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CLASS XI BIOLOGY
Cell Cycle And Cell Division
1. What is the average cell cycle span for a mammalian cell?
Answer: The average cell cycle span of a mammalian cell is 24 hours.
2. Distinguish cytokinesis from karyokinesis.
Answer: The division of cytoplasm is called cytokinesis, while the division of nucleus
is called karyokinesis.
3. Describe the events taking place during interphase.
Answer: The interphase is divided into three further phases:
G1 phase (Gap 1): G1 phase corresponds to the interval between mitosis and
initiation of DNA replication. During G1 phase the cell is metabolically active and
continuously grows but does not replicate its DNA.
S phase (Synthesis): S or synthesis phase marks the period during which DNA
synthesis or replication takes place. During this time the amount of DNA per cell
doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C.
However, there is no increase in the chromosome number; if the cell had diploid or
2n number of chromosomes at G1, even after S phase the number of chromosomes
remains the same, i.e., 2n.
G2 phase (Gap 2): In animal cells, during the S phase, DNA replication begins in
the nucleus, and the centriole duplicates in the cytoplasm. During the G2 phase,
proteins are synthesised in preparation for mitosis while cell growth continues.
4. What is Go (quiescent phase) of cell cycle?
Answer: Some cells in the adult animals do not appear to exhibit division (e.g.,
heart cells) and many other cells divide only occasionally, as needed to replace cells
that have been lost because of injury or cell death. These cells that do not divide
further exit G1 phase to enter an inactive stage called quiescent stage (G0) of the
cell cycle. Cells in this stage remain metabolically active but no longer proliferate
unless called on to do so depending on the requirement of the organism.
5. Why is mitosis called equational division?
Answer: Since the number of chromosomes remains same in parent and daughter
cells so mitosis is also called as equational division.
6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
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Answer: Metaphase
(ii) Centromere splits and chromatids separate.
Answer: Anaphase
(iii) Pairing between homologous chromosomes takes place.
Answer: Metaphase I of meiosis
(iv) Crossing over between homologous chromosomes takes place.
Answer: Prophase I of meiosis
7. Describe the following:
(a) synapsis (b) bivalent (c) chiasmata
Answer: (a) During zygotene of prophase I of meiosis homologous chromosomes
pair together. This pairing is called synapsis.
(b) Bivalent: The complex formed by homologous chromosomes during zygotene is
called a bivalent.
(c) Chiasmata: During diplotene the paired chromosomes make a x-shaped
structure. This is called chiasmata.
Draw a diagram to illustrate your answer.
8. How does cytokinesis in plant cells differ from that in animal cells?
Answer: Due to the presence of a cell wall, cytokinesis in plant cells is significantly
different from that in animal cells. Rather than forming a contractile ring, plant cells
construct a cell plate in the middle of the cell. The Golgi apparatus releases vesicles
containing cell wall materials. These vesicles fuse at the equatorial plane and form a
cell plate. The cell plate begins as a fusion tube network, which then becomes a
tubulo-vesicular network (TVN) as more components are added. The TVN develops
into a tubular network, which then becomes a fenestrated sheet which adheres to
the existing plasma membrane.
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9. List the main differences between mitosis and meiosis.
Answer: Difference Between Mitosis And Meiosis
Mitosis
Takes place in somatic cells
Meiosis
Takes place in gametic cells
Two daughter cells are formed in the end
Four daughter cells are formed in the
end
Number off chromosomes is halved in
daughter cells as compared to those in
parent cells.
Cell division involves multiple and longer
stages
Number of chromosomes remains same
daughter cells as compared to those in
parent cells
Cell division happens in simples steps
10. What is the significance of meiosis?
Answer: Significance of Meiosis
1. Maintaining of genetic identity by maintaining number of chromosomes.
2. Bringing variations to ensure better species.
3. Facilitates sexual reproduction.
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