Module 5

Module 5—Cell Division: The Processes of Mitosis and Meiosis

Lesson Summary

 

This lesson focused on the following questions:

• How do chromosome disorders occur, and why does their occurrence increase with maternal age?
  
• How can embryonic cells be used, and what technologies exist to test the genetic condition of an unborn fetus?

Chromosomes may not separate properly in meiosis I or meiosis II. This is known as nondisjunction. When this occurs in a gamete that is later fertilized and becomes a new individual, the result is a person with a serious condition, such as Klinefelter syndrome.

 

Tests like amniocentesis, cordiocentesis, and chorionic villi sampling can diagnose the presence of abnormal chromosome numbers in an unborn fetus. However, no treatments can be done to change chromosome numbers since the error is found in every cell of that new life.

 

Early embryonic cells are totipotent. As a result, they can become any type of cell present in the human body. It is this characteristic that causes them to be of particular interest to scientists who are trying to find cures for diseases caused by adult cells shutting down or stopping regular function. Society will need to decide upon the guidelines which will govern how scientists are allowed to use these amazing cells.

 

Lesson Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

amniocentesis: a needle sampling of fetal amniotic fluid to gather embryonic cells for karyotyping; determines chromosome number and gender, not presence of specific genes/alleles on chromosomes

 

blastocyst: the embryonic stage about one week after conception; a hollow ball of cells consisting of outer trophoblast that becomes the chorion and inner cell mass that becomes the embryo; implants in the wall of the endometrium

 

chorionic villus sampling (CVS): a sampling of chorionic villi through the vagina, used to obtain embryonic cells for karyotyping

 

cordiocentesis: a sampling of umbilical cord blood of fetus for karyotyping

 

monosomy: offspring that only have one of a particular chromosome rather than two; occurs when a normal n gamete fuses with an n − 1 gamete, producing a 2n − 1 zygote; in humans, an offspring that has a diploid number of 45 rather than 46

 

nondisjunction: non-separation of chromosomes during meiosis; one pair gets dragged to one pole, and no representative of that pair is pulled to the other pole; if occurs in meiosis I, gametes will be two n + 1 gametes and two n – 1 gametes; if occurs in meiosis II, gametes will be n + 1, n – 1, n, and n, results in offspring that have a trisomy or monosomy for that chromosome

 

oocyte: a cell undergoing oogenesis

 

The primary oocyte undergoes meiosis I, producing a large secondary oocyte and the first polar body, which is reabsorbed. The secondary oocyte undergoes meiosis II, forming a large ootid and a second polar body, which is reabsorbed.

 

polar body: a tiny cell that results from each division of oogenesis

 

Meiosis I results in large secondary oocyte and a tiny polar body that is reabsorbed. Meiosis I results in a huge ootid and a tiny second polar body that is reabsorbed. Polar bodies are “garbage cans” that extra nuclear material is dumped into to fulfill the need for one large haploid ovum.

 

prenatal genetic testing: sampling and testing of embryonic or fetal cells to  determine chromosome number and gender

 

spermatids: haploid cells that result from meiosis; swim to epididymis for maturation to a spermatazoan and storage until ejaculation

 

spermatocyte: cells undergoing spermatogenesis; primary spermatocyte undergoes first meiotic division, forming two secondary spermatocytes; each secondary spermatocyte undergoes second meiotic division, forming two spermatids; four spermatids formed from one primary spermatocyte

 

spermatogenesis: the process of producing gametes in males; occurs in walls of seminiferous tubules of testes

 

spermatogonium: initial diploid germ cell in spermatogenesis; undergoes mitosis, forming many primary spermatocytes that each carry out meiosis

 

stem cell: an undifferentiated “generic” cell that can be coaxed into producing a number of different kinds of cells; present in embryonic blastocysts and also recently found in adults; potentially used to repair tissue and build replacement organs

 

Embryonic stem cell research is controversial due to the fact that the creation and termination of human blastocysts is used as a method of obtaining stem cells.

 

trisomy: offspring that have three of one kind of chromosome rather than two; occurs when an n + 1 gamete fuses with a normal n gamete, producing a 2n + 1 zygote; in humans an offspring that has a diploid number of 47 rather than 46

 

Biology 30 Learn EveryWare © 2009 Alberta Education