Oswal Specimen Papers ISC Class 12 Biology Solutions (Specimen Paper - 7)

Section-A 

1. (i) When the number of individuals added to the population becomes equal to the number of individuals lost from the population with in a time period, then the size of the population will remain constant. This is known as zero population growth rate.
(ii) Distillation increases the alcohol content in alcoholic drinks.
(iii) The flower A is chasmogamous flower having exposed anthers and stigma whereas B is cleistogamous flower which do not open at all.
(iv) The initial population at time 0 hr, Pi = 50.
The final population after time 1 hr, PF = 150.
Growth rate of the population = (PF − Pi) ÷ Time
= (150 – 50) ÷ 1
= 100 per hour. 
(v) Cheese is partially degraded flavoured milk curd formed by the activity of micro-organisms. Processed cheese is formed by blending and emulsifying different types of cheese with or without cream.
(vi) a is exponential growth curve or J-shaped curve b is logistic growth curve or S-shaped curve.
(vii) Ti plasmid of Agrobacterium tumefaciens and Ri plasmid of A. rhizogenes have been effectively used as vectors for gene transfer to plant cells.
(viii) The mode of interaction between two species where one is harmed and the other is unaffected is called amensalism.
(ix) (c) its gene pool will be lost for ever.

Explanation :    

A gene pool comprises all the genes and their various alleles in a population of plant or animal species.
When the Bengal tiger becomes extinct, its gene pool will be lost until the end of time. If the Bengal tiger becomes extinct, the total species is lost forever since it is the genes and chromosomes that determine the species. Hyenas and wolves’ populations will increase as the Bengal tiger will not eat them. The wild area is already safe for domestic men and animals if they don’t interfere with the resources of the Bengal tiger.

(x) (d) II and IV

Explanation :    

Influenza, commonly known as the ‘flu’ is an infectious disease of birds and mammals caused by influenza viruses.
Cancer is defined as an uncontrolled division or proliferation of cells without any differentiation. It is a non-infectious disease caused by agents called carcinogens. Smallpox is a serious, highly contagious, and often life-threatening disease characterised by a rash and (blisters) on the face, arms and legs. Allergy is the exaggerated response of the immune system to certain antigens in the environment (pollen, dust, mites, moulds, cloth fibres, animal hair, etc.). It occurs due to the release of chemicals like histamine and serotonin from the mast cells. It is a non-infectious response.

(xi) (a) Both assertion and reason are true and reason is the correct explanation of assertion.

Explanation :    

RNA polymerase also known as DNA-dependent RNA polymerase, is an enzyme that produces primary transcript RNA. It catalyses polymerisation of ribonucleotides only in one direction, i.e., from 5′ → 3′. So, during transcription, only one of the two DNA strands acts as a template. This strand is called the template strand. It has 3′ → 5′ polarity. Thus, both assertion and reason are true and reason is the correct explanation of assertion.

(xii) (b) Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.

Explanation :    

Streptococcus pneumonia and Haemophilus influenza are responsible for causing bacterial infectious disease in human beings. Streptococcus pneumoniae causes pneumonia and H. influenza type b causes bacteremia, pneumonia, epiglottis, and acute bacterial meningitis. These are respiratory disorders and are transmitted through droplet infection. A healthy person acquires the infection by inhaling the droplets released by an infected person.

(xiii) 1. Calotropis plant produces very toxic chemicals called cardiac glycosides that affect the heart; hence cattle avoid browsing on this plant.
2. One example of the Calotropis plant is Giant Milkweed.

(xiv) (a) Trophic level
(b) Food web

(xv) Leguminous plants possess root nodules where nitrogen is fixed by the symbiotic nitrogen-fixing bacteria Rhizobium which fertilises the soil.

(xvi) (a) Many organism occupy position in different food chains and several food chains become inter connected to form a food web.
(b) It is because the biomass available to the consumer for the consumption is a resultant of the primary productivity from plants.

(xvii) G ≡ C, T = A or A = U base pairs of the two nucleotides on the opposite complementary DNA strands is connected via hydrogen bonds.

(xviii) Given cloning vector name is pBR322 and it is inserted in E. coli bacteria.

Section-B

2. (i) (a) (a) – Spermatogonia, (b) – Interstitial cells. (c) – Spermatozoa.
(b) ‘a’ The spermatogonia or male germ cells undergo meiotic division leading to sperm formation.
‘d’ sertoli cells provide nutrition to the germ cells.

OR

(ii) (a) e – Spermatozoa.
(b) f – Sertoli cell. It provides nutrition to the germ cells.
(c) Seminiferous tubule.

3. Synergids have following functions :

  1. Absorption of nourishment from nucellus.
  2. Secretion of chemotaxically active substances from its filiform apparatus for guiding pollen tube.
  3. Act as shock absorber for ingrowing pollen tube.
  4. Forms seat for pollen tube discharge.

4. (i) Each antibody molecule consists of two light chains and two longer chains called heavy chains. So to represent the number of chains it is designated as H2L2.
(ii) It is the response given by the antibodies that are found in blood, such responses are called Humoral Immune Response. 

5. (i) Figure I is proinsulin. Proinsulin is the precursor molecule contains three polypeptide chains known as the chain A, B and Chain C. The mature insulin part contains chain A and B.
(ii) The active insulin is formed through the maturation process when the precursor molecule proinsulin undergoes maturation through the action of endopeptidase known as pro hormone convertases.

6. The benefits that LAB provide are:

  1. It increases the nutritional quality of curd by increasing vitamin B12 content.
  2. It checks the growth of disease-causing organisms in the gut.

7. The methods for calculating diversity are:

  1. Alpha diversity refers to diversity within a particular area, community or ecosystem and it is typically measured as the number of species within that area.
  2. Beta diversity is the species diversity between areas and involves comparing the number of species that are unique to each area. 

8. In a population of diploid organisms,
If frequency of allele A = p and frequency of allele a = q, then the expected genotype frequency under random mating are:
AA = p2 (for AA homozygotes)
aa = q2 (for aa homozygotes)
Aa = 2pq (for Aa heterozygotes)
In the absence of selection, mutation, genetic drift or other forces, p and q are constant through generations.
Therefore, p2 + 2pq + q2 = 1. 

Section-C

9. (i) Grains contain a very small amount of water and remain dormant over a prolonged period. They are the source of protein, starch, and oil reserves.
(ii) The green colour of young fruits helps them to remain hidden and protected from hostile animals. This also helps to protect immature seeds.
(iii) The quiescent seeds can live for many years because they have low water content and hence no metabolic activity. They regain their activity and germinate when conditions are favourable.

10. (i) (a) Endothecium (b) Middle layer (c) Tapetum.
(ii) Tapetum provides nourishment to the developing pollen grains. The tapetal cells secrete ubisch granules that provides sporopollenin and other materials for exine formation.

11. (i) (a) Exponential growth: It is represent by plotline ‘a’.
(b) Logistic growth: It is represented by plotline ‘b’.

(ii) 

S. No. J-Shaped Curve S-Shaped Curve
1. It is divided into three phases i.e., Lag phase, Exponential phase and Senescence. Last phase stops abruptly due to mass starvation and mortality. It has three phases i.e., carry phase of no growth, middle phase of rapid growth and stationary phase of zero growth.
2. When the food supply ceases then mass starvation takes place leading to death. When the food supply ceases, then growth becomes stationary i.e., newly produced cells are approximately equal to the number of dying cells.

12. (i) After each cycle the number of duplexes doubles itself thus after the first cycle there are 2 DNA duplexes. A duplex has 2 DNA strands thus after second cycle there will be 4 Duplexes, after third cycle there will be 8 DNA duplex. Lastly, after the 4th cycle there will be 16 duplexes.
(ii) Amplification is usually carried out by the DNA polymerase I enzyme from Thermus aquaticus. As this organism lives in hot springs, Taq polymerases are thermostable thus they are resistant to high temperature.

Answer

13. (i)

antifoam

OR

(ii) 

origin of resistance

14. (i) 4 linkage groups are present in Drosophila melanogaster.
(ii) Centimorgan is the distance between the linked genes on the same chromosome.
(iii) Homologous chromosomes held together at Chiasmata.

15. 1. The principle of segregation states that an organism possesses two alleles for any one particular trait and that these alleles separate during the formation of gametes. In other words, one allele goes into each gamete.
2. The principle of segregation is important because it explains how the genotypic ratios in the haploid gametes are produced.

Section-D

16. (i) Reproductive health refers to the total well-being in all aspects of reproduction i.e., physical, behavioural, psychological, and social.
(ii) The basic aims of the RCH programmes are creating public awareness regarding reproduction-related aspects and providing facilities to build up a healthy society with added emphasis on the health of mother and child.
(iii) Natural method of contraception:
1. Periodic abstinence
2. Coitus interruptus

17. (i) Significance: The Meselson-Stahl experiment supported Watson and Crick’s hypothesis that DNA replication was semi-conservative. Each of the two new molecules has one strand of new material and one strand of old material.
This enabled the researcher to explain how DNA replicates thereby providing a physical basis for the genetic phenomenon of heredity and diseases, and how organisms preserved and transferred genetic information of DNA to their offspring.
Based on the result, Meselson and Stahl made three conclusions:

  1. Each DNA molecule divided evenly between the two subunits of DNA, and the subunits stayed intact throughout the observed replication cycles.
  2. If a parent passes on one subunit of DNA to its offspring, then half of the parental DNA is conserved in the offspring DNA, and half of the parental DNA is not.
  3. For every parental DNA molecule, two new molecules were made. Therefore, the amount of DNA after each replication increased by a factor of two.

OR

(ii) Semiconservative replication of DNA was proved by the work of Mathew Meselson and Franklin Stahl (1958). They grew Escherichia coli for many generations in a medium having a heavy isotope of nitrogen 15N, till the bacterial DNA became completely labelled with a heavy isotope. The labelled bacteria were then shifted to a fresh medium having normal or 14N nitrogen. Samples were taken for each generation and the DNA was tested for the heavy isotope of nitrogen through density gradient centrifugation using caesium chloride. They found that the DNA of the first generation was hybrid or intermediate between 15N and 14N.

The second generation of bacteria contained two types of DNA, 50% light and 50% intermediate. This observation is possible only if the two strands of DNA duplex separate at the time of replication and act as a template for the synthesis of new complementary strands of DNA having normal or 14N. This will produce two DNA duplexes with one old strand (15N) and one new strand (14N).
During the formation of second-generation, 15N and 14N strands of DNA duplex separate to function as templates so that 50% of new DNA duplexes possess only normal or 14N strands while another 50% have both 15N and 14N strands. 

18. (i) Coat A is made up of protein.
(ii) The enzyme B is reverse transcriptase, which transcribes ‘X’-viral RNA to produce ‘C’- viral DNA.
(iii) The host cell ‘D’ is a macrophage.
(iv) The new virus ‘E’ subsequently attack macrophage and helper T-lymphocytes.

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