Cell Cycle and Cell Division Class 11 Notes CBSE Biology Chapter 10 (Free PDF Download)

Section–A (1 Mark Questions)

1. Where does mitosis take place in plants and animals?

Ans.In plants mitosis takes place in meristematic tissue while in animals it takes place in all somatic cells and gonads.

2. Name the cell division concerned with cancer?

Ans. Mitosis is the type of cell division that is concerned with cancer.

3. Name the enzyme which is involved in crossing over.

Ans. Recombinase enzymes is involved in crossing over.

4. In which type of cell division crossing over occurs?

Ans. Crossing over occurs in prophase I of meiosis. 

5. What is the average cell cycle span for human cell?

Ans. The average cell cycle span for human cell is 24 hours.

Section–B (2 Mark Questions)

6. Give the terms for the following:

(i) The phase between 2 successive mitotic divisions

(ii) Phase in cell cycle where DNA is synthesized.

Ans. (i) The phase between 2 successive mitotic divisions- Interphase

(ii) Phase in cell cycle where DNA is synthesized- S phase (Synthetic phase)

7. Answer the following questions.

(i) What are homologous chromosomes? 

(ii) What happens to homologous chromosomes during meiosis?

Ans. (i) Homologous chromosomes are pairs of similar chromosomes having corresponding genes governing the same set of traits.

(ii) In meiosis, homologous chromosomes pair and allow crossing over of genetic material via the process of recombination. This genetic recombination produces the variation of genes.

8. How does duration affect the cell cycle in organism? 

Ans. The length of the cell cycle is crucial as it determines how quickly an organism can multiply. For single-celled organisms, cell cycle duration determines how rapidly the organism can reproduce new organisms. For multicellular organisms, the duration of the cell cycle determines how long it takes to replace damaged cells. The duration of the cell cycle varies from organism to organism and from cell to cell. Yeast cells divide in 90 minutes only while in humans, the length of the cycle is approximately 24 hours

9. Fill in the blanks.

It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit ___________ phase to enter an inactive stage called ___________ of cell cycle.

Ans. It is observed that heart cells do not exhibit cell division. Such cells do not divide further and exit G1 phase to enter an inactive stage called G0 phase (quiescent stage) of cell cycle.

10. Telophase is the reverse of prophase. Elucidate the statement.

Ans. During telophase, events of prophase occur in reverse sequence. The condensation of chromosomal material initiates prophase. During the process of chromatin condensation, the chromosomal material untangles. At the end of the prophase, the nucleolus disintegrates gradually, and the nuclear envelope disappears. In telophase, the chromosomes decondense and lose their individuality upon arrival at the respective poles. The nuclear envelope at the telophase stage gathers around the cluster of the chromosomes. The Golgi complex, ER, nucleolus reform.

Revision Notes for Class 11 Biology Chapter 10 – Cell cycle and cell division

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. 

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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.  

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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. 

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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. 

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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.  

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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.  

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  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

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• 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.  

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• 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. 

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• 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.

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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.

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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.