A. Detail orally and in writing, the process of oogenesis noting the fate of the chromosomes.
B. Detail orally and in writing, the process of spermatogenesis noting the fate of the chromosomes.
C. Compare these two processes and cite their importance in human development.
Approximate time: one week.
The story of human development begins long before egg and sperm unite in fertilization. The earliest form of these gametes is seen in the primordial germ cells of the early embryo. These primordial cells grow by a series of mitoses to form oogonia in the fetal ovaries and spermatogonia in the fetal seminiferous tubules. The oogonial cells and the spermatogonial cells, which are the precursors of the egg and sperm, however, follow different paths of maturation.
The oogonia continue to divide by mitosis to produce more oogonia or begin to divide by meiosis and undergo the first meiotic division to form primary oocytes. (see Figure 1 Oogenesis). During the first meiotic division the full set of chromosomes,
, called diploid (2N) pair up or synapse as matching pairs or homologues and align themselves gene by gene. While the chromosomes are paired up they undergo crossing over whereby parts are exchanged between the chromatids (a half chromosome) that results in genes changing from one homologous chromosome to the other. The chromosomes at this stage are a tetrad, a four-stranded structure connected by an X-shaped chiasmata. This is prophase I.
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Figure 1 (NOTE: The fate of only one pair of chromosomes is shown.)
By the time a female is born, the primary oocytes have progressed to this point. Some of the oogonial cells have degenerated from “ . . . 6.8 million, found in the five month fetus . . . to 2 million and by puberty less than 200,000 . . . ”
primary oocytes are present. This suspended prophase or dictyotene stage lasts for several years until puberty or beyond when ovulation occurs. Then meiotic division resumes and the first meiotic division (reduction) is completed at the time of ovulation.
When the primary oocyte completes the first meiotic division a secondary oocyte and a polar body are formed each receiving 23 chromosomes, half (haploid) (N). The second meiotic division begins and stops at metaphase as the secondary oocyte passes down the Fallopian tube. If fertilization occurs, the second division is completed and the sperm penetrates the egg with its 23 chromosomes. The act of fertilization activates the ovum to complete meiosis and also restores the diploid number to the zygote. There is only one ovum produced and another polar body. “By this time, the first polar body might also have divided into two tiny cells.”
The male spermatogonia remain in the fetal seminiferous tubules until puberty when they undergo the process of meiosis with regard to the nuclear material and chromosomes. However, the events proceed without interruption once they are started. At about 13-16 years of age, the spermatogonia (which come from the primordial germ cells) in the walls of the seminiferous tubules in the testes enter meiosis as primary spermatocytes with the diploid number (2N). (see Figure 2 Spermatogenesis). These undergo the first meiotic division to form 2 secondary spermatocytes with the haploid number (N). Each of the secondary spermatocytes forms 2 spermatids with a total of four from the original spermatocyte. The four spermatids become changed in the process of spermiogenesis where much of the cytoplasm is lost and sperm emerge with a head and tail. There are 4 functional sperm formed as compared to the production of one functional egg. Another contrasting feature is that males, as far as can be determined, continue to produce sperm for life; females cease ovulating at about 45 years of age. It is noteworthy that the long time that the primary oocyte remains in “suspended prophase,” (can be almost 40 years) may account for some of the birth defects (because of meiotic nondisjunction: i.e. failure of paired chromosomes to separate) occurring in mothers over the age of 40 or in later years of life. In addition, the fact that the sperm undergo so many DNA duplications, since millions of sperm are produced, may account for the mutants discovered in late paternal age.
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