UNIT 9 – Cell Cycle (Q&A) | MZO-001 MSCZOO | IGNOU

SAQ 1

1. (a) Write answers to the following questions in brief.

i. What is the longest phase of the cell cycle?

The longest phase of the cell cycle is Interphase, which occupies about 90% of the total duration of the cycle. It is a highly active stage where the cell prepares for division. Interphase has three sub-phases: G1 phase, where the cell grows and synthesizes proteins; S phase, where DNA replication takes place; and G2 phase, where the cell checks for DNA errors and prepares for mitosis. Though no visible division occurs in interphase, it is crucial for the success of mitosis or meiosis. Cells that do not divide frequently may remain in G1 or enter a resting state called G₀ phase.

ii. During which phase of interphase does DNA replication occur? 

DNA replication occurs during the S phase (Synthesis phase) of Interphase.

In this phase, the entire genetic material of the cell is duplicated so that each daughter cell formed after cell division will receive an identical copy of DNA. Each chromosome is replicated to form two sister chromatids that remain attached at the centromere, preparing the cell for mitosis.

iii. What happens during mitosis (M phase)?

Mitosis, also called the M phase, is the most active and visible part of the cell cycle. In this phase, a single mother cell divides into two daughter cells and each new cell receives the exact copy of genetic material as present in the original cell. This phase is mainly responsible for growth, repair of damaged tissues and maintaining genetic continuity. Mitosis ensures that the chromosome number remains the same in both daughter cells. The entire process is divided into two main stages: karyokinesis, which is the division of the nucleus and cytokinesis, the division of the cytoplasm. These two stages occur in a highly coordinated sequence to ensure the proper distribution of cellular components and genetic material.

1. Karyokinesis (Division of Nucleus)

Karyokinesis is the first major step of mitosis, in which the nucleus of the parent cell divides. It ensures that each daughter cell will receive an equal and exact set of chromosomes. This process is further divided into five clear stages, each with its own specific changes inside the cell:

a. Prophase:

• This is the beginning stage. Here, the long thread-like chromatin condenses and becomes thick and visible as chromosomes. Each chromosome has two identical sister chromatids joined at the centromere. The nucleolus disappears and spindle fibers begin to form as the centrosomes move toward opposite poles of the cell.

b. Prometaphase:

• In this stage, the nuclear envelope completely breaks down. The spindle fibers reach the chromosomes and attach to the centromeres at a region called the kinetochore. The chromosomes begin to move and prepare to line up in the center of the cell.

c. Metaphase:

• This is the stage of perfect alignment. All the chromosomes line up exactly in the middle of the cell, forming what is called the metaphase plate. This proper alignment is important to ensure equal distribution of chromosomes during the next step.

d. Anaphase:

• At this stage, the two sister chromatids of each chromosome are pulled apart by the spindle fibers. They start moving toward opposite poles of the cell. Now each chromatid becomes a separate chromosome. This separation ensures that both sides will receive identical genetic material.

e. Telophase:

• In this final nuclear step, the chromosomes reach the opposite poles and begin to uncoil into thin chromatin again. New nuclear membranes form around each set of chromosomes and the nucleolus reappears. The cell now has two separate nuclei.

2. Cytokinesis (Division of Cytoplasm)

After nuclear division, the cell moves to its final step i.e., cytokinesis. This is the process by which the entire cytoplasm divides, completing the formation of two separate daughter cells. Though it begins during telophase, it is often considered a separate event. Cytokinesis differs slightly between plant and animal cells:

• In animal cells, a contractile ring made of actin filaments forms at the center of the cell, pulling the membrane inward. This process, known as cleavage, deepens until the cell is pinched into two separate daughter cells, each with a full set of organelles and genetic material.

• In plant cells, due to the presence of a rigid cell wall, a cell plate forms between the two daughter nuclei. This plate grows outward until it fuses with the cell membrane, forming a new cell wall that separates the daughter cells.

By the end of mitosis and cytokinesis, two genetically identical daughter cells have been formed, each with an identical set of chromosomes. These cells are now ready to enter the interphase phase of the cell cycle, where they will prepare for their next round of division or differentiation.

iv. What is the function of the G1 phase?

The G1 phase, also known as the first gap phase, is the first phase of interphase in the cell cycle. It occurs after mitosis (M phase) and before the S phase, where DNA replication happens. This phase plays a crucial role in preparing the cell for DNA synthesis and mitosis. The main functions of the G1 phase are:

1. Cell Growth:

• During the G1 phase, the cell increases in size and mass. This phase is critical for the cell to accumulate enough resources, such as proteins, lipids and other macromolecules, needed to proceed with DNA replication and cell division. The cell prepares itself physically by expanding its cytoplasm and increasing the number of organelles, particularly mitochondria and ribosomes, to meet the demands of the next phases.

2. Protein and RNA Synthesis:

• One of the key activities during the G1 phase is the synthesis of proteins and RNA molecules that will be needed for subsequent cell cycle events. Proteins involved in DNA replication, cell signaling and the regulation of the cell cycle (such as cyclins and cyclin-dependent kinases) are produced. Additionally, enzymes such as RNA polymerase are synthesized to aid in gene expression and cell cycle regulation.

3. Preparation for DNA Replication (S Phase):

• The G1 phase is vital for the preparation of the cell's DNA replication machinery. During this phase, the cell synthesizes the necessary proteins and enzymes (such as DNA polymerases and helicases) required for the S phase, where DNA replication occurs. This ensures that all the components needed to duplicate the cell's genetic material are in place.

4. Cell Cycle Checkpoints (G1/S Checkpoint):

• The G1 phase is regulated by critical checkpoints, particularly the G1/S checkpoint. At this point, the cell checks if it has sufficient nutrients, energy and other necessary resources to proceed with DNA replication. The integrity of the DNA is also evaluated, ensuring no damage exists before proceeding to the next stage. If any issues are detected, the cell may either halt progression to the S phase or enter a resting state (G0 phase) to repair or wait for more favorable conditions.

5. Response to External Signals:

• The G1 phase is influenced by various external factors, such as growth factors, hormones and environmental conditions. These signals help the cell decide whether it is appropriate to progress in the cell cycle. For instance, the presence of growth factors can stimulate the cell to enter the S phase, while a lack of such factors or unfavorable conditions can result in the cell entering a non-dividing state (also called as G0 phase), where it remains inactive until conditions are optimal.

6. Metabolic Activity:

• The G1 phase is characterized by increased metabolic activity. The cell requires a large amount of energy and raw materials to support DNA replication and cell division. As a result, the cell increases its metabolic processes, including the production of ATP, lipids, and other molecules that will be needed for growth and division. This ensures that the cell has adequate energy reserves to carry out the necessary functions.

(b) Find the right combination by matching of the columns:

Answer: i)→c;   ii)→b;   iii)→ a;   iv)→d;   v)→e

(c) Find the right combination by matching of the columns:

Answer: i)→c;   ii)→a;   iii)→ b;   iv)→d

SAQ 2

a) Which of the following statements are true or false. Write T for true and F for false.

(i) Mitosis results in two daughter cells with half the number of chromosomes as the parent cell.

 Answer: F

(ii) The spindle fibers help separate sister chromatids during anaphase of mitosis.

Answer: T

(iii) Sister chromatids are identical copies of each other.

Answer: T

(iv) Mitosis is essential for healing wounds by replacing damaged skin cells.

Answer: T

(v) The centrosome plays a role in organising spindle fibers during mitosis.

Answer: T

(vi) The nuclear envelope remains intact throughout mitosis.

Answer: F

b) What are the main stages of Mitosis?

Mitosis is a type of equational cell division in which a single parent cell divides to produce two genetically identical daughter cells. It usually occurs in diploid cells and maintains the same number of chromosomes (2n) in daughter cells as present in the parent cell. Mitosis is commonly found in somatic cells and also in diploid germ cells like spermatogonia and oogonia, where it helps in producing more cells before meiosis begins. It is also seen during growth, repair, regeneration and asexual reproduction in some lower organisms. This division helps in tissue formation, wound healing, and replacement of old or damaged cells. It is a part of the cell cycle and occurs during the M phase. Mitosis has two main stages. The first stage is Karyokinesis which means division of the nucleus, and the second stage is Cytokinesis which means division of the cytoplasm.

1. Karyokinesis

Karyokinesis is the nuclear division part of mitosis. It is a complex process where the chromosomes get separated and move to opposite poles. It is divided into five proper stages. Each stage happens in a sequence and has its own specific role in dividing the chromosomes. These stages are: Prophase, Prometaphase, Metaphase, Anaphase and Telophase.

i. Prophase

• This is the first stage. In this stage, the chromatin present inside the nucleus starts condensing and becomes thick. These change into visible chromosomes. Each chromosome has two sister chromatids which are attached at the centromere. The nucleolus becomes smaller and slowly disappears. The nuclear envelope also starts to break. Outside the nucleus, the centrosomes move to opposite poles and form spindle fibers.

ii. Prometaphase

• This is the second stage. The nuclear membrane completely breaks down. The chromosomes are now free in the cytoplasm. At the centromere of each chromosome, a structure called kinetochore appears. Spindle fibers from opposite poles attach to the kinetochores. These fibers help in moving chromosomes to the middle of the cell.

iii. Metaphase

• This is the third stage. The chromosomes now arrange themselves in a straight line at the center of the cell. This line is called the metaphase plate or equatorial plane. The centromeres of all chromosomes are aligned on this plane. Spindle fibers are fully attached to the kinetochores from both poles. This is the most stable and balanced stage of mitosis. Chromosomes are most visible in this stage under a microscope.

iv. Anaphase

• This is the fourth stage. In this stage, the centromeres divide and the two sister chromatids of each chromosome are separated. These separated chromatids are now called daughter chromosomes. The spindle fibers pull them towards opposite poles. This movement is very fast. Now both poles get equal sets of chromosomes.

v. Telophase

• This is the fifth and last stage of karyokinesis. The daughter chromosomes reach the poles and start uncoiling. They turn back into chromatin. A new nuclear membrane forms around each set of chromosomes. The nucleolus also reappears in each nucleus. Spindle fibers disappear. Now there are two complete nuclei in one cell.

2. Cytokinesis

Cytokinesis is the second and final stage of mitosis. It comes just after karyokinesis is complete. While karyokinesis divides the nucleus, cytokinesis divides the cytoplasm and other cell contents. The main purpose of cytokinesis is to fully separate the two new nuclei into two separate daughter cells, each having its own cytoplasm, organelles and cell boundary.

Cytokinesis is a very important step because without it, the mitotic division will remain incomplete. Even if the nucleus divides properly, the cell would not become two separate cells unless the cytoplasm and organelles also divide equally. Therefore, cytokinesis ensures that both daughter cells are not only genetically identical but also physically separated and ready to function independently.

The process of cytokinesis happens in different ways in animal and plant cells due to the presence or absence of the cell wall.

• In animal cells, cytokinesis starts with the formation of a shallow groove in the middle of the cell surface. This groove is called the cleavage furrow. The furrow is formed by a ring of contractile proteins, mainly actin and myosin, present just under the plasma membrane. These proteins start contracting and pulling the membrane inward. As this furrow deepens, it pinches the cell into two equal parts. This process continues until the cell membrane joins in the middle, fully dividing the parent cell into two daughter cells.

• In plant cells, there is a rigid cell wall, so a cleavage furrow cannot form. Instead, a structure called the cell plate is formed in the center of the dividing cell. The cell plate is made up of vesicles that come from the Golgi apparatus. These vesicles contain materials like pectin and cellulose which help in building the new wall. These vesicles fuse together in the center of the cell and gradually expand outward. As more vesicles join, the cell plate grows until it touches both sides of the cell wall. This forms a complete new dividing wall between the two daughter cells. Later, a new plasma membrane forms on both sides of the plate.

c) What is the significance of Mitosis?

Mitosis is a very important process in eukaryotic organisms. It is a type of cell division in which a diploid (2n) mother cell divides to form two genetically identical diploid daughter cells. It occurs in somatic (body) cells and also in diploid germ cells (before meiosis begins). Mitosis maintains the same number of chromosomes in daughter cells as in the parent cell. This division is highly regulated and accurate so that each new cell gets a full and equal set of genetic material.

The significance of mitosis can be explained under the following points:

1. Genetic Stability and Chromosome Number Maintenance:

This is the most basic and important function of mitosis. It ensures that every daughter cell receives the same number and exact copy of chromosomes as the parent cell. For example, if a human somatic cell with 46 chromosomes divides by mitosis, both daughter cells will also have 46 chromosomes. This helps maintain genetic stability across tissues and organs and ensures organismal identity is preserved during growth and repair.

2. Growth of Multicellular Organisms:

Mitosis is the reason why a single-cell zygote develops into a multicellular body. All cell multiplication during embryonic development and postnatal body growth occurs through mitosis. It increases the number of cells, which helps in tissue formation, organ development, and body size increase in both plants and animals.

3. Tissue Repair and Replacement:

When body tissues are injured or old cells die, new cells are needed to repair or replace them. Mitosis produces genetically identical new cells that restore tissue structure and function. For example, healing of wounds, replacement of skin cells, and renewal of red blood cells from bone marrow all depend on mitosis.

4. Asexual Reproduction in Lower Organisms:

In many unicellular and simple multicellular organisms like Amoeba, Paramecium and Hydra, mitosis is used for asexual reproduction. It allows production of offspring without gametes and all daughter organisms are clones of the parent. This helps in rapid multiplication in favourable conditions.

5. Maintenance of Proper Cell Size and Function:

As cells grow larger, their efficiency decreases because the surface area to volume ratio becomes less favourable. Mitosis helps divide large cells into smaller daughter cells, keeping the ratio in balance. This supports better exchange of nutrients and waste and keeps cellular metabolism normal.

SAQ 3

a) Describe the process of synapsis in meiosis.

Synapsis is a special event that takes place only during meiosis, not mitosis. It refers to the pairing of homologous chromosomes i.e., one chromosome from the mother and one from the father that carry genes for the same traits. These two chromosomes come and lie side by side with each other during the early stages of meiosis. This process of exact alignment is called synapsis. It is very important for the proper distribution of chromosomes in gametes and also for allowing genetic recombination or crossing over to occur.

Synapsis happens only once, during prophase I of meiosis I and does not occur again in meiosis II or mitosis. Without synapsis, homologous chromosomes cannot properly exchange genetic material or segregate correctly, which may lead to genetic disorders.

Process of Synapsis in Meiosis

Synapsis is the pairing of two homologous chromosomes during the early stage of meiosis I, specifically in the zygotene substage of prophase I. It is a highly regulated process that ensures the accurate alignment of genes between two homologous chromosomes and lays the foundation for crossing over and proper chromosomal segregation.

The complete process of synapsis involves the following detailed steps, which occur within the sequence of substages of prophase I:

1. Leptotene Stage (Preparation for Synapsis):

• In this first stage of prophase I, chromosomes begin to condense and become thread-like structures. Each chromosome has two sister chromatids joined at the centromere, but at this stage, they appear as a single structure. The chromosomes begin to move towards the nuclear center, guided by microtubules from the centrosomes. Homologous chromosomes start searching for their corresponding partners, but synapsis has not started yet.

2. Zygotene Stage (Actual Synapsis Begins):

• This is the most important stage for synapsis. Homologous chromosomes begin to recognize each other based on complementary DNA sequences and gradually move closer. Pairing starts from one or more points called synapsis initiation sites and then spreads along the entire length of chromosomes. This pairing is very specific and occurs gene by gene or locus by locus.

• A specialized protein-based structure known as the synaptonemal complex begins to form between homologous chromosomes during meiosis. This complex is composed of three key components:

• Two lateral elements: One is associated with each homologous chromosome.
• One central element: This forms the core of the structure, linking the two lateral elements, resembling a zipper.

• Once the central element joins the lateral elements, it brings the chromosomes very close together, about 100 nanometers apart, maintaining precise alignment for synapsis and recombination. So, the zipper-like configuration is the result of the central element linking the two lateral elements, forming a tight connection between the homologous chromosomes.

3. Pachytene Stage (Synapsis Completion):

• By now, synapsis is fully complete and each pair of homologous chromosomes is fully joined along their entire length. This paired structure is called a bivalent and since each chromosome has two sister chromatids, the bivalent contains four chromatids, also called a tetrad.

• The fully formed synaptonemal complex helps stabilize the structure. This is the stage where genetic recombination or crossing over occurs at specific points called chiasmata.

4. Diplotene Stage (Synapsis Ends):

• The synaptonemal complex starts to break down and the homologous chromosomes begin to separate from each other. However, they remain attached at the chiasmata where crossing over happened. Synapsis is now over, but the result of synapsis, which is chiasma formation and gene exchange, remains visible.

b) Briefly explain independent assortment of chromosomes.

Independent assortment is a key genetic principle that explains how genes are inherited independently from one generation to the next. This concept was first clearly explained by Gregor Johann Mendel in 1865 through his work on pea plants. He described it in his Second Law of Inheritance, called the Law of Independent Assortment. It explains how the alleles of different genes get distributed into gametes separately and randomly, producing genetic variation in offspring. The actual biological basis of independent assortment lies in the behavior of chromosomes during meiosis, particularly during metaphase I and anaphase I of meiosis I.

Cellular Basis of Independent Assortment:

During meiosis I, homologous chromosomes line up at the metaphase plate in a random orientation. Each homologous pair consists of one maternal and one paternal chromosome. The orientation of one pair is completely independent of the orientation of other pairs. As a result, when the homologous chromosomes are pulled apart during anaphase I, different combinations of maternal and paternal chromosomes move into different gametes.

For example, suppose there are two pairs of homologous chromosomes:

1. Pair 1: A (maternal) and a (paternal)
2. Pair 2: B (maternal) and b (paternal)

During metaphase I, the alignment could be: A aligns with B and a aligns with b or A aligns with b and a aligns with B. This results in four possible combinations in the gametes: AB, Ab, aB and ab.

This independent arrangement and separation of chromosome pairs are what cause different gene combinations in gametes.

Importance of Independent Assortment:

Independent assortment plays a vital role in producing genetic diversity among offspring in sexually reproducing organisms. During meiosis, when homologous chromosomes segregate independently, it leads to the formation of gametes with different combinations of parental chromosomes. This randomness ensures that each gamete carries a unique genetic profile, even from the same individual.

As a result, the zygote formed after fertilization inherits new gene combinations, which increases variability in the population. This variation is essential for evolution, as it provides the raw material for natural selection. It also helps populations to adapt better to changing environmental conditions. Without independent assortment, all offspring would be genetically very similar, making the species more vulnerable to diseases or environmental stress. Thus, independent assortment is one of the key mechanisms that maintain the genetic health and adaptability of a species over generations.

Exceptions to Independent Assortment

Mendel’s Law of Independent Assortment explains that different genes located on different chromosomes get separated independently during gamete formation. But this law does not always apply, especially when genes are located on the same chromosome. In such cases, some exceptions are observed.

• Gene Linkage: If two genes are present close together on the same chromosome, they are called linked genes. These genes do not follow independent assortment because they usually pass together into the same gamete. The closer the genes are to each other, the stronger their linkage, and the lower the chance that they will be separated by recombination. This violates Mendel’s second law.

• Crossing Over: During Prophase I of Meiosis, homologous chromosomes may exchange parts of their chromatids. This process is called crossing over. If two genes are far apart on the same chromosome, crossing over can separate them and allow recombination. But if genes are very close, crossing over between them is rare and they are mostly inherited together.

c) Fill in the blank.

i) Meiosis is a cell division process that results in the production of .................... daughter cells.

Answer: four

ii) Crossing over, which allows for genetic exchange, can occur during .................... in Meiosis I.

Answer: prophase I

iii) The process of cell division following Meiosis II is called ..................... .

Answer: cytokinesis 

iv) The structure that attaches to the centromere of a chromosome and helps pull sister chromatids apart during cell division is called the ...................... .

Answer: kinetochore

v) The final stage of Meiosis is called .................... .

Answer: telophase II

d) Which of the following statements are true or false. Write T  for true and F for False.

i) Meiosis reduces the chromosome number by half in the daughter cells.

Answer: T

ii) The main function of meiosis is to increase genetic variation.

Answer: T

iii) Crossing over is guaranteed to happen during every meiosis.

Answer: F

iv) Homologous chromosomes are identical copies of each other.

Answer: F

v) Meiosis reduces the chromosome number by half in the daughter cells.

Answer: T

TERMINAL QUESTIONS

1. What is the product of meiosis, and which type of cells shows this type of division?

Meiosis is a type of cell division that occurs only in diploid germ cells of sexually reproducing organisms. This division is responsible for producing haploid daughter cells (with half the number of chromosomes) from a diploid parent cell. Meiosis occurs in two main stages: Meiosis I and Meiosis II. One round of DNA replication is followed by two successive cell divisions. The major goal of meiosis is to reduce the chromosome number from diploid (2n) to haploid (n), so that the chromosome number remains constant after fertilization.

Meiosis is also responsible for genetic recombination through processes like crossing over and independent assortment. The most important thing is that meiosis produces gametes (in animals) and spores (in plants) which are essential for sexual reproduction.

Product of Meiosis:

The final product of meiosis is four haploid cells, each containing half the chromosome number of the original diploid cell. These haploid cells are:

• Genetically different from each other due to recombination.

• Each of them carries only one set of chromosomes, unlike the parent cell which had two sets.

For example:

In humans, the diploid number of chromosomes is 46 (2n). When a diploid germ cell undergoes meiosis, it forms four haploid cells, each having 23 chromosomes (n).

• In males, these four haploid cells formed after meiosis become four functional sperm cells. Each sperm carries 23 chromosomes and can participate in fertilization.

• In females, however, meiosis is slightly unequal. The diploid germ cell forms one large functional egg (ovum) and three small polar bodies. These polar bodies do not participate in reproduction and usually degenerate. So, although meiosis technically forms four haploid cells in females too, only one becomes a functional gamete, while the others are non-functional.

Thus, the key result of meiosis is that it produces haploid gametes from diploid germ cells, which is essential for maintaining a constant chromosome number during sexual reproduction.

Type of Cells That Show Meiosis

Meiosis occurs only in diploid germ cells that are involved in sexual reproduction. These cells are found in the reproductive organs of both animals and plants. In animals, meiosis takes place in the testes of males, where diploid spermatogonia form haploid sperm, and in the ovaries of females, where oogonia form haploid ova. In plants, meiosis occurs in the anthers, where microsporocytes form pollen, and in the ovules, where megasporocytes form female gametophytes.

These cells are diploid (2n) before meiosis and produce haploid (n) cells after division. Somatic cells, like skin or liver cells, do not undergo meiosis. Only germinal cells divide by meiosis to ensure formation of gametes and genetic diversity. Thus, meiosis is limited to cells of the reproductive system.

Primary oocytes in females

2. Chromosomes can be easily identified at which stage of meiosis?

Chromosomes are most easily identified during Metaphase I and Metaphase II of meiosis. These are the two stages where chromosomes appear most condensed, structured and clearly visible under a microscope.

Reasons Why Chromosomes Are Visible at These Stages

1. Chromosomes are Visible in Metaphase I

In Metaphase I of meiosis, chromosomes are most clearly visible because of maximum condensation. During this stage, homologous chromosomes, which have already paired up during prophase I, align themselves at the equatorial plane of the cell. Each homologous pair, also called a bivalent or tetrad, contains four chromatids (two chromosomes).

At this point, chromosomes are thick, short and highly condensed. Due to this condensed nature, they become clearly distinguishable under the microscope. Also, since the homologous pairs are arranged in a well-organised line along the metaphase plate, it becomes easy to identify individual chromosomes without any overlapping. Scientists often use this stage to study the number and shape of chromosomes because they appear most distinct here.

So, chromosomes are visible in Metaphase I because of maximum condensation and their linear arrangement at the equator.

2. Chromosomes are Visible in Metaphase II

In Metaphase II, each chromosome, made of two sister chromatids joined at a centromere, is again arranged at the metaphase plate. But this time, unlike Metaphase I, homologous pairs are not present. Still, chromosomes remain in a highly condensed state, just like in Metaphase I.

These chromosomes align in a straight line at the cell's equator and since each chromosome is an individual unit, it becomes even easier to observe and count them. The short and thick shape of each chromosome makes them clearly visible and identifiable. This stage is also used in chromosome studies when more individual analysis is needed.

3. What is the cell cycle? Describe its various phases.

The cell cycle refers to a series of events that take place in a cell as it grows and divides to form two daughter cells. The cycle is crucial for the growth, development and maintenance of all living organisms, ensuring that the cells replicate and divide properly. The concept of the cell cycle was first systematically described by Howard and Pelc in 1953. They observed the different stages of cell division and growth, which led to the understanding of the continuous process of the cell cycle. The cycle is highly regulated, and any errors in this process can lead to diseases such as cancer.

Phases of the Cell Cycle

The cell cycle is divided into two main stages:

1. Interphase: The preparatory phase where the cell grows and DNA is replicated.
2. Mitotic Phase (M phase): The phase where the actual division of the cell occurs.

1. Interphase:

Interphase is the longest phase of the cell cycle and is primarily involved in preparing the cell for division. It is divided into three sub-phases:

1. G1 Phase (Gap 1):
• This is the first phase of interphase and marks the cell's growth period. During G1, the cell synthesizes RNA and proteins necessary for DNA replication. It also performs its normal functions in terms of metabolism and energy production. If the cell checks and detects any errors in its environment or internal processes, it can delay further progress in the cycle to repair these issues. This is also the phase where cells decide whether to continue through the cycle or enter G₀ phase (G Zero Phase).
2. S Phase (Synthesis):
• In this phase, the cell's DNA is replicated. Each chromosome is duplicated, creating two sister chromatids joined at the centromere. This phase ensures that after cell division, each daughter cell will have an identical set of chromosomes.
3. G2 Phase (Gap 2):
• In the G2 phase, the cell continues to grow and prepares for the mitotic phase. During this stage, the cell synthesizes proteins required for mitosis and checks the replicated DNA for any errors. If any issues are found, the cell can halt the process to correct the mistakes before proceeding to division.

• G₀ Phase (Resting Stage):
• The G₀ phase is also referred to as the resting stage, but this doesn't mean that the cell is inactive. It's a phase where some cells exit the cell cycle after the G₁ phase and may stay in this phase temporarily or permanently. Cells in the G₀ phase do not divide but continue performing normal biological functions. For example, nerve cells and cardiac muscle cells often enter the G₀ phase and may never divide again.

2. Mitotic Phase (M phase):

The mitotic phase is the part of the cell cycle where the actual division occurs. This phase is further divided into Karyokinesis and Cytokinesis.

1. Karyokinesis (Nuclear Division):

Karyokinesis refers to the division of the cell's nucleus. This is broken down into four stages:

1. Prophase:
• This is the first stage. The chromatin inside the nucleus becomes thick and changes into visible chromosomes. Each chromosome has two sister chromatids joined at the centromere. The nucleolus becomes smaller and disappears. The nuclear envelope starts breaking. Outside the nucleus, centrosomes move to opposite sides and begin forming spindle fibers.
2. Prometaphase:
• The nuclear membrane breaks fully. Chromosomes become free in the cytoplasm. A structure called the kinetochore appears on each centromere. Spindle fibers from opposite poles attach to these kinetochores.
3. Metaphase:
• Chromosomes line up at the center of the cell on the metaphase plate. Their centromeres are aligned. Spindle fibers are fully attached. Chromosomes are most clearly seen in this stage.
4. Anaphase:
• The centromeres divide. Sister chromatids separate and move to opposite poles. They are now called daughter chromosomes.
5. Telophase:
• Daughter chromosomes reach the poles and turn back into chromatin. A new nuclear membrane and nucleolus form. Spindle fibers disappear.

2. Cytokinesis (Cytoplasmic Division):

Cytokinesis is the process where the cell's cytoplasm and other organelles are divided into two daughter cells. 

• In animal cells, a contractile ring forms around the center of the cell, pinching the cell membrane and forming two distinct daughter cells.
• In plant cells, a cell plate forms in the center of the cell, eventually turning into a new cell wall, thus dividing the cell into two.

4. What is bivalent in meiosis-I?

In meiosis I, the process of chromosome segregation requires the pairing of homologous chromosomes. This pairing forms a structure known as a bivalent. A bivalent is essentially a pair of homologous chromosomes, each consisting of two sister chromatids. The formation of bivalents occurs during prophase I, a crucial phase of meiosis. These structures are key to ensuring the correct distribution of chromosomes into daughter cells, as well as contributing to genetic diversity.

Formation of Bivalent

During prophase I of meiosis, homologous chromosomes come together and pair up through a process called synapsis. These paired chromosomes are held together by the synaptonemal complex, a protein structure that stabilizes the connection. The bivalent consists of two homologous chromosomes, each with two sister chromatids, making a total of four chromatids in the structure. This pairing is crucial for crossing over, a process where genetic material is exchanged between non-sister chromatids, enhancing genetic diversity. The bivalent (or tetrad) is clearly visible under a microscope by the end of prophase I, setting the stage for proper chromosome segregation during meiosis.

Role of Bivalent

1. Genetic Recombination (Crossing Over):

• The alignment of homologous chromosomes in a bivalent allows for crossing over, where parts of non-sister chromatids are exchanged between homologous chromosomes. This exchange of genetic material increases genetic diversity, which is crucial for evolution and variation among offspring.

2. Segregation of Chromosomes:

• In anaphase-I of meiosis, the homologous chromosomes of each bivalent are pulled apart towards opposite poles. This ensures that each daughter cell gets only one chromosome from each homologous pair. As a result, the chromosome number is halved, leading to the formation of haploid cells from the original diploid cell.

Importance of Bivalent

• Bivalents are crucial for ensuring the proper separation of chromosomes during meiosis. Without proper pairing and segregation, the daughter cells may end up with incorrect numbers of chromosomes, leading to genetic disorders.

• They play a significant role in genetic variation because of the crossing over that occurs within the bivalent, making each gamete genetically unique.

5. Name some of the theories that explain process of crossing over.

Crossing over is a very important process that takes place during prophase-I of meiosis, especially during the pachytene sub-stage, where homologous chromosomes exchange genetic material. This process increases genetic variation and is a key part of sexual reproduction. To explain how crossing over happens, many scientists have proposed different theories over time. These theories developed step-by-step, with later theories giving more accurate and detailed explanations. There are mainly four important theories that explain the process of crossing over. These are:

1. Copy Choice Theory

Copy choice theory was proposed by J. Belling in 1931. This is one of the oldest theories. It suggested that new chromosomes are formed by copying some parts from one chromosome and some from the other during DNA replication. According to this theory, the copying machinery switches templates while making the new DNA strand. However, this theory was later rejected because no physical exchange between chromosomes happens as per this idea, which is not true according to cytological evidence.

2. Break and Exchange Theory

Break and exchange theory proposed by C. D. Darlington in 1937. This theory suggested that homologous chromosomes break at the same point and then exchange the broken parts with each other. This theory was better than the previous one because it involved actual physical exchange. But still, it lacked proper details about how exactly the exchange takes place at the molecular level.

3. Breakage and Reunion Theory

Breakage and reunion theory proposed by Stern and Hotta in 1969. According to this theory, crossing over happens due to the breakage of chromatids at identical locations. Then, the broken ends of non-sister chromatids of homologous chromosomes join with each other. This theory gave more support to the idea that exchange happens during the pachytene stage of meiosis. It could be observed under a microscope and this made the theory more acceptable than the earlier ones.

4. Double-Strand Break Repair Model (DSBR Model)

DSBR model proposed by Resnick in 1976 and improved by Szostak et al. in 1983. This is the most accepted and modern theory of crossing over. It explains the process at the molecular level. According to this model, crossing over starts with a double-strand break (DSB) in one chromatid. The broken DNA ends are then processed and a strand from the homologous chromosome invades the broken site, forming a Holliday junction. This structure helps in the proper exchange and later gets resolved to complete the crossing over. This model is supported by genetic and biochemical experiments and is used in modern textbooks.

6. The non-dividing cells are most likely in which stage of the cell cycle?

The non-dividing cells are most likely in the G₀ phase of the cell cycle.

The G₀ phase (Gap zero phase) is a resting or quiescent stage where the cell exits the active cell cycle. In this phase, the cell does not prepare for division and stops progressing through the cycle. It is a reversible or permanent resting stage, depending on the cell type.

Cells enter G₀ from the G₁ phase when conditions are not favorable for division or when the cell has reached full maturity. Some cells can re-enter the cell cycle from G₀ when needed, while others remain permanently in G₀ and never divide again. For example, nerve cells and cardiac muscle cells stay permanently in G₀ and do not divide after maturation. On the other hand, liver cells can re-enter the cell cycle from G₀ when there is damage or a need for regeneration.

This phase is very important for controlling unnecessary cell division, especially in multicellular organisms where not all cells need to divide continuously. G₀ helps maintain tissue stability and prevent diseases like cancer that are caused by uncontrolled cell division.

7. Describe the various sequences of events that occur during mitosis.

Mitosis is a type of cell division that occurs in somatic cells of eukaryotic organisms. It is known as equational division because the number of chromosomes remains unchanged from the parent cell to the daughter cells. This process is very important for the growth of the body, replacement of old or damaged cells and asexual reproduction in many organisms. In humans and most animals, mitosis helps in increasing cell numbers without changing the genetic material. Each daughter cell formed after mitosis has the same number and type of chromosomes as the mother cell.

The whole mitotic process happens in a stepwise manner and is completed in two main stages:

1. Karyokinesis (division of the nucleus)
2. Cytokinesis (division of the cytoplasm)

1. Karyokinesis (Division of the Nucleus)

Karyokinesis is the first stage of mitosis in which the nucleus of the cell divides properly to ensure that each daughter cell receives an exact copy of the genetic material. The word karyokinesis is made up of "karyo" meaning nucleus and "kinesis" meaning movement or division.

In many earlier books and older classification systems, karyokinesis was described in four phases: prophase, metaphase, anaphase and telophase. But according to modern cell biology and updated textbooks, it is now divided into five phases, with prometaphase added as a distinct stage between prophase and metaphase.

Now let us understand all five phases one by one in the correct sequence:

i. Prophase:

• This is the beginning stage of karyokinesis. In this phase, the chromatin present in the nucleus starts condensing and forms visible thread-like chromosomes. Each chromosome consists of two sister chromatids attached at a centromere. The nucleolus begins to disappear and the centrosome starts moving towards opposite poles of the cell. Spindle fibres start forming from the centrosomes. The spindle apparatus is very important for the later stages of mitosis.

ii. Prometaphase:

• Prometaphase is a newly accepted separate phase that comes after prophase. In this phase, the nuclear envelope breaks down completely, which allows spindle fibres to contact the chromosomes. Special disc-shaped structures called kinetochores form on each chromatid at the centromere. Spindle fibres attach to these kinetochores and help in the movement of chromosomes. The chromosomes now start moving towards the center of the cell.

iii. Metaphase:

• Metaphase is the stage where the chromosomes become most clearly visible under a microscope. The chromosomes align themselves along the metaphase plate, an imaginary line at the centre of the cell. This alignment ensures that the separation of chromatids in the next phase will be equal. At this point, the spindle assembly checkpoint also occurs to make sure that all chromosomes are properly attached to the spindle fibres.

iv. Anaphase:

• Anaphase begins when the centromeres split, and the two sister chromatids of each chromosome are pulled apart. Now, each chromatid becomes an independent daughter chromosome. These daughter chromosomes are pulled by the spindle fibres towards opposite poles of the cell. Anaphase ensures equal distribution of chromosomes between the two forming daughter cells.

v. Telophase

• This is the final phase of karyokinesis. In telophase, the chromosomes that have reached the poles begin to uncoil and become less visible. A new nuclear envelope starts forming around each set of chromosomes. The nucleolus also reappears in each daughter nucleus. The spindle fibres disappear and now the cell has two separate nuclei, each with the same genetic material.

2. Cytokinesis (Division of Cytoplasm)

After karyokinesis is completed, the cell enters cytokinesis. In this phase, the cytoplasm and the remaining cell contents are divided between the two daughter cells. The method of cytokinesis differs slightly in plant and animal cells:

• In animal cells, a cleavage furrow forms at the centre of the cell. This furrow deepens and finally pinches the cell into two separate daughter cells.
• In plant cells, due to the presence of a rigid cell wall, a cleavage furrow cannot form. Instead, a cell plate forms in the centre of the cell from Golgi vesicles. This cell plate gradually becomes the new cell wall between the daughter cells.

8. When and how do homologous chromosomes separate during meiosis?

Homologous chromosomes separate during the first division of meiosis, which is called meiosis I. More specifically, this separation happens in the anaphase I stage of meiosis I.

Before this stage, in the earlier phase of meiosis I called prophase I, homologous chromosomes come close to each other and form pairs. This process is called synapsis. Each pair has two chromosomes (one from the mother and one from the father) that carry similar types of genes. These pairs are called bivalents or tetrads, because they have four chromatids.

Then, in metaphase I, these homologous chromosome pairs align themselves at the centre of the cell (equator). They are attached to spindle fibres from opposite poles of the cell.

Now comes the most important step i.e., anaphase I. In this stage, the spindle fibres pull the homologous chromosomes apart. One chromosome from each pair is pulled to one side of the cell and the other chromosome goes to the opposite side. This is the point where homologous chromosomes separate. It is important to note that the sister chromatids of each chromosome do not separate at this stage. They remain attached to each other at the centromere.

As a result, each new cell formed after meiosis I gets only one chromosome from each homologous pair. This reduces the chromosome number by half, which is why meiosis I is called a reductional division.

The separated chromosomes then reach the poles during telophase I and two haploid cells are formed after cytokinesis.

In short:

• When homologous chromosomes separate during meiosis?
• The answer is during anaphase I of meiosis I

• How do homologous chromosomes separate during meiosis?
• The answer is by pulling of spindle fibres that move one chromosome from each homologous pair to opposite poles

9. DNA replication occurs in which phase of cell growth.

DNA replication takes place during the S phase, which stands for Synthesis phase of the cell cycle. This phase is part of the Interphase, which is the period between two cell divisions when the cell prepares itself for the next mitotic or meiotic division.

The Interphase has three main stages:

1. G1 phase (Gap 1) – The cell grows and carries out normal functions.
2. S phase (Synthesis) – This is the stage where DNA replication occurs.
3. G2 phase (Gap 2) – The cell prepares for division by synthesising proteins and other components.

What Happens in the S Phase:

In this phase, the entire genetic material (DNA) of the cell is copied. Each chromosome makes an exact duplicate of itself, forming two sister chromatids joined at the centromere. These chromatids will later be separated into daughter cells during mitosis or meiosis.

This replication is semi-conservative, meaning each new DNA molecule contains one old strand and one newly synthesized strand. The process is highly regulated and involves enzymes like DNA helicase, DNA polymerase and ligase.

By the end of S phase, the amount of DNA in the nucleus doubles, but the chromosome number remains the same. For example, in humans, cells remain diploid with 46 chromosomes, but now each chromosome has two chromatids.

10. If a cell has twice as much DNA as a normal functional cell, the cell undergoes ................... ?

Answer: S phase of interphase.

11. Define Aneuploidy. What is the cause of Down's syndrome and Turner's syndrome?

Aneuploidy

Aneuploidy is a type of chromosomal abnormality in which the number of chromosomes in a cell is not exactly 46, which is the normal diploid number in humans. In this condition, the cell may have either one extra chromosome or one missing chromosome. So, the total number of chromosomes becomes either 45 or 47.

This abnormality happens mostly due to non-disjunction during meiosis, which means that chromosomes fail to separate properly during the formation of gametes (sperm or egg). Because of this, the gamete may carry an extra or missing chromosome. When such a gamete fuses with a normal gamete, the resulting zygote has an abnormal chromosome number.

There are two major types of aneuploidy:

1. Trisomy – where one extra chromosome is present (total becomes 2n + 1, that is 47)
2. Monosomy – where one chromosome is missing (total becomes 2n − 1, that is 45)

Down's Syndrome

Down's syndrome is a genetic disorder that occurs due to trisomy of chromosome number 21, which means the person has three copies of chromosome 21 instead of the normal two. So, the total number of chromosomes becomes 47 instead of 46. This condition is also known as Trisomy 21.

It was first described in 1866 by a British physician Dr. John Langdon Down, who noticed a pattern of similar physical and mental features among some patients. Later, in 1959, the chromosomal cause of the syndrome was discovered by a French geneticist Jerome Lejeune, who identified the extra chromosome 21 as the reason behind the disorder.

The condition results from non-disjunction during meiosis, where chromosome 21 does not separate properly. As a result, one of the gametes carries two copies of chromosome 21, and when it combines with a normal gamete, the zygote ends up with three copies of chromosome 21.

People with Down's syndrome usually show a group of signs like a flat face, upward slanting eyes, small ears, short neck, poor muscle tone and intellectual disability. Some also have congenital heart defects or other medical conditions. The severity of symptoms can vary from person to person.

Turner's Syndrome

Turner's syndrome is a genetic disorder that affects only females, caused due to the absence of one X chromosome, leading to a total of 45 chromosomes instead of 46. The genetic formula becomes 45, XO, where "O" means the second sex chromosome is missing. This condition is a type of monosomy involving the sex chromosomes.

It was first described by Dr. Henry Turner in 1938, who observed a set of common physical features in certain female patients and linked them to a chromosomal issue.

The main reason for Turner's syndrome is non-disjunction during meiosis, which leads to the formation of gametes that lack one sex chromosome. When such a gamete is involved in fertilisation, the resulting zygote ends up with only one X chromosome.

Girls with Turner's syndrome usually have short stature, a broad chest with widely spaced nipples, webbed neck, low hairline at the back of the neck, and underdeveloped secondary sexual characteristics. Most of them are infertile due to the absence of ovaries (gonadal dysgenesis), and they may also have heart and kidney defects.

12. Describe the following terms: Bouquet stage, Chiasma, Kinetochore, Synapsis and Crossing over.

Bouquet Stage

The bouquet stage is a special arrangement of chromosomes during the early part of zygotene stage of prophase I in meiosis. In this stage, the ends of all chromosomes (called telomeres) gather together at one side of the nuclear envelope, making a shape that looks like a bouquet of flowers. This arrangement helps in bringing homologous chromosomes closer, so that they can easily pair with each other. The exact function of this stage is not fully understood, but it is believed to help in proper alignment for synapsis and crossing over. This stage is temporary and happens only during early meiosis.

Chiasma (Plural: Chiasmata)

A chiasma is the visible point where two homologous non-sister chromatids exchange genetic material during crossing over. It looks like an X-shaped structure under the microscope. Chiasmata are seen during the diplotene stage of prophase I in meiosis. At this point, homologous chromosomes are still attached at these crossing points even though they begin to repel each other. The number of chiasmata can vary depending on the length of chromosomes. Chiasmata ensure genetic recombination and help maintain the connection between homologs till anaphase I, which ensures proper segregation.

Kinetochore

A kinetochore is a disc-shaped protein structure found on the centromere of each chromosome. It forms during cell division (both mitosis and meiosis). It is the place where spindle fibres attach to the chromosome to pull them apart. Each chromosome has two kinetochores, one on each sister chromatid and they face opposite directions. In meiosis I, homologous chromosomes are pulled apart, so the kinetochores act together. In meiosis II and in mitosis, sister chromatids are separated. Kinetochores play a key role in chromosome movement, alignment and segregation, and they are also involved in cell cycle checkpoints to prevent errors.

Synapsis

Synapsis is the pairing of homologous chromosomes during the zygotene stage of prophase I in meiosis. Each chromosome comes close to its corresponding homolog and forms a pair called a bivalent or tetrad. The process of synapsis is very precise and involves the formation of a protein structure called the synaptonemal complex, which helps in holding the homologs tightly together. Synapsis is important for crossing over, as it brings the homologs close enough to exchange segments. If synapsis does not happen properly, it can lead to errors in chromosome segregation.

Crossing Over

Crossing over is the process in which non-sister chromatids of homologous chromosomes exchange segments of genetic material. It takes place during the pachytene stage of prophase I in meiosis. This process increases genetic variation among offspring, which is important for evolution. The enzyme recombinase helps in breaking and rejoining DNA strands. After crossing over, the chromosomes carry genes from both parents. This is why siblings look similar but are not exactly the same. The points where crossing over occurs later become visible as chiasmata.

13. Name an event that restores the normal chromosome's number in the life stage?

The event that restores the normal chromosome number in the life cycle is called fertilization.

In sexually reproducing organisms, gametes such as sperm and egg are formed by a special type of cell division known as meiosis. Meiosis reduces the chromosome number from diploid (2n) to haploid (n), meaning the gametes (sperm in males and eggs in females) contain only one set of chromosomes. This reduction is very important because if both parents contributed diploid sets during reproduction, the chromosome number would double in every generation, which would lead to genetic imbalance and abnormalities.

Fertilization is the process where the male gamete (n) fuses with the female gamete (n) to form a zygote (2n). This zygote contains a complete set of chromosomes, with one set coming from the mother and one from the father. In this way, the diploid number of chromosomes is restored, ensuring that the species maintains a constant chromosome number from one generation to the next.

For example, in humans, the haploid number is 23. During fertilization, the sperm with 23 chromosomes and the egg with 23 chromosomes fuse, forming a zygote with 46 chromosomes, which is the normal human diploid number. This diploid zygote then undergoes mitotic divisions to develop into a multicellular individual.

Besides restoring the chromosome number, fertilization also introduces genetic variation by combining genes from two different individuals. This variation is further increased by processes like crossing over and independent assortment during meiosis. Thus, fertilization is not only important for chromosome balance but also for the diversity and survival of the species.

Therefore, fertilization is a crucial event in the life cycle, playing a central role in both genetic continuity and variation, which are essential for reproduction, evolution and proper development.

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