All questions of Cellular Reproduction for Grade 9 Exam

Correct option is D. In mitosis the sequence is prophase(chromatids compact to form chromosome) metaphase (chromosomes atrange at metacentruc plate)anaphase (cromosomes separate )and then telophase(move towards opposite poles and Endoplasmic reticulum , cell wall, etc is again formed.)

Prophase and telophase are stages involved in mitosis or meiosis. During G2 phase division of centrioles, mitochondria and chloroplasts occurs.

Most organisms that undergo sexual reproduction contain two types of cells in their body – haploid and diploid cells. Diploid cells are seen in most parts of the body and contain two copies of every chromosome. Therefore, they contain two genes to determine the sequence of nearly every RNA or protein. Haploid cells are usually male or female gametes formed as a result of meiosis and are seen in sexual organs. They contain only one complete set of chromosomes and are designed to fuse with another haploid gamete to produce a diploid zygote – with one paternal and one maternal set of chromosomes. Though they code for the same genes, their DNA sequences can vary slightly from one another. For instance, the paternal chromosome may contain the genetic information to result in antigen A being present on red blood cells, while the maternal chromosome may code for antigen B. These two antigens are similar in function, but differ from each other marginally in their amino acid sequence.

Mitosis is called equational division because during mitosis the cell divides equally into two identical daughter cells. 

Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects haploid cells also divide by mitosis. It is very essential to understand the significance of this division in the life of an organism. Are you aware of some examples where you have studied about haploid and diploid insects? Mitosis results in the production of diploid daughter cells with identical genetic complement usually. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio. A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues – the apical and the lateral cambium, result in a continuous growth of plants throughout their life.

Same mitosis is also called equavational division becz. before and after mitosis ploidy is same..

Anaphase -The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibres attached to the kinetochore regions.

The crossing over takes place during the pachytene stage of prophase I of meiosis.

Explanation:
Prophase I is the first stage of meiosis I, which is further divided into five substages: leptotene, zygotene, pachytene, diplotene, and diakinesis. During the prophase I, homologous chromosomes pair up and exchange genetic material through a process known as crossing over. This results in the formation of recombinant chromosomes that carry a combination of genetic material from both parents.

The crossing over occurs during the pachytene stage, which is characterized by the following events:

1. Homologous chromosomes pair up: The homologous chromosomes come together and form a bivalent or a tetrad.

2. Synapsis: The paired homologous chromosomes undergo synapsis, which is the physical association of the homologs.

3. Crossing over: The chromatids of the homologous chromosomes exchange genetic material at specific sites known as chiasmata. This leads to the formation of recombinant chromosomes.

4. Formation of the synaptonemal complex: The synaptonemal complex holds the homologous chromosomes together and facilitates the exchange of genetic material.

5. Nuclear envelope breakdown: The nuclear envelope breaks down, and the spindle fibers attach to the chromosomes.

In conclusion, the crossing over takes place during the pachytene stage of prophase I of meiosis.

The correct answer is option 'A', the cell plate.

Plant cells have a unique feature called the cell wall, which provides support and protection to the cell. During cell division, a new cell wall is formed to separate the two daughter cells. This process is known as cytokinesis.

- Cell Division in Plant Cells:

1. Prophase: In the first stage of cell division, the nuclear membrane starts to disintegrate, and the chromosomes become visible. Each chromosome consists of two identical copies called chromatids, which are held together by a structure called the centromere.

2. Metaphase: The chromosomes align at the center of the cell, forming a metaphase plate. This alignment ensures that each daughter cell receives an equal number of chromosomes during division.

3. Anaphase: The chromatids separate and move towards opposite poles of the cell. This movement is facilitated by the spindle fibers, which attach to the centromere of each chromatid.

4. Telophase: As the chromatids reach the opposite poles, the nuclear membranes start to reform around them. At this stage, the cell begins to prepare for cytokinesis.

- Cytokinesis in Plant Cells:

In animal cells, cytokinesis occurs through a process called cleavage furrow formation, where a contractile ring pinches the cell membrane inwardly. However, plant cells have a rigid cell wall that cannot be constricted in the same way.

1. Cell Plate Formation: Instead of a cleavage furrow, plant cells form a structure called the cell plate during cytokinesis. The cell plate is made up of vesicles containing cell wall materials, such as cellulose and other polysaccharides.

2. Golgi Vesicles: Golgi vesicles containing cell wall materials accumulate at the center of the cell, between the two daughter nuclei.

3. Fusion of Vesicles: The Golgi vesicles fuse together, forming a continuous structure known as the cell plate. This structure gradually expands towards the periphery of the cell, eventually reaching the cell membrane.

4. Cell Wall Deposition: Enzymes are secreted into the cell plate, which catalyze the deposition of cellulose and other components of the cell wall. As the cell plate expands and fuses with the existing cell wall, it forms a new cell wall between the two daughter cells, separating them completely.

In conclusion, the new cell wall in plant cells begins to form through the process of cytokinesis, specifically through the formation and expansion of the cell plate. The Golgi vesicles fuse together to create the cell plate, which then undergoes cell wall deposition to form a new cell wall between the daughter cells.

Plant cells divide by a cell plate that eventually becomes the cell wall whereas animal cells divide by a cleavage furrow.

Mitosis is the mechanism by which the chromosome content of a somatic cell (haploid or diploid) is kept constant through successive cell divisions. The division of the cell is initiated by division of the nucleus i.e. Karyokinesis followed by division of cytoplasm i.e. Cytokinesis.

The nuclear membrane, also called the nuclear envelope, is a double membrane layer that separates the contents of the nucleus from the rest of the cell. It is found in both animal and plant cells. A cell has many jobs, such as building proteins, converting molecules into energy, and removing waste products. The nuclear envelope protects the cell’s genetic material from the chemical reactions that take place outside the nucleus. It also contains many proteins that are used in organizing DNA and regulating genes.

1)Microfilaments collect in the middle region of the cell below the cell membrane. The furrow deepens centripetally and cleaves the cell into two daughter cells. This method is called cleavage cytokinesis. 2)Cytokinesis in plants occurs by cell plate formation. 3)The phragmoplast is a plant cell specific structure that forms during late cytokinesis. Hope it helps!!!!!

The arms of chromosome are known as chromatids. These arms are held together at a point called the centromere (or primary constriction). Centromere occurs any where along the length of chromosome. During ceIl division spindle fibres are attached to centromere and help in the movement of chromosomes towards the poles.

Crossing over occurs between prophase 1 and metaphase 1 and is the process where homologous chromosomes pair up with each other and exchange different segments of their genetic material to form recombinant chromosomes. It can also happen during mitotic division, which may result in loss of heterozygosity.

During metaphase, the chromosomes get arranged in the form of a plate called the equatorial plate or metaphase plate at the equator of the spindle. This plate is at right angles to the axis of the spindle and is formed of the kinetochores, the arms of chromatids trailing away. The centromeres are drawn to the equator by the equal pull of two chromosomal fibres which connect the sister kinetochores to the opposite poles. The process of drawing the chromosomes onto the equator of the spindle is known as congression.

The correct answer is option 'C': Number of chromosomes remain the same.

Meiosis is a type of cell division that occurs in sexually reproducing organisms. It involves the division of cells to produce gametes (sperm and egg cells) with half the number of chromosomes as the parent cell. This reduction in chromosome number is important for maintaining the correct number of chromosomes in the offspring.

Let's discuss each option in detail to understand why option 'C' is not a significance of meiosis division:

a) Formation of spores and gametes:
- One of the significant outcomes of meiosis is the formation of spores and gametes. In organisms that undergo sexual reproduction, meiosis ensures the production of gametes (sperm and egg cells) with half the number of chromosomes as the parent cell.
- Spores are produced in some non-reproductive structures of plants and fungi through meiosis. These spores can then germinate and develop into new individuals.

b) New recombination of genes:
- Meiosis allows for the recombination of genes, leading to genetic variation in offspring.
- During meiosis, homologous chromosomes pair up and exchange segments of genetic material through a process called crossing over. This exchange of genetic material results in new combinations of genes, contributing to genetic diversity.

c) Number of chromosomes remains the same:
- This statement is incorrect. The main significance of meiosis is the reduction in chromosome number. In meiosis I, the chromosome number is halved when homologous chromosomes separate. In meiosis II, sister chromatids separate, resulting in the production of four haploid cells (gametes) with half the number of chromosomes as the parent cell.

d) Number of chromosomes reduced to half:
- This statement is correct. Meiosis is responsible for reducing the number of chromosomes in gametes to half of the parent cell's chromosome number.
- This reduction is crucial for sexual reproduction because when the gametes fuse during fertilization, the resulting zygote will have the correct number of chromosomes for the species.

In conclusion, the correct answer is option 'C': Number of chromosomes remains the same. Meiosis is not associated with maintaining the same number of chromosomes; instead, it reduces the number of chromosomes by half in the gametes.

Overview of Cell Cycle Phases
The cell cycle consists of several phases, including G1, S, G2, and M. Each phase has distinct roles in cell growth and division.
Statement I: G1 Phase
- The G1 phase is critical for cell growth.
- During this phase, the cell is metabolically active, producing RNA and synthesizing proteins.
- Importantly, DNA replication does not occur in G1; instead, it prepares the cell for the S phase, where DNA synthesis takes place.
Statement II: G2 Phase
- The G2 phase follows DNA synthesis and focuses on preparing the cell for mitosis.
- During G2, protein synthesis continues, particularly those proteins necessary for cell division, including components of the mitotic spindle.
- Cell growth also continues in this phase, ensuring that the cell is ready for the M phase (mitosis) where actual division occurs.
Conclusion
Both statements accurately describe their respective phases:
- G1 Phase: Metabolic activity and growth occur without DNA replication.
- G2 Phase: Preparations for mitosis take place with ongoing protein synthesis and further cell growth.
Thus, the correct answer is option 'C': Both I and II are correct. Understanding these phases is crucial for grasping cellular processes, especially in the context of the NEET syllabus.

In late prophase, the nuclear envelope, nucleolus, Golgi complex, and endoplasmic reticulum disappear, allowing the chromosomes to become fully condensed and visible. This marks the transition into metaphase, where the mitotic spindle fully forms.