Chapter 10: Cell Cycle and Cell Division

Q1Written answer

What is the average cell cycle span for a mammalian cell?

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Ans: The average cell cycle span for a mammalian cell is \[24\] Hours.

Q2Written answer

Distinguish between cytokinesis and karyokinesis.

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Ans: The differences between cytokinesis and karyokinesis are—

Cytokinesis	Karyokinesis
‘Cyto’ word stands for the cytoplasm and kinesis for division. So, Cytokinesis is the physical method of cell division, where the cytoplasm of a parental cell is divided into two daughter cells.	Karyon denotes nucleus and kinesis denotes division. So karyokinesis is the physical method of the nucleus division. Karyokinesis precedes cytokinesis.
It occurs at the end of the M-phase after the nuclear division is over.	It takes place during the M-phase of the cell cycle.

Q3Written answer

Explain the events that take place throughout the inter-phase.

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Ans: Below mentioned are the events that take place during the inter-phase:

The interphase is also known as the preparatory phase of the cell cycle.
During this period although the cell is said to be in a resting phase, it is metabolically quite active.
It is the time during which the cell prepares itself for division by enduring both DNA replication and cell growth in a sequential and orderly manner and it is completed in three steps.

(i) G1 (Gap 1) phase- This phase basically involves the synthesis of RNA and protein.

(ii) S (Synthesis) phase – It involves DNA replication. Throughout this time the amount of DNA doubled per cell. In animal cells, during the S phase DNA replication takes place in the nucleus, and the centriole replicates in the cytoplasm.

(iii) G2 (Gap 2) phase – It also involves RNA and protein synthesis.

Diagram showing Interphase of the cell cycle

Q4Written answer

What is the G0 (quiescent phase) of the cell cycle?

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Ans: Quiescent Phase-

The G0 phase is characterised by the inactivation of the cell cycle due to the lack of mitogens and energy-rich substances.
Cells during this phase tend to stay metabolically active however they no longer proliferate. i.e., do not undergo splitting unless called on to do so dependent on the requirement of the organism.
For example, the Nerve and heart cells of chordates are in the permanent Go phase.

Q5Written answer

Why is mitosis called equational division?

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Ans: Mitosis is called equational division as:-

Mitosis is a cell division process wherein chromosomes multiply and become evenly distributed in two daughter nuclei, resulting in the daughter cells having the same number and kind of chromosomes as that of the mother cell.
So mitosis is called equational division.

Q6Written answer

Name the phase of the cell cycle at which each one of the following events takes place:

i. When chromosomes are transferred to the spindle equator.

ii. When a centromere splits and chromatids separate.

iii. When a pairing between homologous chromosomes takes place.

iv. When a crossing over among homologous chromosomes occurs.

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Ans: Metaphase

Ans: Anaphase

Ans: Zygotene of prophase-I of meiosis

Ans: Pachytene of prophase-I of meiosis

Q7Written answer

Describe the following.

(a) Synapsis

During the zygotene phase of prophase-I of meiosis, homologous chromosomes begin to join up, a process known as synapsis.
Electron micrographs of this phase reveal that chromosome synapsis is accompanied by the formation of a complex structure known as synaptonemal complex among the synapsed chromosomes.
Pairing is taking place in a zipper-like fashion.

(b) Bivalent

The complex established by a pair of synapsed homologous chromosomes is known as a tetrad or a bivalent i.e., a pair of chromosomes or four chromatids.
A bivalent contains two centromeres and four chromatids.

(c) Chiasmata

Chiasmata are cross-like structures that appear in synapsed chromosomes. They characterize sites at which crossing over has taken place.
Crossing over is the exchange of gene segments among non-sister chromatids of homologous chromosomes.
Crossing over takes place at the pachytene stage, however, chiasmata are visible in the diplotene stage of prophase 1 of meiosis.

Draw a diagram to illustrate your answer.

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Ans: Following diagram shows the synapsis, bivalent and chiasma formation.

Q8Written answer

In what way does cytokinesis in animal cells vary from that in plant cells?

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Ans: Cytokinesis in animal cells vary from that in plant cells:-

Cytokinesis in plant cells takes place by cell plate formation whereas in animal cells cytokinesis is accomplished by furrow formation.
Plant cells have a rigid cell wall. So In-plant cell wall development begins in the middle of the cell and develops outward to connect the existing lateral walls.
This involves the laying of a cell plate that characterises the middle lamella among the cell walls of adjacent cells.
Cell plate develops centrifugally. Animal cells lack a cell wall. Hence cytokinesis involves the development of a furrow in the cell membrane. This grows centripetally and connects ultimately in the centre of the cell and the cytoplasm is split into two.

Diagram showing differences between animal and plant cell cytokinesis

Q9Written answer

Illustrate with examples where the four daughter cells from meiosis are equivalent in size and where they are found uneven in size.

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Ans: The four daughter cells created during meiosis during the generation of male gametes (i.e. spermatozoa) in a human being are similar in size. Then, during the creation of a female gamete (i.e., ovum), in a human being, the four daughter cells are unequal in size.

Q10Written answer

Is DNA replication possible without cell division?

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Ans: Yes. DNA replication is possible without cell decisions. Endomitosis is the proliferation of chromosomes found in a set in the nucleus without cytokinesis and karyokinesis leads to several copies within each cell. It is of two kinds, which are described below.

i. Polyteny:

Here chromosomes split and redivide without partition of chromatids so that such chromosomes turned into multi-stranded with several copies of DNA.
Such polytene (several stranded) chromosomes persist in the permanent prophase stage and do not endure the cell cycle e.g., polytene salivary glands) the chromosome of Drosophila.
Here several sets of chromosomes do not change.

ii. Polyploidy (endo-duplication)

Here all chromosomes in a set split and its chromatids are isolated but the nucleus does not split.
This raises the number of sets of chromosomes in the nucleus (\[4\]x, \[8\]x…..).
This rise in sets of chromosomes is known as polyploidy. It can be stimulated by colchicine (alkaloid obtained from Colchicum autumnale (family –Liliaceae). These chromosomes are normal and endure a cell cycle.

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