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Lecture 3, part 4 of 4
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III. Meiosis 2

Prophase II.

This second meiotic division is very similar to a mitotic division. Prophase II differs from Prophase I in appearance as the sister chromatids show a very striking repulsion so that the chromatid arms are widely separated from each other. This makes the dyad arms look like crosses and the shortness of the partially coiled chromosomes makes it possible for chromosome counts.
 
 


Prophase II in Rye anthers

Image from Dept. of Plant Science, University of Manitoba


Metaphase II

Anaphase II

Metaphase II in Rye anthers

Image from Dept. of Plant Science, University of Manitoba




Anaphase II in Rye anthers

Image from Dept. of Plant Science, University of Manitoba

Telophase II


 

Telophase II in Rye anthers

Image from Dept. of Plant Science, University of Manitoba
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Click here for a QuickTime animation (5.7Mb)  of the later steps of meiosis ,  by Maura Jess, Neuroscience Research Institute University of California, Santa Barbara. obtained from http://www.science.smith.edu/Biology/Bio112/meiosislab.html

IV. C Values

 
Fig. 7.3. Increase and decrease of the C-value during mitotic and meiotic cycles of animals and plants. S-synthesis; A-anaphase. (Redrawn from Swanson et al., 1967, Prentice-Hall, Inc., Englewood Cliffs, N.J.).

The relationship between mitosis and meiosis can be expressed in terms of total DNA in a haploid gamete before fertilization, a gamete has 1C and a zygote has 2C. During the S phase of premeiotic meiosis, the DNA content rises to 4C and at the mitotic anaphase goes back to 2C.

Anaphase I reduces the DNA content to 2C and anaphase II to gametic level of 1C.

V. DURATION OF MEIOSIS

The duration of meiosis is positively correlated with DNA content per nucleus in diploid species and with mitotic cycle time (Table 3.3). Meiotic duration is also influenced by chromosomal organization, DNA structure and the developmental pattern of the species.
Table 3.3 Duration of meiosis (h) in diploid species
Species 2n Meiotic 

cycle (hr.)

DNA per cell 

(picograms)

Antirrhinum majus 16 24.0 5.5
Haploppapus gracilis 4 36.0 5.5
Secale cereale 14 51.2 28.7
Allium cepa 16 96.0 54.0
Tradescantia paludosa 12 126.0 59.0
Tulbaghia violacea 12 130.0 58.5
Lilium henryi 24 170.0 100.0
Lilium longiflorum 24 192.0 106.0
Trillium erectum 10 274.0 120.0

Note: Results from several authors; data taken at different temperatures, however, provide convincing evidence.  From Bennett, M.D. 1971. Proc. Royal Soc. London Ser. B., 178:277-299.

VI. GAMETOGENESIS

Evolution is constantly reinventing the mechanisms of sexual reproduction. We will only discuss one example here. In this example, we will detail one of the many variants of the process of gametogenesis across the higher eukaryotes, as found in flowering plants.

  

Microsporogenesis - male meiosis in flowering plants
In higher plants, the two adjacent cells undergo a synchronized procedure of chromosome congression, orientation and distribution. The two equatorial plates line up across the separating cell wall. The kinetochores double, the monad chromosomes move towards the four poles of the duet cells of microsporogenesis and form a telophase II cell. The cell walls form at telophase II to produce the so-called radial quartet cells from four adherent cells resulting from two meiotic divisions. These quartets then differentiate into four haploid microspores, end products of microsporogenesis.

Megasporogenesis - femaile meiosis in flowering plants
Oogenesis and megasporogenesis are different (unequal) in that only one large haploid egg and three small polar bodies in animals and only one large megaspore in plants are produced. Three mitotic divisions occur during megagametogenesis. In the first division, a megaspore nucleus divides to give rise to two primary nuclei. The second division produces two nuclei at each end of the embryo sac. The third mitosis results in four nuclei at each of the opposite poles of the nuclear sac.


One nucleus from each pole known as polar nuclei migrates to the middle of the embryo sac and the two nuclei fuse to give rise to a secondary nucleus or polar fusion nucleus with 2n chromosome constitution. At the micropylar end of the embryo sac, one nucleus differentiates into an egg cell and the remaining nuclei become synergids, part of the egg apparatus. The other three nuclei are called the antipodal nuclei.

During pollination in higher plants, the tube nucleus of the pollen grain directs the growth of the pollen tube down the style, through the micropyle and into the nucellus. The two sperms are released into the nuclear sac. One sperm unites with the egg cell to produce the 2n zygote. The other sperm fuses with the secondary polar nucleus to form the endosperm nucleus with triploid  (3N) chromosome number. From this double fertilization, the seed develops.



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previous page PLNT3140 Introductory Cytogenetics
Lecture 3, part 4 of 4
first page