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Problems of Cell Cycle
If the most basic function of the cell cycle is to duplicate accurately the DNA in the
chromosomes and then distribute the copies precisely to the daughter cells, why are
there gaps between S phase and M phase?
Many cell-cycle genes from human cells function perfectly well when expressed in
yeast cells. Why do you suppose that is considered remarkable? After all, many
human genes encoding enzymes for metabolic reactions also function in yeast, and no
one thinks that is remarkable.
What determines the length of S phase? One possibility is that it depends on how
much DNA the nucleus contains. As a test, you measure the length of S phase in
dividing cells of a lizard, a frog, and a newt, each one of which has a different amount
of DNA. As shown in Table 17-1, the length of S phase does increase with increasing
DNA content.
Even though these organisms are similar, they are different species. You recall that it
is possible to obtain haploid embryos of frogs and repeat your measurements with
haploid and diploid frog cells. Haploid frog cells have the same length S phase as
diploid frog cells. Further research in the literature shows that in plants, tetraploid
strains of beans and oats have the same length S phase as their diploid cousins.
Propose an explanation to reconcile these apparently contradictory results. Why do
you suppose the length of S phase increases with increasing DNA content in different
species, but remains constant with increasing DNA content in the same species?
The budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharomyces pombe provide facile genetic systems for studying a wide range of
eukaryotic cell biological processes. If cell-cycle progression is essential for cell
viability, as it is in these yeasts, how is it possible to isolate cells that are defective in
cell-cycle genes?
You have isolated a new Cdc mutant of budding yeast that forms colonies at 25°C but
not at 37°C. You would now like to isolate the wild-type gene that corresponds to the
defective gene in your Cdc mutant. How might you isolate the wild-type gene using a
plasmid-based DNA library prepared from wild-type yeast cells?
Fertilized eggs from the frog Xenopus, which contain 100,000 times more cytoplasm
than a typical mammalian cell, are a favorite choice for studying the biochemistry of
the cell cycle. Why isn't it just as easy to study these biochemical questions by
growing large numbers of mammalian cells, which is a straightforward process?
As genes encoding cyclin-dependent kinases were cloned from fission yeast, budding
yeast, and animal cells, investigators working in plant systems designed experiments
to determine whether plants also possessed CDKs that functioned as the catalysts for
cell cycle progression. To isolate a cDNA encoding a CDK in maize, investigators
aligned the predicted amino acid sequences of human cdc2, S. pombe cdc2, and S.
cerevisiae cdc28 genes (see the figure below). Degenerate oligonucleotide PCR
primers corresponding to the boxed amino acids were generated and a DNA
fragment of the expected size was synthesized by PCR using these primers and maize
cDNA library as a template. Why were the boxed regions selected as the basis for
primer design?
The PCR product was then used as a probe to screen the maize cDNA library, and a
full-length cDNA clone was isolated. The clone was sequenced, and the predicted
amino acid sequence was 64% identical to human cdc2 and 63% identical to S.
pombe cdc2 and to S. cerevisiae cdc28. What does the level of sequence similarity
suggest about the evolution of cdc2 genes?
A complementation experiment was performed with S. cerevisiae cells possessing a
temperature-sensitive cdc28 mutation. Wild-type cells, cdc28ts cells, and cdc28ts cells
transformed with the maize cdc2 cDNA under the influence of a strong promoter
were grown at the permissive (25 C) or restrictive (37 C) temperatures. Cell
proliferation was monitored by the growth of colonies on the culture plates (shown
in the figure below).
Why do the cdc28ts cells form colonies at 25 C but not 37 C? What is the
significance of colony formation of the cdc28ts + maize cdc2 cells at 37 C? What does
this experiment tell us about the functional homology of cyclin-dependent kinase
genes among eukaryotic species?
Order the following events in animal cell division.
A. Alignment of chromosomes at the spindle equator. B. Attachment of micro
tubules to chromosomes.
C. Breakdown of nuclear envelope.
D. Condensation of chromosomes.
E. Decondensation of chromosomes.
F. Duplication of centrosome.
G. Elongation of the spindle.
H. Pinching of cell in two.
I. Re-formation of nuclear envelope.
J. Separation of centrosomes.
K. Separation of sister chromatids.