Test: Cell Cycle (NCERT) - NEET MCQ

Cell cycle consists of two basic phases, long non-dividing I phase and a short-dividing M phase. In the average duration of 24 hours in the cell cycle of a dividing human cell, M phase lasts for one hour (less than 5% of total ) while I phase or interphase occupies 75-95% of total generation time.

Line in NCERT: "The M Phase represents the phase when the actual cell division or mitosis occurs and the interphase represents the phase between two successive M phases." "In the 24 hour average duration of cell cycle of a human cell, cell division proper lasts for only about an hour." "The interphase lasts more than 95% of the duration of cell cycle."

S or synthesis phase marks the period during which DNA synthesis or replication takes place. DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles, however, there is no increase in the chromosome number.

Line in NCERT: "However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G₁, even after S phase the number of chromosomes remains the same, i.e., 2n."

(i) Cell division is the process by which a cell divides to form to nearly equal daughter cell which resembles the parent. The cell cycle is the sequence of events which occur during the cell growth and cell division. It is completed into two steps interphase and M-phase.
(ii) In the cell cycle interphase is the period between the end of one division to the beginning of next cell division. It is known as the resting phase. In the case of human beings it is around 24 hours then the M phase takes place.

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

• The cell cycle consists of several distinct phases, including interphase and mitosis. Interphase, which is the longest phase of the cell cycle, can be further divided into three subphases: G₁ (gap phase 1), S (synthesis phase), and G₂ (gap phase 2).
• During the S phase, DNA synthesis occurs. This is the period when the cell replicates its DNA to ensure that each daughter cell produced during cell division receives a complete set of genetic information.
• The DNA replication process involves unwinding and separating the two strands of the DNA double helix and synthesising new complementary strands using the existing strands as templates. As a result, each chromosome is duplicated, and sister chromatids are formed, which remain attached at the centromere.
• After the S phase, the cell progresses into the G₂ phase, where it prepares for cell division. The subsequent phase is mitosis, which includes prophase, prometaphase, metaphase, anaphase, and telophase, leading to the separation of sister chromatids and the formation of two identical daughter cells.
• Therefore, DNA synthesis occurs specifically during the S phase of the somatic cell cycle.

Line in NCERT: "S phase marks the period during which DNA synthesis or replication takes place."

The cell cycle starts from interphase. Interphase is the phase in the cell cycle that prepares a cell nucleus for division. It has three stages — G₁, S and G₂ G₁, Phase is the longest stage of interphase, also first growth phase or post-mitotic gap phase. Both and its nucleus grow in size. There is synthesis proteins, nucleotides, amino acids for histones an rich compounds. In S-Phase chromosomes along with their DNAs replicate. DNA content doubles during this phase. In G₂-Phase there is increased synthesis of RNA and proteins. Cell organelles or their precursorsmultiply. Cell grows in Size.

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. S or synthesis phase marks the period during which DNA synthesis or replication takes place. 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. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G₁, even after S phase the number of chromosomes remains the same, i.e., 2n."

Interphase in a cell cycle has three stages —G₁,S and G₂.M− phase is not a part of interphase.

Line in NCERT: "The stages through which a cell passes from one division to the next is called the cell cycle. Cell cycle is divided into two phases called (i) Interphase – a period of preparation for cell division, and (ii) Mitosis (M phase) – the actual period of cell division."

The phase in which cells fail to divide further (do not enter S- phase after G₁ -phase) and undergo differentiation is known as G₀ phase or quiescent stage. It occurs due to non availability of mitogen and energy rich compounds. The cells remain metabolically active, grow in size and differentiate for particular function after attaining a particular shape.

Line in NCERT: "These cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G) of the cell cycle."

• The Assertion is true because it correctly states that certain adult animal cells, such as heart cells, do not divide and enter a quiescent stage.
• The Reason is also true as it accurately describes the nature of cells in the G0 phase, indicating they are metabolically active but not proliferative.
• Furthermore, the Reason correctly explains the Assertion by providing the functional context of why these cells do not divide.

Line in NCERT: "Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G₁ phase to enter an inactive stage called quiescent stage (G) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism."

• The Assertion is true; it accurately reflects that in the animal kingdom, mitotic cell division predominantly occurs in diploid somatic cells, with exceptions such as haploid male honey bees.
• The Reason is also true, as it correctly states that plants can undergo mitotic divisions in both haploid and diploid cells.
• However, the Reason does not explain the Assertion directly, as the flexibility of cell division in plants does not relate to the primary mitotic nature of animal cells.

Line in NCERT: "In animals, mitotic cell division is only seen in the diploid somatic cells. However, there are few exceptions to this where haploid cells divide by mitosis, for example, male honey bees. Against this, the plants can show mitotic divisions in both haploid and diploid cells."

• Statement i is incorrect because DNA replication only occurs during a specific stage of the cell cycle (S phase), not continuously throughout the cycle.
• Statement ii is correct as the cell cycle includes phases (G1, S, G2, and M) that collectively lead to cell division.
• Statement iii is correct since cell growth (cytoplasmic increase) does happen continuously throughout the cell cycle.
• Statement iv is incorrect because the distribution of replicated chromosomes to daughter nuclei involves complex events, particularly during mitosis.

Thus, the correct statements are ii and iii, making Option A the correct answer.

Line in NCERT: "Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle."