Chapter 10: Cell Cycle and Cell Division

Q11Written answer

List the major differences between meiosis and mitosis.

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Ans: Differences between meiosis and mitosis:

Mitosis	Meiosis
It occurs in all somatic cells and may persist all over life.	It occurs in reproductive cells at specific times.
It involves a single division, which leads to two daughter cells only.	It involves two successive divisions, which leads to four daughter cells.
Consequent mitotic divisions are similar to the previous ones.	Two meiotic divisions are dissimilar, the first is reductional while the second is equational.
Prophase is relatively short and simple.	Prophase I is very long and elaborate, which comprises five subphases like leptotene, pachytene, zygotene, diplotene, and diakinesis.
There is no pairing of homologous chromosomes.	Homologous chromosomes pair and often undergo crossing over in prophase I.
Chromatids are genetically similar to the chromosomes they arise from.	Chromatids may vary genetically from the chromosomes they emerge from because of crossing over.
No synaptonemal complex formed.	Synaptonemal complex forms among synapsed homologous chromosomes.
Chromosomes do not unfold, and no protein synthesis and transcription take place in prophase.	Chromosomes unfold, n and protein synthesis and transcription may take place in diplotene of prophase I (oocytes of specific animals).
Daughter cells have a diploid number (2N) of chromosomes like the parent cell.	Daughter cells have a haploid number (N) of chromosomes, unlike the parent cell.

Q12Written answer

Distinguish anaphase of mitosis from anaphase 1 of meiosis.

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Ans: Difference between anaphase of mitosis from anaphase 1 of meiosis:

During mitosis, chromatids separate from one another, but during meiosis, synapsed homologous chromosomes split from one another during anaphase I.
It is important to recollect here that during the metaphase stage of prophase chromosomes lie on the equator with their centromere lying on the metaphase plate. The centromere separates and the chromatids travel to opposite poles throughout anaphase.
In metaphase I of Meiosis, the bivalents arrange themselves on the equator, and centromeres of synapsed homologous chromosomes do not lie on the equator.
Anaphase I involves the separation of paired homologous chromosomes to the opposite poles of the equator. This is known as a disjunction of chromosomes.

Diagram showing anaphase I of Meiosis

Diagram showing anaphase of Mitosis

Q13Written answer

What is the significance of meiosis?

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Ans: The significance of meiosis is given below:

Formation of gametes: Meiosis is a process that produces gametes, which are necessary for sexual reproduction.
Genetic information: It activates genetic information for the production of gametophytes or gametes while turning off saprophytic information.
Maintenance of chromosome number: Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms by producing gametes that have half of the chromosomes as compared to the mother cell. It is vital since the chromosome number develops into a double after fertilization
Assortment of chromosomes: In meiosis maternal and paternal chromosomes separate independently. It leads to the reordering of chromosomes and the characteristics controlled by them. The variations help the breeders in enhancing the races of domesticated animals and plants.
Crossing over: It leads to the recombination of genes that brings about variations.
Mutations: Chromosomal and genomic mutations can occur by irregularities of meiotic divisions. Some of these mutations are useful to the organism and are propagated by natural selection.
Evidence of basic relationship of organisms: Details of meiosis are basically similar in the vast majority of organisms indicating their basic resemblance and relationship.

Q14Written answer

Discuss with your teacher about:

i. Haploid insects and lower plants where cell division occurs, and

ii. Some haploid cells in higher plants where cell division does not occur.

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Ans: Cell division tends to happen in haploid insects, including drones of the honey bee and lower plant-like gametophytes of bryophytes, algae, and pteridophytes.

Ans: Synergids and antipodals in the embryo sac of the ovule are haploid cells where cell division does not occur.

Q15Written answer

Can there be mitosis without DNA replication in the ‘S’ phase?

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Ans: No, there cannot be any mitotic division without – DNA replication in the ‘S’ phase.

Q16Written answer

Analyze events that took place during each phase of the cell cycle and observe in what way the following two parameters change.

i. A number of chromosomes (N) per cell.

ii. Amount of DNA content (C) per cell.

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Ans: Number of chromosomes (N) per cell:

In mitotic division, the number of chromosomes remains the same as the parent cell at prophase and metaphase. At the anaphase stage of mitosis when the sister chromatids separate and form two nuclei in telophase, each of them contains the same number of chromosomes as the parent cell.
In meiotic division, the homologous chromosomes get separated into two different poles during anaphase-I. Thus during prophase-I, metaphase-I and anaphase-I, the cell contains the same chromosome number as the parent cell. As two separate nuclei are formed in telophase, each contains half the number of chromosomes compared to the parent cell.
During meiosis-II, the sister chromatids are separated in anaphase-II. Thus the daughter cells retain their chromosome number the same as cells after meiosis I. So, the daughter cells contain N number of chromosomes if the parent cell has \[2\]N chromosomes.

Ans: Amount of DNA content (C) per cell:

S or synthesis phase indicates the period during which DNA replication or synthesis takes place. Throughout this time the amount of DNA per cell becomes double.
If the initial amount of DNA is signified as \[2\]C then it rises to \[4\]C. During prophase, metaphase and anaphase of mitosis, the DNA content remains \[4\]C.
After the telophase, as the sister chromatids are separated into two separate nuclei, each nucleus contains \[2\]C amount of DNA same as the parent cell in G\[1\]phase.
In meiosis, the daughter cells after first division contain \[2\]C volume of DNA which gets halved i.e., C after second division.

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