Samacheer Kalvi Class 11 Bio Botany Solutions Chapter 7 Cell Cycle

Choose The Correct Answer:

 

Question 1. The correct sequence in cell cycle is
(a) S M G1 G2
(b) S G1 G2 M
(c) G1 S G2 M
(d) M G1 G2 S
Answer: (c) G1 S G2 M
In simple words: The cell cycle follows a specific order: first G1 phase (growth), then S phase (DNA replication), followed by G2 phase (more growth and preparation), and finally M phase (mitosis or cell division). This sequence ensures proper cell reproduction.

🎯 Exam Tip: Remember the mnemonic "Go, Study, Grow, Mitosis!" for G1, S, G2, and M phases to recall the correct sequence of the cell cycle.

 

Question 2. If mitotic division is restricted in G phase the cell cycle, then the condition is known as
(a) S Phase
(b) G2 Phase
(c) M Phase
(d) Go phase
Answer: (d) Go phase
In simple words: If a cell stops dividing and doesn't prepare for mitosis in the G1 phase, it enters a resting state called the Go phase. In this phase, the cell is still active but not dividing.

🎯 Exam Tip: Understand that the Go phase is a temporary or permanent exit from the cell cycle, where cells carry out their normal functions without dividing, unlike cells actively progressing through G1, S, G2, and M phases.

 

Question 3. Anaphase promoting complex APC is a protein degradation machinery necessary for proper mitosis of animal cells. If APC is defective in a human cells. Which of the following is expected to occur?
(a) Chromosomes will be fragmented
(b) Chromosomes will not condense
(c) Chromosomes will not segregate
(d) Recombination of Chromosomes will occur
Answer: (c) Chromosomes will not segregate
In simple words: APC helps to separate chromosomes during anaphase. If APC does not work correctly, the chromosomes will not pull apart and move to opposite ends of the cell. This would stop proper cell division.

🎯 Exam Tip: Focus on the role of key enzymes and protein complexes like APC in regulating critical checkpoints and events during the cell cycle; defects often lead to failure of specific processes like segregation.

 

Question 4. In the S phase of cell cycle
(a) Amount of DNA doubles in each cell
(b) Amount of DNA remain same in each cell
(c) Chromosome number is increased
(d) Amount of DNA is reduced to half in each cell
Answer: (a) Amount of DNA doubles in each cell
In simple words: During the S phase, the cell makes an exact copy of all its DNA. This means the total amount of DNA in the cell becomes twice as much as before, getting ready for cell division.

🎯 Exam Tip: The S phase is vital for heredity as it ensures that each daughter cell receives a complete and identical set of genetic information.

 

Question 5. The centromere is required for
(a) Transcription
(b) Crossing over
(c) Cytoplasmic cleavage
(d) movement of chromosome towards the pole
Answer: (d) movement of chromosome towards the Pole
In simple words: The centromere is like a handle on the chromosome that spindle fibers grab onto. This helps pull the chromosomes to the opposite ends of the cell during division.

🎯 Exam Tip: Differentiate between the functions of centromeres (chromosome movement) and telomeres (chromosome protection) for clarity in cell biology questions.

 

Question 6. Synapsis occurs between
(a) m RNA and ribosomes
(b) spindle fibres and centromeres
(c) two homologous chromosomes
(d) a male and a female gemale
Answer: (c) two homologous chromosomes
In simple words: Synapsis is when two matching chromosomes (one from each parent) come together and pair up very closely during meiosis. This pairing is important for genetic exchange.

🎯 Exam Tip: Synapsis is a unique event in meiosis I (specifically prophase I) that leads to the formation of bivalents, crucial for genetic variation through crossing over.

 

Question 7. In meiosis crossing over is initiated at
(a) Leptotene
(b) Pachytene
(c) Zygotene
(d) Diplotene
Answer: (b) Pachytene
In simple words: Crossing over, which is the exchange of genetic material between homologous chromosomes, starts during the Pachytene stage of meiosis. This process helps create genetic diversity.

🎯 Exam Tip: Remember the order of substages in Prophase I of meiosis (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis) and the key events in each, especially Pachytene for crossing over.

 

Question 8. Colchicine prevents the mitosis of the cells at which of the following stage
(a) Anaphase
(b) Metaphase
(c) Prophase
(d) Interphase
Answer: (a) Anaphase
In simple words: Colchicine is a substance that stops cell division by preventing the formation of spindle fibers. This interruption mostly happens during anaphase, where chromosomes should separate.

🎯 Exam Tip: Colchicine specifically targets microtubule formation, which are essential components of the spindle fibers that separate chromosomes during mitosis.

 

Question 9. The pairing of homologous chromosomes on meiosis is known as
(a) Bivalent
(b) Synapsis
(c) Disjunction
(d) Synergids
Answer: (b) Synapsis
In simple words: When homologous chromosomes come together and line up side-by-side during meiosis, this process is called synapsis. It's a key step before they exchange genetic material.

🎯 Exam Tip: Synapsis is a characteristic event of Prophase I in meiosis, essential for genetic recombination and accurate chromosome segregation.

 

Question 10. Anastral mitosis is the characteristic feature of
(a) Lower animals
(b) Higher animals
(c) Higher plants
(d) All living Organism
Answer: (c) Higher plants
In simple words: Anastral mitosis is a type of cell division found in higher plants where the spindle fibers form without the help of asters. Asters are star-shaped structures found in animal cells.

🎯 Exam Tip: Remember that astral spindles are common in animal cells, while anastral spindles are characteristic of plant cells, highlighting a key difference in their mitotic machinery.

 

Question 11. Write any three significance of mitosis
Answer:
1. Genetic stability: Daughter cells created by mitosis are exactly like the parent cell, carrying the same genetic information. This helps keep the organism's characteristics consistent.
2. Tissue repair and growth: Mitosis produces new cells to replace damaged ones or to help multicellular organisms grow in size. For example, your skin cells are constantly replaced by mitosis.
3. Asexual reproduction: Some organisms, like yeast, reproduce by mitosis, creating offspring that are identical copies of themselves.
In simple words: Mitosis helps living things grow, repair damaged parts, and keep their genetic information the same when new cells are made. It is like making exact copies.

🎯 Exam Tip: When asked for the significance of mitosis, always mention genetic stability, growth, and repair, as these are its most fundamental roles.

 

Question 12. Differentiate between Mitosis and Meiosis.
Answer:

In simple words: Mitosis makes two identical cells for growth and repair, keeping the chromosome number the same. Meiosis makes four different cells (like sperm or egg cells) for reproduction, and these cells have half the usual number of chromosomes. This difference is key for genetic diversity.

🎯 Exam Tip: When differentiating, always compare the number of divisions, outcome (number and type of cells), and genetic identity, as these are the core differences between mitosis and meiosis.

 

Question 13. Differentiate Cytokinesis in plant cells & animal cells
Answer:

In simple words: In animal cells, the cell divides by pinching inwards, forming a cleavage furrow. In plant cells, a new cell wall (cell plate) grows from the inside out to split the cell into two. The cell plate grows outwards, creating a new boundary between the two daughter cells.

🎯 Exam Tip: Focus on the presence of a cell wall in plants versus its absence in animals; this structural difference dictates whether a cleavage furrow (animals) or a cell plate (plants) forms during cytokinesis.

 

Question 14. Given an account of the Go phase.
Answer: Some cells leave the G1 phase and enter a resting state known as the G0 (Go) phase. In this phase, the cells remain active but do not divide or grow. Cells can stay in the Go phase for long periods, reducing their rate of RNA and protein production. For instance, mature nerve cells and skeletal muscle cells remain permanently in the Go phase. Many animal cells stay in Go unless specific growth factors or other signals prompt them to divide again. It's important to remember that Go cells are not inactive; they just pause their division.
In simple words: The Go phase is a resting stage where cells are active but do not divide. Some cells stay here for a long time, like nerve cells, until they get a signal to divide again.

🎯 Exam Tip: Highlight that the Go phase is a state of quiescence, not dormancy, where cells are metabolically active but have exited the cell cycle to perform specialized functions.

 

Question 15. Write about the Pachytene and Diplotene stage of Prophase I.
Answer: Prophase I, the longest stage of meiosis, is divided into five substages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
3. Pachytene stage:

• Chromosomes appear as bivalents or tetrads.
• There are four chromatids and two centromeres visible.
• The pairing of homologous chromosomes (synapsis) completes, except at chiasmata where crossing over occurs.
• Recombination, which is the exchange of chromosomal parts, is completed by the end of this stage. However, chromosomes remain linked at the sites where crossing over happened.
• An enzyme called recombinase helps mediate this entire process.

4. Diplotene stage:

• The synaptonemal complex, which held the homologous chromosomes together, disassembles and dissolves.
• Homologous chromosomes start to separate but remain attached at points where crossing over occurred. These attachment points form 'X'-shaped structures called chiasmata.
• This substage can last for days or even years, depending on the organism and its sex.
• In females, chromosomes become very active in transcribing RNA, storing materials for the embryo.
• In animals, these chromosomes show prominent loops and are known as lampbrush chromosomes.

In simple words: In Pachytene, matching chromosomes fully pair up and swap bits of their genetic material (crossing over). Then, in Diplotene, these chromosomes start to move apart but stay connected at the places where they swapped genes.

🎯 Exam Tip: For Prophase I substages, remember Pachytene is for crossing over and Diplotene is characterized by the visible chiasmata (X-shaped structures) due to the separation of homologous chromosomes.

 

11th Bio Botany Guide Cell Cycle Additional Important Questions and Answers

 

I. Choose The Correct Answer: (1 Marks)

 

Question 1. Most of the neurons in the brain are in ............... stage.
(a) G1
(b) S
(c) G2
(d) Go
Answer: (d) Go
In simple words: Most brain cells (neurons) are in the Go phase, meaning they are active but have stopped dividing. They perform their jobs without making new cells.

🎯 Exam Tip: Cells like neurons and muscle cells are often terminally differentiated, meaning they exit the cell cycle and permanently reside in the Go phase, performing specialized functions.

 

Question 2. Un differentiated cells include
(a) Stem cells in animals
(b) Meristematic cells in plants
(c) RBC which carry out the transportation of oxygen
(d) Mesophyll cells which carry out photosynthesis
(i) a & b
(ii) c & d
(iii) a & c
(iv) b & c
Answer: (i) a & b
In simple words: Undifferentiated cells are like blank slate cells that can turn into many different types of cells. Stem cells in animals and meristematic cells in plants are examples of these because they haven't specialized yet.

🎯 Exam Tip: Undifferentiated cells are crucial for growth and repair, possessing the ability to divide and differentiate into various cell types.

 

Question 3. Robert Brown discovered the nucleus in the cells of ............... roots.
(a) Mirabilis
(b) Orchid
(c) Moringa
(d) Oryza
Answer: (b) Orchid
In simple words: Robert Brown, a famous scientist, was the one who first found and named the nucleus inside plant cells, specifically when he looked at orchid roots.

🎯 Exam Tip: Remember key historical discoveries in biology, such as Robert Brown's identification of the nucleus, as they are often tested for general knowledge.

 

Question 4. The proteins that activate the cell to perform cell division are
(a) Actin and Myosin
(b) Kinases and Cyclin
(c) Histamine and Cyclin
(d) Tubulin and Actin
Answer: (b) Kinases and Cyclin
In simple words: Cell division is controlled by special proteins called cyclins and enzymes called kinases. Together, they make sure the cell moves through its growth and division steps correctly.

🎯 Exam Tip: Cyclins and cyclin-dependent kinases (CDKs) are the primary regulators of the cell cycle, ensuring that each phase is completed before the next begins.

 

Question 5. The number of chromosomes in the onion cell is ...............
(a) 8
(b) 16
(c) 32
(d) 64
Answer: (Missing from source content)
In simple words: (The specific number of chromosomes for an onion cell is an important biological fact that helps us understand how its genes are organized. Knowing this helps to understand plant genetics.)

🎯 Exam Tip: Always remember the diploid chromosome number (2n) for common experimental organisms, as it is a frequent factual recall question.

 

Question 6. Mitosis is called
(a) Direct cell division
(b) Indirect cell division
(c) Mitotic Meiotic cell division
(d) Reduction division
Answer: (b) Indirect cell division
In simple words: Mitosis is called indirect cell division because it involves several distinct steps and changes within the nucleus before the cell finally divides. It's not a simple, direct splitting.

🎯 Exam Tip: Direct cell division (amitosis) is simpler and less common, primarily seen in prokaryotes and some specialized eukaryotic cells, contrasting with the complex stages of mitosis.

 

Question 7. During Anaphase
(I) The daughter chromosome move to the opposite poles of the phragmoplast
(II) due to the thickening of chromosomes
(III) Shortening of microtubules
(IV) Shortening of asters
Answer: (III) Shortening of microtubules
In simple words: During anaphase, the microtubules, which are like tiny ropes, get shorter. This pulling action helps separate the sister chromatids and move them towards opposite ends of the cell.

🎯 Exam Tip: Focus on the role of spindle fibers and microtubules in anaphase; their shortening is the primary mechanism for chromosome movement.

 

Question 8. Cell cycle was discovered by ...............
(a) Singer & Nicolson
(b) Prevost & Dumans
(c) Schleider & Schwann
(d) Boveri
Answer: (b) Prevost & Dumans
In simple words: Two scientists named Prevost and Dumans were the first to identify and describe the repeating process of growth and division that cells go through, known as the cell cycle.

🎯 Exam Tip: Associating scientists with their major contributions, like Prevost and Dumans with the cell cycle, is important for foundational knowledge.

 

Question 9. The cells without nucleus are
(a) RBC – platelets, tracheids & vessels
(b) RBC – sievetube, companion cells thrombocytes
(c) WBC – platelets, companion cells & vessels
(d) RBC – platelets, companion cells & neurons
Answer: (a) RBC – platelets, tracheids & vessels
In simple words: Red blood cells (RBCs), platelets, tracheids (in plants), and vessels (in plants) are all types of cells that do not have a nucleus when they are mature. This helps them perform their specific functions better.

🎯 Exam Tip: Be aware of exceptions to typical cell structures; for example, mature RBCs lack a nucleus to maximize oxygen-carrying capacity.

 

Question 10. The stage between two meiotic division is called
(a) Cytokinesis
(b) Interphase I
(c) Inter kinesis
(d) Interphase II
Answer: (c) Inter kinesis
In simple words: Interkinesis is a short resting period that happens between meiosis I and meiosis II. During this time, the cell does not duplicate its DNA again.

🎯 Exam Tip: Distinguish interkinesis from interphase; interkinesis is short and lacks DNA replication, while interphase involves a full cycle of growth and DNA synthesis before mitosis or meiosis I.

 

Question 11. Replication of DNA occurs at ............... phase.
(a) Go
(b) G1
(c) S
(d) G2
Answer: (c) S
In simple words: The S phase, which stands for "synthesis" phase, is when a cell makes an exact copy of all its DNA. This process ensures that both new cells get a full set of genetic material.

🎯 Exam Tip: The S phase is a crucial checkpoint; accurate DNA replication is vital to prevent mutations and ensure genetic stability in daughter cells.

 

Question 1.
(I) Nucleolus disappear during the metaphase stage of mitosis
(II) The microtubules arrange to form asters in plant cells
(III) In plant cells phragmoplast is formed prior to the formation of cell plate
(IV) Mitosis is responsible for the Regeneration of lost arms of starfish
Answer: (b) False - False - True - True

In simple words: The nucleolus usually disappears during prophase, not metaphase. Plant cells do not form asters during division; asters are specific to animal cells. The phragmoplast does indeed form a cell plate in plant cells, and mitosis is vital for regeneration, like a starfish growing a new arm.

🎯 Exam Tip: Pay close attention to the specific phases of mitosis and key differences between plant and animal cell division when evaluating true/false statements.

 

Question 3.
(i) C Value is the amount in picograms of DNA contained within a haploid Nucleus
(ii) Nucleolan membrane disappear during Ana Phase stage of mitosis
(iii) The arrangement of microtubules is called to form Asters -, which is a unique feature of plant cells
(iv) One of the protein synthesis in G2 – phase is known as Maturation Promoting Factor
Answer: (b) True - False - True - False

In simple words: The C-value is indeed the amount of DNA in a haploid nucleus. The nucleolar membrane disappears in prophase, not anaphase. Asters are structures formed by microtubules mainly in animal cells, not plant cells. Maturation Promoting Factor (MPF) is a protein complex that helps start mitosis, but its synthesis happens in G2, not as *the* protein synthesis itself.

🎯 Exam Tip: Carefully review the definitions of C-value, the timing of nuclear envelope breakdown, the differences in cytoskeletal structures between plant and animal cells, and the regulatory molecules of the cell cycle.

III. Find Out The Correct Match From The Following.

 

Question 1. Find out the correct match from the following:
Zygotene - Chromosomes appear as tetrads
Pachytene – Synapsis of homologous chromosomes occur
Diplotene – Condensation of chromosomes takes place
Diakinesis – Terminalisation of chiasmata occur & Nucleolus Disappear
Answer: Diakinesis -Terminalisation of chiasmata occur & Nucleolus Disappear
In simple words: The correct event is that during Diakinesis, the chiasmata (crossing-over points) move to the ends of the chromosomes, and the nucleolus vanishes. This is an important step before metaphase begins.

🎯 Exam Tip: Remember the order of prophase I substages (Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis) and the key event in each to accurately match them.

 

Question 2. Duration of different Phases of cell cycle given find out the correct match
(I) S Phase – 12 Hours
(II) G1 Phase – 11 Hours
(III) G2 Phase – 4 Hours
(IV) M Phase – 2 Hours
Answer: (III) G2 Phase – 4 Hours
In simple words: The G2 phase of the cell cycle, which is a period of cell growth and preparation for division, typically lasts for about 4 hours. This phase ensures the cell is ready to divide.

🎯 Exam Tip: While exact timings can vary, knowing the typical relative durations of each cell cycle phase (G1, S, G2, M) helps in identifying the correct match.

IV. Find Out The Wrong Match

 

Question 1. Find out the wrong match:
(I) The chromosome does not divide as chromatids, for centromere does not divide – Anaphase I
(II) The chromatids move to the opposite poles by the splitting of the centromere – Anaphase
(III) Sister chromatids get separated by splitting of Centromere – Anaphase II
(IV) Homologous chromosomes appear as bivalent or tetrad – Metaphase II
Answer: (IV) Homologous chromosomes appear as bivalent or tetrad – Metaphase II
In simple words: In Metaphase I, homologous chromosomes pair up to form bivalents (or tetrads) at the metaphase plate. By Metaphase II, these bivalents have already separated, so the statement that they appear in Metaphase II is incorrect.

🎯 Exam Tip: Understand the distinct events of Meiosis I and Meiosis II, especially how chromosomes behave in Metaphase I versus Metaphase II, to accurately identify incorrect statements.

 

Question 2. Most neurons remain in G-'o' stage do not divide. Which of the following statements is correct regarding this?
(I) This technique can be applied to replace neurons in dementia patients
(II) neurons can be activated by giving electric shocks
(III) neurons can be replaced by surgical procedures
(IV) Dead or injured neurons can be replaced by stem cell therapy
Answer: (IV) Dead or injured neurons can be replaced by stem cell therapy
In simple words: Many neurons stay in a resting phase called G0, meaning they don't divide. However, if neurons get damaged or die, new ones can be introduced using stem cell therapy. This helps in repairing the nervous system.

🎯 Exam Tip: Remember that G0 phase is a quiescent state where cells are metabolically active but no longer dividing. Stem cell therapy is a promising area for replacing non-dividing cells like neurons.

V.

 

Question 1. Match the following and find the correct option.
(I) Robert Brown - A) Coined the word cell
(II) Robert Hook – B) Coined the word Mitosis
(III) Schleiden & Schwann – C) Studied the presence of Nucleus in cells
(IV) Walther Flemming – D) Cell theory

Answer: (d) C-A- D- B
In simple words: This match identifies the correct contributions of different scientists to cell biology. Robert Brown discovered the nucleus (C), Robert Hooke coined the word 'cell' (A), Schleiden & Schwann proposed the Cell Theory (D), and Walther Flemming coined 'Mitosis' (B).

🎯 Exam Tip: Knowing the key discoveries and contributions of important scientists in cell biology is essential for matching questions.

 

Question 2. Find out the correct match from the following.
(I) Cell cycle – A) division that follows the nuclear division
(II) Restriction point – B) Longest part but not resting stage
(III) Interphase – C) A series of events leading to the formation of a new Cell
(IV) Cytokinesis – D) The checkpoint at the end of G1 determine a cell's fate

Answer: (c) C-D- B- A
In simple words: This match correctly pairs each term with its definition. The Cell Cycle (I) is a series of events for a new cell (C). The Restriction point (II) is a G1 checkpoint determining cell fate (D). Interphase (III) is the longest part, not a resting stage (B). Cytokinesis (IV) is the division following nuclear division (A).

🎯 Exam Tip: Clearly distinguish between the stages of the cell cycle and related processes like cytokinesis and checkpoints. The restriction point is crucial for cell cycle regulation.

VI.

 

Question 1. Read the following Assertion and Reason. Find the correct answer.
Assertion A: The mitochondrial inheritance in higher animals is uniparental
Reason R: The mitochondria from the male partner either undergo degeneration or rejected and only mitochondria from egg or ova is accepted.
(a) Assertion and Reason are correct Reason is explaining assertion
(b) Assertion and Reason are correct but the reason is not explaining assertion
(c) Assertion is true but Reason is wrong
(d) Assertion is true but Reason is wrong
Answer: (a) Assertion and Reason are correct. Reason is explaining assertion
In simple words: In higher animals, mitochondria are passed down only from the mother (uniparental inheritance). This happens because the father's mitochondria are either destroyed or not used, while the mother's egg provides all the mitochondria for the offspring. This explains why mitochondrial DNA can be traced through the maternal line.

🎯 Exam Tip: For assertion-reason questions, first check if both statements are individually true. Then, see if the reason directly explains the assertion. Mitochondrial inheritance is a classic example of maternal inheritance.

 

Question 2. Read the following Assertion and Reason. Find the correct answer.
Assertion A: In Meiosis Prophase I is a longer but significant phase
Reason R: Chiasma formation and crossing over takes place and recombination takes place
(a) Assertion and Reason are correct Reason is explaining Assertion
(b) Assertion and Reason are correct but Reason is not explaining Assertion
(c) Assertion is true but Reason is wrong
(d) Assertion is true but Reason not explaining Assertion
Answer: (b) Assertion and Reason are correct but Reason is not explaining Assertion
In simple words: Both the assertion (Prophase I is long and important) and the reason (chiasma formation, crossing over, and recombination happen) are true statements. However, the reason describes *what* happens in Prophase I, not *why* it is long or significant. The significance comes from these events leading to genetic variation, which is why it's long.

🎯 Exam Tip: Distinguish between a true statement and a true explanation. A reason only correctly explains an assertion if it provides the 'why' or 'how' behind it.

 

Question 3. Read the following Assertion and Reason. Find the correct answer.
Assertion A: Interphase is the longest part of cell division and the cell actively involved protein synthesis & DNA synthesis
Reason: The Interphase is also known as the resting phase, & the cell takes rest between successive cell division
(a) Assertion and Reason are correct Reason is explaining Assertion
(b) Assertion and Reason are correct but Reason is not explaining Assertion
(c) Assertion is true but Reason is wrong
(d) Assertion is true but Reason not explaining assertion
Answer: (c) Assertion is true but Reason is wrong
In simple words: The assertion is true because interphase is indeed the longest part of the cell cycle where the cell grows, synthesizes proteins and DNA. However, the reason is wrong; interphase is an active period of growth and preparation, not a "resting phase." The term "resting phase" is misleading for interphase.

🎯 Exam Tip: While historically called a "resting phase," interphase is highly metabolically active. This misconception is a common trap in cell biology questions.

 

Question 4. Read the following Assertion and Reason. Find the correct answer.
Assertion A: The sister chromatids of homologous chromosomes exchange chromosomal bits
Reason R: This process of exchange of chromosomal bits is known as crossing over
(a) Assertion and Reason are correct Reason is explaining Assertion
(b) Assertion and Reason are correct but Reason is not explaining Assertion
(c) Assertion is true but Reason is wrong
(d) Assertion is true but Reason not explaining assertion
Answer: (d) Assertion is true but Reason not explaining assertion
In simple words: The assertion states that sister chromatids exchange parts, which is false; crossing over happens between non-sister chromatids of homologous chromosomes. The reason defines crossing over, which is true, but it's not relevant to a false assertion about sister chromatids. Since the assertion is false, the option should reflect that.

🎯 Exam Tip: A common misunderstanding in meiosis is confusing sister and non-sister chromatids. Crossing over always occurs between non-sister chromatids of homologous chromosomes, not sister chromatids.

VII.

 

Question 1. See the diagram & label the parts.

Answer:
A – Leptotene
B – Chromosomes are visible under a light microscope
C – Paired sister chromatids begin to condense
In simple words: The diagram shows a chromosome, which is a structure that becomes visible in the leptotene stage (A) of cell division. At this stage, chromosomes are indeed visible under a microscope (B), and the paired sister chromatids start to condense, becoming shorter and thicker (C).

🎯 Exam Tip: For diagram-based questions, identify the key features shown and relate them to specific stages or processes of cell division. Even if a diagram is generic, the accompanying text can provide clues.

 

Question 2. Label the different phases of the cell cycle based on the diagram.

Answer: (a) Cell growth – Interphase – Mitotic phase – Cyto kinesis
In simple words: This question asks to match the labels on the cell cycle diagram with their correct descriptions. Option (a) correctly assigns "Cell growth" to A, "Interphase" to B, "Mitotic phase" to C, and "Cytokinesis" to D, representing the stages and processes of the cell cycle. The actual segments A, B, C, D in the diagram correspond to M, G1, S, and G2 phases, respectively, and the options provide broader functional descriptions for these.

🎯 Exam Tip: When labeling diagrams, pay close attention to what each letter represents in the options versus its position on the diagram. Sometimes, the options describe broader processes rather than specific labeled parts.

 

Question 3. See the diagram & label Find out the correct labelling.

Answer: (b) Centromere – Chromatin – Secondary constriction – Kinetochore
In simple words: This question asks to correctly identify the parts of a chromosome. Label A points to the centromere, which is the constricted region of the chromosome. Label B identifies the main chromatin material of the chromosome arm. Label C indicates a secondary constriction, often associated with nucleolus formation. Label D points to the kinetochore, a protein structure on the centromere where spindle fibers attach.

🎯 Exam Tip: Accurately identify the centromere, chromatin, secondary constrictions, and kinetochores as key structural components of a chromosome. Understanding their roles helps in diagram labeling.

 

Question 4. Label the diagram properly.

Answer:
A – Non-sister Chromatids
B - Centromere
C - Chiasma
D - Bivalent or Tetrad
In simple words: The diagram illustrates homologous chromosomes undergoing crossing over. A points to the non-sister chromatids, which are chromatids from different homologous chromosomes. B indicates the centromere, the constricted region holding sister chromatids together. C highlights the chiasma, the visible point of exchange between non-sister chromatids. The entire structure shown, consisting of two homologous chromosomes paired together, is called a bivalent or tetrad (D).

🎯 Exam Tip: Familiarize yourself with the structure of homologous chromosomes, sister chromatids, non-sister chromatids, centromeres, and chiasmata, especially during prophase I of meiosis.

 

Question 5. Label the diagram properly.

Answer:
A - Outer kinetochore
B - Fibrous corona
C - Inner Kinetochore
D - Microtubules
In simple words: This diagram shows the detailed structure of a kinetochore, which is where spindle fibers attach during cell division. A points to the outer kinetochore, B to the fibrous corona (a halo of fibers), C to the inner kinetochore, and D illustrates the microtubules extending from the spindle. This structure is vital for chromosome movement.

🎯 Exam Tip: Understand the three main regions of the kinetochore (inner, outer, and fibrous corona) and their role in microtubule attachment for accurate chromosome segregation.

 

Question 1. Name the two types of nuclear division.
Answer: The two main types of nuclear division are:
1. Mitosis
2. Meiosis
These processes ensure that genetic material is properly distributed to new cells. Mitosis creates identical cells for growth and repair, while meiosis produces sex cells for reproduction with half the chromosomes.
In simple words: There are two ways a cell's nucleus can divide: Mitosis and Meiosis. Mitosis makes exact copies of cells, and Meiosis makes special cells for reproduction.

🎯 Exam Tip: Clearly state both mitosis and meiosis. Briefly knowing their primary functions (growth vs. reproduction, identical vs. genetically diverse cells) enhances the answer.

 

Question 2. What are the reasons for the arresting growth of cell during G1 Phase?
Answer: Cell growth can stop during the G1 phase due to several reasons:
• Lack of nutrients
• Absence of necessary growth factors or density-dependent inhibition (when cells get too crowded)
• Certain metabolic changes that cause the cell to enter the G0 stage, a resting phase where cells are not actively dividing.
This G0 state is like a pause button for cell division, allowing the cell to wait for better conditions or specialize.
In simple words: Cells stop growing in the G1 phase if there isn't enough food, if they don't get the right growth signals, or if they are too close together. They might also go into a resting state called G0.

🎯 Exam Tip: Remember that the G1 phase is a critical checkpoint. Factors like nutrient availability, growth factors, and cell density are key regulators that can halt the cell cycle.

 

Question 3. Point out the reasons responsible for the arresting of the cell in the G1 phase?
Answer: Cells stop their progression in the G1 phase for these key reasons:
• Nutrient deprivation: The cell lacks enough food to grow and divide.
• Lack of growth factors or density-dependent inhibition: The cell doesn't receive signals to divide or is too crowded by other cells.
• Undergo metabolic changes and enter into G0 state: The cell makes internal changes that lead it to a non-dividing, resting phase. This ensures cells only divide when conditions are favorable.
In simple words: A cell might stop in the G1 phase if it doesn't have enough food, is missing growth signals, or if it decides to enter a special resting stage called G0.

🎯 Exam Tip: When explaining cell cycle arrest in G1, always mention the primary regulatory factors: nutrition, growth signals, and cell density, which all influence entry into the G0 phase.

 

Question 4. Differentiate between Karyokinesis and Cytokinesis of Amitosis.

In simple words: In amitosis, karyokinesis is when the nucleus splits into two by simply pinching in the middle, forming a dumbbell shape. Cytokinesis is when the cell's outer membrane also pinches inward, dividing the rest of the cell into two, following the nuclear division. This is a direct, simple way for cells to divide.

🎯 Exam Tip: For differentiation questions, focus on specific characteristics unique to each process. In amitosis, the key is the direct, non-spindle-based division of both nucleus (karyokinesis) and cytoplasm (cytokinesis).

 

Question 5. Define amitosis & write about its drawbacks.
Answer: Amitosis is a type of direct cell division, sometimes called incipient cell division. In amitosis, there is no formation of a spindle apparatus, and the chromatin material does not condense into distinct chromosomes.
The process of amitosis typically involves two steps:
1. Karyokinesis: The nucleus simply constricts and divides into two.
2. Cytokinesis: The cytoplasm then divides, splitting the cell into two daughter cells.
A major drawback of amitosis is that it causes an unequal distribution of chromosomes, which can lead to abnormalities in:
• Metabolism
• Reproduction
This unequal distribution can have serious consequences for the cell's function and the organism's health.
In simple words: Amitosis is a simple way cells divide, where the nucleus and then the cell just pinch in half without forming special structures like spindles. The problem is, the new cells don't get an even amount of genetic material, which can cause issues with how they work and reproduce.

🎯 Exam Tip: When defining amitosis, highlight the absence of spindle formation and chromosome condensation. Emphasize that its main drawback is the unequal distribution of genetic material, leading to potential cellular dysfunction.

 

Question 6. Distinguish between Closed and Open mitosis.

In simple words: In closed mitosis, the nucleus stays whole while the chromosomes move inside it, like in yeast. In open mitosis, which happens in most plants and animals, the nucleus breaks apart first, and then new nuclear envelopes form around the separated chromosomes. This is a key difference in how cells manage their genetic material during division.

🎯 Exam Tip: The critical distinction is the integrity of the nuclear envelope. Closed mitosis keeps it intact, while open mitosis involves its breakdown and reformation. Provide clear examples for each type.

 

Question 7. Differentiate between Anastral and Amphiastral cell division.
Answer: The differences between Anastral and Amphiastral cell division are as follows:
• Anastral mitosis occurs in plant cells. No asters (star-like structures of microtubules) or centrioles are formed; only spindle fibers are present. This ensures plant cells can divide efficiently without these structures.
• Amphiastral mitosis occurs in animal cells. Asters and centrioles are formed at each pole of the spindle during cell division.
In simple words: Anastral cell division happens in plants; it doesn't have star-shaped asters or centrioles. Amphiastral cell division happens in animals and does have asters and centrioles at the poles. These structures are important for setting up the spindle during cell division.

🎯 Exam Tip: Focus on the presence or absence of asters and centrioles as the primary distinguishing features. This difference is a classic marker for plant versus animal cell division.

 

Question 8. What happens to plant cells at the end of Telophase in Mitosis?
Answer: At the end of telophase in plant cells, a phragmoplast forms between the two daughter nuclei. This phragmoplast then develops into a cell plate, which grows outwards and eventually fuses with the existing cell wall. The cell wall is reconstructed, and the organelles and macromolecules are evenly distributed as the cells finally separate into two distinct daughter cells. This process builds a new cell wall to fully divide the cell.
In simple words: In plant cells, after telophase, a cell plate grows in the middle, which turns into a new cell wall. This divides the cell into two new daughter cells, making sure everything inside is shared.

🎯 Exam Tip: Emphasize the formation of the phragmoplast and cell plate as unique to plant cytokinesis. Contrast this with animal cells where a cleavage furrow forms.

 

Question 9. Differentiate meiosis in Plants & Animals.

In simple words: In plants, meiosis happens to make spores in parts like the anther and ovule. In animals, meiosis creates sex cells (gametes) like sperm and eggs in the reproductive organs. Both processes aim to produce cells with half the number of chromosomes for reproduction.

🎯 Exam Tip: For this comparison, focus on the structures where meiosis occurs (anther/ovule in plants vs. reproductive organs in animals) and the resulting products (spores vs. gametes).

 

Question 10. Define Mitogens.
Answer: Mitogens are biochemical substances or factors that encourage and speed up the cell cycle, leading to cell proliferation (increase in cell number). They are also known as growth promotors, helping cells to divide. Examples include auxins, gibberellins, cytokinins, and sometimes specific acids like Acelic Acid and Kinetin which can trigger cell division.
In simple words: Mitogens are chemicals that tell cells to divide and grow faster. They are like signals that promote the cell cycle to make more cells.

🎯 Exam Tip: When defining mitogens, emphasize their role in promoting cell cycle acceleration and proliferation, and mention that they are also referred to as growth promotors.

 

Question 11. Explain briefly about Endomitosis.
Answer: Endomitosis is a special type of cell division where chromosomes replicate but the cell's nucleus and cytoplasm do not divide. This results in cells having many copies of their chromosomes, making them polyploid. During endomitosis, the chromonema (thread-like chromatin) doesn't separate to form individual chromosomes but stays closely associated. The nuclear membrane remains intact, and no spindle fibers form. This process is commonly seen in the salivary glands of fruit flies (Drosophila), where cells contain giant polytene chromosomes, each made up of thousands of closely associated or synapsed chromatids.
In simple words: Endomitosis is when a cell's DNA makes copies, but the cell itself doesn't divide. So, the cell ends up with many sets of chromosomes. This happens because the nucleus doesn't break down, and no spindle forms to pull the chromosomes apart.

🎯 Exam Tip: Key points for endomitosis are chromosome replication without nuclear or cytoplasmic division, leading to polyploidy, and the absence of spindle formation. Polytene chromosomes in Drosophila salivary glands are a classic example.

3 Mark Questions

 

Question 1. Write down the significance of Meiosis.
Answer: Meiosis is a crucial process for sexual reproduction and has several significances:
• It maintains a constant chromosome number across generations by halving the chromosome count in gametes (sex cells).
• Crossing over, which is the exchange of genetic material between homologous chromosomes, leads to genetic variations among offspring.
• This genetic variation is the raw material for evolution, allowing species to adapt to changing environments.
• Meiosis produces genetic variability by sorting different combinations of genes into gametes through independent assortment.
• It is responsible for enabling organisms to adapt to various environmental stresses due to the increased genetic diversity. This ensures species survival and diversity.
In simple words: Meiosis is important because it keeps the number of chromosomes the same in new generations and creates variety among offspring. This variety comes from mixing genes (crossing over) and helps living things adapt and change over time.

🎯 Exam Tip: When discussing the significance of meiosis, always include maintaining chromosome number and generating genetic variation (through crossing over and independent assortment) as the core points.

 

Question 2. Differentiate between Mitosis in Plants & Animals.

In simple words: Plant and animal cell mitosis differ mainly in structures and how the cell splits. Plant cells don't have centrioles or asters, and they form a cell plate to divide. Animal cells have centrioles and asters, and they use a cleavage furrow to split the cell. These differences reflect their different cellular structures.

🎯 Exam Tip: Key differences in plant vs. animal mitosis include the presence/absence of centrioles/asters and the method of cytokinesis (cell plate vs. cleavage furrow). Mentioning these points will earn full marks.

 

Question 3. Explain Endomitosis.
Answer: Endomitosis is a process where chromosomes replicate within the nucleus, but the cell itself does not undergo nuclear division (karyokinesis) or cytoplasmic division (cytokinesis). This leads to an increase in the number of chromosome sets within the cell, resulting in polyploidy.
During endomitosis:
• Chromonema (the chromatin thread) does not separate to form distinct, individual chromosomes.
• The nuclear membrane does not rupture, remaining intact throughout the process.
• There is no formation of a spindle apparatus, which is typically required for chromosome segregation.
• Each chromosome consists of thousands of synapsed chromatids, as seen in the salivary gland chromosomes of Drosophila (known as Polytene chromosomes). This process allows certain cells to become very large and metabolically active.
In simple words: Endomitosis is when a cell's genetic material multiplies, but the cell doesn't divide into two. This means the cell ends up with many copies of its chromosomes inside one nucleus. This happens without the nucleus breaking open or forming a spindle, and it helps some cells grow very large.

🎯 Exam Tip: For endomitosis, remember the key events: chromosome replication, absence of nuclear/cytoplasmic division, intact nuclear membrane, and no spindle formation. Providing an example like polytene chromosomes in Drosophila strengthens the explanation.

 

Question 1. Explain in detail the various stages of Prophase I.
Answer: Prophase I is the longest part of meiosis and is divided into five smaller stages. These stages are Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. Each stage helps organize the chromosomes for proper cell division.
(1) **Leptotene:** Chromosomes become visible as thin threads under a light microscope and begin to condense. They start to become shorter and thicker as they coil.
(2) **Zygotene:** Homologous chromosomes pair up closely, aligning gene by gene, in a process called synapsis. This pairing forms a structure known as the synaptonemal complex, and the paired chromosomes are called bivalents or tetrads.
(3) **Pachytene:** The bivalent chromosomes are clearly visible as tetrads, each containing four chromatids. During this stage, crossing over occurs, where non-sister chromatids exchange genetic material. Recombination nodules appear at the sites of this exchange, and an enzyme called recombinase mediates the process.
(4) **Diplotene:** The synaptonemal complex dissolves, and homologous chromosomes start to separate. However, they remain attached at the chiasmata, which are the 'X'-shaped points where crossing over occurred. This stage can be long, lasting for days or even years in some organisms, and chromosomes are actively transcribed.
(5) **Diakinesis:** The chiasmata move to the ends of the chromosomes (terminalisation). The nuclear envelope breaks down, the nucleolus disappears, and the spindle fibers fully assemble. Chromosomes become maximally condensed, preparing for metaphase I.
In simple words: Prophase I is a long first step in meiosis with five mini-stages. It's all about chromosomes pairing up, swapping genetic parts, and getting ready to divide.

🎯 Exam Tip: Remember the order of Prophase I substages: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis (LeZaPaDiDia). Focusing on the key event in each stage (e.g., synapsis in Zygotene, crossing over in Pachytene) will help you describe them accurately.

 

Question 2. Explain karyokinesis in mitosis of plant cell.
Answer: Karyokinesis is the division of the nucleus during mitosis, leading to two identical nuclei. In plant cells, it involves four main stages: Prophase, Metaphase, Anaphase, and Telophase.
**Prophase:**
• Chromosomes, which look like long threads, condense into thick, visible structures. They become shorter and thicker as they coil.
• Spindle fibers, which help move chromosomes, begin to form.
• The nucleolus, a small dense structure inside the nucleus, disappears.
• The nuclear envelope (the membrane around the nucleus) breaks down.
• The Golgi apparatus and endoplasmic reticulum (ER) are not seen during this stage.
**Metaphase:**
• Sister chromatids (two identical copies of a chromosome) attach to the spindle fibers at a region called the kinetochore on their centromere.
• All chromosomes line up perfectly at the cell's center, forming the metaphase plate.
• A checkpoint ensures that all chromosomes are correctly attached to the spindle fibers before the next stage begins.
**Anaphase:**
• The centromere of each chromosome splits, separating the sister chromatids.
• The now-separated daughter chromatids are pulled towards opposite poles of the cell as the spindle fibers shorten.
• This movement ensures each new cell gets a complete set of chromosomes.
**Telophase:**
• The separated chromosomes arrive at the opposite poles of the cell and begin to decondense, becoming long and thin again.
• New nuclear envelopes form around each set of chromosomes at the poles.
• The nucleolus reappears in each new nucleus, and the genetic material division is complete.
• Each pole now has its own set of chromosomes, enclosed within a new nucleus.
In simple words: Karyokinesis is how a plant cell's nucleus divides into two. It has four main steps: Prophase (chromosomes get ready), Metaphase (chromosomes line up in the middle), Anaphase (chromosomes split and move apart), and Telophase (two new nuclei form).

🎯 Exam Tip: When explaining karyokinesis in plant cells, always mention the key events like chromosome condensation, spindle fiber formation, metaphase plate alignment, and the reappearance of nuclear structures. Remember that plant cells do not form asters during mitosis.

 

Question 3. Explain S phase & G2 phase.
Answer: The cell cycle includes interphase, which prepares the cell for division. S phase and G2 phase are crucial parts of interphase, ensuring the cell has all necessary components before mitosis or meiosis.
(i) **S Phase (Synthesis Phase):**
• This is known as the synthetic phase of interphase during mitosis.
• The cell continues to grow during this time.
• DNA replication takes place, meaning the cell makes an exact copy of its entire DNA.
• Histone proteins, which help package DNA, are synthesized and attach to the newly copied DNA.
• Centrioles, which are important for spindle formation in animal cells, also duplicate.
• As a result, the DNA content in the cell doubles, changing from 2C to 4C.
(ii) **G2 Phase:**
• At the start of G2, the DNA amount is 4C, having doubled in S phase.
• The cell continues to grow, preparing for division.
• Many organelles, including mitochondria and chloroplasts (in plant cells), are synthesized and multiply.
• Tubulin, a protein that forms microtubules, is synthesized to build the spindle fibers.
• The spindle begins to organize and form.
• This phase directly precedes nuclear division.
• Maturation Promoting Factor (MPF) is formed, which helps trigger the condensation of interphase chromosomes into the mitotic form.
In simple words: In S phase, the cell makes an exact copy of its DNA and grows. In G2 phase, the cell grows more, makes extra cell parts, and gets ready for the main division.

🎯 Exam Tip: Remember that DNA replication is the defining event of the S phase, while the G2 phase is primarily about growth and final preparations, including synthesis of proteins for spindle formation.

 

Question 5. What are the significances of Mitosis.
Answer: Mitosis is a fundamental process of cell division that produces two genetically identical daughter cells from a single parent cell. It plays several critical roles in living organisms, ensuring stability and growth.
(1) **Genetic Stability:** Mitosis ensures that daughter cells receive an exact, identical copy of the parent cell's chromosomes. This maintains the genetic makeup across cell generations, which is vital for tissue function and overall organism health.
(2) **Growth:** For multicellular organisms, mitosis is essential for increasing the number of cells, allowing the organism to grow in size and develop from a single fertilized egg into a complex being.
(3) **Repair of Tissues:** When tissues are damaged or old cells need replacement, mitosis produces new, identical cells to repair the injury or replenish the cell population. This constant renewal helps maintain tissue integrity.
(4) **Asexual Reproduction:** In many single-celled organisms and some multicellular ones, mitosis is the basis for asexual reproduction. It creates new individuals that are genetically identical to the parent, ensuring rapid propagation.
(5) **Plant Propagation:** In flowering plants, structures like bulbs, corms, tubers, rhizomes, and runners are formed through mitotic division. These structures can then separate from the parent plant and grow into new, independent individuals, effectively allowing the plant to reproduce clonally. The ability to quickly generate many genetically uniform offspring is a key advantage.
(6) **Regeneration:** Mitosis is crucial for regeneration, where organisms can regrow lost or damaged body parts, such as the arms of a starfish or the tail of a lizard. This process relies on generating new cells to reconstruct complex structures. This ability is also used in tissue culture for biotechnology.
In simple words: Mitosis is important for growing bigger, fixing damaged body parts, making exact copies for reproduction, and keeping the same genetic information in all new cells.

🎯 Exam Tip: When listing the significance of mitosis, always connect each point to either growth, repair, reproduction, or genetic continuity, as these are its core functions.