Paper II — Q2
(a) Describe types and composition of chromatin. How is DNA packaged in a eukaryotic cell ? 10+10=20 (b) Explain coupling and…
Describe types and composition of chromatin. How is DNA packaged in a eukaryotic cell ? 10+10=20
Explain coupling and repulsion hypothesis in linkage. Give a brief account of procedure used in preparing a chromosome map with the help of three-point test cross. 5+10=15
What do you understand by standard deviation and coefficient of variation ? Discuss their significance. 10+5=15
हिंदी में प्रश्न पढ़ें
क्रोमेटिन के प्रकारों तथा संयोजन का वर्णन कीजिए । यूकेरियोटिक कोशिका में डी.एन.ए. कैसे पैकेज होता है ? 10+10=20
सहलग्नता में युग्मन और प्रतिकर्षण परिकल्पना की व्याख्या कीजिए । त्री-पॉइंट परीक्षार्थ संकरण की सहायता से गुणसूत्र मानचित्र तैयार करने में प्रयुक्त प्रक्रिया का संक्षिप्त विवरण दीजिए । 5+10=15
मानक विचलन और विचरण गुणांक से आप क्या समझते हैं ? इनके महत्व की चर्चा कीजिए । 10+5=15
Model answer
Drafted with AI by UPSC Answer Check against this question's marking rubric, to the expected length. UPSC does not publish answers for Mains — this is one way to score well, not an official key. How we build rubrics and answers.
(a) Chromatin: types, composition and DNA packaging Chromatin is the DNA–protein complex of interphase nuclei. Euchromatin is lightly stained, decondensed, gene-rich and transcriptionally active; heterochromatin is densely stained, condensed and mostly inactive (constitutive at centromeres and telomeres, facultative elsewhere). Composition is roughly DNA : histone : non-histone protein : RNA = 1 : 1 : 0.5–1 : small. Five histones occur: H2A, H2B, H3 and H4 (core) and H1 (linker). Packaging occurs in levels. About 146 bp of DNA wraps 1¾ turns around a histone octamer (two each of H2A, H2B, H3, H4) to form a nucleosome (“beads on a string”, 10 nm fibre, ~7-fold compaction). H1 binds the linker DNA and folds the string into the 30 nm solenoid fibre (~40-fold). This is organised into 300 nm looped domains attached to a nuclear scaffold, then into 700 nm condensed chromatids and the ~1,400 nm metaphase chromosome, giving 10,000-fold overall compaction.
(b) Coupling–repulsion hypothesis and three-point test cross Bateson and Punnett (sweet pea) noted that dominant alleles tended to stay together. In coupling (cis) the dominants lie on one chromosome (AB/ab); in repulsion (trans) they lie on different homologues (Ab/aB). Linked genes give parental types in excess and recombinants in a fixed low frequency, so the ratio departs from 9:3:3:1 or 1:1:1:1. In a three-point test cross a heterozygote (A B C / a b c) is crossed to the triple recessive. Of eight progeny classes, the two commonest are parental; the two rarest are double crossovers. The gene whose position differs between parental and double-crossover classes is in the middle. Recombination frequencies (RF = recombinants/total × 100) for each interval give map distances in centimorgans; the outer distance is the sum of the inner ones plus twice the double crossovers. Interference = 1 − coefficient of coincidence (observed/expected double crossovers).
(c) Standard deviation and coefficient of variation Standard deviation measures spread of observations around the mean: SD = √[Σ(x − x̄)² / (n − 1)] for a sample (n for a population). It is in the original units and is the square root of variance. For a normal curve, about 68%, 95% and 99.7% of values fall within 1, 2 and 3 SD of the mean. Coefficient of variation expresses SD as a percentage of the mean: CV = (SD / x̄) × 100. It is unitless, so variability of traits measured in different units (height in cm, yield in kg) or with widely different means can be compared; the lower the CV, the more uniform or consistent the character.
How this answer will be evaluated
Framework: Botany Paper 2: Define > Structure/Process > Labelled Diagram > Significance
What earns full marks: Complete definitions, accurate diagrams, stepwise processes, specific examples, and clear significance
Evaluation rubric
Each sub-part is marked on its own, against the marks and word limit printed on the paper.
- (a) Define chromatin types/composition and explain eukaryotic DNA packaging hierarchy. 20 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Distinguish euchromatin and heterochromatin
- Identify histone proteins (H1-H4) and nucleosome structure
- Describe DNA packaging levels (beads-on-string to 30nm fiber)
- Include a labelled diagram of chromatin structure
Loses marks
- Unlabelled or missing diagram
- Confusing prokaryotic and eukaryotic packaging
- Omitting histone composition details
Earns more
- Mention specific histone modifications
- Reference specific eukaryotic model organism
- Link to gene expression regulation
- Mention recent biotech application in chromatin engineering
Extra mark
- Named specific histone variant
- Recent CRISPR-based chromatin modification example
- (b) Explain coupling/repulsion in linkage and describe three-point test cross mapping procedure. 15 marks
explain— definition/context → points in order → small example → short close
Must cover
- Define coupling and repulsion phases
- Explain three-point test cross procedure stepwise
- Describe recombination frequency calculation
- Show how gene order is determined
Loses marks
- Confusing coupling with repulsion
- Omitting recombination frequency calculation
- Not showing how gene order is determined
Earns more
- Include worked example with specific genes
- Mention interference and coefficient of coincidence
- Reference specific organism used in classic experiments
- Link to modern genetic mapping applications
Extra mark
- Named specific classic experiment (e.g., Sturtevant's Drosophila work)
- Recent application in crop genome mapping
- (c) Define standard deviation and coefficient of variation, discuss their significance in biological data. 15 marks
discuss— intro → 3-4 dimensions → example → balanced close
Must cover
- Define standard deviation with formula
- Define coefficient of variation with formula
- Explain significance of each measure
- Provide biological example or application
Loses marks
- Missing or incorrect formulas
- Not distinguishing between SD and CV
- No biological context or example
Earns more
- Compare when to use SD vs CV
- Reference specific biological dataset or study
- Link to experimental design or statistical analysis
- Mention limitations of each measure
Extra mark
- Named specific statistical software or package
- Recent application in genomic or phenotypic data analysis
What "Describe" is asking you to do
Give a full, ordered account of the thing named — its parts, stages or mechanism — in the sequence in which it actually exists or occurs. Most describe questions come from the science optionals, where the marks sit in correct technical detail and, where the stem says so, a labelled diagram.
Structure that answers it
One-line identification of the subject → the parts or stages in their real order, each with its defining detail → labelled diagram where the subject is structural → closing line on function or significance
Where marks are lost
Loose general prose where the examiner is ticking named parts, correct terminology and their sequence; and in the General Studies papers, turning to evaluation before the description is finished.
Practice this exact question
Write your answer and it is marked point by point against the model answer above — what you covered, what you missed, what you got wrong.
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