Cell Cycle and Cell Division: Class 11 NCERT Guide

Understanding the Phases of the Cell Cycle

The cell cycle is an ordered series of events that take place in a cell, leading to its division and the production of two daughter cells. It is not a continuous process but is divided into two main periods: Interphase and M Phase (Mitosis phase). Think of Interphase as the 'living' or 'preparatory' phase, where the cell spends about 95% of its time. It is further subdivided into three stages:

• G1 Phase (Gap 1): This is the interval between mitosis and the initiation of DNA replication. During G1, the cell is metabolically active and continuously grows. It synthesizes RNA and proteins. The cell's fate is decided here – it either proceeds to divide or enters a quiescent stage called G0.
• S Phase (Synthesis): This is the most critical stage of interphase. The primary event here is the replication of DNA. The amount of DNA per cell doubles (from 2C to 4C), but importantly, the number of chromosomes remains the same. For example, a human cell in G1 has 46 chromosomes, and after S phase, it still has 46 chromosomes, but each chromosome now consists of two sister chromatids.
• G2 Phase (Gap 2): Following the S phase, the cell enters the G2 phase. Here, protein synthesis continues, and the cell prepares for mitosis by producing proteins like tubulin, which are needed for spindle formation. The cell continues to grow until it enters the M phase.

The M Phase represents the actual cell division. It begins with nuclear division (karyokinesis) and usually ends with the division of the cytoplasm (cytokinesis).

A Step-by-Step Guide to Mitosis

1. Prophase — This is the first stage of mitosis. The chromatin material, which is like a tangled mess of threads, starts to condense and becomes visible as distinct chromosomes. Each chromosome consists of two sister chromatids joined at a point called the centromere. The nuclear envelope and nucleolus begin to disintegrate, and the mitotic spindle, made of microtubules, starts to form.
2. Metaphase — The chromosomes, now fully condensed, align themselves at the cell's equator. This imaginary line is called the metaphase plate. Each chromosome is attached to spindle fibers from opposite poles via its kinetochore (a protein structure on the centromere). This alignment is a key characteristic and makes metaphase the best stage to study chromosome morphology.
3. Anaphase — This is a dramatic and rapid stage. The centromeres of each chromosome split simultaneously, and the sister chromatids separate. Now, each chromatid is considered an independent chromosome. These newly formed daughter chromosomes are then pulled towards opposite poles of the cell by the shortening spindle fibers.
4. Telophase — The separated chromosomes reach the opposite poles and begin to decondense, reverting to their chromatin form. A new nuclear envelope forms around each set of chromosomes, and the nucleolus reappears. In essence, Telophase is the reverse of Prophase. Two distinct nuclei are now formed within the same cell.
5. Cytokinesis — This process, which often begins during late anaphase or telophase, involves the division of the cell's cytoplasm into two separate daughter cells. In animal cells, a cleavage furrow forms and deepens to pinch the cell in two. In plant cells, a cell plate forms in the middle and grows outwards to become a new cell wall, dividing the cell.

Comparison: Mitosis vs. Meiosis

Exam Traps: Chromosome vs. Chromatid Counting

A very common area for confusion and a favorite for MCQ questions is counting chromosomes, chromatids, and DNA content at different stages. Remember this simple rule: Count the number of centromeres to find the number of chromosomes.

• G1 Phase: A diploid cell (2n) has, for example, 46 chromosomes. The DNA content is 2C.
• S/G2 Phase: After DNA replication, the cell still has 46 chromosomes because the sister chromatids are joined by a single centromere. However, each chromosome has two chromatids. So, you have 46 chromosomes, 92 chromatids, and the DNA content is now 4C.
• Anaphase: This is the tricky part! When the centromeres split and sister chromatids separate, each chromatid becomes an independent chromosome. For a brief period, the cell contains double the normal number of chromosomes. So, the human cell in anaphase has 92 chromosomes (each with one chromatid) and a DNA content of 4C, temporarily.
• After Mitosis: Each daughter cell returns to the G1 state: 46 chromosomes and 2C DNA content.

Practice Questions with Solutions

• Q: A biologist observes a cell from an onion root tip under a microscope. She sees chromosomes aligned at the equatorial plate. Which stage of mitosis is she observing, and what will happen next? A: Step 1: Identify the key feature described. The alignment of chromosomes at the equatorial plate (metaphase plate) is the defining characteristic of Metaphase. Step 2: Recall the sequence of mitosis. The stage immediately following Metaphase is Anaphase. Step 3: Describe the key event of the next stage. In Anaphase, the centromeres will split, and the sister chromatids will separate, moving towards opposite poles of the cell. Final answer: The stage observed is Metaphase. The next event will be Anaphase, where sister chromatids separate and move to opposite poles.
• Q: If a diploid cell has 20 chromosomes (2n=20) and undergoes meiosis, what will be the chromosome number in each of the four daughter cells? A: Step 1: Understand the function of meiosis. Meiosis is a reductional division, meaning it halves the chromosome number. Step 2: The parent cell is diploid (2n), meaning it has two sets of chromosomes. Here, 2n = 20. Step 3: Meiosis produces haploid (n) daughter cells, which contain only one set of chromosomes. Step 4: Calculate the haploid number. If 2n = 20, then n = 20 / 2 = 10. Final answer: Each of the four daughter cells will have 10 chromosomes.
• Q: A cell is in the G1 phase of the cell cycle. If it has a DNA content of 2C = 10 picograms (pg), what will be its DNA content in the G2 phase and after meiosis I? A: Step 1: Recall the event in the S phase. The S phase, which lies between G1 and G2, involves DNA replication. This doubles the DNA content. Step 2: Calculate the DNA content in G2. If the G1 content is 2C = 10 pg, after replication in S phase, the G2 content will be 4C. Therefore, 4C = 2 * 10 pg = 20 pg. Step 3: Understand Meiosis I. Meiosis I separates homologous chromosomes, not sister chromatids. This halves the chromosome number and also the DNA content from the G2 state (4C). Step 4: Calculate the DNA content after Meiosis I. The cells become haploid in terms of chromosome sets but each chromosome still has two chromatids. The DNA content is halved from 4C to 2C. So, the DNA content will be 10 pg. Final answer: The DNA content in the G2 phase will be 20 pg (4C), and after Meiosis I, it will be 10 pg (2C).
• Q: Why is crossing over important, and in which specific stage does it occur? A: Step 1: Define crossing over. Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes. Step 2: Identify the stage where it occurs. This event is a hallmark of Meiosis I, specifically during the Pachytene substage of Prophase I. Step 3: Explain its importance. The primary significance of crossing over is that it creates new combinations of alleles on the chromosomes. This process of 'recombination' is a major source of genetic variation among offspring produced by sexual reproduction. Final answer: Crossing over is important because it creates genetic variation by recombining alleles. It occurs during the Pachytene stage of Prophase I of meiosis.

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