossing over creates a new combination of genes on one chromosome.
Reduction and fusion of gametes – when haploid gametes fuse at fertilization, they combine different genetic material of the two parents
Independent (random) assortment – during metaphase II, homologous chromosomes arrange themselves randomly and so they are re-sorted in the daughter cells.

CONCLUSION:

Pollen grains are produced in pollen sacs. The diploid pollen mother cells divide by meiosis to form four haploid cells, each of which will develop into a pollen grain.

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