ANSWERS TO REVIEW QUESTIONS – CHAPTER 08
1. How does a bacterial cell divide? (p. 168)
A bacterial cell has a single, circular, double-stranded molecule of DNA attached to one point on the
plasma membrane. This molecule is replicated between cell divisions and the new molecule, an
identical copy of the other, attaches to a separate point on the plasma membrane. When the cell reaches
a certain size, growth of the plasma membrane and cell wall between the attachment points of the
molecules separates them. The plasma membrane and cell membrane grow inwards across the middle
of the cell, ultimately creating two new cells, each with one of the two molecules of DNA.
2.
Describe the following structures and their role in cell division: chromatid, centromere,
kinetochore, centriole, aster, microtubule. (pp. 170–176)
Chromatid—At the onset of cell division in eukaryotic cells, each chromosome consists of two
identical chromatids, held together at a constricted region known as the centromere. During anaphase
of cell division the chromatids will separate, with one of each pair moving to each pole of the cell.
Centromere—A constricted region of the chromosome joining the two chromatids. It consists of two
kinetochores, each of which is connected to microtubules. Centromeres attach chromosomes to the
spindle during cell division.
Kinetochore—Two protein discs or kinetochores comprise the centromere. During prometaphase of
cell division, the kinetochores of each chromosome interact with the spindle fibres to move the
chromosomes. When the chromosomes divide during anaphase, the kinetochores move each chromatid
to opposite poles.
Centriole—Centrioles are located near the nucleus of animal and protist cells and are associated with
the spindle during cell division.
Aster—Asters are arrays of radial fibres (bundles of microtubules) arising from the centrosomes that
occupy the poles of the spindle during cell division.
Microtubule—These are tubular scaffolding components of the cytoskeleton, forming hollow cylinders.
They are associated with plasma membranes and with the formation of the spindle during cell division.
3. What are homologous chromosomes? (p. 169)
The term ‘homologous’ means ‘similar but not identical’. Thus homologous chromosomes are a pair,
one inherited from the egg and one from the sperm, which physically appear identical but have subtle
differences between them. They associate at the first stage of meiosis and one of each pair is passed to
each of the daughter cells.
4. What is meant by haploid and diploid? (p. 178)
The diploid state is the most common in eukaryotic organisms. Each cell contains pairs of homologous
chromosomes (2n, where n is the number of homologous pairs of chromosomes, also known as the
haploid number). Diploid cells have two copies of each gene, one on each chromosome of a
homologous pair. Thus a deleterious mutation in one of the genes can be masked by the presence of the
other. Haploid cells contain only one of each of the homologous pairs (n). The haploid state is most
commonly restricted to the gametes (sperm and ova), which unite at fertilisation to produce a diploid
zygote.
5.
State when each of the following occurs during the mitotic cell cycle of eukaryotes. (pp. 169–
171)
(a) DNA replication
This occurs during the S phase of the cell cycle. At the end of the S phase the nucleus is enlarged and
holds double the amount of DNA.
(b) Breakdown of the nuclear envelope
This occurs during prometaphase, a stage of cell division itself.
(c) Chromosome aggregation in the middle of the spindle
Metaphase.
(d) Centromere division
Anaphase.
6.
Describe how sister chromatids separate from one another and move to opposite poles
during anaphase. (pp. 171–176)
During anaphase, the two kinetochores in the centromere of each chromosome divide. The sister
chromatids then move apart, forming two identical groups. The sister chromatids are then drawn to
opposite poles of the cell.
7.
Some of the processes observed in prophase and early prometaphase are reversed during
telophase. What are they? (pp. 171–176)
Chromosomes clumped together at the poles decondense into chromatin, a nuclear envelope forms
around the chromosomes, and the nucleoli reform and become conspicuous.
8. Compare and contrast cytokinesis in animal and plant cells. (pp. 171–176)
Cell division in animal and plant cells is more remarkable for its great similarity than for its
differences. Those differences relate mainly to the absence of centrioles in plant cells and to the
presence of the cell wall in plants, which influences the development of the final cell wall.
The absence of centrioles means that plant cells lack astral spindles, with fibres focused upon discrete
poles. Rather, they have broad poles and the fibres form a barrel-shaped spindle. The presence of a cell
wall means that the cleavage characteristic of animal cells cannot occur. Instead, fibres between the
separated chromosomes thicken to form a phragmoplast, which in turn gives rise to a cell plate that
thickens and separates the two cells. Vesicles within the cell plate fuse and fibrous wall material
develops between them.
9.
Describe the purpose of cell cycle checkpoint controls. How do they work during the cell
cycle? (pp. 177–178)
Checkpoint controls are the mechanisms that sense whether there is sufficient or undamaged DNA to
undergo cell division and then determine whether or not to proceed with mitosis. Checkpoint
mechanisms active during the G2 phase cause inhibition of MPF and thereby prevent progression into
the M phase. An additional checkpoint during mitosis determines if the centromeres are attached and
inhibits activation of protease complex, which prevents sister chromatid separation and cyclin
degradation.
10. What major events occur during meiosis I? What is its end result? (p. 179)
Meiosis I ends with two sets of nuclei formed, each of which is haploid with one set of chromosomes
(n). This is achieved by moving randomly one member of each homologous pair of chromosomes to
opposite poles of the cell, a process known as independent assortment. The reduction in chromosome
number and the random separation of homologous chromosomes are two significant events of the first
division. A third major event is crossing over. During prophase I, homologous chromosomes align
along their lengths. Chromatids of these homologous pairs may exchange portions of their genetic
material, allowing new combinations of genetic information.