CBSE Class 11 Biology Revision Notes Chapter 10

CBSE Class 11 Biology Revision Notes Chapter 10 – Cell Cycle and Cell Division

The cell is commonly acknowledged as life’s most fundamental structural and functional unit. A cell can be a standalone creature (such as an amoeba) or a component of bigger multicellular organisms such as ourselves. But how can this basic unit multiply to expand or repair? All these important concepts are discussed in Class 11 Biology Chapter 10.

Students can access the Revision Notes for this chapter offered by Extramarks for help with their preparation of this chapter. The Revision Notes clearly explain all the major concepts in the chapter in a concise manner.

Revision Notes for Class 11 Biology Chapter 10 – Cell Cycle and Cell Division

Mentioned below are some of the important topics covered in Revision Notes for Class 11 Biology Chapter 10 – Cell cycle and cell division.

Cell cycle

The cell cycle is described as events that cause a cell to divide into two daughter cells and grow. Interphase and mitosis are the two stages of the cell cycle. It takes 24 hours for a cell to complete its two phases. Mitosis lasts only one hour. As a result, interphase takes up the bulk of a cell cycle.

Interphase

It is a stage in which the cell undergoes many modifications to prepare for cell division. It is further broken into phases.

The G1 phase, also known as the first gap phase, is concerned with biosynthetic processes that occur at a rapid rate. The number of mitochondria and ribosomes increases as the cell synthesises more proteins. In addition, the cell prepares for DNA replication.

DNA replication occurs during the S phase. Each chromosome will contain two sister chromatids after DNA replication. As a result, DNA content doubles, but ploidy stays the same.

The G2 phase, during which the cell prepares for mitosis, involves protein synthesis and leads to cell development.

Mitosis

The number of chromosomes in the daughter and parent cells stays the same, known as equational division. It all starts with nuclear fusion. That is referred to as karyokinesis. During mitosis, chromosomes are spread evenly. Mitosis occurs only in somatic cells. It is broken down into the following stages.

Prophase

Chromatin condenses to create chromosomes during prophase. Each chromosome has two identical copies connected to the centromere. The nucleolus disappears after this phase. Finally, the nuclear membrane disintegrates. The centrosome shifts from one pole to the other. Spindle fibres begin to emerge.

Metaphase

On the equatorial plate, chromosomes are aligned. Metaplate is another name for it. Analysing metaphase chromosomes is useful in cytogenetics and cancer research.

Anaphase

It is the cell cycle’s shortest stage. The duplicated chromosomes separate during this phase, and daughter chromatids travel to opposing poles. During late anaphase, chromosomes become condensed. While travelling towards the poles in separate locations, the chromosomes become Y-shaped.

Telophase

Telophase results in the development of two daughter nuclei. The nuclear membrane and nucleolus resurface. 

Cytokinesis is a sign of late telophase. Mitosis has reached its conclusion. The chromosomes finally have the poles.

Cytokinesis

Cytokinesis is the division of the cytoplasm, its organelles, and the membrane into two cells. It causes a cell to divide into two daughter cells identical to their parent.

Importance of Mitosis

• Produce genetically similar offspring cells.
• Mitosis is the process through which the body grows.
• Mitosis is also responsible for cell repair and replacement.
• The nucleocytoplasmic ratio must be maintained.

Cell Cycle Exit

Some cells divide quickly, whereas others divide slowly or do not divide once they have been established. Those cells that do not divide once they have been created enter the GO phase. Neuronal cells, for example, do not divide after they have been established.

Meiosis

Meiosis is also known as reductional division since the progenies have half the number of chromosomes as the parent cell. Meiosis is divided into two types: I and II, which produce gametes such as sperm or eggs.

Meiosis I

It is broken down into the following stages.

• Prophase I 

Homologous chromosome pairs and DNA segments are exchanged during the period of meiosis I. Recombination is another name for this process. It is further subdivided into the following sections:

• Leptotene

1. Leptotene is the first step of meiosis.
2. There are two sister chromatids on each chromosome.
3. The synaptonemal complex comes together.
4. Chromosome coiling and condensation occur in leptotene.

• Zygotene

1. Chromosomes are arranged in homologous pairs.
2. There are homologous chromosomal synapses.
3. Because of their appearance, paired chromosomes are referred to as bivalent or tetrad.

• Pachytene

1. During the pachytene stage, homologous recombination and crossing over occur.
2. Chiasmata are X-shaped structures formed when homologous chromosomes are kept together.

• Diplotene

1. The separation of homologous chromosomes begins.
2. The synaptonemal complex disintegrates.
3. The chiasmata keep the chromosomes linked.

• Diakinesis

1. Chromosomes compress even further, revealing four components of the tetrads.
2. The nucleoli vanish, and the nuclear membrane breaks down.
3. The mitotic spindle begins to take shape.

• Metaphase I

1. On the meta plate, homologous chromosomes stay aligned.
2. Cohesin is a protein that binds the duplicated chromosomes together as a whole.

• Anaphase I 

The shortening of microtubules causes homologous chromosomes to be dragged to different poles. The cohesin (protein complex) degrades in the chromosomal arms but is safe around the centromere. As a result, the sister chromatids coexist while the homologous separate.

• Telophase I 

The number of chromosomes in the daughter cell is half that of the parent cell. The microtubule-based spindle begins to fade away. Once more, chromosomes arise from chromatin. Sister chromatids continue to be linked. 

Meiosis II

The second division of meiosis is known as meiosis II. Although the process is similar to mitosis, the genetic outcomes are not. The two haploid cells generated after meiosis are divided into four haploid cells. Prophase II, Metaphase II, Anaphase II, and Telophase II are the four phases of Meiosis 2.

The nucleoli and nuclear membrane vanish in prophase II, while the chromatids shorten and thicken. Centrosomes migrate to the poles for the second meiotic division, and spindle fibres emerge.

The centromeres are present in metaphase II, with two kinetochores connected to centrosome spindle fibres at opposing poles. Compared to meiosis I, the metaplate is 90 degrees perpendicular to the preceding plate.

Sister chromatid segregation determines Anaphase II. The remaining protein cohesin degrades, allowing sister chromatids to be separated.

The de-condensation of chromosomes occurs in telophase II, which is comparable to telophase I. Nuclear envelopes cleave and rearrange. Four daughter cells are formed from the cell plate, each with a haploid pair of chromosomes.

Significance of Meiosis:

• In the event of sexual reproduction, the chromosomal number is maintained from generation to generation.
• Increases the population’s diversity