This MCQ module is based on: Nucleus Chromosomes
TOPIC 34 OF 45
Nucleus Chromosomes
🎓 Class 11
Biology
CBSE
Theory
Ch 8 – Cell: The Unit of Life
⏱ ~14 min
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Nucleus Chromosomes
8.16 Nucleus — The Control Centre
The nucleus, as a cell organelle, was first described by Robert Brown in 1831. Later the material of the nucleus stained by the basic dyes was given the name chromatin by Flemming. Interphase nucleus (nucleus of a cell when it is not dividing) has highly extended and elaborate nucleoprotein fibres called chromatin, nuclear matrix and one or more spherical bodies called nucleoli.
Electron microscopy has revealed that the nuclear envelope, which consists of two parallel membranes with a space between (10 to 50 nm) called the perinuclear space, forms a barrier between the materials present inside the nucleus and that of the cytoplasm. The outer membrane usually remains continuous with the endoplasmic reticulum and also bears ribosomes on it.
At a number of places the nuclear envelope is interrupted by minute pores, which are formed by the fusion of its two membranes. These nuclear pores are the passages through which movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm. Normally, there is only one nucleus per cell, variations in the number of nuclei are also frequently observed.
Some mature cells even lack nucleus, e.g., erythrocytes of many mammals and sieve tube cells of vascular plants. The nuclear matrix or the nucleoplasm contains nucleolus and chromatin. The nucleoli are spherical structures present in the nucleoplasm. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
8.17 Chromosomes — From Chromatin to Visible Bodies
The nucleoplasm consists of nucleolus and chromatin. Every chromosome essentially has a primary constriction or the centromere on the sides of which disc shaped structures called kinetochores are present. Based on the position of the centromere, the chromosomes can be classified into four types:
- Metacentric chromosome has middle centromere forming two equal arms of the chromosome.
- Sub-metacentric chromosome has centromere slightly away from the middle of the chromosome resulting in one shorter arm and one longer arm.
- Acrocentric chromosome has centromere situated close to its end forming one extremely short and one very long arm.
- Telocentric chromosome has terminal centromere.
Sometimes a few chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the satellite. Such chromosomes are called SAT-chromosomes.
8.18 Karyotype and Idiogram
The number of chromosomes is constant in a species. A picture of all the chromosomes of a cell, paired and arranged in order of decreasing size, is called the karyotype. An idealised diagrammatic karyotype is called an idiogram.
Humans have 46 chromosomes = 22 pairs of autosomes + 1 pair of sex chromosomes (XX in females, XY in males). Other examples: Onion 16, Rice 24, Maize 20, Dog 78.
| Organism | Diploid (2n) chromosome number |
|---|---|
| Human (Homo sapiens) | 46 |
| Dog | 78 |
| Cat | 38 |
| Fruit fly (Drosophila) | 8 |
| Maize (Zea mays) | 20 |
| Pea (Pisum sativum) | 14 |
| Onion (Allium cepa) | 16 |
| Rice | 24 |
Interactive: Identify the Chromosome Type
Move the slider to position the centromere along the chromosome (0 = top end; 100 = bottom end). See which type is formed.
50
Type: Metacentric
Centromere in the middle, two equal arms. Examples: human chromosomes 1, 3.
Activity 8.4 — Onion Root Tip Squash for Chromosomes
Setup: Onion root tips (germinated overnight); acetocarmine stain; HCl; microscope slide; coverslip; compound microscope.
Predict: Will you see chromosomes in every cell? Why or why not?
My prediction: …
- Cut ~5 mm of growing root tip. Place in 1N HCl for 5 minutes to soften.
- Transfer to a slide. Add a drop of acetocarmine stain. Wait 5-10 minutes.
- Place a coverslip and squash gently with thumb pressure. The cells spread in one layer.
- Observe at 100× oil immersion. Search for cells in different stages of mitosis.
- Sketch a metaphase cell with visible chromosomes lined up at the equator.
Observation: Most cells are in interphase — you see a dark-stained nucleus but no individual chromosomes. About 5-10% of cells are caught in mitosis. In metaphase cells, you can count 16 condensed chromosomes lined up at the cell's equator (onion 2n = 16). In anaphase, chromatids move apart toward the poles. In telophase, two new nuclei form.
The reason chromosomes are only visible during division is that during interphase chromatin is dispersed and unwound to allow gene expression. During division, chromatin condenses into discrete chromosomes for safe distribution to daughter cells. This is the practical proof that chromatin = chromosome in different states.
The reason chromosomes are only visible during division is that during interphase chromatin is dispersed and unwound to allow gene expression. During division, chromatin condenses into discrete chromosomes for safe distribution to daughter cells. This is the practical proof that chromatin = chromosome in different states.
8.19 Worked Examples
Worked Example 1: From DNA to chromosome
How does a 2-metre long DNA strand fit inside a nucleus only ~6 µm across?
Through hierarchical compaction:
1. DNA double helix (2 nm wide) wraps twice around a core of 8 histone proteins (octamer) → nucleosome (10 nm 'beads on a string').
2. Nucleosomes pack together → 30 nm fibre (solenoid).
3. 30 nm fibre forms loops anchored to a protein scaffold → 300 nm fibre.
4. Looped fibre folds further → 700 nm chromatid.
5. Sister chromatids paired at centromere → ~1400 nm metaphase chromosome.
The total compaction is about 10,000-fold. Without this packing, just one chromosome would be longer than the cell! This is why DNA needs histones — they're not just packaging, but also regulators of gene access.
1. DNA double helix (2 nm wide) wraps twice around a core of 8 histone proteins (octamer) → nucleosome (10 nm 'beads on a string').
2. Nucleosomes pack together → 30 nm fibre (solenoid).
3. 30 nm fibre forms loops anchored to a protein scaffold → 300 nm fibre.
4. Looped fibre folds further → 700 nm chromatid.
5. Sister chromatids paired at centromere → ~1400 nm metaphase chromosome.
The total compaction is about 10,000-fold. Without this packing, just one chromosome would be longer than the cell! This is why DNA needs histones — they're not just packaging, but also regulators of gene access.
Worked Example 2: Why do some mature cells lack a nucleus?
Mature mammalian red blood cells (RBC) lack a nucleus, mitochondria, and most organelles. Why does this make biological sense?
RBCs are specialised oxygen-delivery 'bags'. Without internal organelles, they are:
1. Smaller and more flexible — can squeeze through capillaries narrower than the RBC itself.
2. More space for haemoglobin — about 280 million haemoglobin molecules per RBC. With organelles in the way, they could not pack so much oxygen-carrying pigment.
3. No mitochondria → no O₂ consumption — every oxygen molecule the RBC carries can be delivered to body tissues; the RBC itself doesn't consume any.
The cost: an RBC cannot synthesise proteins (no ribosomes) or divide (no nucleus). So mature RBCs have a fixed lifespan of about 120 days before being recycled by the spleen.
This is an extreme case of functional specialisation.
1. Smaller and more flexible — can squeeze through capillaries narrower than the RBC itself.
2. More space for haemoglobin — about 280 million haemoglobin molecules per RBC. With organelles in the way, they could not pack so much oxygen-carrying pigment.
3. No mitochondria → no O₂ consumption — every oxygen molecule the RBC carries can be delivered to body tissues; the RBC itself doesn't consume any.
The cost: an RBC cannot synthesise proteins (no ribosomes) or divide (no nucleus). So mature RBCs have a fixed lifespan of about 120 days before being recycled by the spleen.
This is an extreme case of functional specialisation.
Worked Example 3: Karyotype detective
A karyotype shows 47 chromosomes with three copies of chromosome 21. What diagnosis can you make and what features would the person likely show?
Diagnosis: Trisomy 21, also known as Down's syndrome. Normal humans have 2 copies of chromosome 21 (one from each parent). Trisomy means three copies due to non-disjunction during meiosis (most often in the maternal egg).
Features may include: distinctive facial features (flat profile, upward-slanted eyes, small ears), short stature, hypotonia (low muscle tone), intellectual disability (mild to moderate), increased risk of heart defects, hearing/vision issues, and Alzheimer-type symptoms at younger age.
Note: Many people with Down's syndrome lead happy, fulfilling lives. Karyotype analysis from amniotic cells can detect this prenatally. It's diagnosed by counting 47 chromosomes total and three copies of the small acrocentric chromosome 21.
Features may include: distinctive facial features (flat profile, upward-slanted eyes, small ears), short stature, hypotonia (low muscle tone), intellectual disability (mild to moderate), increased risk of heart defects, hearing/vision issues, and Alzheimer-type symptoms at younger age.
Note: Many people with Down's syndrome lead happy, fulfilling lives. Karyotype analysis from amniotic cells can detect this prenatally. It's diagnosed by counting 47 chromosomes total and three copies of the small acrocentric chromosome 21.
Competency-Based Questions
Q1. The chromosome with centromere at its very end is called: L1 Remember
Answer: (d) Telocentric. Only one arm; centromere is terminal. NOT found in normal humans.
Q2. The structure that allows materials to pass between nucleus and cytoplasm is: L1 Remember
Nuclear pores. Each pore is a large protein complex that allows selective passage of mRNA out and proteins/transcription factors in.
Q3. Apply: The nucleolus of an oocyte (egg cell) is very large compared to that of a muscle cell. Predict the reason and what activity it reflects. L3 Apply
The nucleolus is the site of rRNA synthesis and ribosomal subunit assembly. A large nucleolus = high ribosome production rate.
An oocyte needs to stockpile millions of ribosomes to feed early embryonic protein synthesis after fertilisation (the first cleavage divisions happen without transcription). So the oocyte ramps up ribosome production by enlarging its nucleolus — sometimes the nucleolus is visible even to the naked eye in amphibian oocytes.
A muscle cell has lower protein turnover and can rely on a small nucleolus.
An oocyte needs to stockpile millions of ribosomes to feed early embryonic protein synthesis after fertilisation (the first cleavage divisions happen without transcription). So the oocyte ramps up ribosome production by enlarging its nucleolus — sometimes the nucleolus is visible even to the naked eye in amphibian oocytes.
A muscle cell has lower protein turnover and can rely on a small nucleolus.
Q4. Analyse: Compare prokaryotic 'DNA region' (nucleoid) with eukaryotic nucleus. What advantages does the eukaryotic compartmentalisation provide? L4 Analyse
Prokaryotic nucleoid: Naked circular DNA in cytoplasm; transcription and translation are coupled (mRNA is translated while still being made).
Eukaryotic nucleus: Membrane-bound; linear DNA + histones; mRNA undergoes processing (splicing, capping, polyadenylation) BEFORE export to cytoplasm for translation.
Advantages of compartmentalisation:
1. Quality control — only mature, properly spliced mRNAs leave the nucleus.
2. Gene regulation — transcription factors and chromatin modifiers can act in a controlled environment.
3. Allows complex regulation — alternative splicing produces multiple proteins from one gene.
4. Protects DNA — physically separated from harsh cytoplasmic enzymes.
This compartmentalisation is one reason eukaryotes can produce vastly more diverse and complex proteins than prokaryotes.
Eukaryotic nucleus: Membrane-bound; linear DNA + histones; mRNA undergoes processing (splicing, capping, polyadenylation) BEFORE export to cytoplasm for translation.
Advantages of compartmentalisation:
1. Quality control — only mature, properly spliced mRNAs leave the nucleus.
2. Gene regulation — transcription factors and chromatin modifiers can act in a controlled environment.
3. Allows complex regulation — alternative splicing produces multiple proteins from one gene.
4. Protects DNA — physically separated from harsh cytoplasmic enzymes.
This compartmentalisation is one reason eukaryotes can produce vastly more diverse and complex proteins than prokaryotes.
Q5. Create: Design a simple model to show students the four types of chromosomes using everyday materials. L6 Create
Materials: Two equal-length pipe cleaners (or strips of stiff paper) per chromosome model, one small bead, glue.
How to build each type:
1. Metacentric: Join two pipe cleaners side by side; place a bead exactly in the middle of one cleaner.
2. Sub-metacentric: Same, but place the bead 30% from one end (so one arm is shorter).
3. Acrocentric: Place the bead just 10% from one end (long arm + tiny short arm).
4. Telocentric: Place the bead exactly at one end (only one arm visible).
Activity: Mix all four types in a bag. Students draw one at random and identify the type by measuring the arm ratio. They can then arrange a model 'karyotype' for an imaginary species with these chromosomes.
How to build each type:
1. Metacentric: Join two pipe cleaners side by side; place a bead exactly in the middle of one cleaner.
2. Sub-metacentric: Same, but place the bead 30% from one end (so one arm is shorter).
3. Acrocentric: Place the bead just 10% from one end (long arm + tiny short arm).
4. Telocentric: Place the bead exactly at one end (only one arm visible).
Activity: Mix all four types in a bag. Students draw one at random and identify the type by measuring the arm ratio. They can then arrange a model 'karyotype' for an imaginary species with these chromosomes.
Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: The nucleolus is the site of ribosomal RNA synthesis.
R: Larger nucleoli are found in cells actively making proteins.
Answer: (A). Both true; R explains A. Cells needing more ribosomes synthesise more rRNA in the nucleolus, which enlarges visibly.
A: Mature mammalian red blood cells lack a nucleus.
R: Without a nucleus, RBCs have more room for haemoglobin and a flexible biconcave shape.
Answer: (A). Both true; R explains A. Losing the nucleus is a specialised adaptation for oxygen carrying.
A: Telocentric chromosomes are common in humans.
R: Telocentric chromosomes have one terminal centromere.
Answer: (D). Assertion is FALSE — humans do NOT have telocentric chromosomes; we have metacentric, sub-metacentric and acrocentric. Reason is TRUE (correct definition).
Frequently Asked Questions - Nucleus Chromosomes
What is the main concept covered in Nucleus Chromosomes?
In NCERT Class 11 Biology Chapter 8 (Cell: The Unit of Life), "Nucleus Chromosomes" covers the core biological structures, functions, and classifications students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and ecological/physiological significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Nucleus Chromosomes useful in real-life or applied biology?
Real-life applications of "Nucleus Chromosomes" from NCERT Class 11 Biology Chapter 8 include medical diagnostics, agriculture, food preservation, biotechnology, ecological monitoring, and public health. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems, not just textbook content.
What are the key terms students should memorize for Nucleus Chromosomes?
Key terms in "Nucleus Chromosomes" (NCERT Class 11 Biology Chapter 8 Cell: The Unit of Life) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of Chapter 8?
NCERT Class 11 Biology Chapter 8 (Cell: The Unit of Life) is structured so each part builds biological understanding sequentially. "Nucleus Chromosomes" connects to neighbouring parts via shared classifications, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Nucleus Chromosomes?
CBSE board questions from "Nucleus Chromosomes" typically include: (1) 1-mark MCQs on definitions and classification, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining structure + function + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Nucleus Chromosomes" lesson allows students to explore biological structures, classifications, or processes using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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