Cell Cycle and Cell Division Class 11 Notes: CBSE Biology Chapter 10

Cell cycle 

The cell cycle is defined as the series of changes that a cell undergoes which results in the division of cells into two daughter cells and its growth. The cell cycle is divided into two phases- interphase and mitosis. The average duration of a cell completing its two phases in humans is 24 hours. Of this, the duration of mitosis is for only 1 hour. Therefore, the majority of time of a  cell cycle is spent in interphase.

Interphase 

It is a phase in which different changes occur to prepare the cell for cell division. It is further divided into the following phases 

The G1 phase also called the first gap phase deals with the biosynthetic activities which occur at a very fast rate. Cell synthesizes more proteins, increasing the number of mitochondria and ribosomes. The cell also prepares itself for DNA  replication.  

In the S phase, DNA is replicated. At the end of DNA replication, each chromosome will have two sister chromatids. So the content of DNA gets doubled, but ploidy remains the same. 

G2 phase, where the cell prepares itself for mitosis involves protein synthesis and leads to further growth of the cell. 

Mitosis 

Called equational division as the number of chromosomes in the daughter cell and the parent cell remains the same. It begins with nuclear division. This is known as karyokinesis. Chromosomes get distributed equally during mitosis. Only somatic cells undergo mitosis.  

It is divided into the following phases 

Prophase 

In prophase, chromatin condenses to form chromosomes. Two identical copies of each chromosome are attached to the centromere. During the end of this phase, the nucleolus dissolves. The nuclear membrane also disintegrates at the end. The centrosome moves to the opposite poles. Spindle fibers start to appear.

Metaphase 

Here chromosomes get aligned on the equatorial plate. It is also called metaplate. The method of analyzing the metaphase chromosomes helps in cytogenetics and cancer studies. 

Anaphase 

It is the shortest stage of the cell cycle. During this phase, the replicated chromosomes move apart and daughter chromatids move to the opposite poles. Chromosomes get condensed during the late anaphase. The chromosomes become Y-shaped while moving towards the poles on opposite sites.

Telophase 

The end result of telophase is the formation of two daughter nuclei. Nucleolus and nuclear membrane reappear. The late telophase is marked by cytokinesis. This is the last phase of mitosis. Finally, the chromosomes reach the poles.

Cytokinesis 

The division of the cytoplasm, its organelles, and membrane to form two cells is known as cytokinesis. It results in the division of a cell into two daughter cells that are identical to its parent.  

  Importance of Mitosis 

• Produce genetically identical daughter cells. 

• Growth of the body takes place as a result of mitosis. 

• Cell repair and replacement also occur as a result of mitosis. 

• Maintenance of nucleo-cytoplasmic ratio.  

Cell Cycle Exit 

Some cells divide at a fast rate whereas some divide slowly and some even do not divide once they are formed.  Those cells which do not divide once formed, enter into a phase known as GO phase. For example, neuronal cells once formed will not divide.  

Meiosis 

Meiosis is also called reductional division as it reduces the number of chromosomes in the progenies to half the parent cell. There are two types of meiosis; I and II which results in gametes such as sperm or egg.  

Meiosis I 

It is divided into the following phases 

• Prophase I 

The longest phase of meiosis I is where homologous chromosome pairs and DNA segments are exchanged. This process is also known as recombination. It is further divided into following 

• Leptotene 

First stage of meiosis is leptotene

Individual chromosomes subsist with two sister chromatids.  

Elements of synaptonemal complex assemble. 

Condensation and coiling of chromosomes takes place in leptotene

• Zygotene 

• Chromosomes align together as homologous pairs

• Synapses of homologous chromosomes occur. 

• The paired chromosomes are known as bivalent or tetrad due to its appearance

• Pachytene

• Crossing over and homologous recombination occurs during the pachytene stage 

• Chiasmata are X-shaped structures as a result where homologous chromosomes remain in contact.

•  Diplotene 

• Homologous chromosomes start to separate. 

• Synaptonemal complex disassemble. 

• Chromosomes remain attached to the chiasmata. 

• Diakinesis 

• Chromosomes condense further so that four parts of the tetrads are visible. 

• The nucleoli disappear, and the nuclear membrane disintegrates. 

• Mitotic spindle starts to form. 

• Metaphase I 

• Homologous chromosomes remain aligned on the meta plate. 

• The replicated chromosomes are collectively bound together with a protein known as cohesin.

• Anaphase I 

Homologous chromosomes are pulled to the opposite poles as the result of the shortening of microtubules. The cohesin  (protein complex) degrades from the chromosome arms and remains safe around the centromere. Thus the sister chromatids exist together while the homologus segregate. 

• Telophase I 

The daughter cell contains half the number of chromosomes as compared to the parent cell. The spindle begins to disappear which was formed of microtubules. Once again chromosomes from chromatin. Sister chromatids remain attached together.

Meiosis II 

Meiosis II is the second meiotic division. Similar to mitosis in process, but the genetic results are different. Products formed are four haploid cells from the two haploid cells produced in meiosis I.  

The four main steps of Meiosis II are:- Prophase II, Metaphase II, Anaphase II, and Telophase II. 

In prophase II nucleoli and nuclear membrane disappear, shortening and thickening of the chromatids occur. Centrosomes move to the poles and spindle fibers are formed for the second meiotic division.

In metaphase II, the centromeres are present with two kinetochores attached to spindle fibers from the centrosomes at opposite poles. When compared to meiosis I, the metaplate is rotated perpendicular to the previous plate by 90 degrees. 

Anaphase II is determined by sister chromatid segregation. The remaining protein cohesin degrades to allow segregation of sister chromatids.  

Telophase II which is similar to telophase I, results in the de-condensation of chromosomes. Nuclear envelopes reorganize and cleavage.  Cell plate formation produces four daughter cells, each with a haploid set of chromosomes.

Significance of Meiosis:

• Maintenance of chromosome number generation after generation in case of sexual reproduction. 

• Increases variation in the population. 

5 Important Topics of Biology Class 11 Chapter 10 You Shouldn’t Miss!

Importance of Cell Cycle and Cell Division Class 11 Short Notes

• Cell Division Class 11 Notes PDF provides a strong understanding of the basic concept of cell cycle and cell division, which are essential for learning more complex biological processes in higher classes.

• Cell cycle and Cell division a fundamental chapters for NEET and other competitive exams, making these notes crucial for exam preparation.

• The notes break down complex concepts into simple, easy-to-understand points, helping students grasp the material quickly.

• Well-organised Cell Cycle and Cell Division Short Notes allow for efficient revision, covering key points, important reactions, and frequently asked structures in exams.

• Diagrams and examples included in the notes enhance understanding and retention of the subject matter.

• Cell Division Class 11 Notes PDF covers all important topics and subtopics, ensuring that students have a complete understanding of the chapter.

Tips for Learning the Class 11 Biology Chapter 10 Cell Cycle and Cell Division

• Familiarise yourself with key terms like interphase, mitosis, meiosis, cytokinesis, chromatin, chromosomes, and spindle fibres.

• Review and draw diagrams of the cell cycle, phases of mitosis, and meiosis to help with memorization.

• Learn the cell cycle and mitosis/meiosis phases step-by-step, focusing on what happens in each phase and its purpose.

• Understand how cell division impacts growth, repair, and reproduction by relating concepts to real-life examples.

• Solve end-of-chapter questions and previous year’s papers to test your understanding and application of concepts.

• Periodically review the material to reinforce your memory and understanding.

Conclusion

The Cell Cycle and Cell Division are fundamental processes essential for life. They enable growth, repair, and reproduction by ensuring that cells divide correctly and pass on genetic information accurately. Understanding the phases of the cell cycle, including interphase and mitosis, as well as the process of meiosis, helps in grasping how organisms develop and maintain their functions. These processes are crucial for both single-celled and multicellular organisms. By regularly reviewing Cell Cycle and Cell Division Class 11 Notes and practising with diagrams and key points, you will strengthen your grasp of the material. 

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