WARM UP #6: Meiosis / Nondisjunction
With selected student responses

QUESTION 1: = Why do cells undergoing mitosis require one set of divisions, but cells undergoing meiosis require 2 sets of divisions? What is the end result of meiosis?

From Amy: Q1 = Cells undergoing mitosis just divide once because they are forming two new genetically identical cells where as in meiosis cells require two sets of divisions because they need to make the cell a haploid cell which only has half of the total number of chromosomes.The end rsult of meiosis is 4 haploid cells.

From Rachel: Q1 = In mitosis the chromosomes in a cell are copied, line up at metaphase plate, split apart at the centromere, and segregate into two new cells. This is to grow, repair and develop cells that are in the body. Meiosis occurs in the chromosomes , but they are reduced from 46 to 23. A pair of 23 are taken from the two reproduction cells one set from each cell. The end result is a baby.

From LM: Q1 = Cells undergoing mieosis require 2 sets of divisions because only half of the cromosomes from each parent are needed. This is so half of the offspring's genes come from each parent. This process generates the diversity of all sexually reproducing organisms. Meiosis produces sex cells eggs and sperm.

From Lily: Q1 = During mitosis cells are creating exact duplicates of themselves through copy and division. That is their goal. To create more of themselves as identical as possible to be used as replacement/development cells throughout the body. The purpose of meiosis, however, is to create a gamete that carries 23 chromosomes, half many chromosomes as the parent cell. Each gamete ends with half of a mix-matched set of chromosomes that when fertilized with another gamete combine to make a unique human blueprint.

From Dr. Marrs: Excellent answers - In either case, the chromosomes (46) are duplicated to sister chromatids (46; held together at the centromere) during S phase.
Mitosis: The cell divides ONCE (1 cell becomes 2), pulling the sister chromatids apart at the centromere, so that each cell ends up with a full set of chromosomes (46, 23 pairs).
Meiosis: The cell divides TWICE (1 cell becomes 4), the first time lining up and then pulling the homologues apart (now 23 sister chromatids in each cell), and the second time pulling the sister chromatids apart, so that each cell ends up with a half set of chromosomes (23, one of each pair).

QUESTION 2: = A common error in meiosis in an egg (or a sperm) can result in trisomy, in which each somatic cell of the of baby has 3 copies of one particular chromosome. Trisomy 21 is when an individual has not 2 but 3 copies of Chromosome 21 in every one of their cells. Trisomy 21 is also called Down Syndrome. This is not in the book - but what do you think happens in meiosis that would result in 3 copies of Chromosome 21 in an embryo? Do you think there are other trisomys in humans?

From Amy: Q2 = I am sure that there are other trisomys in humans but I am not sure what they are. They are probably formed when something goes wrong with the replication or the division the second time.

From PP Q2 = I definitely think there are other trisomys out there. The original cell must not have divided correctly possibly because of outside factors. When one of the daughter haploids divided then it took two Chromosome 21 and left the other sister without any. I hope that makes sense.

From RV: Q2 = The prefix tri means three. My guess would be that instead of getting a pair of chromosomes in number 21 that something goes wrong like a cell not splitting and the other copy attaches to that chromosome anyway which ends up with three instead of a pair in that particular area. (I was curious about this after answering it so I went searching the net. Found this info : one cell has two 21st chromosomes instead of one, so the resulting fertilized egg has three 21st chromosomes. Hence the scientific name, trisomy 21 there is a triplication of the 21st chromosome material.) I do think that other trisomys happen in humans.

From Des: Q2 = Yes, I think that there are many other trisomys in humans. If meiosis would result in 3 copies of Chromosome 21 in an embryo, I think the results of other trisomys would be awful!

Fron Walker: Q2 = Just as in any cell, the opportunity for error is great. In the case of Down Syndrome, the cell must have had the 21st pair of chromosome attract an additional/misplaced chromosome.(?)I'm assuming that the genetic material on this additional chromosome is what attributes to the unique characteristics of a Down's child. Where this chromosome came from I don't know.

From Dr. Marrs: Trisomy 21 is one of the only viable trisomys in humans. Many children with Trisomy 21 die in utero, but those born have the condition called Down Syndrome, and have an extra copy (3 total) of chromosome 21 in each of their cells. This condition happens when an egg (or sperm) does not go through meiosis properly, and both chromosome 21 homologues end up in one egg (leaving the other egg or sperm with no copies of chromosome 21). Fertilization results in 3 copies of that chromosome instead of 2. Advanced maternal age correlates with frequency of Down syndrome. Two other common trisomys are Trisomy 18 and Trisomy 13; both these trisomys are typically fatal in early childhood. Other trisomys occur, but the pregnancy cannot continue.

QUESTION 3: = Chemicals such as alcohol and nicotine can be teratogens. What are teratogens and why are they so dangerous for a developing baby? What does the name teratogen mean? How can heat (like from a fever or use of a hot tub in early pregnancy) be a teratogen?

From Pint-size: Q3 = Teratogens are toxins that are transmitted from the pregnant woman to her fetus. They are so dangerous for a developing baby because all the vital organs are forming and the toxins can severely hurt or even kill the baby.

From Ah: Q3 = Teratogens are substances that cause birth defects. Teratogens can cause abnormalties such as mental retardation, heart defects, brain damage, cancer, low birth weight, spina bifida, and FAS, etc. Teratology is the study of abnormal formations in animals or plants.

From BT: Q3 = I think teratogens are poisons. They are dangerous for the baby's development because these toxins can inflict serious damage to a fetus. I have absolutely no idea what the name means but just a stab in the dark I'll say in means posion. Heat can be a teratogen because high temperatures can cause proteins to unravel and as the saying goes, "you can't unboil an egg."

From Panda: Q3 = Teratogens are things that are dangerous for a baby because they can cause birth defects. According to Websters, teratogens, teras means monster + gen = create. I think that heat from a fever or hot tub would be a teratogen in early pregnancy because it would raise the temp of the embryo so much that it could literally cook the cells that are forming. The result would be mutations in certain cells that could cause birth defects.

From Atlman: Q3 = As far as I can tell, teratogens are toxins such as alcohol and cigarette tar that inhibit the growth and development of the fetus during its crucial period of organ growth.

QUESTION 4: Misc

From sal: Q4 = I am sure that it will have an affect on my life somewhere down theline. How do I know, I might end up a biology professor.
From Lil: Q4 = I don't think i will ever use this stuff. I won't be teaching elementary students in depth biology. I am just hoping that i pass this class so i never have biology again!!!!
From Nick: Q4 = can we review the whole mitosis splitting process again.