Class 9 Biology – Chapter 4: Cell Cycle

Board: Federal Board (FBISE)

Section I: Multiple Choice Questions

1. A bivalent consists of:

A) Two chromatids and one centromere
B) Four chromatids and two centromeres
C) Four chromatids and four centromeres
D) Two chromatids and two centromeres

Correct Answer: B) Four chromatids and two centromeres

2. During cell division, spindle fibres attach to a chromosome at the:

A) Centromere
B) Telomere
C) Upper arm of chromosome
D) Lower arm of chromosome

Correct Answer: A) Centromere

3. Some SSC students observed a thin cross-section of an onion root tip under a microscope. They found dividing cells at different stages of their life cycle. One student found a cell at late prophase and counted 28 chromosomes in it. What will be the number of chromosomes in the daughter cells?

A) 14
B) 28
C) 56
D) 7

Correct Answer: B) 28

4. Crossing over results in genetic recombination. It occurs between:

A) Two chromatids of the same chromosome
B) Two chromatids of any two non-homologous chromosomes
C) Two chromatids of opposite gametes
D) Two chromatids of homologous chromosomes

Correct Answer: D) Two chromatids of homologous chromosomes

More precisely, crossing over occurs between non-sister chromatids of homologous chromosomes.

5. The spindle apparatus of plants differs from that of animals because it does not have:

A) Microtubules
B) Equator of spindle
C) Centrioles
D) Centromere

Correct Answer: C) Centrioles

6. Substances and energy required for DNA replication are accumulated in the cell during:

A) \(G_1\) phase
B) \(G_0\) phase
C) S phase
D) M phase

Correct Answer: A) \(G_1\) phase

7. All of the following events take place in both mitosis and meiosis except:

A) Condensation of chromatin to form chromosomes
B) Formation of spindle apparatus
C) Disappearance of the nuclear envelope and nucleolus
D) Pairing of chromosomes for crossing over

Correct Answer: D) Pairing of chromosomes for crossing over

8. The cell shown in the diagram is passing through:

A) Prophase I of meiosis
B) Prophase of mitosis
C) Telophase of meiosis I
D) Anaphase of meiosis II

Correct Answer: A) Prophase I of meiosis

Note: The diagram was not included in the supplied material.

9. The longest phase of meiosis is:

A) Interphase I
B) Prophase I
C) Interphase II
D) Prophase II

Correct Answer: B) Prophase I

10. What causes the number of chromosomes to reduce to half when a cell divides by meiosis?

A) Replication of DNA during Interphase I
B) Separation of homologous chromosomes during meiosis I
C) Separation of sister chromatids of all chromosomes during meiosis I
D) Crossing over during meiosis I

Correct Answer: B) Separation of homologous chromosomes during meiosis I

11. Which of the following cells is at the Metaphase I stage?

A) Homologous pairs aligned at the metaphase plate
B) Anaphase pulling chromosomes apart
C) Interphase nucleus
D) Telophase division

Correct Answer: A) Homologous pairs aligned at the metaphase plate

12. The diagram shows different stages of crossing over. Which stage contains a chiasma?

A) Stage A
B) Stage B
C) Stage C
D) Stage D

Correct Answer: B) Stage B

Note: The diagram was not included in the supplied material.

13. Which process involves meiosis?

A) Region A
B) Region B
C) Region C
D) Region D

Correct Answer: B) Region B

Note: The referenced diagram is required to identify the process shown in Region B.

14. The diagram shows the replication of chromosomes. What is the total number of chromosomes in this diagram?

A) 2
B) 3
C) 4
D) 6

Correct Answer: A) 2

Note: The diagram was not included. The supplied answer key gives option A.

15. The chromosome number of a fruit fly is 8. The gametes of a fruit fly contain:

A) 2 chromosomes
B) 4 chromosomes
C) 8 chromosomes
D) 16 chromosomes

Correct Answer: B) 4 chromosomes

Section II: Short Answer Questions

1. Explain the spindle apparatus in detail.

The spindle apparatus is a temporary cytoskeletal structure made of microtubules. It forms during cell division (mitosis and meiosis) and helps separate chromosomes towards opposite poles of the cell.

It consists of three main types of fibres:

• Kinetochore fibres: Attach to chromosomes at their kinetochores and help move them during cell division.

• Polar fibres: Overlap with fibres from the opposite pole and help push the poles apart.

• Astral fibres: Extend towards the cell membrane and help anchor and position the spindle apparatus in animal cells.

2. What is the significance of crossing over?

Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I of meiosis.

Its significance includes:

1. Genetic Variation: It produces new combinations of genes in chromosomes.

2. Evolutionary Adaptation: Genetic diversity provides raw material for natural selection and evolution.

3. Population Survival: It increases variation within populations, helping them adapt to changing environments.

3. Enlist the events taking place during the \(G_1\) phase.

The main events of the \(G_1\) phase are:

1. Intensive cellular growth and protein synthesis.

2. Accumulation of energy reserves and building blocks, such as nucleotides and amino acids, required for DNA replication.

3. Synthesis of enzymes and structural proteins needed for the subsequent S phase.

4. Increase in the number of cytoplasmic organelles.

4. What will be the difference in the materials synthesized by cell A and cell B?

Cell A (\(G_1\) phase):

It synthesizes enzymes, nucleotides, and proteins required for DNA replication during the S phase and for continued cell cycle progression.

Cell B (\(G_0\) phase):

It enters a resting or quiescent state and synthesizes specialized functional proteins required for its particular tissue function rather than proteins needed for cell division.

5. Why do epithelial cells of the skin divide continuously?

Epithelial cells of the skin are constantly exposed to friction, wear and tear, and external damage. Continuous mitotic division replaces dead or damaged cells and maintains the protective skin barrier.

6. Give differences between meiosis I and meiosis II.

7. In rapidly dividing cells, which phase of the cell cycle is reduced? Explain.

The \(G_1\) phase is drastically shortened or may be skipped in rapidly dividing cells, such as early embryonic cells. These cells bypass extensive growth and preparation periods to increase the speed of cell proliferation and DNA replication.

8. What is the difference between cytokinesis in a plant cell and an animal cell?

Animal Cells:

Cytokinesis occurs through the formation of a cleavage furrow. The plasma membrane pinches inward from the edges until the cell divides into two daughter cells.

Plant Cells:

Because plant cells have a rigid cell wall, cytokinesis occurs through the formation of a cell plate. Golgi-derived vesicles collect at the centre of the cell and fuse to form the cell plate, which expands outward and eventually develops into a new cell wall.

9. Both skin cells and cancerous cells divide rapidly. Why are cancerous cells harmful but skin cells are not?

Skin Cells:

Skin cells divide in a controlled and regulated manner to replace damaged or dead cells. They respond to normal physiological signals and stop dividing when the required tissue density is reached.

Cancerous Cells:

Cancerous cells divide uncontrollably due to mutations that disrupt normal cell cycle regulation. They ignore regulatory signals, may form tumours, damage surrounding tissues, and can spread to other parts of the body through metastasis.

10. How do haploid organisms produce gametes?

Haploid organisms already contain a single set of chromosomes (\(n\)). Therefore, they cannot undergo the usual reductional meiosis to produce haploid gametes. Instead, they produce gametes through mitosis or specialized mitotic divisions.

11. Why do people with Down syndrome have 47 chromosomes?

Down syndrome is usually caused by nondisjunction, which occurs when chromosome 21 fails to separate properly during meiosis.

As a result, one gamete receives an extra copy of chromosome 21 and contains 24 chromosomes instead of 23. When it fuses with a normal gamete containing 23 chromosomes, the resulting zygote has 47 chromosomes.

This condition is called trisomy 21 and is represented as \(2n+1\).

Section III: Extensive Answer Questions

1. If a cell completes meiosis I but meiosis II fails to occur, what type of anomalies will appear in the two daughter cells thus formed?

If meiosis I is completed but meiosis II does not occur, two daughter cells are formed. Each cell has a haploid number of chromosomes, but every chromosome still consists of two attached sister chromatids.

The resulting cells therefore have:

1. A haploid chromosome number (\(n\)).

2. Chromosomes that still consist of two sister chromatids.

3. An increased DNA content compared with normal haploid cells after meiosis II.

4. An inability to produce normal haploid gametes through the usual completion of meiosis.

2. A cell is shown in the diagram. Answer the following questions after observing it.

a) Identify whether it is an animal or plant cell.

It is an animal cell because clearly visible centrioles are present at the poles.

b) Identify the stage of cell division.

The cell is in Anaphase of mitosis.

c) Enlist the reasons for your identification.

1. Sister chromatids have separated at their centromeres.

2. Spindle fibres are pulling the separated chromatids towards opposite poles.

3. The chromosomes form characteristic V-shaped or U-shaped structures as they move towards the poles.

Note: The original diagram was not included; the identification above follows the description supplied in the source.

3. Meiosis II is identical to mitosis. Explain.

Meiosis II resembles mitosis because both involve the separation of sister chromatids.

The similarities are:

1. Sister Chromatid Separation: In both processes, sister chromatids separate at the centromere during anaphase.

2. Equational Division: Both are equational divisions because the chromosome number remains unchanged in the resulting cells.

3. Similar Stages: Both follow four main stages:

• Prophase

• Metaphase

• Anaphase

• Telophase

4. Spindle Apparatus: In both processes, spindle fibres help move chromosomes towards opposite poles.

Difference in Chromosome Number: Mitosis usually occurs in diploid cells and produces diploid daughter cells, whereas meiosis II begins with haploid cells produced by meiosis I and produces haploid daughter cells.

Notes by Sir Dilawar

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