Ploidy After meiosis, germ cells contain a haploid (1n) genetic complement. After fertilization, a 46,XX or 46,XY diploid DNA complement is restored. Restoration of the normal cellular DNA content is crucial to normal function. Abnormalities of cellular DNA content cause distinct phenotypic abnormalities, as exemplified by hydatidiform molar pregnancy (see Chapter 39). In a complete hydatidiform mole, an oocyte without any nuclear genetic material (e.g., an empty ovum) is fertilized by one sperm. The haploid genetic content of the fertilized ovum is then duplicated, and the diploid cellular DNA content is restored, resulting in a homozygous 46,XX gamete. Less often, a complete hydatidiform mole results from the fertilization of an empty ovum by two sperm, resulting in a heterozygous 46,XX or 46,XY gamete. In complete molar pregnancies, the nuclear DNA is usually paternally derived, embryonic structures do not develop, and trophoblast hyperplasia occurs. Rarely, complete moles are biparental. This karyotype seems to be found in patients with recurrent hydatiform moles and is associated with a higher risk of persistent trophoblastic disease.
A partial hydatiform mole follows the fertilization of a haploid ovum by two sperm, resulting in a 69,XXX, 69,XXY, or 69,XYY karyotype. A partial mole contains paternal and maternal DNA, and both embryonic and placental development occur. Both the 69,YYY karyotype and the 46,YY karyotype are incompatible with embryonic and placental development. These observations demonstrate the importance of maternal genetic material, in particular the X chromosome, in normal embryonic and placental development.
In addition to total cellular DNA content, the chromosome number is an important determinant of cellular function. Abnormalities of chromosome number, which often are caused by nondisjunction during meiosis, result in well-characterized clinical syndromes such as trisomy 21 (Down syndrome), trisomy 18, and trisomy 13.
Genetic Control of the Cell Cycle
Cellular proliferation must occur to balance normal cell loss and maintain tissue and organ integrity. This process requires the coordinated expression of many genes at discrete times during the cell cycle (3). In the absence of growth factors, cultured mammalian cells are arrested in the G0 phase. With the addition of growth factors, these quiescent cells pass through the so-called restriction point 14 to 16 hours later and enter the S phase 6 to 8 hours thereafter. The restriction point or G1/S boundary marks the point at which a cell commits to proliferation. A second checkpoint is the G2/M boundary, which marks the point at which repair of any DNA damage must be completed (4–7). To successfully complete the cell cycle, a number of cell division cycle (cdc) genes are activated.