Oswal Practice Papers CBSE Class 12 Biology Solutions (Practice Paper - 8)


1. (c) Synergids

Explanation :    

The synergids have special cellular thickening at the micropylar tip called filiform apparatus, which plays an
important role in guiding the pollen tube into the synergids.

2. (b) exine, sporopollenin, germ pores

Explanation :    

Pollen grain has a two-layered wall i.e., exine and intine. The hard outer layer of pollens, named exine, is made of sporopollenin. It has prominent apertures called germ pores where sporopollenin is absent.

3. (d) the alleles of two genes are segregating independently.

Explanation :    

Cross involving two contrasting characters is called a dihybrid cross. The two flowers of each trait assort at random and independent of their traits and get randomly as well as independently rearranged in the offspring.

4. (b) Phosphodiester bonds

Explanation :    

In DNA molecule, the nucleotides are linked by phosphodiester bonds whereas the nitrogen bases are held by hydrogen bonds.

5. (d) II and IV

Explanation :    

Influenza, is an infectious disease caused by influenza viruses. Cancer is a non-infectious disease. Smallpox is a serious, highly contagious disease. It is caused by the Variola virus. Allergy is the exaggerated response of the immune system to certain antigens. It is a non-infectious response.

6.(b) CO2

Explanation :    

The dough which is used for making foods such as dosa and idli is fermented by using bacteria like Saccharomyces. The puffed-up appearance of this dough is due to the production of CO2 gas.

7. (d) 100 per hour

Explanation :    

Population of Paramecium shows 100 per hour growth, i.e. two individuals are produced by one.

8. (d) (A) - (ii), (B) - (iii), (C) - (iv), (D) - (v)

Explanation :    

The cutting of DNA by restriction endonucleases results in fragments of DNA. These fragments are separated by technique known as gel-electrophoresis. In colony hybridisation, selected cells with desired genes are grown in culture. In gradient centrifugation, cells/DNA are spinned at very high speed to purify DNA. PCR is amplification or gene clonning process used to amplify gene in large amount.

9. (c) A symbiotic relationship with the algae

Explanation :    

Lichens are a well-known combination of an algae and a fungus where the fungus has a symbiotic relationship with the algae. The fungus depends on the algae for food, as fungi cannot perform photosynthesis. In return, they provide shelter to the algae.

10. (d) (C)

Explanation :    

In forest ecosystem, it is a partly upright of spindle shaped because producers are lesser in number and support a greater number of herbivores and which in turn support a fewer number of carnivores. After that the number goes down at each successive level.

11. (d) single stranded RNA

Explanation :    

A’ refers to single stranded RNA. HIV is a retrovirus having a rounded outline. The core has two single strands of genomic RNA, enzyme reverse transcriptase, protein P-15 associated with genomic RNA, inner covering of P-24 and outer covering of P-17.

12. (b) I, II and III are true
13. (d) A is false but R is true.

Explanation :    

Tapetum is the innermost wall layer of a microsporangium. It nourishes the developing pollen grains. The tapetal cells enlarge radically and become filled with dense protoplasmic contents as well as nutrients. Microsporogenesis refers to the process of formation of haploid microspores mother cell or pollen mother cell through meiosis. Thus, assertion is false, but reason is true.

14. (b) Both A and R are true and R is not the correct explanation of A.

Explanation :    

In Mirabilis jalapa, red and white-coloured flowers are seen. When they are crossed, in the F1 generation, all flowers are pink coloured. This is because of the incomplete dominance of red-coloured flowers. Thus, both assertion and reason are true but reason is not the correct explanation of asssertion.

15. (c) A is true but R is false.

Explanation :    

Malignant malaria caused by Plasmodium falciparum is the most serious one and can even be fatal. The parasites initially multiply within the liver cells and then attack the red blood cells (RBCs) resulting in their rupture. The rupture of RBCs is associated with the chill and high fever recurring every three to four days. Thus, assertion is true but reason is false.

16. (b) Both A and R are true and R is not the correct explanation of A.

Explanation :    

Fleming obtained an extract from the mold, naming its active agent penicillin and penicillin was the first antibiotic. Thus, both assertion and reason are true but reason is not the correct explanation of the assertion.


17. (a) (i) Micropyle : It is small opening at the apex of the integuments in the ovule. It serves the function of providing entry to the pollen tube containing male gametes, into the ovule.
(ii) Integuments : Nucellus is surrounded by one or two cellular coats called integuments.

(b) The function of the antipodal cells is to provide nourishment to the egg cell. It is rich in lipid content. They also form the basis of endosperm production. The synergids consist of filiform apparatus which guides the pollen tube and the pollen tube discharges its content into one of the synergids.

18. If the father has blood group A and mother has blood group B, then the possible genotype of the parents will be: Father–Mother: IA IA / IAi — IB IB/IB

(a) A cross between homozygous parents will produce progeny with AB blood group.

AB Blood Group

(b) A cross between heterozygous parents will produce progenies with AB blood group (IAIB), O blood group (ii), A blood group (IAi) and B blood group (IBi).

heterozygous parents

19. According to the symptoms given, the disease that the patient is suffering from is amoebiasis. The pathogen responsible for this disease is Entamoeba histolytica. The pathogen lives in the large intestine of the human body. The pathogens are carried by houseflies from one place to another and are also directly contacted by the usage of contaminated water and food.

20. (a) Figure I is proinsulin. Proinsulin is the precursor molecule containing three polypepride chains known as the chain A, B and chain C. The mature insulin part contains chain A and B.

(b) The active insulin is formed through the maturation process when the precursor molecule proinsulin undergoes maturation through the action of endopeptidase known as prohormone convertase.

21. (a) W1 = Woert
express exponential growth in plants
W1 = final size, Wo = Initial size
r = growth rate, t = time of growth
and e = base of natural logarithm 

(b) The size of the population is regulated by the factors like availability of food, predation pressure, adverse climatic conditions and competition. All these entities are dynamic in nature. Also the four processes that regulate the growth of population are natality, mortality, immigration and emigration.
So, all these entities are never static. The size of the population changes as according to these factors, hence it is never static.


Death rate = Number of deaths / Total populations
Death rate = 8/80 = 0.1 individuals per butterfly per week.


22. (a) Germ pore is a prominent aperture on exine of the pollen grain. It is the place where sporopollenin is absent.
Function: The contents of the pollen grain move into the pollen tube through the germ pore.

(b) Sporopollenin is a major component of the tough outer (exine) walls of plant spores and pollen gains. It is chemically very stable and is usually well preserved in soils and sediments.
Function: Sporopollenin can withstand high temperatures and strong acids and alkali.
Pollen grains are well preserved as fossils because of the presence of sporopollenin. 

23. (a) It is responsible for secretion of progesterone during menstrual cycle. High level of progesterone inhibits FSH and LH and hence prevents ovulation.
(b) It is an inner lining of a uterus. This part undergoes changes during menstrual cycle and gets ready for implantation of an embryo.
(c) It is a cap like structure on the head of the sperm. It contains hyaluronidase enzyme which hydrolyses the outer membrane of an egg and allows the entry of the sperm.

24. (a) The diagram explains about the steps in PCR technique to amplify the gene of interest.
(b) At the end of PCR the result is an amplified DNA segment at the rate of 1 billion copies. Now, the amplified fragment can be used to ligate with the vector for further cloning.
(c) These are small, chemically synthesized oligonucleotides which are complementary to the regions of DNA.

25. (a) The two alternative factors (alleles) of a pair representing a character are separated from each other at the time of gamete formation, so that a gamete contains only one factor (allele) for the character and paired condition is restored during fertilisation.
(b) The colouration of human skin follows quantitative inheritance. It is regulated by at least three genes. There can be seven allelic combinations in the gametes of a person heterozygous for all three genes. Therefore, as a result, sixty four different combinations of colours are possible. On the
other hand, the colour of the flower in pea is controlled by non-allelic complementary genes, which independently shows a simple effect.

26. An Operon is a part of DNA which acts as a single regulated unit of one or more structural genes, an operator gene, a promoter gene, a regulator gene, a repressor and an inducer or a compressor gene. In lac operon when there is an addition of lactose, it enters the cell with the help of permease. The lactose binds itself to active repressor and changes its structure. The repressor now fails to bind to the operator. Then the RNA polymerase starts the transcription of the operon by binding to the promoter site. All the three enzymes for digestion of lactose molecules are produced in the whole process of the induction. It can be understood with the help of the given diagram.


After some time, when the entire lactose is consumed, there is no inducer present so as to bind to the repressor. So the repressor once again activates and attaches itself to the operator and finally switches off the operon.


(a) The arrangement where, a polycistronic structural gene is regulated by a common promoter and regulatory gene. This pattern of arrangement is known as operon. Example: lac operon, trp operon, ara operon, his operon, val operon.

(b) Here, the lactose acts as an inducer. It binds with the repressor protein, free operator gene. Further RNA polymerase freely moves over the structural genes transcribing the lac mRNA which in turn produces the enzymes that are able to digest the lactose.

27. (a) The above given diagram is of the E. coli cloning vector pBR322 depicting the restriction sites.
(b) A is Hind III and B shows BamH1 which are the restriction sites on the vector.
(c) Rop codes for the proteins that are involved in the replication of the plasmid.

28. (a) A larger biomass or large volume of culture can be processed leading to higher yields of desired specific products (proteins or enzymes), under controlled conditions.
(b) Stirring type of biorector is most commonly used.
(i) Mixing of reactor contents evenly with an agitator system or a stirrer.
(ii) Facilitates O2 availability.
(iii) Temperature, pH and foam are controlled under optimum conditions. 


29.(a) The above representation shows the sex chromosomes in male and female.
(b) Henking carried out experiments on certain entomological species (Insects). During his experiments he focused on the nuclear structure during the process of spermatogenesis. He observed that only fifty percent of sperms received this nuclear structure through spermatogenesis. But he was unable to prove the significance of this nuclear structure and he termed it as “X Body”.
(c) Grasshopper is an example of XO type of sex determination where, males possess single X chromosome whereas females have pair of X chromosome.


(c) The mechanism followed by human beings in determining the sex is of XY type. Out of 23 pairs of chromosome present, 22 are autosomes. A pair of X chromosomes is present in females on the other hand; X and Y chromosome are present in males. If ovum fuses with sperm carrying X chromosome
then the offspring will be a female (XX) and if the ovum fuses with the sperm carrying Y chromosome then the offspring will be male (XY). Hence in human beings, father is said to be the sex determiner and not the mother.

30. (a) The chemical structure is of Morphine.
(b) Heroin or Smack.
(c) It is crystalline in nature, bitter in taste, white in colour and is odourless.


(c) It is obtained from Papaver somniferum, commonly called as poppy plant.


31. Pollination is a mechanism by which pollen grains are transferred to the receptive stigma of same or another flower, located on same or another plant. Pollination can be categorised into three major types.


(a) Geitonogamy: It is the transfer of pollen grains from anther to receptive stigma of another flower of the same plant. Though it functionally means cross pollination, but it is genetically similar to autogamy because pollen grains are from the same plant on which stigma is present.

(b) Autogamy: It is a type of pollination that enables pollen grains to transfer to stigma of same flower. Autogamy may be further categorised as:

  1. Chasmogamy: It is transfer of pollen grains from anther to stigma of other species in which both anther and stigma are open and exposed. For example: Oxalis and Commelina.
  2. Cleistogamy: It is transfer of pollen from anthers to receptive stigma present in a flower that does not open at all. Such flowers produce assured seed set even in absence of the pollinating agents.

(c) Xenogamy: It is a kind of pollination where pollen grains from anther to stigma are transferred on the flower present on another plant. It is the only kind of pollination that assures fusion of genetically different anthers and stigma.


Basis Spermatogenesis Oogenesis
(a) Time of initiation of the process. At puberty During foetal or embryonic state.
(b) Site of completion of the process Semininferous tubule. Ampullery isthmic juction of fallopian tube.
(c) Nature of meoitic division undergone by gamate mother cell. Equal cell division.
Continuous cell division.
Formation of 4 spermatide.
Unequal cell division.
Arrested at early embryonic stage.
Formation of one egg.

32. (a) E. coli prefers to use glucose as an energy source when both glucose and lactose are available. Lactose is an alternative energy source that can be used if glucose is absent. The overall rate of messenger RNA synthesis from the lac operon, and from other operons is indirectly regulated by the
concentration of glucose in the cell. cAMP is the second messenger used directly in the regulation of lac operon expression in response to changing levels of glucose.

The lac operon consists of :

  1. Regulatory gene i - It codes for the repressor protein.
  2. Structural gene

Normally, the lac operon is turned off. A repressor protein binds the operator (control) region upstream of the operon preventing transcription. When lactose is present outside the cell, it crosses the cell membrane and acts as an inducer of the operon. It does so once lactose is broken down to create allolactose. The lac operon is then membrane facilitating lactose transport into the cell, and b-galactosidase, which eats up lactose to make glucose moleclues and beta-galactosidase also makes allolactose. This leads to a positive feedback loop.

(b) (i) z gene — It codes for beta-galactosidase which catalyzes the hydrolysis of lactose into glucose and galactose.
(ii) y gene — It codes for permease which regulates the lactose permeability in the cell.
(iii) a gene — It codes for transacetylase which assists the enzyme betagalactosidase.


In birds-ZZ-ZW type


In Birds, the females are heterogametic (ZW) and produce two types of gametes whereas in Humans the males are heterogametic (XY) and produce two types of gametes.

33. (a) Population: Total number of organisms of a species in a particular area at a particular time.
(b) The size of a population for any species is not a static parameter because of factors like:

  • Birth Rate or Natality: Number of births during a given period.
  • Death Rate or Mortality: Number of deaths during a given period.
  • Immigration: Number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration.
  • Emigration: Number of individuals of the population who have left the habitat and gone elsewhere during the time period under consideration.


(a) Chemotrophs are the organisms that manufacture organic compounds from simple inorganic compounds without using sunlight.
(b) Energy flow in an ecosystem is always unidirectional or one way, i.e., solar radiations – producers – herbivores – carnivores. It cannot pass in the reverse direction. There is a decrease in the content and flow of energy with the rise in trophic levels.
(c) Photosynthetically active radiation (PAR) is light of wavelengths 400-700 nm and is the portion of the light spectrum utilised by plants for photosynthesis. 2-10 % of PAR is captured by plants.

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