Test: Cell Cycle and Cell Division - Assertion and Reasoning - NEET MCQ

Previously interphase was called resting stage because there is no apparent activity related to cell division. The interphase cell is metabolically quite active. Interphase consist of three subphases (G₁ , G₂ and S). Synthesis of DNA occurs in S phase. G₁ is the period between the end of mitosis and the start of S phase. G₂ is the interval between S phase and start of mitosis. As the synthesis of DNA occurs in S phase so, it is considered as metabolically active phase.

Chromosome contains equal amounts of DNA and histone. The DNA and histone octamer forms a nucleosome. Histone is the protein found in eukaryotic chromosomes. There are 5 types of histone namely  H₁, H₂A, H₂B, H₃ and H₄.

The synthesis of DNA occurs only in a restricted portion of the interphase during S period, which is preceded and followed by two "gap" periods of interphase (G₁ and G₂) in which there is no DNA synthesis. G₁ is the period between the end of mitosis I and the start of DNA synthesis. S is the period of DNA synthesis and G₂ is the interval between the end of DNA synthesis and the start of mitosis. During G₁, a cell contains two times (4C) the amount of DNA present in the original diploid cell (2C). Following mitosis, the daughter cell again enter is the G1 period and have a DNA content equivalent to 2C.

Let's analyze both statements:

1. Assertion: Some cells enter the G0 phase due to inactivation of the cell cycle. This is true. The G0 phase is a resting state where cells exit the cell cycle, often in response to lack of growth signals or nutrients.

2. Reason: G0 phase occurs due to non-availability of mitogen and energy-rich compounds. This is also true. Cells can enter the G0 phase when there is a lack of mitogens (growth signals) and energy-rich compounds (nutrients).

Since both statements are true and the reason provided correctly explains why cells enter the G0 phase due to inactivation of the cell cycle, the correct option is: If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

G₁ phase is also known as first growth phase or post mitotic gap phase. It is the phase between end of mitotic phase of previous cell and initiation of S-phase of next mitotic phase. During G₁ phase, the cell is metabolically active and continuously grows. In this phase, different types of RNA (mRNA, tRNA and rRNA) and proteins are synthesised.

Mitosis causes the growth of multicellular organism. The nucleus controls the functions of cell. The size of nucleus does not change, but cytoplasm increases during cell growth. Increase in size disturbs the nucleo –cytoplasmic ratio. This ratio is restored to efficient level through cell division.

DNA synthesis takes place only during one specific stage in cell cycle (interphase).

In S phase or synthetic phase chromosomes replicates, DNA content doubles, i.e., 1C to 2C for haploid cells and 2C to 4C for diploid cells. Along with replication of DNA, new chromatin fibres are formed which remain attached in pairs and the number of chromosomes does not increase i.e. 1n to 1n and 2n to 2n. As the chromatin fibres are elongated chromosomes, each chromosomes comes to have two chromatin threads which are attached at centromere. During metaphase, every chromosome have two chromatids. The chromosomes separate from each other during anaphase.

Mitosis keeps all the somatic cells of an organism genetically similar, resembling the fertilized egg. Mitosis involves replication and equitable distribution of all the chromosomes so that all the cells of a multicellular organism have the same number and type of chromosomes. This helps in proper co-ordination among different cells.

M-phase represents the phase of actual division. It consists of karyokinesis (the division of nucleus) followed by cytokinesis (the division of cytoplasm). Cell divisions stop after M-phase.

Prophase which is the first stage of mitosis follows the S and G₂ phases of interphase. In the S and G₂ phases, the new DNA molecules formed are not distinct but interwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation.

In anaphase, centromeres of chromosomes start to divide into two, forming daughter chromatids with centromere in each. Daughter chromosomes are repulsive so, migrate towards opposite poles. Spindle fibres attached to the centromeres shorten and pull the chromosomes, the centromeres lead the path while the limbs trail behind. So, anaphasic chromosomes appear as V, L, J and L shaped.

Mitosis is often said as an indirect division. It is the method of equal division of nuclei. It is an elaborate process which includes a series of important changes in the nucleus as well as in cytoplasm. Therefore, it is called indirect method of division. In direct division, the nucleus becomes constricted at the center, assuming an hour glass shape, and then divides into two. This is followed by cleavage or division of cytoplasm to form 2 daughter cells. It does not involve doubling of chromosomes. Also called amitosis.

Mitosis results in the production of diploid daughter cells with identical genetic complement. In multicellular organism, growth 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 and to restore the nucleocytoplasmic ratio.

In an animal cell, cytokinesis is marked by the appearance of a furrow in the plasma membrane. The furrow slowly deepens and ultimately joins in the centre dividing the cell cytoplasm into two. Plants cells however, are enclosed by a relatively inextensible cell wall, therefore they undergo cytokinesis by a different mechanism. In plant cells, wall formation starts in the centre 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 that represents the middle lamella between the walls of two adjacent cells.

Synapsis occurs during zygotene stage. Synapsis is the pairing of homologous chromosomes which leads to the formation of bivalents.

Meiosis II is known as equational or homotypic division like mitosis. It ensures the maintenance of constant number of chromosomes from generation to generation of a species.

Interkinesis or interphase II is a period of rest that cells of some species enter during meiosis, between meiosis I and meiosis II. No DNA replication occurs during interkinesis however, it does occur during the interphase I stage of meiosis. Interkinesis is generally short lived.

Diplotene is the longest and most active subphase of prophase I of meiosis. The beginning of diplotene is recognized by the dissolution of synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. Diplotene can last for months and years in oocytes of some vertebrate.

The points of attachment between the homologous chromosomes after the partial dissolution of nucleoprotein complex are called chiasmata. It occurs during diplotene substage of prophase I.