Cell Cycle and Cell Division

Cell Cycle and Cell Division is a fundamental biological process that enables organisms to grow, develop, and repair tissues. It involves a series of events that lead to the division of a cell into two daughter cells.

Phases of the Cell Cycle

1. Interphase: This is the preparatory phase where the cell grows, synthesizes DNA, and prepares for division. It consists of three stages:
   • G1 phase: The cell grows in size and synthesizes proteins.
   • S phase: DNA replication occurs, resulting in the formation of identical copies of DNA.
   • G2 phase: The cell continues to grow and synthesizes proteins needed for cell division.
2. Mitotic (M) Phase: It consists of two stages:
   • Cytokinesis: The division of the cytoplasm, resulting in the formation of two daughter cells.

Types of Cell Division

1. Mitosis: This type of cell division results in the formation of two daughter cells that are genetically identical to the parent cell.
2. Meiosis: This type of cell division results in the formation of four daughter cells that are genetically different from the parent cell. It is used for sexual reproduction.

Significance of Cell Division

Cell division is essential for:

• Repair: Cell division is used to repair damaged tissues.
• Reproduction: Asexual reproduction involves cell division, while sexual reproduction involves a combination of meiosis and fertilization.
• Development: Cell division plays a crucial role in the development of embryos and the formation of different tissues and organs.

Exercise

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

Ans : The average cell cycle span for a mammalian cell is 24 hours. However, this can vary depending on factors such as cell type, organism, and environmental conditions. For example, rapidly dividing cells, such as those in the intestinal lining, may have shorter cell cycles, while slower-dividing cells, such as nerve cells, may have longer cell cycles.

2. Distinguish cytokinesis from karyokinesis.

Ans : 

Feature	Karyokinesis	Cytokinesis
Process	Division of the nucleus	Division of the cytoplasm
Timing	Occurs after DNA replication	Follows karyokinesis
Result	Formation of two identical sets of chromosomes	Formation of two daughter cells
Animal Cells	Cleavage furrow formation	Cleavage furrow formation
Plant Cells	Cell plate formation	Cell plate formation

3. Describe the events taking place during interphase

Ans : 

1. G1 phase (Gap 1):
   
   • The cell grows in size and mass.  
   • Organelles are duplicated.  
   • Proteins necessary for DNA replication and cell division are synthesized.  
   • The cell accumulates nutrients and energy.  
2. S phase (Synthesis):
   
   • The cell’s DNA is replicated, creating identical copies of chromosomes.  
   • Centrosomes, which organize microtubules during cell division, are also duplicated.  
3. G2 phase (Gap 2):
   
   • The cell continues to grow and synthesize proteins.  
   • The cell prepares its organelles and cytoskeleton for mitosis.  

4. What is Go (quiescent phase) of cell cycle?

Ans : 

The G0 phase, also known as the quiescent phase, is a resting phase of the cell cycle. They may remain in this phase for a long period of time, or they may re-enter the cell cycle under certain conditions.

Some cells, such as nerve cells and muscle cells, permanently enter the G0 phase and do not divide again after they are fully developed. Other cells, such as liver cells and fibroblasts, can re-enter the cell cycle in response to signals, such as injury or growth factors.

The G0 phase is important for maintaining tissue homeostasis and preventing uncontrolled cell growth, which can lead to cancer.

5. Why is mitosis called equational division?

Ans : 

Mitosis is called equational division because it results in the formation of two daughter cells that are genetically identical to the parent cell. 

During mitosis, the chromosomes are replicated and then equally divided between the two daughter cells. This ensures that each daughter cell receives a complete and identical set of chromosomes, hence the term “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. 

(iv) Crossing over between homologous chromosomes takes place.

Ans : 

Event	Stage of Cell Cycle
(i) Chromosomes are moved to spindle equator	Metaphase
(ii) Centromere splits and chromatids separate	Anaphase
(iii) Pairing between homologous chromosomes takes place	Prophase I (meiosis)
(iv) Crossing over between homologous chromosomes takes place	Prophase I (meiosis)

7. Describe the following: 

(a) synapsis (b) bivalent (c) chiasmata 

Draw a diagram to illustrate your answer

Ans : 

Synapsis: This is the pairing of homologous chromosomes during the zygotene stage of prophase I in meiosis. 

Bivalent: A bivalent, also known as a tetrad, is a pair of synapsed homologous chromosomes. It is formed during the zygotene stage of prophase I.

Chiasmata: Chiasmata are the points where the non-sister chromatids of homologous chromosomes cross over. They are formed during the pachytene stage of prophase I. 

8. How does cytokinesis in plant cells differ from that in animal cells?

Ans : 

Cytokinesis in plant cells differs from that in animal cells primarily due to the presence of a cell wall in plant cells.

Animal Cells:

• Cytokinesis occurs through a cleavage furrow.  
• A ring of contractile proteins (actin and myosin) pinches inward at the equator of the cell, forming a furrow that eventually divides the cell into two.  

Plant Cells:

• Cytokinesis occurs through the formation of a cell plate.  
• Vesicles from the Golgi apparatus fuse at the center of the cell, forming a cell plate.  
• The cell plate extends outwards until it reaches the cell wall, dividing the cell into two daughter cells.  
• Each daughter cell then forms a new cell wall on either side of the cell plate.

9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Ans : 

Equal-Sized Daughter Cells:

• Most animals: In most animals, the four daughter cells produced through meiosis are approximately equal in size. This ensures that each daughter cell receives a balanced set of genetic material.
• Some plants: In certain plant species, such as ferns and gymnosperms, the four daughter cells formed through meiosis can be relatively equal in size.

Unequal-Sized Daughter Cells:

• Angiosperms: In flowering plants (angiosperms), the four daughter cells produced through meiosis are typically unequal in size. One of the daughter cells, called the megaspore, is larger than the other three, which are called microspores. The megaspore develops into the female gametophyte (ovule), while the microspores develop into pollen grains.
• Some algae: In certain algae, the four daughter cells produced through meiosis can also be unequal in size. This can be due to differences in the distribution of cytoplasm during cell division.

10. Distinguish anaphase of mitosis from anaphase I of meiosis.

Ans : 

Feature	Anaphase of Mitosis	Anaphase I of Meiosis
Homologous Chromosomes	Not present	Present
Separation	Sister chromatids separate	Homologous chromosomes separate
Result	Each daughter cell receives a complete set of chromosomes	Each daughter cell receives only one member of each homologous pair
Ploidy	Remains diploid	Becomes haploid

11. List the main differences between mitosis and meiosis.

Ans : 

Feature	Mitosis	Meiosis
Purpose	Growth, repair, asexual reproduction	Sexual reproduction
Number of Divisions	One	Two
Number of Daughter Cells	Two	Four
Ploidy of Daughter Cells	Diploid (same as parent)	Haploid (half the number of chromosomes as the parent)
Genetic Variation	No genetic variation	Significant genetic variation due to crossing over and independent assortment
Homologous Chromosomes	Do not pair	Pair and undergo crossing over
Separation	Sister chromatids separate	Homologous chromosomes separate
Occurrence	Somatic cells	Germ cells

12. What is the significance of meiosis?

Ans : 

Meiosis is significant because it is essential for sexual reproduction and genetic diversity.

• Sexual Reproduction: Meiosis produces haploid gametes (sperm and egg cells) that can fuse during fertilization to form a diploid zygote. This process allows for the creation of genetically unique offspring, which increases the genetic diversity of a population.
• Genetic Diversity: Meiosis contributes to genetic diversity through two mechanisms:
  • Independent Assortment: The random alignment of homologous chromosomes during metaphase I and II results in the independent assortment of genes, further increasing genetic diversity.
• Adaptation: Genetic diversity is essential for adaptation to changing environmental conditions. Organisms with greater genetic diversity are more likely to have individuals with traits that allow them to survive and reproduce in new environments.

13. 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

Ans : 

(i) Haploid Insects and Lower Plants Where Cell-Division Occurs

• Haploid Insects: While most insects are diploid, some species have haploid males. In these species, the male is produced from an unfertilized egg. Cell division occurs in the male to produce sperm cells.
• Lower Plants: In some lower plants, such as mosses and liverworts, the gametophyte generation is haploid. Cell division occurs in the gametophyte to produce gametes, which then fuse to form a diploid sporophyte.

(ii) Haploid Cells in Higher Plants Where Cell-Division Does Not Occur

• Pollen Grains: Pollen grains are haploid male gametophytes. They do not undergo cell division after they are released from the anther.
• Ovules: Ovules are haploid female gametophytes. They do not undergo cell division after they are formed.

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

Ans : 

No, DNA replication is a crucial step in the cell cycle, ensuring that each daughter cell receives a complete and identical copy of the genetic material. If DNA replication does not occur, the daughter cells would have incomplete or incorrect genetic information, leading to severe consequences or even cell death.

Therefore, DNA replication in the S phase is an essential prerequisite for mitosis to proceed.

15. Can there be DNA replication without cell division?

Ans : 

Yes, DNA replication can occur without cell division.

While DNA replication is typically followed by cell division (mitosis or meiosis), there are certain scenarios where DNA replication takes place without subsequent division:

1. Endoreduplication: This is a process where a cell undergoes multiple rounds of DNA replication without dividing its cytoplasm. This results in a cell with multiple copies of its genome, which can be beneficial for certain cell types, such as muscle cells and liver cells.
2. Formation of polyploid cells: Polyploid cells contain more than two sets of chromosomes. They can arise through endoreduplication or through the fusion of multiple cells. Polyploidy can be beneficial in some plants and animals, as it can increase cell size and metabolic activity.
3. Development of gametes: During the formation of gametes (sperm and egg cells) through meiosis, DNA replication occurs without cell division. This results in the production of haploid gametes, which have half the number of chromosomes as the parent cell.

16. 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

Ans : 

The cell cycle is divided into two major phases: interphase and mitotic (M) phase. Let’s analyze how the number of chromosomes (N) and the amount of DNA content (C) change during each stage.

Interphase

• G1 phase:
  • N remains constant.
  • C remains constant.
• S phase:
  • N remains constant.
  • C doubles as DNA replication occurs.
• G2 phase:
  • N remains constant.
  • C remains constant (after DNA replication).

Mitotic (M) Phase

• Prophase:
  • N remains constant.
  • C remains constant (after DNA replication).
• Metaphase:
  • N remains constant.
  • C remains constant.
• Anaphase:
  • N doubles as sister chromatids separate.
  • C remains constant.
• Telophase:
  • N remains constant.
  • C remains constant.
• Cytokinesis:
  • N remains constant in each daughter cell.
  • C remains constant in each daughter cell.

Stage	Chromosome Number (N)	DNA Content (C)
G1	N	C
S	N	2C
G2	N	2C
Prophase	N	2C
Metaphase	N	2C
Anaphase	2N	2C
Telophase	2N	2C
Cytokinesis	2N (in each daughter cell)	2C (in each daughter cell)