Download 10. Cell Cycle and Cell Division.

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1. What is the average cell cycle span for a mammalian cell?
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Solution:
The average cell cycle span for mammalian cell is 24 hours.
2. Distinguish cytokinesis from karyokinesis.
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Solution:
Cytokinesis
Karyokinesis
The division of cytoplasam during mitotic phase is The division and separation of chromosomes to
called cytokinesis.
daughter chromosomes is called karyokinesis.
3. Describe the events taking place during interphase.
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Solution:
This phase is called ‘resting stage’, but it is in fact a period of great activity. Three important
processes, which are preparatory to cell division, take place during interphase. They are,
(i) Replication of DNA along with the synthesis of nuclear proteins such as histones.
(ii) In animal cells, duplication of a centriole takes place by the outer growth of daughter centrioles
from the parent centrioles, which are at right angles to each other.
(iii) Synthesis of energy-rich compounds, which provide energy for mitosis and synthesis of proteins
at the end of interphase.
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Interphase can be divided into three periods,
(i) the post mitotic gap phase (G1) takes place at the end of one cell division. RNA and protein are
synthesized during this period, but there is no synthesis of DNA.
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(ii) During the synthesis phase (S), DNA is formed form nucleotides and the DNA content of the
nucleus is doubled.
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(iii) During the pre-mitotic gap phase (G2), synthesis of RNA and protein continues, but DNA
synthesis stops. The duration of the S phase, the G2phase and mitosis is generally constant in most
cell types.
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The length of G1 phase is usually variable. Cells that do not divide frequently have a longer G1 phase,
while frequently dividing cells have a shorter phase. During G1 phase, a cell may follow one of the
three options; (a) cell may continue on the cycle and divide, (b) the cell can permanently stop
division and enter G0 or quiescent stage and the (c) the cell cycle may be arrested at a specific point
of G1 phase. The cell in the arrested condition is said to be in the G0 state. The cell in the G0 state
may be considered to be withdrawn from the cell cycle. When conditions change and growth is
resumed, the cell re-enters the G1 period. Eukaryotic chromosomes undergo condensation - decondensation cycle during interphase. G1chromosomes are completely dispersed.
The most important point in the regulation of the cell cycle occurs in the G1 phase, during which it
must decide whether the cell wall starts a new cycle or will become arrested in the G0 phase. Once
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this G1 check point has been passed, the cell goes on to complete a new cycle.
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The G1 phase can be terminated by various stimuli. Once a higher eukaryotic cell enters the S phase
and has begun DNA replication, it usually begins to divide. During interphase, replication of
chromosomes takes place so that each chromosome now consists of two chromatids. Following this,
the cell enters into the mitosis (M) phase. Hence, the cell cycle is divided into four phases: G1 phase,
S phase, G2 phase and M phase.
4. What is G0 (quiescent phase) of cell cycle?
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Solution:
In adult animals some cells do not exhibit division and many other cells divide occasionally as to
replace cells, which have been lost due to injury. These cells that do not divide further exit G1 phase
to enter an inactive stage called quiescent stage (G0) of the cell cycle. Depending on the
requirement of the organism the cells proliferate other wise the cells remain metabolically active but
they do not proliferate.
5. Why is mitosis called equational division?
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Solution:
In mitosis, the process of cell division occurs whereby the chromosomes are duplicated and
distributed equally to the daughter cells. This type of a division is called equational division.
6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
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(iv) Crossing over between homologous chromosomes takes place.
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(ii) Centromere splits and chromatids separate during the anaphase.
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Solution:
(i) Chromosomes are moved to spindle equator during the metaphase stage.
(iii) Pairing between homologous chromosomes takes place during zygotene stage of prophase I in
meiosis.
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(iv) Crossing over between homologous chromosomes takes place during the pachytene stage of
prophase I in meiosis.
7. Describe the following:
(a) synapsis
(b) bivalent
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(c) chiasmata.
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Solution:
(a) The process of association during the crossing over of chromosomes in the zygotene stage is
called synapsis.
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(b) At the end of synapsis a synaptonemal complex is formed. This complex formed by a pair of
synapsed homologous chromosomes is called a bivalent or tetrad.
(c) The beginning of the diplotene stage is marked by the beginning of separation of the paired
homologous chromosomes. The separation of homologous chromosomes is however not completed.
They remain attached at one or more points where crossing over has occurred. These points of
attachment are called chiasmata.
8. How does cytokinesis in plant cells differ from that in animal cells?
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Solution:
Cytokinesis is different in an animal and a plant cell. In animal cells a cleavage furrow appears at the
beginning of telophase. This furrow or constriction becomes progressively deeper as the spindle
breaks down. Eventually, the in-growing constrictions join and separate two daughter cells. This
division of cytoplasm is called cytokinesis. When nuclear division takes place without cytoplasmic
division, it results in the formation of syncytium, which is a condition where a large number of nuclei
are present in a single cell.
In plants, there is a formation of cell plate between the two daughter nuclei. This grows from the
middle towards the periphery and finally joins the cell wall. The cell plate represents the middle
lamella between the walls of two adjacent cells. During cell division, cell organelles like mitochondria,
plastids, golgi complex, lysosomes and the cytoplasmic matrix are distributed into the two daughter
cells. Of these, mitochondria and plastids reproduce themselves, but details of the fate of other
organelles are not yet known.
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9. Find examples where the four daughter cells from meiosis are equal in size and where
they are found unequal in size.
Solution:
Meiosis takes place during gametogenesis in plants and animals. Under gametogenesis the formation
of sperm is called spermatogenesis at the end of which four daughter cells of equal size is formed.
Formation of ovum is called oogenesis, which results in the formation of four unequal daughter cells.
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Solution:
Anaphase of mitosis
The splitting of centromere separates the
chromatids.
The chromatids move to the opposite poles
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10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Anaphase I of meiosis
The sister chromatids remain associated at their
centromere.
The homologous chromosome without splitting at
the centromere separate and move to the
opposite poles.
11. List the main differences between mitosis and meiosis.
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Solution:
Meiosis
1. There are two successive cell divisions; the first
and the second meiotic divisions.
2. Meiosis takes place in the germ cells.
3. It occurs only in sexually reproducing organisms.
4. The DNA replicates once for two cell divisions.
5. Prophase is comparatively longer and may take
days.
6. Prophase is complicated and is divided into
leptotene, zygotene, pachytene, diplotene and
diakinesis.
7. Synapsis of homologous chromosomes takes
place during prophase I.
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Mitosis
1. The cell divides only once after one round of
DNA replication.
2. Mitosis takes place in the somatic cells.
3. It occurs in both sexually as well as asexually
reproducing organisms.
4. The DNA replicates once for one cell division.
5. The duration of prophase is short; usually for
a few hours.
6. Prophase is comparatively simple.
7. There is no synapsis.
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Solution:
(i) It maintains a definite and constant number of chromosomes in the organisms.
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12. What is the significance of meiosis?
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8. The two chromatids of a chromosome do not 8. The chromatids of two homologous chromosomes
exchange segments during prophase.
exchange homologous segments during the
pachytene stage of prophase.
9. During prophase and metaphase, each
9. During prophase and metaphase, homologous
chromosomes form bivalents. Each bivalent has four
chromosome consists of two chromatids held
together by a centromere.
chromatids and two centromeres.
10. The arms of the chromatids are close to one 10. The arms of the chromatids are separated
another during prophase.
widely during prophase II.
11. Division of centromeres takes place during 11. There is no centromeric division during
anaphase.
anaphase I. Centromeres divide only during
anaphase II.
12. Spindle fibres disappear completely in
12. Spindle fibres do not disappear completely
telophase.
during telophase I.
13. Nucleoli reappear at telophase.
13. Nucleoli do not reappear at telophase I.
14. The chromosome number remains constant 14. The chromosomal number is reduced from
at the end of mitosis.
diploid to haploid.
15. The genetic constitution of daughter cells is 15. The genetic constitution of the daughter cells
identical to that of the parent cell.
usually differs from that of the parent cell due to
crossing over. Each chromosome of daughter cells
usually contains a mixture of maternal and paternal
genes.
(ii) By crossing over, meiosis provides an opportunity for the exchange of genes and thus, causes
genetic variation within the species. The variation serves as raw material in the evolutionary process.
(iii) Meiosis has an impact on genetic consequences due to events such as paring of homologous
chromosomes, process of crossing over and recombination and segregation of homologous
chromosomes.
13. Can there be mitosis without DNA replication in ‘S’ phase?
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Solution:
In mitosis ‘S’ phase is the period during which DNA synthesis or replication takes place. In animals
DNA replication begins in the nucleus and centriole duplicates in the cytoplasm.
14. Can there be DNA replication without cell division?
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Solution:
Replication of DNA takes place only during the ‘S’ phase of cell division. There cannot be a DNA
replication without cell division.
15. Analyse the events during every stage of cell cycle and notice how the following two
parameters change
(i) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell
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Solution:
(i) The number of chromosomes (N) remains constant throughout the different stages in mitosis. In
meiosis the number of chromosome is reduced to half that is from diploid to haploid. During the
anaphase I stage this half number is carried on till the end of meiosis.
(ii) The amount of DNA (C) becomes double during the interphase stage which is followed by mitosis.
This doubling occurs due to the replication of the DNA.
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