2004 Part 1

1. Meiosis reduces chromosome number and rearranges genetic information.

a. Explain how the reduction and rearrangement are accomplished in

meiosis.

b. Several human disorders occur as a result of defects in the meiotic process. Identify ONE such chromosomal abnormality; what effects does it have on the phenotype of people with the disorder?

c. Production of offspring by parthenogenesis or cloning bypasses the typical meiotic process. Describe either parthenogenesis or cloning and compare the genomes of the offspring with those of the parents.

1. Meiosis is a type of cell division in which the chromosomes are not completely reproduced. Instead of full cell division, the chromosome number is halved so that these cells can be used in sexual reproduction.

2. The joining of two sex cells will produce a cell with the full number of chromosomes. That cell will then continue to divide in order to create a new organism.

3. Meiosis is divided up into several stages. The broadest of which are Meiosis I and Meiosis II. The basics of meiosis are very similar to regular mitosis, but four cells are produced instead of the two by normal cell division.

4. In Meiosis I, the homologous pairs in a diploid cell separate, producing two haploid cells. Haploid cells are cells with a single set of chromosomes. Diploid (di) cells, have two sets of an organism’s chromosomes.

5. For example, a regular human cell—a muscle cell, a skin cell, etc—contains 46 chromosomes with a pair of every chromosome. Haploid cells in humans, sex cells, have 23 chromosomes with a single chromosome instead of a pair.

6. This is followed by prophase I, where homologous chromosomes pair and crossing over occurs. This step is unique to meiosis and allows for various combinations of traits. The paired and replicated chromosomes are called bivalents or tetrads which have two chromosomes.

7. Crossing over is the process by which two chromosomes pair up and exchange sections of their DNA. During this process, matching regions on matching chromosomes break and then reconnect to the

other chromosomes. The result of this process is an exchange of genes. This allows for variations of the same trait.

8. During metaphase I, homologous pairs of chromosomes move together along the metaphase plate. Microtubules emerge from both centrioles and attach the homologous pairs which then line up along an equatorial plan that bisects the spindle.

9. Anaphase I involves the separating of the homologous chromosomes by the microtubules from metaphase I. This forms two haploid sets of the organism’s chromosomes which can be used in two sex cells. The cell elongates in preparation for division down the center.

10. The final step of Meiosis I is telophase I which is the last meiotic division. This step 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 disappear, a new nuclear membrane surrounds each haploid set, the chromosomes uncoil back into chromation, and cytokinesis occurs.

11.Cytokinesis is the pinching of the cell membrane in animal cell division. Due to this process, the two sets of chromosomes are separated and the organelles equally divide themselves along with the cytoplasm. In plant cells, this process involves the creation of an additional cell wall.

12. Upon completion of meiosis I, meiosis II begins where the two new haploid cells divide again. The division that occurs in this process is almost identical to mitosis. The end result is four fully functioning haploid cells, or sex cells.

13. In prophase II, the nucleoli disappear along with the nuclear envelope while the chromatids shorten and thicken. The centrioles move to the polar regions of the cell and arrange spindle fibers for the second meiotic division.

14. Metaphase II is identical to metaphase I. The chromosomes move together along the metaphase plate while attached by the microtubules that emerge from the centrioles. The chromosomes then line up along the new equatorial metaphase plate which is perpendicular to the previous plate.

15.During anaphase II, the centromeres are cleaved and the microtubules pull the sister chromatids apart. As they move toward the opposing poles to prepare for cytokinesis, the sister chromatids become sister chromosomes.

16.Meiosis II ends with telophase II which is marked by the uncoiling and lengthening of the chromosomes and the disappearance of the spindle. Cytokinesis occurs again creating four daughter cells, each with a haploid set of chromosomes. Thus ends meiosis.

17.Occasionally, meiosis is not successful and results in mismatched chromosomes. The chromosomes may be paired unequally

sometime during the division and results in chromosomal disorders. These include disorders like Down’s syndrome, Klinefelter syndrome, Turner syndrome, and XYY syndrome.

18.Down’s syndrome is also known as trisomy 21. From the name, one can tell what the actual disorder is. Instead of the required two, there are three (tri) chromosome 21’s. This is the smallest human chromosome .

19. Trisomy 21 is caused by a meiotic nondisjunction event. In nondisjunction, a gamete is produced with an extra copy of chromosome 21 causing the gamete to contain 47 chromosomes. Most cases of trisomy 21 result from nondisjunction in the materal gamete.

20. In rare cases, some cells in the body have 46 chromosomes while others have trisomy 21. this is called mosaic Down syndrome. It occurs due to a nondisjunction event during an early cell division in the embryo instead of the nondisjunction in a sex cell.

21.Down syndrome is estimated to occur at somewhere between 1 in 800 to 1 in 1000 births. Maternal age greatly influences the changes of conceiving a baby with Down syndrome. For women 20 to 24 the probability is 1 in 1562. This rockets to 1 in 19 for women over 45.

22. Parthenogenesis, is an asexual form of reproduction found in females where growth and development of embryos or seeds occurs without fertilization by a male. The offspring produced by parthenogenesis are always female. While this process occurs naturally in some species, it has also been induced artificially in other species.

23. In this process, females produce eggs that develop without fertilization and develop without a Y chromosome thus producing only females. Parthenogenesis occurs naturally in aphids, daphnia, rotifers, some other invertebrates, and in many plants. Some vertebrates like komodo dragons and hammerhead-sharks also complete this process naturally.

24.Only in the XY sex-determination system will all offspring be female. In the less common ZW sex-determination system, both chromosomes passed on will be those associated with males. The process involves the inheritance and subsequent duplication of only a single sex chromosome.

25.When parthenogenesis occurs naturally, the offspring is capable of sexual reproduction. This process of asexual reproduction has existed since the beginning of life on Earth which implies that is successful.

26. As with all types of asexual reproduction, there are costs and benefits. The cons comprise of low genetic diversity and susceptibility to adverse mutations that might occur. The main pro is that of reproduction without the need for a male. A vital characteristic in perhaps a dying species.

This came up on google and I thought it was kinda funny