Test: Cell Cycle & Cell Division - 1 - ACT 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.

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.

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.

Chromosomes appeared beaded during the leptotene stage of the prophase of meiosis I, also the longest stage of meiosis cell division.
Chromatin materials start condensing after that stage.

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

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

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

The number of chromosomes and amount of DNA are changed in anaphase and S phase respectively.(i) In S phase, number of chromosomes remain same but each chromosome replicates so that it comes to have two chromatids. Due to this the DNA contents doubles from 1C to 2C or 2C to 4C depending upon haploid/diploid nature of cell.(ii) In Anaphase. In mitotic anaphase, number of chromosome remains the same. It is only sister chromatids which move towards their respective poles. DNA contents remain unchanged. In Anaphase-I of meiosis, number of chromosomes are reduced to half, i.e., from 2N to 1N. DNA contents decrease to one half in Anaphase-I of Meiosis-I from 2C to 1C

G0-phase

Explanation: The G0 phase, also known as the "resting phase," is a period in the cell cycle where cells are metabolically active but not actively dividing. During this phase, cells are considered to be in an invisible or quiescent state because they are not involved in cell division or preparation for cell division. The G0 phase can be temporary, with cells returning to the cell cycle to divide, or it can be permanent, such as in the case of some differentiated cells that no longer divide.

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

The figure given above shows diakinesis phase of prophase of meiosis I as terminalisation of chiasmata is  visualized in it.

Rudolf Virchow was the first scientist who stated that new cells are formed from division of pre-existing cells.

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

Coenocyte is a multinucleate mass of protoplasm which is formed as a result of repeated nuclear division which is not accompanied by the cell fission, whereas the Syncytium is the multinucleated cell which is formed by the fusion of the multiple unicellular cell.

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

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.

The M Phase represents the phase when the actual cell division or mitosis occurs

The mechanism of distributing chromosomes by centriole pairs to the daughter cells is called astral division. This kind of division occurs in animal cells.

Fig: Image showing Astral Division

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. 

Meiosis is the production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes.

Gametes fuse to produce zygote.

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.

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.

Gametogenesis is the formation of gametes from some specialized diploid cells. Since gametes are haploid, gametogenesis involves the reduction of chromosome number by half through meiosis.

Fig: Process of Gametogenesis in males and females