Structure of Chromosome - Competency-Based Question Bank (with Answers)

Section A: Case-Based Questions

Case Study 1: A student is examining a micrograph of a nucleus during Interphase. He sees a tangled network of thread-like structures. His teacher explains that this material is composed of DNA and histones.

1. Identify the thread-like structure observed by the student.
   • Answer: Chromatin.
2. What transformation will this structure undergo as the cell begins to divide?
   • Answer: It will undergo condensation (coiling and thickening) to form distinct chromosomes.
3. Analysis: Why is the association of DNA with histone proteins crucial for the packaging of genetic material inside the nucleus?
   • Answer: DNA is very long (approx 2m). Histones provide a structural framework around which DNA wraps, allowing it to be compacted millions of times to fit into a microscopic nucleus.
4. Designing: Design a simple model using a long string and beads to demonstrate the concept of a "Nucleosome". Explain what the string and beads represent.
   • Answer: Beads represent Histone Octamers. The string represents the DNA double helix. Wrapping the string twice around each bead demonstrates the formation of nucleosomes ("beads on a string").
5. If the histone proteins were negatively charged, would they still bind effectively to DNA? Justify your answer.
   • Answer: No. DNA is negatively charged (due to phosphate groups). Histones must be positively charged to create an electrostatic attraction and bind the DNA tightly.

Case Study 2: During a forensic investigation, DNA profiling is used to identify a suspect. The analysis focuses on specific segments of DNA that vary greatly between individuals. 6. What is the fundamental unit of heredity located on the chromosome?

• Answer: Gene.

7. Differentiate between "Chromatin" and "Chromosome" in terms of visibility and condensation.
   • Answer: Chromatin is a diffused, thread-like network visible during interphase. Chromosomes are highly condensed, distinct structures visible only during cell division.
8. Creating: Create an analogy to explain the relationship between DNA, Genes, and Chromosomes.
   • Answer: The Chromosome is like a Book. The Genes are the Chapters or paragraphs that provide specific instructions. The DNA is the Alphabet/Ink used to write everything.
9. Critical Thinking: Why are specific regions of chromatin (Heterochromatin) more darkly stained than others?
   • Answer: Heterochromatin is more tightly packed/condensed, allowing it to take up more stain. This density usually means the genes in that region are inactive (silenced).
10. The backbone of the DNA helix is made of sugar and phosphate. How does this structure contribute to stability?
    • Answer: The strong covalent (phosphodiester) bonds in the backbone protect the nitrogenous bases (the actual code) inside the helix from chemical damage.

Section B: Assertion-Reasoning Questions

Directions: (a) Both A and R true, R explains A; (b) Both true, R doesn't explain A; (c) A true, R false; (d) A false, R true.

11. Assertion (A): The two sister chromatids of a chromosome are genetically identical. Reason (R): They are formed by the replication of a single DNA molecule during the S-phase.
    • Answer: (a) Both A and R are true and R is the correct explanation of A.
12. Assertion (A): Histones are basic proteins. Reason (R): They are rich in basic amino acids like lysine and arginine which help in binding to acidic DNA.
    • Answer: (a) Both A and R are true and R is the correct explanation of A.
13. Assertion (A): A nucleotide is the building block of chromatin. Reason (R): A nucleotide consists of a phosphate group, a pentose sugar, and a nitrogenous base.
    • Answer: (b) Both A and R are true but R is not the complete explanation of how chromatin is built (it involves nucleosomes/histones too).
14. Assertion (A): The centromere is always located in the exact center of the chromosome. Reason (R): The position of the centromere determines the shape of the chromosome during anaphase.
    • Answer: (d) A is false but R is true. (Centromere can be at different positions).
15. Assertion (A): Human males are heterogametic. Reason (R): They possess two different sex chromosomes, X and Y.
    • Answer: (a) Both A and R are true and R is the correct explanation of A.

Section C: Creating and Designing (Application & Analysis)

16. Designing: Draw a schematic diagram of a DNA double helix.
    • Answer: [Description: Two twisting strands with A-T and G-C pairs as rungs, sugar-phosphate sides].
17. Creating: "Wanted Poster" for a Nucleosome.
    • Answer: "Wanted: Nucleosome. Description: 8 Histone proteins wrapped in 146 base pairs of DNA. Last seen: Inside the nucleus. Function: Compacting DNA."
18. Analysis: If a DNA molecule has 20% Adenine, calculate the % of Cytosine.
    • Answer: A=20%, so T=20%. Total A+T=40%. Remaining G+C=60%. Since G=C, Cytosine = 30%.
19. Designing: Experiment to prove DNA is genetic material.
    • Answer: Hershey-Chase experiment using radioactive P-32 (labels DNA) and S-35 (labels protein) in bacteriophages. Only P-32 enters the bacteria.
20. Visualisation: Hierarchy of packaging.
    • Answer: DNA -> Nucleosome -> Solenoid (30nm fiber) -> Supercoiled Loops -> Chromatid -> Chromosome.
21. Application: Why is the Y chromosome smaller than the X?
    • Answer: It carries fewer genes, mostly those related to male sex determination (SRY gene) and sperm production.
22. Creating: Impact of a missing Gene.
    • Answer: A missing gene means the cell cannot produce a specific protein (e.g., melanin), leading to a change in trait (e.g., Albinism).
23. Analysis: Why is DNA better for storage than RNA?
    • Answer: DNA is double-stranded (stable) and contains Thymine instead of Uracil (more resistant to mutation/deamination).
24. Designing: Crossword clue for "Karyotype".
    • Answer: "A photograph of an individual's complete set of chromosomes arranged in pairs."
25. Creating: Histone speech.
    • Answer: "I am the core of the library; without my octameric structure, the scrolls of life would be a tangled mess."

Section D: Competency & Critical Thinking

26. Nucleotide vs Nucleoside.
    • Answer: Nucleoside = Sugar + Base. Nucleotide = Sugar + Base + Phosphate.
27. 47 chromosomes (XXY). Syndrome?
    • Answer: Klinefelter Syndrome. Feature: Sterile male, feminized traits (e.g., gynaecomastia).
28. Why is chromosome number constant?
    • Answer: To maintain the genetic identity and characteristics of a species across generations.
29. Significance of Telomere.
    • Answer: They protect the ends of chromosomes from deterioration or fusing with neighbors. They shorten with each division.
30. Autosomes vs Allosomes.
    • Answer: Autosomes (22 pairs in humans) control somatic traits. Allosomes (1 pair, XX/XY) determine sex. Sperm has 22 autosomes and 1 allosome (X or Y).
31. Diagram Based: A (Centromere), B (p-arm), C (q-arm), D (Telomere).
    • Answer: Centromere: Attachment for spindles. p-arm: Short arm. q-arm: Long arm. Telomere: Protective tip.
32. Why is G-C bond stronger than A-T?
    • Answer: G-C has 3 hydrogen bonds; A-T has only 2.
33. How does structure facilitate equal distribution?
    • Answer: Replication creates two identical chromatids, and the centromere allows the spindle to pull them to opposite poles.
34. "Key and Lock" analogy for base pairing.
    • Answer: Only specific bases fit together (A with T) due to their molecular shape and hydrogen bonding sites.
35. DNA denaturation/renaturation.
    • Answer: Demonstrates the complementary nature of the two strands and the strength of the hydrogen bonds.
36. DNA with 6 types of bases.
    • Answer: Increased complexity/storage capacity for genetic information.
37. Classify chromosomes: Metacentric (V), Sub-metacentric (L), Acrocentric (J), Telocentric (I).
38. Sex determination probability.
    • Answer: Father (XY) x Mother (XX) -> 50% XX (Female), 50% XY (Male).
39. Mutation leading to non-functional protein.
    • Answer: Change in DNA sequence -> Change in mRNA -> Change in Amino acid sequence -> Faulty folding of protein.
40. Why "antiparallel"?
    • Answer: One strand runs 5' to 3' and the other runs 3' to 5'. This allows base pairs to align for hydrogen bonding.

Section E: Advanced Competency

41. Gametogenesis X-Y failure.
    • Answer: Sperm could be XY or O (neither). Result: XXY (Klinefelter) or XO (Turner).
42. 2m DNA in nucleus model.
    • Answer: Like a very long thread wound onto multiple spools (histones), then coiled into a spring (solenoid).
43. Homologous Pair genes.
    • Answer: They carry the same genes at the same loci, but the alleles (versions) might be different (e.g., Brown eyes vs Blue eyes).
44. Timeline of DNA.
    • Answer: 1952: Franklin's Photo 51. 1953: Watson & Crick model.
45. Two centromeres.
    • Answer: Unstable. The chromosome might be pulled in two directions during anaphase and break.
46. 'Rungs of the ladder' metaphor.
    • Answer: The sugar-phosphate backbones are the vertical rails; the nitrogenous base pairs are the horizontal steps.
47. Linkage Groups for 2n=12.
    • Answer: 6 linkage groups (Equal to the haploid number 'n').
48. DNA amount constant?
    • Answer: No. It doubles in S-phase and is halved during cytokinesis.
49. Quiz questions.
    • Answer: (T/F) DNA is a right-handed helix? (True). A-G are pyrimidines? (False - Purines).
50. Why mitochondrial DNA is circular?
    • Answer: Reflects its evolutionary origin from endosymbiotic bacteria.