ISC Class 12 Biology Board Exam Question Paper 2015 with Solutions

ISC Class 12 Biology Board Exam Question Paper with Solutions 2015

 

Part - I

 

Question 1.
(a) Give a brief answer for each of the following : [4]
(i) What is heterosis?
(ii) Why is non-cyclic photophosphorylation considered as a non-cyclic pathway?
(iii) Define test cross.
(iv) What are introns?

Answer:
(i) Heterosis is the increased hybrid vigour displayed by the offspring from a cross between genetically different parents.
(ii) Non-cyclic photophosphorylation is considered as a non-cyclic pathway because electrons expelled by the excited photo centre do not return to it. It produces both ATP and NADPH.
(iii) Test cross is a special type of back cross made between the individual with a dominant trait and its recessive parent in order to know whether it is homozygous or heterozygous for the trait.
(iv) Introns are the segments of DNA that do not code for the gene product. They are transcribed in primary transcript but are subsequently removed from the transcript before translation.

Teacher's Note:
a) Heterosis leads to superior offspring characteristics compared to parents, often utilized in agriculture.
b) Students must clearly state that electrons in non-cyclic flow do not return to the same photosystem, unlike cyclic photophosphorylation.

 

(b) Each of the following question(s)/statement(s) has four suggested answers. Choose the correct option in each case. [4]
1. Triple Fusion involves :
(i) Fusion of one male gamete with female gamete
(ii) Fusion of tube nucleus with generative nucleus
(iii) Fusion of two polar nuclei
(iv) Fusion of second male gamete with two polar nuclei

Answer: (iv) Fusion of second male gamete with two polar nuclei

Triple fusion results in the formation of the primary endosperm nucleus (PEN).

Teacher's Note:
a) Triple fusion is a characteristic feature of angiosperms and forms triploid endosperm.
b) Do not confuse triple fusion with syngamy, which involves the fusion of one male gamete with the egg cell.

 

2. An EEG represents spontaneous electrical activity of the :
(i) Kidney
(ii) Spinal cord
(iii) Heart
(iv) Brain

Answer: (iv) Brain

Electroencephalogram (EEG) measures electrical activity in the brain.

Teacher's Note:
a) An ECG records heart activity, whereas an EEG records brain waves.
b) Remember the full forms and applications of diagnostic tests like EEG, ECG, and EMG to avoid confusion.

 

3. The genotype of a person with Turner's syndrome will be :
(i) 44 + XXY
(ii) 44 + XYY
(iii) 44 + XO
(iv) 44 + XXYY

Answer: (iii) 44 + XO

Turner's syndrome is caused by the absence of one X chromosome (monosomy).

Teacher's Note:
a) Affected individuals are sterile females with short stature and underdeveloped secondary sexual characters.
b) Distinguish this from Klinefelter's syndrome, which has the genotype 44 + XXY.

 

4. Transcription is the transfer of genetic code from a DNA molecule to :
(i) RNA molecule
(ii) Second DNA molecule
(iii) Ribosomal subunit
(iv) Sequence of amino acids in a protein molecule

Answer: (i) RNA molecule

Transcription synthesizes messenger RNA (mRNA) from a DNA template.

Teacher's Note:
a) Transcription is the first step of gene expression where genetic information flows from DNA to RNA.
b) Translation is the subsequent step from RNA to protein sequence.

 

(c) Give a scientific term for each of the following: [4]
(i) The first formed category of photosynthetic organisms.
(ii) The surgical removal of a section of fallopian tube.
(iii) An animal behaviour which benefits others but is of no advantage to itself.
(iv) The hydrostatic pressure developed inside the cell on the cell wall due to endosmosis.

Answer:
(i) Photoautotrophic bacteria
(ii) Tubectomy
(iii) Altruism (or Commensalism as accepted in key)
(iv) Turgor pressure

Teacher's Note:
a) Tubectomy is a permanent method of female sterilization.
b) Turgor pressure is vital for maintaining the rigidity and shape of plant tissues.

 

(d) Expand the following abbreviations : [4]
(i) STD
(ii) NADP
(iii) MRI
(iv) DDT

Answer:
(i) Sexually Transmitted Disease
(ii) Nicotinamide Adenine Dinucleotide Phosphate
(iii) Magnetic Resonance Imaging
(iv) Dichloro Diphenyl Trichloroethane

Teacher's Note:
a) Spelling of scientific abbreviations must be precise to secure full marks.
b) DDT is a persistent organic pollutant and synthetic pesticide.

 

(e) Name the scientists who are associated with the following : [4]
(i) Discovered the fossil of Australopithecus
(ii) Microspheres
(iii) Coined the term Diffusion Pressure Deficit
(iv) Invented the CT scan

Answer:
(i) Raymond Dart
(ii) Sidney Fox
(iii) Meyer
(iv) Godfrey Hounsfield

Teacher's Note:
a) Raymond Dart discovered the Taung Child fossil of Australopithecus africanus in 1924.
b) Memorizing key scientists associated with evolutionary and physiological concepts is essential for scoring in this section.

 

Part - II
Section - A
(Answer any two questions)

 

Question 2.
(a) Give any three characters that have developed during human evolution. [3]
(b) Explain the term homogeny. [1]
(c) Give any two distinctive features of Dryopithecus. [1]

Answer:
(a) Characters developed during human evolution:
1. Bipedal locomotion with erect posture.
2. Free grasping hands with opposable thumbs.
3. Large cranial capacity, well-developed brain, speech, and complex memory.
(b) Chemogeny or chemical evolution refers to the process in which elements in the early atmosphere combined to produce simple and compound molecules, ultimately leading to the formation of complex organic molecules.
(c) Distinctive features of Dryopithecus:
1. Walked semi-erect on knuckles.
2. Brow ridges were absent (or snout was slightly projecting / forelimbs and hindlimbs were of the same size).

Teacher's Note:
a) Human evolutionary trends emphasize brain expansion, orthognathism, and bipedalism.
b) Ensure precise keywords like bipedalism and cranial capacity are mentioned.

 

Question 3.
(a) Explain the evolution of giraffe's neck according to Lamarck's theory of evolution. [3]
(b) Give two chromosomal similarities between man and apes. [1]
(c) Name any two temporary embryonic structures in vertebrates which provide evidence for evolution. [1]

Answer:
(a) According to Lamarck, the ancestral giraffe was initially a deer-like animal browsing on low herbs and shrubs. Due to environmental changes leading to famine and scarcity of ground vegetation, food became scarce. To reach leaves on high shrubs and trees, ancestors of the giraffe stretched their necks and forelimbs. This continuous stretching was an acquired character passed on to successive generations, gradually resulting in the long neck and forelimbs of modern-day giraffes.
(b) Chromosomal similarities between man and apes:
1. Total amount of DNA in both is more or less similar.
2. The banding pattern of individual human chromosomes is very similar to the corresponding chromosomes in apes.
(c) Temporary embryonic structures in vertebrates:
1. Gill clefts
2. Tail

Teacher's Note:
a) Lamarck's theory is based on the inheritance of acquired characters, which was later disproved by Weismann's germplasm theory.
b) Embryonic gill clefts in terrestrial vertebrate embryos strongly support the theory of common ancestry (recapitulation theory).

 

Question 4.
(a) Persons suffering from sickle cell anaemia are at an advantage in Malaria infested areas. Explain. [3]
(b) Define the term gene flow. [1]
(c) What are analogous organs? Describe with one example from the plant kingdom. [1]

Answer:
(a) Sickle cell anaemia is caused by an abnormal haemoglobin (Haemoglobin-S) where glutamic acid is replaced by valine at the 6th position of the beta-globin chain. Under low oxygen tension, erythrocytes assume a sickle shape. Heterozygous individuals (HbA HbS) do not suffer from severe anaemia, but their red blood cells inhibit the penetration and reproduction of the malaria parasite (*Plasmodium*). Consequently, these individuals have a survival advantage against malaria in endemic tropical regions.
(b) Gene flow is the addition or removal of alleles from a population due to the migration or interbreeding of a section of a population with another.
(c) Analogous organs are those that have similar outward form and function but different basic anatomical structure and embryonic origin. Example from the plant kingdom: Tendrils for climbing can be modified leaves (leaf-tendril in *Pisum*), modified leaflets, or modified stem tendrils (*Cucurbita*).

Teacher's Note:
a) This is a classic example of natural selection maintaining a deleterious allele due to balanced polymorphism (heterozygote advantage).
b) Clearly distinguish between homologous organs (same structure, different function) and analogous organs (different structure, same function).

 

Section - B
(Answer any two questions)

 

Question 5.
(a) With the help of diagrams, name and describe the different types of placentation seen in angiosperms. [4]
(b) Give four points of anatomical differences between a monocot stem and a dicot stem. [4]
(c) Define the following terms : [2]
(i) Racemose inflorescence
(ii) Osmotic pressure

Answer:
(a) Placentation is the arrangement of ovules within the ovary. The major types are:
1. Marginal: Ovules are borne on a single longitudinal placenta along the ventral suture of a unilocular ovary (e.g., Pea).
2. Parietal: Ovules develop on the inner wall of a syncarpous unilocular ovary, often becoming bilocular due to a false septum (replum) (e.g., Mustard).
3. Axile: Multilocular ovary where ovules are attached to the central axis formed by the fusion of septa (e.g., China rose).
4. Free central: Ovules are borne on a central axis not connected to the ovary wall in a unilocular ovary (e.g., *Dianthus*).
5. Basal: A single ovule is attached at the base of a unilocular ovary (e.g., Sunflower).
[Figure: Diagrams of Marginal, Parietal, Axile, Free central, and Basal placentation types as in standard botanical texts]
(b) Anatomical differences between Monocot and Dicot stems:

Monocot Stem	Dicot Stem
1. Epidermal hairs are absent.	1. Multicellular epidermal hairs are present.
2. Ground tissue is undifferentiated.	2. Ground tissue is differentiated into cortex, endodermis, and pericycle.
3. Vascular bundles are scattered.	3. Vascular bundles are arranged in a ring.
4. Vascular bundles are conjoint, collateral, and closed.	4. Vascular bundles are conjoint, collateral, and open (with cambium).

(c) Definitions:
(i) Racemose inflorescence: An indefinite inflorescence where the main axis continues to grow indefinitely and bears flowers laterally in an acropetal succession.
(ii) Osmotic pressure: The maximum pressure that can develop in a solution separated from pure water by a semipermeable membrane when it is subjected to a state of equilibrium.

Teacher's Note:
a) Placentation types must be supported by neat diagrams and correct botanical examples.
b) For stem differences, mentioning the presence or absence of cambium (open vs closed vascular bundles) is mandatory.

 

Question 6.
(a) Draw a diagram of the internal structure of the human ovary.
(b) Define the term water potential. What are its components? Explain.
(c) Give definition and importance of: [2]
(i) Imbibition
(ii) Parturition

Answer:
(a) [Figure: Diagram showing internal structure of mammalian ovary with primary follicles, growing follicles, mature Graafian follicle, corpus luteum, and ovum release]
(b) Water potential (\( \Psi \)) is the chemical potential of water, representing the difference between the free energy of water molecules in pure water and that of water in any other system. Its components are:
1. Solute potential (\( \Psi_s \)): Effect of dissolved solutes, always negative.
2. Pressure potential (\( \Psi_p \)): Hydrostatic pressure, usually positive.
3. Matric potential (\( \Psi_m \)): Binding of water to matrix surfaces, often negligible in plant cells.
Formula: \( \Psi = \Psi_s + \Psi_p \).
(c) Definitions and importance:
(i) Imbibition: The adsorption of water by hydrophilic colloidal solids without forming a solution. Importance: It is the initial step in seed germination and water absorption by root hairs.
(ii) Parturition: The act of expelling the fully developed fetus from the mother's uterus at the end of gestation. Importance: It marks the successful completion of pregnancy and initiates lactation.

Teacher's Note:
a) Water potential equation must be clearly written with standard psi notation.
b) Mention that pure water at standard temperature and pressure has a water potential of zero.

 

Question 7.
(a) Give four adaptations in flowers pollinated by insects. [4]
(b) Describe the mass flow hypothesis for translocation of organic solutes (food) in plants. [4]
(c) Write a brief note on the causes of infertility. [2]

Answer:
(a) Adaptations in insect-pollinated (entomophilous) flowers:
1. Flowers are large, showy, and brightly coloured to attract insects.
2. They possess nectar glands to provide a nutritional reward.
3. Pollen grains are sticky or spiny to easily adhere to insect bodies.
4. Stigmas are sticky and positioned inside the flower to receive pollen from visiting insects.
(b) Mass Flow Hypothesis (Munch, 1927): Organic solutes flow in mass from a region of high osmotic pressure (source, like leaves) to a region of low osmotic pressure (sink, like roots) driven by a turgor pressure gradient. Sugars actively loaded into sieve elements decrease water potential, causing water to enter from adjacent xylem, generating high turgor pressure that drives the bulk flow of sap towards sinks where solutes are unloaded.
(c) Causes of infertility: Inability to conceive after one year of unprotected intercourse. Causes can be in males (e.g., oligospermia, cryptorchidism, blockage of vas deferens) or females (e.g., anovulation, blocked fallopian tubes, uterine abnormalities).

Teacher's Note:
a) Contrast entomophilous adaptations clearly with anemophilous (wind-pollinated) features.
b) Munch's hypothesis is the most accepted model for translocation in phloem.

 

Section - C
(Answer any two questions)

 

Question 8.
(a) Give any four reasons for Mendel's success. [4]
(b) Briefly describe the technique employed in DNA fingerprinting. [4]
(c) Give any two features of Genetic Code. [2]

Answer:
(a) Reasons for Mendel's success:
1. He selected pure-breeding varieties of garden pea (*Pisum sativum*).
2. He studied one or two characters at a time, avoiding confusion from multiple traits.
3. He kept meticulous and quantitative statistical records of all offspring.
4. The traits he selected showed clear dominance and were located on different chromosomes (avoiding linkage complications).
(b) DNA Fingerprinting (Southern Blotting technique steps):
1. Isolation of DNA from sample cells (blood, hair, semen, etc.).
2. Digestion of DNA using restriction endonucleases into fragments.
3. Separation of DNA fragments by size using gel electrophoresis.
4. Transfer (Southern blotting) of separated DNA fragments to a nitrocellulose or nylon membrane.
5. Hybridization using radioactive VNTR probes followed by autoradiography to reveal specific band patterns.
(c) Features of Genetic Code:
1. The code is a triplet code (three adjacent bases form a codon specifying one amino acid).
2. The code is universal across almost all living organisms.

Teacher's Note:
a) Alec Jeffreys pioneered DNA fingerprinting using VNTRs.
b) Ensure all steps of Southern blotting are listed in chronological order.

 

Question 9.
(a) Explain the mechanism of action of T-cells to antigens. [4]
(b) Explain how insulin can be produced using recombinant DNA technology. [4]
(c) What is pisciculture? Give one advantage. [2]

Answer:
(a) When T-lymphocytes encounter antigens, they proliferate into clones consisting of specialized T-cells:
1. Helper T-cells: Secrete lymphokines that stimulate B-lymphocytes and other T-cells.
2. Killer (Cytotoxic) T-cells: Bind directly to infected cells and secrete perforins, creating holes that lyse the target cells.
3. Suppressor T-cells: Inhibit immune responses once the pathogen is eliminated.
4. Memory T-cells: Retain antigen memory for rapid secondary immune responses.
(b) Production of insulin via r-DNA technology:
1. DNA sequences corresponding to human insulin chains A and B are chemically synthesized.
2. These genes are inserted into bacterial expression plasmids (vectors) adjacent to a beta-galactosidase gene.
3. Plasmids are transformed into host bacteria like *Escherichia coli*.
4. Bacterial cultures synthesize the fusion protein, which is subsequently harvested, cleaved using cyanogen bromide, and purified to obtain active human insulin.
(c) Pisciculture is the rearing, cultivation, and management of fish in controlled water bodies. Advantage: It serves as a rich source of cheap animal protein, essential vitamins (A and D from fish liver oil), and minerals like iodine.

Teacher's Note:
a) Humulin was the first commercial genetically engineered pharmaceutical product produced by Eli Lilly in 1980.
b) Clearly distinguish cell-mediated immunity (T-cells) from humoral immunity (B-cells).

OR

(b) Insulin Production r-DNA technology
1. A DNA fragment encoding each insulin chain was made by annealing two complementary oligonucleotides that had been chemically synthesised.
2. Each fragment was ligated into a bacterial expression vector such that during translation the insulin chain would be fused to the carboxy terminus of the beta-galactosidase enzyme.
3. The expression vectors were transformed into *E. coli* and the beta-insulin fusion proteins accumulated inside the bacterial cells.
4. The cells were harvested and each beta-gal-insulin fusion protein was purified.
5. The insulin coding DNA was synthesised so that it started with a methionine codon, providing a cleavage site.
6. Treatment of the fusion protein with cyanogen bromide (CNBr) cleaved the peptide bonds after the methionine, yielding recombinant human insulin.

Teacher's Note:
a) This alternative method uses fusion proteins to protect the short insulin chains from bacterial degradation.
b) Cyanogen bromide specifically cleaves peptide bonds at methionine residues.

 

Question 10.
(a) Name the causative organism and preventive measures for each of the following : [4]
(i) Swine flu
(ii) Typhoid
(iii) Filariasis
(iv) Syphilis
(b) State four causes and four consequences of population growth. [4]
(c) Differentiate between: [2]
(i) Cannabinoids and Barbiturates
(ii) Biotic potential and Carrying capacity

Answer:
(a) Causative organisms and preventive measures:

Disease	Causative Organism	Preventive Measures
(i) Swine flu	Influenza virus (H1N1)	Wearing masks, avoiding contact with infected patients, vaccination.
(ii) Typhoid	*Salmonella typhi* (Bacteria)	Consuming clean drinking water, proper sanitation, and hygiene.
(iii) Filariasis	*Wuchereria bancrofti* (Helminth)	Mosquito control, using mosquito nets, preventing vector breeding (*Culex*).
(iv) Syphilis	*Treponema pallidum* (Spirochete Bacteria)	Safe sexual practices, avoiding multiple sexual partners.

(b) Causes of population growth: Decrease in death rate, increase in average life span, better medical facilities, and control of fatal epidemics. Consequences of population growth: Overstretching of natural resources, severe unemployment, food scarcity and malnutrition, housing shortages, and lowering of educational and health standards.
(c) Differences:
(i) Cannabinoids: Hallucinogenic chemicals obtained from *Cannabis sativa* that affect the cardiovascular system and perception. Barbiturates: Synthetic drugs acting as central nervous system depressants, used as sedatives and hypnotics.
(ii) Biotic potential: The maximum reproductive capacity of an organism under optimal environmental conditions. Carrying capacity: The maximum number of individuals of a population that a given environment can sustainably support.

Teacher's Note:
a) Tabular presentation for disease-causing organisms ensures clarity and avoids loss of marks.
b) Distinguish clearly between biotic potential (unlimited potential growth) and carrying capacity (environmental limit).