Test: Cell Cycle & Cell Division - 1 - NEET MCQ

Mitosis is the type of cell division that ensures equal distribution of genetic material in daughter cells. Mitosis can occur both in diploid and haploid cells.
The main function of mitosis is to make copies of cells for growth and regeneration.

Line in NCERT: "Against this, the plants can show mitotic divisions in both haploid and diploid cells."

In plants, cytokinesis or division of cytoplasm occurs due to the formation of the cell plate.

In animals, cytokinesis occurs due to the formation of constriction in the middle cell membrane.

Line in NCERT: "The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells."

Cell growth results in disturbing the ratio between the nucleus and the cytoplasm.
Therefore, it becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.

Line in NCERT: "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."

The transition period between the M-phase of Meiosis I and M-phase II during which no replication of DNA occurs is called interkinesis.

Line in NCERT: "The stage between the two meiotic divisions is called interkinesis and is generally short lived. There is no replication of DNA during interkinesis."

The average cell cycle span for a mammalian cell is approximately 24 hours.

Line in NCERT: "A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours."

Bivalent or tetrad formation takes place during the zygotene stage of prophase I, after synapsis has occurred and homologous chromosomes form pairs. However, the bivalent is distinct only in the next stage, pachytene.

Line in NCERT: "During this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads."

In some lower plants and in some social insects haploid cells also divide by mitosis.

Line in NCERT: "Mitosis usually results in the production of diploid daughter cells with identical genetic complement. However, in some lower plants and in some social insects haploid cells also divide by mitosis."

Pachytene: During the third substage of meiosis, the chromosomes continue to condense. Crossing over takes place at this stage and at each point of crossing over a chiasma is formed (singular: chiasma) between non-sister chromatids of homologous chromosomes.

Fig: Stages of Prophase I of Meiosis

Line in NCERT: "During this stage, the four chromatids of each bivalent chromosomes becomes distinct and clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes."

Since, in mitosis the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division.

Line in NCERT: "Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell."

Yeast can progress through the cell cycle in only about 90 minutes.

Line in NCERT: "Yeast for example, can progress through the cell cycle in only about 90 minutes."

In plant cells, wall formation starts in the center of the cell and grows outward to meet the existing lateral walls.
The formation of the new cell wall begins with the formation of a simple precursor, called the cell plate.

Fig: Plant Cell

Line in NCERT: "The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells."

In meiosis I, during pachytene, crossing over occurs between the non-sister chromatids of the homologous chromosomes. The exchange of genetic material (DNA) between the non-sister chromatids of the homologous chromosomes is known as crossing over. It leads to the recombination of genetic material which involves a mutual exchange of the corresponding segments of non-sister chromatids of homologous chromosomes. It takes place by breakage and reunion of chromatid segments.

Line in NCERT: "the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes."

Line in NCERT: "At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles."

The middle lamella serves as a cementing layer between the primary walls of adjacent cells.
The primary wall is the cellulose-containing layer laid down by cells that are dividing and growing. 

Line in NCERT: "The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells."

Cell cycle is divided into 2 phase. (i) Interphase (a period of preparation for cell division), (ii) M phase (the actual period of cell division). The interphase cell is metabolically quite active. Interphase is the long non dividing phase further divided into G1, S and G2. It occupies 75 to 90 % of entire cell division time.

Line in NCERT: "The interphase lasts more than 95% of the duration of cell cycle."

If the DNA content of an onion tip cell is 2C at the end of the M-phase, then its DNA content would be 4C at the end of the S-phase. During the S-phase of the cell cycle, DNA replication takes place, and the amount of DNA in the cell doubles. At the end of the S-phase, each chromosome consists of two identical sister chromatids, which are held together at the centromere. Thus, the DNA content of the cell is doubled from the G1 phase (1C) to the end of the S-phase (2C), and it remains the same through the G2 phase. Therefore, if the DNA content of an onion tip cell is 2C at the end of the M-phase, it means that the cell has completed the mitotic division and has two sets of chromosomes. If the cell then enters the S-phase, it will replicate its DNA, resulting in the DNA content of 4C at the end of the S-phase.

Line in NCERT: "During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C."

Statement I is correct: During the G1 phase, the cell grows, is metabolically active, but does not replicate its DNA. DNA replication occurs in the S phase.
Statement II is also correct: During the G2 phase, the cell synthesizes proteins needed for mitosis and continues growing in preparation for cell division.

Line in NCERT: "G1 phase corresponds to the interval between mitosis and initiation of DNA replication. During G1 phase the cell is metabolically active and continuously grows but does not replicate its DNA. During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues."

• The synaptonemal complex is a protein structure that forms between homologous chromosomes during meiosis, facilitating their pairing and recombination.
• It begins forming during the Zygotene stage of prophase I as homologous chromosomes start pairing (synapsis).
• It dissolves during the Diplotene stage when the homologous chromosomes begin to separate, although they remain connected at chiasmata.

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.

Line in NCERT: "Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope."