Before we discuss all the complicated terms of Class 11 Cell Cycle and Cell Division, let's dive into the simple terms that this chapter is about. Let's understand the cell cycle and cell division in general terms. 

 

Define Cell Cycle Class 11 NCERT Chapter. 

Cell division is the series of events wherein the cell splits into more than its initial number for propagation or reproduction. It increases the population.

Cell Cycle refers to the series of events that occur simultaneously in a cell as it proceeds towards division. 

Cell division is an important factor of Biology. It defines the course of life on Earth. If you can understand all the concepts under the Cell Cycle chapter well, then you can find Cell Cycle and Cell Division Class 11 NCERT Solutions. Let us navigate through all the phases or stages that are involved in the process of the cell cycle.

The cell cycle is closely related to cell division. Cell division is the process of division from the parent cells to the daughter cells. The process is continuous, and the result is different for prokaryotic cells and eukaryotic cells. We'll talk about that in detail later. 

Now, coming to the cell cycle. A cell cycle can be simply defined as the processes involved in cell growth, synthesis of DNA, and division of cells. It has two main phases: interphase and mitosis. 

Let's have a look at how the cell cycle is carried out in prokaryotic cells and eukaryotic cells.

Prokaryotic Cell Cycle 

The cell cycle in prokaryotic cells is extremely simple. In prokaryotic organisms or prokaryotes, the cell grows, the replication of DNA takes place, and then the cell divides. The cell cycle is completed then. This is called asexual reproduction. 

 

Eukaryotic Cell Cycle

Due to the presence of a nucleus and several other organelles, the process of the cell cycle is complicated here. We will learn about the process in detail later.

 

It is the first step towards mitosis in eukaryotic cells. In this particular stage, the cells are in preparation mode for division. It is further divided into numerous other phases: 

• • G1 Phase: The growth phase or first gap phase is the most important as it is the longest phase of the cell cycle. During this phase, the cell performs all its routine activities. It synthesizes more proteins and amino acids. If the cell does not divide in this phase, it enters the G0 phase.

  • G0 Phase: This is the resting phase of the cycle. The cell stops dividing here. A cell may or may not enter this phase depending on the division. They can stay here for a longer duration. 

  • S Phase: This is known as the synthesis phase. The replication of DNA happens here. This means that each chromosome is with two sister chromatids. Henceforth, DNA gets doubled in the S phase. However, the ploidy(number of complete sets of chromosomes) remains the same.

  • G2 Phase: In this phase, the cell starts preparing itself for mitosis that it will undergo later.

The M Phase or Mitosis is also known as the Equational Division. It goes by that name since the number of chromosomes remain the same in the daughter cell as in the parent cell. The nuclear division occurs here. It goes by the name Karyokinesis. 

 

It is further divided into:

• Prophase 

In this phase, we see the formation of chromosomes by the condensation of chromatin. The nucleolus also dissolves at the end of the Prophase. The centromere is moved to the opposite poles. The formation of spindle fibres occurs.

• Metaphase 

In this phase, the chromosomes align themselves along the line of the equatorial plate. This is used in the study of cancerous cells.

• Anaphase 

In this phase, the replicated chromosomes separate and move away from each other. The daughter chromatids move to the opposite poles. Since the separation occurs, they appear in Y-shape. This is the shortest stage.

• Telophase 

In this phase, we see the formation of two daughter nuclei. The nucleolus and nuclear membrane that had disappeared are in sight again. This is officially the last stage in mitosis. However, it is followed by cytokinesis.

• Cytokinesis

As the name suggests, it is the stage where the two daughter cells are finally formed. It is characterized by the division of the organelles, cytoplasm, and even cell membrane. The daughter cells formed are identical to their parent cells.

 

 

It is as follows: 

1. It helps in equal division. It means that the daughter cells have the same number of chromosomes after the division.

2. It aids in the repair of damaged tissues.

3. It helps in the growth and development of multicellular organisms.

4. It affects the process of sexual reproduction indirectly.

5. It forms the backbone of the evolution of a single cell into a sexually mature individual. 

6. It helps in maintaining the skin cells.

It is also called the Reductional Division. In this type of division, the number of chromosomes is halved. It is divided into Meiosis I and Meiosis II.

Meiosis I 

It is divided into: 

• Prophase I

This is the longest phase present in Meiosis I. In this phase, the recombination of homologous pairs and DNA segments take place. 

Prophase goes on to proceed under the following stages: 

               1. Leptotene 

It is the first stage. In this stage, the chromosomes exist with a sister chromatid. The assembly of synaptonemal complexes occurs. We also observe the condensation of chromosomes.

      2. Zygotene

The chromosomes start aligning, and the synapsis of homologous chromosomes occurs. This pair is called Bivalent or Tetrad. 

               3. Pachytene

The crossing over and recombination occurs in this stage. Chiasmata is formed here. 

               4. Diplotene 

The homologous chromosomes start separating in this stage. Chromosomes remain bound at chiasmata. 

               5. Diakinesis 

Condensation of chromosomes takes place. Tetrads are visible. Nucleoli disappear, and the nuclear membrane also disintegrates. We see the formation of mitotic spindles.

• Metaphase I 

There is the alignment of the homologous chromosomes on the metaphase plate. They're joined by Cohesin.

• Anaphase I 

The sister chromatids are attached, and the homologous chromosomes are separated.

• Telophase I 

The daughter cells contain half its chromosomes now. The microtubules forming the spindle start disappearing. Chromatin is formed. The sister chromatids remain attached.

Meiosis II 

This is the second meiosis that takes place. It is extremely similar to the first one except for its result. This one results in four haploid cells by the division of the two haploid cells produced in the first meiosis. 

 

The stages involved are: 

• • Prophase II 

The chromatids are shortened and thickened. We see the disappearance of nucleoli and the nuclear membrane. The centrosomes take a position at the poles. 

• • Metaphase II

The metaphase plate is rotated at an angle of 90° as compared to meiosis I.

• • Anaphase II 

The sister chromatids separate here. 

• • Telophase II 

It is similar to telophase I. We see the condensation of chromosomes. Nuclear envelopes are visible again. Now, four daughter cells are formed as a result of a haploid set of chromosomes.

 

• It is essential for sexual reproduction. 

• It forms gametes. 

• It maintains the chromosome numbers.

• It introduces new variations. 

• It helps in the transfer of genetic information.

 

1. What are the events taking place during Interphase? 

The events involved in interphase are Growth Phase 1 (G1), Synthesis Phase (S), and Growth Phase 2 (G2).

2. Define what stage G0 is? 

In this stage, no division of cells takes place. It is a dormant stage.

3. How many changes take place in chromosomes and the amount of DNA content per cell during every part of the cell cycle? 

It takes place as the following: