Maharashtra Board Class 11 Biology Chapter 7 Cell Division Solutions

Choose Correct Option

Question A. The connecting link between Meiosis - I and Meiosis - II is ..........
(a) interphase - I
(b) interphase - II
(c) interkinesis - III
(d) anaphase - IV
Answer: (c) interkinesis - III
In simple words: Meiosis is a two-stage cell division. The interkinesis stage is the resting period that occurs between Meiosis I and Meiosis II, where no DNA replication happens.

🎯 Exam Tip: Focus on distinguishing the stages between Meiosis I and Meiosis II for precise answers.

Question B. Synapsis is pairing of ..........
(a) any two chromosomes
(b) non - homologous chromosomes
(c) sister chromatids
(d) homologous chromosomes
Answer: (d) homologous chromosomes
In simple words: Synapsis is the precise alignment and pairing of homologous chromosomes during prophase I of meiosis.

🎯 Exam Tip: Understand the specific types of chromosomes involved in synapsis to avoid common misconceptions.

Question C. Spindle apparatus is formed during which stage of mitosis?
(a) Prophase
(b) Metaphase
(c) Anaphase
(d) Telophase
Answer: (b) S-phase
In simple words: The spindle apparatus, crucial for chromosome separation, begins to form during prophase of mitosis, while S-phase is for DNA synthesis.

🎯 Exam Tip: Carefully recall the key events of each mitotic phase, especially the initiation of structural components like the spindle.

Question D. Chromosome number of a cell is almost doubled up during ..........
(a) G₁ - phase
(b) S - phase
(c) G2-phase
(d) Go-phase
[Note: Due to DNA replication the DNA content of cell doubles during S-phase. But the number of chromosomes remain the same.]
Answer: (b) S - phase
In simple words: During the S-phase of the cell cycle, the DNA content within a cell doubles due to replication, preparing for cell division.

🎯 Exam Tip: Differentiate between the doubling of DNA content (S-phase) and the actual chromosome number, which typically remains constant until anaphase in mitosis.

Question E. How many meiotic divisions are necessary for formation of 80 sperms?
(a) 80
(b) 40
(c) 20
(d) 10
Answer: (c) 20
In simple words: Each meiotic division typically produces four sperm cells from one primary spermatocyte, so 20 meiotic divisions would yield 80 sperms.

🎯 Exam Tip: Remember that one primary spermatocyte undergoes meiosis to produce four functional sperm.

Question F. How many chromatids are present in anaphase - I of meiosis - I of a diploid cell having 20 chromosomes?
(a) 4
(b) 6
(c) 20
(d) 40
Answer: (d) 40
In simple words: In anaphase-I of meiosis, homologous chromosomes separate, but each chromosome still consists of two chromatids. With 20 chromosomes (each with 2 chromatids), there would be 40 chromatids.

🎯 Exam Tip: Distinguish between chromosome number and chromatid number at different stages of meiosis; in Anaphase I, chromosomes still have two chromatids.

Question G. In which of the following phase of mitosis chromosomes are arranged at equatorial plane?
(a) Prophase
(b) Metaphase
(c) Anaphase
(d) Telophase
Answer: (b) Metaphase
In simple words: During metaphase, chromosomes align precisely along the equatorial plate of the cell, forming the metaphase plate, which is crucial for their equal segregation.

🎯 Exam Tip: The alignment of chromosomes at the metaphase plate is a defining characteristic of metaphase in both mitosis and meiosis II.

Question H. Find incorrect statement.
(a) Condensation of chromatin material occurs in prophase.
(b) Daughter chromatids are formed in anaphase.
(c) Daughter nuclei are formed at metaphase.
(d) Nuclear membrane reappears in telophase.
Answer: (c) Daughter nuclei are formed at metaphase.
In simple words: Daughter nuclei are formed during telophase, not metaphase, as metaphase is characterized by chromosome alignment.

🎯 Exam Tip: Clearly understand the distinct events of each stage of mitosis, especially the timing of nuclear membrane changes and daughter nuclei formation.

Question I. Histone proteins are synthesized during
(a) G₁ phase
(b) S - phase
(c) G2 - phase
(d) Interphase
Answer: (b) S - phase
In simple words: Histone proteins, essential for DNA packaging into chromosomes, are synthesized during the S-phase, concurrent with DNA replication.

🎯 Exam Tip: Recall that S-phase is primarily dedicated to DNA replication and the synthesis of associated proteins like histones.

Answer The Following Questions

Question A. While observing a slide, student observed many cells with nuclei. But some of the nuclei were bigger as compared to others but their nuclear membrane was not so clear. Teacher inferred it as one of the phase in the cell division. Which phase may be inferred by teacher?
Answer: Prophase.
In simple words: The teacher inferred prophase because during this stage, the nuclear membrane starts to disintegrate and chromosomes begin to condense, making the nucleus appear larger and less distinct.

🎯 Exam Tip: Recognize key morphological changes in cells during different phases of mitosis, such as nuclear membrane dissolution and chromosome condensation in prophase.

Question B. Students prepared a slide of onion root tip. There were many cells seen under microscope. There was a cell seen under microscope. There was a cell with two groups of chromosomes at opposite ends of the cell. This cell is in which phase of mitosis?
Answer: Anaphase.
In simple words: The cell described is in anaphase, a mitotic stage where sister chromatids separate and move towards opposite poles of the cell.

🎯 Exam Tip: Associate the migration of separated chromatids to opposite poles with the anaphase stage of mitosis.

Question C. Students were shown some slides of cancerous cells. Teacher made a comment as if there would have been a control at one of its cell cycle phase, there wouldn't have been a condition like this. Which phase the teacher was referring to?
Answer: The phase teacher was referring would be G¡ phase.
In simple words: The teacher was likely referring to the G1 phase checkpoint, where cell growth is normally controlled, and errors can lead to uncontrolled division seen in cancerous cells.

🎯 Exam Tip: Understand the role of cell cycle checkpoints, particularly the G1 checkpoint, in regulating cell division and preventing abnormal growth.

Question D. Some Mendelian crossing experimental results were shown to the students. Teacher informed that there are two genes located on the same chromosome. He enquired if they will be ever separated from each other?
Answer:
1. Genes are located on chromosomes at specific distance and position.
2. The greater this distance, the greater the chance that a crossover can occur between the genes and the greater the chances of recombination.
3. The chances of recombination are less between the genes that are placed closed to each other on the chromosome.
4. Therefore, due to recombination the two genes located on the same chromosome have possibility of separating from each other.
In simple words: Genes on the same chromosome can be separated by crossing-over during meiosis, with the frequency of separation depending on their distance apart.

🎯 Exam Tip: Grasp the concept of gene linkage and crossing-over, and how recombination frequency is used to map gene distances on a chromosome.

Question E. Students were observing a film on Paramoecium. It underwent a process of reproduction. Teacher said it is due to cell division. But students objected and said that there was no disappearance of nuclear membrane and no spindle formation, how can it be cell division? Can you clarify?
Answer:
1. Paramoecium is a unicellular organism. The division in Paramoecium occurs by amitosis.
2. It is the simplest mode of cell division.
3. In amitosis, nucleus elongates and a constriction appears. This constriction deepens and divides the nucleus in two daughter nuclei followed by the division of cytoplasm.
In simple words: The division observed in Paramecium is amitosis, a simple form of cell division that doesn't involve nuclear membrane disappearance or spindle formation, unlike mitosis or meiosis.

🎯 Exam Tip: Be aware of different types of cell division, including amitosis, and their distinct characteristics, especially the absence of typical mitotic events.

Question F. Is the meiosis responsible for evolution? Justify your answer.
Answer:
1. Meiosis ensures that organisms produced by sexual reproduction contain correct number of chromosomes.
2. Meiosis exhibits genetic variation by the process of recombination.
3. Variations increase further after union of gametes during fertilization creating offspring with unique characteristics. Thus, it creates diversity of life and is responsible for evolution.
In simple words: Yes, meiosis is crucial for evolution because it introduces genetic variation through recombination and independent assortment, leading to diverse offspring that can adapt to changing environments.

🎯 Exam Tip: Emphasize the role of genetic variation (recombination and independent assortment) in meiosis as the driving force behind evolution through natural selection.

Question G. Why mitosis and meiosis - II are called as homotypic division?
Answer:
1. In mitosis, the chromosome number and genetic material of daughter cells remain same as that of the parent cell.
2. In meiosis - II, two haploid cells formed during first meiotic division divide further into four haploid cells. This division is identical to mitosis. The daughter cells formed in second meiotic division are similar to their parent cells with respect to the chromosome number formed in meiosis - I. Hence mitosis and meiosis - II are called homotypic division.
In simple words: Mitosis and Meiosis II are considered homotypic divisions because in both, the chromosome number in the daughter cells remains the same as in the parent cell undergoing that specific division.

🎯 Exam Tip: Understand that "homotypic" implies an equational division where chromosome number is maintained, distinguishing it from the reductional division of Meiosis I.

Question H. Write the significance of mitosis.
Answer:
1. As mitosis is equational division, the chromosome number is maintained constant.
2. It ensures equal distribution of the nuclear and the cytoplasmic content between the daughter cells, both quantitatively and qualitatively. Therefore, the process of mitosis also maintains the nucleo-cytoplasmic ratio.
3. The DNA is also equally distributed.
4. It helps in growth and development of organisms.
5. Old and worn-out cells are replaced through mitosis.
6. It helps in the asexual reproduction of organisms and vegetative propagation in plants.
In simple words: Mitosis is vital for growth, repair, and asexual reproduction, ensuring that daughter cells receive an identical set of chromosomes and maintaining genetic consistency.

🎯 Exam Tip: List the key roles of mitosis in multicellular organisms, focusing on cell replacement, growth, and genetic stability.

Question I. Enlist the different stages of prophase - I.
Answer:
1. Prophase -I:
It is the most complicated and longest phase of meiotic division.
It is further divided into five sub-phases viz. leptotene, zygotene, pachytene,
diplotene and diakinesis.
a. Leptotene:
The volume of the nucleus increases.
The chromosomes become long distinct and coiled.
They orient themselves in a specific fonn known as bouquet stage. This is
characterized with the ends of chromosomes converged towards the side of
nucleus where the centrosome lies. j Lep
The centriole duplicates into two and migrates to opposite poles. [Note: Centrioles
divide during Gj phase of interphase.]
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र लेप्टोटीन अवस्था को दर्शाता है, जिसमें क्रोमोसोम लंबे, अलग-अलग और कुंडलित दिखाई देते हैं। सेंट्रियोल दो में विभाजित होकर विपरीत ध्रुवों की ओर बढ़ते हैं, जबकि न्यूक्लियर मेम्ब्रेन, सेंट्रोमियर और न्यूक्लियोलस भी स्पष्ट रूप से लेबल किए गए हैं।
b. Zygotene:
Pairing of non-sister chromatids of homologous chromosomes takes place by
formation of synaptonemal complex. This pairing is called synapsis.
Each pair consists of a maternal chromosome and a paternal chromosome.
Chromosomal pairs are called bivalents or tetrads.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र जाइगोटीन अवस्था को दर्शाता है जहाँ समजात गुणसूत्र (homologous chromosomes) एक-दूसरे के साथ युग्मन (pairing) करते हैं, जिससे एक बाइवेलेंट या टेट्राड बनता है। इसमें सेंट्रोमियर, न्यूक्लियोलस और सिनैप्टोनीमल कॉम्प्लेक्स स्पष्ट रूप से लेबल किए गए हैं जो युग्मन की प्रक्रिया को दर्शाते हैं।
c. Pachytene:
Each individual chromosome begins to split longitudinally into two similar
chromatids. Therefore, each bivalent now appears as a tetrad consisting of four
chromatids.
The homologous chromosomes begin to separate but they do not separate
completely and remain attached to one or more points. These points are called
chiasmata (Appear like a cross-X).
Chromatids break at these points and broken segments are exchanged between
non-sister chromatids of homologous chromosomes resulting in recombination.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र पैकीटीन अवस्था को दर्शाता है, जहाँ समजात गुणसूत्रों के बीच क्रॉसिंग ओवर होता है। इस चरण में न्यूक्लियोलस गायब होने लगता है और गुणसूत्रों के बीच आनुवंशिक सामग्री का आदान-प्रदान स्पष्ट रूप से दिखाया गया है, जिससे पुनर्संयोजन होता है।
d. Diplotene:
The chiasma becomes clearly visible in diplotene due to beginning of repulsion
between synapsed homologous chromosomes. This is known as desynapsis.
Synaptonemal complex also starts to disappear.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र डिप्लोटीन अवस्था को दर्शाता है, जहाँ गुणसूत्रों के बीच प्रतिकर्षण (repulsion) शुरू होने के कारण कियास्मेटा (chiasma) स्पष्ट रूप से दिखाई देते हैं। प्रत्येक गुणसूत्र में अभी भी दो क्रोमेटिड होते हैं, जैसा कि लेबल में दिखाया गया है।
e. Diakinesis:
The chiasmata begin to move along the length of chromosomes from the
centromere towards the ends of chromosomes. The displacement of chiasmata is
termed as terminalization.
The terminal chiasmata exist till the metaphase.
The nucleolus and nuclear membrane completely disappear and spindle fibres
begin to appear.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र डाइकाइनेसिस अवस्था को दर्शाता है, जहाँ कियास्मेटा गुणसूत्रों की लंबाई के साथ सिरों की ओर बढ़ने लगते हैं, जिसे टर्मिनलाइजेशन कहते हैं। इस चरण में न्यूक्लियोलस और परमाणु झिल्ली पूरी तरह से गायब हो जाते हैं।
In simple words: Prophase I of meiosis is a complex and lengthy stage divided into five sub-phases: leptotene, zygotene, pachytene, diplotene, and diakinesis, each with distinct chromosomal behaviors leading to genetic recombination.

🎯 Exam Tip: Memorize the distinct events of each sub-stage of Prophase I, as they are fundamental to understanding meiotic recombination and genetic variation.

Draw Labelled Diagrams And Write Explanation

Question A. With the help of suitable diagram, describe the cell cycle.
Answer:
1. Series of events occurring in the life of a cell is called cell cycle. Interphase and
M - phase are the two phases of cell cycle.
2. Interphase: It is the stage between two successive cell divisions. It is the longest
phase of a cell cycle during which the cell is highly active and prepares itself for
cell division.
The interphase is subdivided into three sub-phases as G₁ - phase, S-phase and G2-
phase.
a. G₁ - phase (First gap period/First Gap Phase):
It begins immediately after cell division.
RNA (mRNA, rRNA and tRNA) synthesis, protein synthesis and synthesis of
membranes take place during this phase.
b. S - phase (Synthesis phase):
In this phase DNA is synthesized (replicated), so that amount of DNA per cell
doubles.
Synthesis of histone proteins takes place in this phase.
c. G2 - phase (Second growth phase/Second Gap Phase):
Metabolic activities essential for cell division occur during this phase.
Various proteins which are necessary for the cell division are also synthesized in
this phase.
Apart from this, RNA synthesis also occurs during this phase.
In animal cells, a daughter pair of centrioles appears near the pre-existing pair.
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र कोशिका चक्र (Cell Cycle) को दर्शाता है, जिसमें इंटरफेज (G1, S, G2) और माइटोसिस (प्रोफेज, मेटाफेज, एनाफेज, टेलोफेज, साइटोकाइनेसिस) शामिल हैं। विभिन्न चरणों में कोशिका वृद्धि, डीएनए प्रतिकृति, प्रोटीन संश्लेषण, अंग दोहराव और कोशिका विभाजन जैसी महत्वपूर्ण प्रक्रियाएं होती हैं, जिससे बेटी कोशिकाओं का निर्माण होता है।
In simple words: The cell cycle is the ordered series of events that a cell undergoes to grow and divide into two daughter cells, comprising interphase (G1, S, G2) and the M-phase (mitosis/meiosis and cytokinesis).

🎯 Exam Tip: A well-labeled diagram illustrating the different phases and sub-phases of the cell cycle is essential, along with a clear description of the key events in each stage.

Question B. Distinguish between mitosis and meiosis.
Answer:

🎯 Exam Tip: A clear, comparative table highlighting the differences in cell type, number of divisions, chromosome number, and outcomes between mitosis and meiosis is crucial for understanding.

Question C. Draw labelled diagrams and write explanation Draw the diagram of metaphase.
Answer:
Metaphase:
a. Chromosomes are completely condensed and appear short.
b. Centromere and sister chromatids become very prominent.
c. All the chromosomes are arranged at equatorial plane of cell. This is called
metaphase plate.
d. Mitotic spindle is fully formed in this phase.
e. Centromere of each chromosome divides horizontally into two, each being
associated with a chromatid. [Note: The centromeres divide at the beginning of
anaphase so that the two chromatids of each chromosome become separated
from each other.
Source: Cell Division, Donald B. McMillan, Richard J. Harris, in An Atlas of
Comparative Vertebrate Histology, .]
ℹ️ चित्र व्याख्या (Diagram Explanation): [मेटाफेज का एक स्पष्ट, लेबल वाला चित्र जिसमें गुणसूत्र कोशिका के मध्य में मेटाफेज प्लेट पर संरेखित होते हुए, और उनसे जुड़े स्पिंडल फाइबर दिखाई दें।]
In simple words: Metaphase is a key stage of mitosis where condensed chromosomes align perfectly at the cell's equatorial plane, forming the metaphase plate, ensuring equal distribution to daughter cells.

🎯 Exam Tip: The formation of the metaphase plate is the hallmark of metaphase; diagrams should clearly show chromosomes aligned at the center, attached to spindle fibers.

Question 4. Match the following column - A with column - B
Answer:

🎯 Exam Tip: A thorough understanding of the characteristic events in each sub-stage of Prophase I is essential for correctly matching these terms.

Question 5. Is the given figure correct? Why?
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र एक कोशिका को मेटाफेज अवस्था में दिखाता है, जहाँ गुणसूत्र मेटाफेज प्लेट पर व्यवस्थित हैं, लेकिन यह गलत है क्योंकि गुणसूत्रों के सेंट्रोमियर से स्पिंडल फाइबर जुड़े हुए नहीं दिखाए गए हैं।
Answer:
1. The given figure is incorrect as the spindle fibres are not attached to centromere
of the chromosomes.
2. During metaphase, chromosomes are attached to spindle fibres with the help of
centromeres.
In simple words: The figure is incorrect because a defining feature of metaphase is the attachment of spindle fibers to the centromeres of chromosomes to facilitate their movement.

🎯 Exam Tip: Always ensure that diagrams of metaphase accurately depict spindle fibers attached to the centromeres of chromosomes aligned at the equatorial plate.

Question 6. If an onion has 16 chromosomes in its leaf cell, how many chromosomes will be there in its root cell and pollen grain.
Answer:
1. The chromosomes in root cell will be 16 as root cell is a diploid cell.
2. The chromosomes in pollen grain will be 8 as pollen grain is a haploid cell.
In simple words: Onion leaf cells and root cells are diploid (2n), so they have 16 chromosomes. Pollen grains are haploid (n), containing half the chromosome number, which is 8.

🎯 Exam Tip: Understand the difference between diploid (somatic cells) and haploid (gametes) chromosome numbers in an organism.

Question 7. Identify the following phases of mitosis and label the 'A' and 'B' given in diagrams.
Answer:
(i)
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र माइटोसिस की मेटाफेज अवस्था को दर्शाता है, जहाँ स्पिंडल फाइबर स्पष्ट रूप से दिखाई दे रहे हैं। लेबल 'A' उन गुणसूत्रों की ओर इशारा करता है जो कोशिका के भूमध्यरेखीय तल पर मेटाफेज प्लेट पर व्यवस्थित हैं।
The diagram shown is of Metaphase.
A: Chromosomes arranged on metaphase plate

(ii)
ℹ️ चित्र व्याख्या (Diagram Explanation): यह चित्र माइटोसिस की एनाफेज अवस्था को दर्शाता है, जहाँ क्रोमेटिड्स अलग होकर कोशिका के विपरीत ध्रुवों की ओर बढ़ रहे हैं। लेबल 'B' इन अलग होते हुए क्रोमेटिड्स को दर्शाता है।
The diagram shown is of Anaphase.
B: Chromatids moving to opposite poles.
In simple words: Question 7 requires identifying two mitotic phases from diagrams: (i) Metaphase, where chromosomes align at the equatorial plate (A); and (ii) Anaphase, where chromatids separate and move to opposite poles (B).

🎯 Exam Tip: Accurately identifying mitotic stages relies on recognizing characteristic chromosomal arrangements and movements, and correct labeling demonstrates a strong understanding of cell division morphology.

Practical / Project

Question 1. Fix the onion root tips at different durations of the day starting from 6am up to 9am at the intervals of half an hour. Prepare the slide of each fixed root tip and analyse the relation between time and phase of mitosis.
Answer:
Mitotic division is an equational division in which one parent cell give rise to two daughter cells with equal number of chromosomes in daughter cells and mother cell. It has four sub phases: prophase, metaphase, anaphase, telophase.
Mitosis is affected by temperature and time. Mitotic index is high in morning so the mitosis is observed clearly in the morning. (Mitotic index is defined as the ratic between the number of cells in a population undergoing mitosis to the total number of cells in a population.)
[Note: Students catt use above information for reference and perform this activity on their own.]
In simple words: This project aims to study how the timing of mitosis in onion root tips varies throughout the day, recognizing that environmental factors like temperature and time influence mitotic activity.

🎯 Exam Tip: Practical experiments like observing onion root tips reinforce theoretical knowledge of mitotic phases and cell cycle regulation.

11th Biology Digest Chapter 7 Cell Division Intext Questions And Answers

Can You Recall? (Textbook Page No. 76)

Question. How do your wounds heal?
Answer:
a. A wound is an injury to living tissue.
b. Healing of wound take place by mitosis.
c. Repetitive mitotic divisions near the site of injury results in healing of wound.
In simple words: Wounds heal through mitosis, where new cells are continuously produced by repetitive cell division near the injury site to replace damaged tissue.

🎯 Exam Tip: Relate the concept of mitosis to real-life biological processes like wound healing and regeneration.

Can You Tell? (Textbook Page No. 79)

Question. What is cell cycle?
Answer:
1. Sequential events occurring in the life of a cell is called cell cycle.
2. Interphase and M - phase are the two phases of cell cycle.
3. Cell undergoes growth or rest during interphase and divides during M -
phase.
In simple words: The cell cycle is the series of events a cell goes through from its formation to its division into two daughter cells, consisting of interphase (growth) and M-phase (division).

🎯 Exam Tip: A fundamental concept, ensure you can define the cell cycle and identify its primary phases.

Discuss With Teacher (Textbook Page No. 76)

Question. Some cells do not have gap phase in their cell cycle whereas some cells spend maximum part of their life in gap phase. Search for such cells. Some cells are said to be in G₀ phase. What is this G₀ phase?
Answer:
1. G₀ is the phase of the cell cycle in eukaryotes in which many cell types stop dividing. It is also called a quiescent stage.
2. If cells are deprived of appropriate growth factors, they stop at the G₁ checkpoint of the cell cycle. Their growth and division are arrested and they remain in G₀ phase.
3. Mature neurons and muscle cells remain in G₀ phase.
In simple words: The G₀ phase is a resting state where cells are not actively dividing or preparing to divide, often due to lack of growth signals or being fully differentiated. Many specialized cells like neurons enter this phase permanently.

🎯 Exam Tip: Understanding the G₀ phase is crucial for distinguishing between actively dividing cells and those in a quiescent state, which is important for topics like tissue repair and cancer biology.

Question 5.
Answer:
1. Series of events occurring in the life of a cell is called cell cycle. Interphase and M - phase are the two phases of cell cycle.
2. Interphase: It is the stage between two successive cell divisions. It is the longest phase of a cell cycle during which the cell is highly active and prepares itself for cell division.
The interphase is subdivided into three sub-phases as G₁ - phase, S-phase and G₂-phase.
a. G₁ - phase (First gap period/First Gap Phase):
It begins immediately after cell division.
RNA (mRNA, rRNA and tRNA) synthesis, protein synthesis and synthesis of membranes take place during this phase.
b. S - phase (Synthesis phase):
In this phase DNA is synthesized (replicated), so that amount of DNA per cell doubles.
Synthesis of histone proteins takes place in this phase.
c. G₂ - phase (Second growth phase/Second Gap Phase):
1. Metabolic activities essential for cell division occur during this phase.
In simple words: The cell cycle is the ordered series of events a cell undergoes to grow and divide, involving interphase (growth and DNA replication) and M-phase (cell division).

🎯 Exam Tip: Memorizing the distinct events and purposes of each sub-phase (G₁, S, G₂) of interphase is vital for accurately describing the cell cycle in descriptive questions.

Internet My Friend (Textbook Page No. 77)

Question. What is Karyogram or Karyotype?
Answer:
1. A karyotype is a representation of condensed chromosomes arranged in pairs.
2. Analysis of the karyotype of a particular individual indicates whether the individual has a normal set of chromosomes or whether there are abnormalities in number or appearance of individual chromosomes.
In simple words: A karyotype is an organized display of a cell's chromosomes, used to identify chromosomal abnormalities or determine an individual's chromosomal makeup.

🎯 Exam Tip: Karyotypes are fundamental in genetics for diagnosing chromosomal disorders and understanding evolutionary relationships between species.

Can You Tell? (Textbook Page No. 79)

Question. Which are the steps of mitosis?
Answer:
Steps in mitosis are Karyokinesis and Cytokinesis. Karyokinesis includes four stages - Prophase, Metaphase, Anaphase and Telophase.
In simple words: Mitosis involves two main steps: karyokinesis (nuclear division with phases like prophase, metaphase, anaphase, telophase) and cytokinesis (cytoplasmic division).

🎯 Exam Tip: Listing all phases of karyokinesis in the correct order is a common scoring point. Remember to include cytokinesis as the final step of overall cell division.

Internet My Friend (Textbook Page No. 79)

Question. How the life span of a cell is decided?
Answer:
1. Life span of different cells vary greatly.
2. Life span of a cell is decided by its growth rate, metabolic activities and cell size.
3. The life span of a cell can be analysed in laboratory by applying carbon-14 technique to DNA.
4. This method is commonly used in archaeology and paleontology to find the age of fossils. Same can be applied to determine the life span of a cell.
In simple words: A cell's lifespan is influenced by its growth rate, metabolic activity, and size, and can be indirectly estimated using techniques like carbon-14 dating on its DNA.

🎯 Exam Tip: While carbon-14 dating is primarily for fossils, remember the biological factors (growth rate, metabolic activities, cell size) that determine a cell's natural life cycle.

Do Yourself (Textbook Page No. 80)

Question. Write down the explanation of prophase I in your own words.
Answer:
1. Prophase -I:
It is the most complicated and longest phas0e of meiotic division.
It is further divided into five sub-phases viz. leptotene, zygotene, pachytene, diplotene and diakinesis.
a. Leptotene:
1. The volume of the nucleus increases.
2. The chromosomes become long distinct and coiled.
3. They orient themselves in a specific fonn known as bouquet stage. This is characterized with the ends of chromosomes converged towards the side o nucleus where the centrosome lies.
4. The centriole duplicates into two and migrates to opposite poles. [Note: Centrioles divide during G₁ phase of interphase.]
b. Zygotene:
1. Pairing of non-sister chromatids of homologous chromosomes takes place by formation of synaptonemal complex. This pairing is called synapsis.
2. Each pair consists of a maternal chromosome and a paternal chromosome. Chromosomal pairs are called bivalents or tetrads.
c. Pachytene:
1. Each individual chromosome begins to split longitudinally into two similar chromatids. Therefore, each bivalent now appears as a tetrad consisting of four chromatids.
2. The homologous chromosomes begin to separate but they do not separate completely and remain attached to one or more points.
3. These points are called chiasmata (Appear like a cross-X).
4. Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes resulting in recombination.
d. Diplotene:
The chiasma becomes clearly visible in diplotene due to beginning of repulsion between synapsed homologous chromosomes. This is known as desynapsis.
e. Diakinesis:
1. The chiasmata begin to move along the length of chromosomes from the centromere towards the ends of chromosomes. The displacement of chiasmata is termed as terminalization.
2. The terminal chiasmata exist till the metaphase.
3. The nucleolus and nuclear membrane completely disappear and spindle fibres begin to appear.
In simple words: Prophase I is the first and longest stage of meiosis, characterized by five sub-phases (leptotene, zygotene, pachytene, diplotene, diakinesis) where homologous chromosomes pair, exchange genetic material (crossing over), and prepare for separation.

🎯 Exam Tip: Detailed knowledge of each sub-stage of Prophase I, especially synapsis, chiasmata formation, and crossing over, is critical for understanding genetic variation and scoring well.

Curiosity Box: (Textbook Page No. 81)

Question. (i) What is exact structure of synaptonemal complex?
Answer:
Synaptonemal complexes are zipper like structures assembled between homologous chromosomes during the prophase of first meiotic division.
In simple words: The synaptonemal complex is a ladder-like protein structure that forms between homologous chromosomes, facilitating their precise alignment and genetic exchange during meiosis I.

🎯 Exam Tip: Focus on the "zipper-like" and "protein structure" aspects to describe the synaptonemal complex's role in holding homologous chromosomes together.

Question. (ii) What is structure of chiasma?
Answer:
Chiasma is a X-shaped point of attachment between two non-sister chromatids of a homologous chromosomes.
In simple words: A chiasma is the visible, X-shaped point where non-sister chromatids of homologous chromosomes remain temporarily joined after crossing over has occurred.

🎯 Exam Tip: Clearly state that chiasmata are X-shaped points of connection between *non-sister* chromatids after recombination has taken place.

Question. (iii) Which type of proteins are involved in formation of spindle fibres?
Answer:
Spindle fibres are formed from microtubules with many accessory proteins.
In simple words: Spindle fibers are primarily made of microtubules, which are polymers of tubulin protein, along with various associated proteins.

🎯 Exam Tip: Remember that tubulin is the key protein subunit of microtubules, which form the structural basis of spindle fibers.

Question. (iv) Why and how spindle fibres elongate and some contract?
Answer:
a. Spindle fibres elongate for assembly of chromosomes at equatorial plane of the cell during metaphase and spindle fibres contract for pulling chromosomes towards opposite poles during anaphase.
b. The spindle fibres elongate (polymerize) by incorporating subunits of the protein tubulin and contract
In simple words: Spindle fibers elongate through the polymerization of tubulin to align chromosomes at the metaphase plate and contract via depolymerization and motor proteins to pull chromatids apart during anaphase.

🎯 Exam Tip: Distinguish between polymerization (elongation in metaphase) and depolymerization (contraction in anaphase) of tubulin as the mechanisms for spindle fiber movement.

Question. (v) What is the role of centrioles in formation of spindle apparatus?
Answer:
Centriole plays an important role in cell division. Centrioles help organize microtubule assembly and forms spindle apparatus that separate the chromosomes during cell division.
In simple words: Centrioles organize the formation of the spindle apparatus in animal cells, which is essential for properly segregating chromosomes during cell division.

🎯 Exam Tip: Highlight that centrioles act as the organizing centers for microtubules, which then form the spindle fibers responsible for chromosome separation.

Curiosity Box (Textbook Page No. 81)

Question. What would have happened in absence of meiosis?
Answer:
1. Gametes are produced by the process of meiosis which are essential for sexual reproduction.
2. Diploid organisms have two set of chromosomes (one paternal and one maternal).
3. For a diploid organism to undergo sexual reproduction it needs to create gametes that contain only one set of chromosomes so the number of chromosomes remains same in the next generation.
4. In absence of meiosis, the chromosome number of parents and their offsprings will differ in every generation; hence no species will hold its characters.
5. Also, there will be no crossing over of homologous chromosomes. Thus, there will be no variations with respect to the changing environment in progeny to maintain their existence, which may lead to extinction of species.
In simple words: Without meiosis, sexual reproduction would lead to an uncontrolled doubling of chromosome numbers in each generation, causing genetic instability and a lack of genetic variation, which is crucial for adaptation and survival.

🎯 Exam Tip: Emphasize the two main consequences: maintenance of chromosome number and generation of genetic variation through crossing over, both critical for species survival.

Can You Tell? (Textbook Page No. 82)

Question. (i) What is the difference between mitosis and meiosis?
Answer:

🎯 Exam Tip: For differentiation questions, always compare criteria such as cell type involved, number of divisions, chromosome number in daughter cells, and genetic outcome (identical vs. varied).

Question. (ii) What is difference between meiosis - I and meiosis - II?
Answer:

🎯 Exam Tip: Focus on the key differences: Meiosis I is reductional (chromosome number halves), while Meiosis II is equational (chromosome number stays the same as cells entering this stage).

Question. (iii) Elaborate the process of recombination.
Answer:
a. Recombination is exchange of genetic material between paternal and maternal chromosomes during gamete formation.
b. The points where crossing over takes place is known as chiasmata.
c. Chromatids acquire new combinations of alleles by physically exchanging segments in crossing-over.
d. The exchange of genetic material between homologous chromosomes involves accurate breakage and joining of DNA molecules through a complex mechanism.
e. It is catalyzed by enzymes.
In simple words: Recombination, or crossing over, is the enzymatic process during meiosis where homologous chromosomes exchange segments of genetic material at chiasmata, leading to new combinations of alleles on chromatids.

🎯 Exam Tip: Ensure to mention homologous chromosomes, non-sister chromatids, chiasmata, and the role of enzymes in the process of genetic recombination.

Do Yourself (Textbook Page No. 82)

Question. Prepare a concept map on cell division in following box.
Answer:
Refer Quick Review
In simple words: A concept map for cell division should visually organize the main types (mitosis, meiosis), their phases, and key events like DNA replication and chromosome segregation.

🎯 Exam Tip: When asked to "refer Quick Review" or similar, it implies integrating information from the chapter. A concept map should visually represent the hierarchy and relationships of different cell division stages and their outcomes.

Internet My Friend (Textbook Page No. 82)

Question. Different types of proteins like cyclins, maturation promoting factor (MPF), cyclosomes, enzymes like cyclin dependent kinases (CDK) play important role in control of cell cycle. Collect more information about these proteins and enzymes from internet, prepare a power-point presentation and present it in the class.
Answer:
1. The regulation of the cell cycle involves an internal control system consisting of proteins called cyclins and enzymes called cyclin-dependent kinases.
2. A Cdk is a protein kinase. When the kinase of the Cdk is activated upon binding to a cyclin, it phosphorylates target proteins in the cell, regulating their activities.
3. Those proteins play important roles in initiating or regulating significant events of the cell cycle, such as DNA replication, mitosis, and cytokinesis.
4. Maturation Promoting Factor (MPF) triggers the cell's passage into the mitotic phase.
[Note: Students are expected to perform the above activity by their own with the help of information provided in the answer.]
In simple words: The cell cycle is tightly controlled by a system of regulatory proteins, primarily cyclins and cyclin-dependent kinases (CDKs), which activate at specific checkpoints to ensure proper progression through growth, DNA replication, and cell division.

🎯 Exam Tip: For questions on cell cycle regulation, understanding the interaction between cyclins and CDKs as a complex that phosphorylates target proteins is key. MPF's role in initiating mitosis is also a frequently tested concept.