All questions of Cell Cycle & Cell Division for Grade 11 Exam

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.

Crossing over is the exchange of genes between two chromosomes, resulting in non-identical chromatids that comprise the genetic material of gametes. This process occurs during Prophase I of Meiosis, just prior to chromosome alignment and splitting of the cell.

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.

As previously mentioned, the first round of nuclear division that occurs during the formation of gametes is called meiosis I. It is also known as the reduction division because it results in cells that have half the number of chromosomes as the parent cell.

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.

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.

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!!!!!

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.

Pachytene Stage in Meiosis

The process of meiosis is divided into several stages, one of which is the Pachytene stage. This stage occurs during the first meiotic division or meiosis I. During this stage, homologous chromosomes pair up and form structures called bivalents or tetrads. The Pachytene stage is characterized by the following events:

Synapsis

During the Pachytene stage, homologous chromosomes pair up and form structures called bivalents or tetrads. This process is called synapsis. The synaptonemal complex plays a crucial role in this process. The complex is composed of proteins that hold the homologous chromosomes together.

Crossing over

Crossing over occurs during the Pachytene stage. It is the exchange of genetic material between homologous chromosomes. This process results in the creation of new combinations of genes, which contributes to genetic diversity.

Chiasmata formation

During the Pachytene stage, the homologous chromosomes that have paired up undergo crossing over. This process results in the formation of chiasmata, which are visible under a microscope. Chiasmata are the points where the homologous chromosomes crisscross and exchange genetic material.

Conclusion

When synapsis is complete all along the chromosome, the cell is said to have entered a stage called Pachytene. During this stage, homologous chromosomes pair up and form bivalents or tetrads. Synapsis, crossing over, and chiasmata formation occur during this stage. The Pachytene stage is an essential process in meiosis as it contributes to the genetic diversity of offspring.

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.

Interphase, Prophase I, Metaphase I, Anaphase I, Telophase I, Cytokinesis, Prophase II, Metaphase II, Anaphase II, Telophase II, Cytokinesis