bcl-2 has been associated with the increased incidence and severity of
prostate carcinoma in African-American males. Cell division kinase
inhibitors block activation of cyclin-CDk [cyclin dependent kinase com-
plexes(answer b)] and cell cycle progression (answer a).p53 and Rb are
tumor suppressor genes. In the absence of Rb or p53, tumor suppression
and normal control are lost. p53 increases in the presence of DNA damage,
resulting in the inhibition of cell division. p53 mutations inhibit cell divi-
sion kinase (Cdk) inhibitors such as p21, resulting in uncontrolled cell
division. The absence of p53 also permits proliferation of damaged cells.
For more details on the regulation of the cell cycle, see “High-Yield Facts.”
68.The answer is c.(Alberts, pp 1130–1139. Ross and Pawlina, pp 86–87.)
Pachytene begins as soon as the synapsis is complete and includes the
period of crossover. The fully formed synaptonemal complex is present
during the pachytene stage. At each point where crossover has occurred
between two chromatids of the homologous chromosomes, an attachment
point known as a chiasma forms. Meiosis is the mechanism used by the
reproductive organs to generate gametes—cells with the haploid number
of chromosomes. DNA synthesis occurs before meiotic prophase I begins
and is followed by a G 2 phase. Cells then enter meiotic prophase I. During
meiotic prophase I, maternal and paternal chromosomes are precisely
paired, and recombination occurs in each pair of homologous chromo-
somes. The first meiotic prophase consists of five substages: leptotene,
zygotene, pachytene, diplotene, and diakinesis. During metaphase I, there
is random segregation of maternal and paternal chromosomes. Homolo-
gous chromosomes are aligned on the metaphase plate of the meiotic spin-
dle in metaphase I. The second meiotic division is responsible for the
reduction in the chromosome content of the cell by 50%. In meiotic divi-
sion II, metaphase consists of daughter chromatids of single homologous
chromosomes aligned on a metaphase plate (metaphase II). Condensation
of the chromatids occurs in leptotene (answer a).In zygotene (answer b),
the synaptonemal complex begins to form, which initiates the close associ-
ation between chromosomes known as synapsis. The bivalent is formed
between the two sets of homologous chromosomes (one set maternal and
one set paternal equals a pair of maternal chromatids and a pair of paternal
chromatids). The four chromatids form a tetrad (bivalent). The formation
of chiasmata and desynapsing (separation of the axes of the synaptonemal
complex) occurs in the diplotene stage (answer d).Diakinesis(answer e)
150 Anatomy, Histology, and Cell Biology