All questions of The Mitotic Cell Cycle for A Level 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.

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

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 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..

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.

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.

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

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.

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.

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.

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.

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.

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!!!!!

As we know, Prophase is associated with condensation of chromosomes, Metaphase is associated with the arrangement of Chromosomes as Metaphase plate, Anaphase is associated with the separation of chromosomes whereas Telophase is associated with the movement of chromosomes to their respective poles and reformation of cell organelles

Explanation:

Given: DNA content of an onion tip cell is 2C at the end of the M-phase.

During S-phase:
- The DNA content of a cell doubles during the S-phase.
- The DNA content of a cell at the end of the S-phase can be calculated by multiplying the DNA content at the end of the M-phase by 2.

Calculation:
- DNA content at the end of the S-phase = 2C x 2 = 4C
Therefore, the DNA content of an onion tip cell at the end of the S-phase would be 4C.

The Synaptonemal Complex in Meiosis
The synaptonemal complex (SC) is a crucial structure in meiosis that facilitates the pairing of homologous chromosomes. Understanding its formation and dissolution is essential for grasping the process of genetic recombination.
Stages of Meiosis
- Leptotene: Chromosomes begin to condense.
- Zygotene: Homologous chromosomes start to pair, and the synaptonemal complex forms, facilitating close alignment.
- Pachytene: The SC is fully developed, and crossing over occurs.
- Diplotene: The SC begins to dissolve, and homologous chromosomes start to separate.
- Diakinesis: The chromosomes condense further, and the nuclear envelope breaks down.
Explanation of the Correct Answer (Option D)
- A - Pachytene:
- The synaptonemal complex is fully formed during this stage.
- This is when homologous chromosomes are tightly paired, allowing for genetic exchange (crossing over) to occur.
- B - Diplotene:
- The SC begins to dissolve during diplotene.
- Homologous chromosomes start to separate, and the complex is no longer needed for chromosome alignment.
Importance of the Synaptonemal Complex
- The SC is vital for ensuring accurate chromosome pairing and exchange of genetic material.
- Its formation and dissolution are integral to the proper progression of meiosis, leading to genetic diversity in gametes.
In conclusion, the synaptonemal complex is formed during the pachytene stage and dissolves during the diplotene stage, making option D the correct answer. Understanding these processes is crucial for comprehending meiosis and its role in reproduction and genetics.