Students can access the CBSE Sample Papers for Class 12 Biology with Solutions and marking scheme Term 2 Set 7 will help students in understanding the difficulty level of the exam.
CBSE Sample Papers for Class 12 Biology Standard Term 2 Set 7 with Solutions
Time Allowed: 2 Hours
Maximum Marks: 40
General Instructions:
- All questions are compulsory.
- The question paper has three sections and 13 questions. All questions are compulsory.
- Section-A has 6 questions of 2 marks each; Section-B has 6 questions of 3 marks each; and Section-C has a case-based question of 5 marks.
- There is no overall choice. However, internal choices have been provided in some questions. A student has to attempt only one of the alternatives in such questions.
- Wherever necessary, neat and properly labeled diagrams should be drawn.
Section – A
Question 1.
After getting vaccinated, you develop immunity to that disease, without having to get the disease first. How does a vaccine for a particular disease immunise the body against that disease ?
Answer:
A vaccine is a suspension of killed or attenuated pathogenic microorganisms which act as antigen. This, when administered in the body, leads to the stimulation of T-cells and B-cells which produce antibodies for fighting the antigenic preparation. In the process, some T-cells form memory cells which may fight the infection if occurred in future.
Question 2.
The graph below compares the primary and secondary immune responses to the same pathogen. The secondary response may eliminate the pathogens before any damage occurs.
In reference to the graph above, which peak A or B represents secondary response?
OR
Biofertilizers are living microbes that enhance plant nutrition by either by mobilizing or increasing nutrient availability in soils. How does mycorrhiza act as biofertilizer? Explain. Name a genus of a fungus that forms a mycorrhizal association with the plants
Answer:
PeakB
During the primary immune response to the first encounter with a specific pathogen, some lymphocytes called memory cells develop with the ability to confer long-lasting immunity to that pathogen, often for life. These memory cells recognise antigens on the pathogens they have encountered before, triggering the immune system to respond faster and more effectively than on the first exposure. Hence, the response is more effective as the antibodies produced in second exposure are much larger as compared to the first exposure to same antigen.
OR
The mycorrhiza is a symbiotic association between the fungus and the roots of higher plants. Such fungi in the association tend to absorb phosphorus from the soil and pass it on to the plant for completing its metabolic and catabolic processes. The genus Glomus forms a mycorrhizal association with the plants.
Question 3.
What is the function of organ labelled A?
Answer:
Site of T-lymphocyte differentiation.
The thymus is a lymphoid organ located in the mediastinal space. Thymus is the site of T-lymphocyte differentiation. The mature T-cells leave the thymus and migrate to secondary lymphoid organs like the spleen, lymph nodes, and other lymphoid tissues where they control cell-mediated immune responses. Thymus provides micro-environment for development and maturation of T-lymphocytes.
Question 4.
At which point of time in the graph is when the population size reaches the carrying capacity for ecosystem?
Answer:
Carrying capacity refers to the maximum size of population of a species that a certain environment can support for an extended period of time. In given graph, 12 represent the point with maximum population size.
Question 5.
What is an ecological principle behind the biological control method of managing with the insects?
Answer:
The basic principle behind the biological control method is predation. Predation is an act of capturing and preying on the other organism. This maintains the population of prey. Accordingly, the insect pests are controlled by using another insect that prey on them. For example, larvae of green lace wing feeds on the aphids reducing their population to the minimum.
Question 6.
Certain animal species maintain homeostasis escaping the time period of unfavourable stress. Such animals are unable to migrate to another habitat. How do organisms like fungi, zooplanktons and bears overcome the temporary short-lived climatic stress conditions? Explain.
OR
What were the observations of Alexander Von Humboldt?
Answer:
The fungal species form thick-walled spores which help them survive in unfavourable conditions. On the availability of suitable environment, they germinate. The zooplanktons under stressful conditions enter diapause. The bears in extreme low temperature escape the winter by entering the state of hibernation.
OR
A German naturalist and a geographer named Alexander Von Humboldt quoted some observations during his exploration in the forests of South America. These observations were:
- Within a region, species richness increased with an increase in area but only up to certain limit.
- When plotted on graph the species area relation shows rectangular hyperbola.
- There are wide range of factors that determine the slope of curve namely, predation-prey dynamics, immigration, extinction and clustering of the species.
Section – B
Question 7.
The entry of an antigen is sensed by the B-cells. The B-cells become active and synthesize specific protein molecule called the antibody, which is secreted into the plasma. Upon encountering the antigen, the antibodies bind with the antigen molecules and deactivate them. This whole process takes from days to weeks during first exposure but will be rapid upon second exposure due to the memory which was built up during first encounter with the pathogen. This phenomenon of recognizing an antigen is called as immunization and this lead to the basis of vaccination.
Vaccination is the process by which an individual’s immune system is strengthened against a particular infectious agent.
(i) The condition where immune system mistakes part of body like tissues, joints, skin, as foreign and attack them is:
(ii) Vaccines set off immune response to pathogens by producing and storing:
(iii) The antibody most likely be found in mucus is:
OR
Any substance (other than food) that is used to prevent, diagnose, treat, or relieve symptoms of a disease or abnormal condition. Drugs can also affect how the brain and the rest of the body work and cause changes in mood, awareness, thoughts, feelings, or behavior. Name an opioid drug and its source plant. How does the drug affect the human body?
Answer:
(i) Autoimmunity: Autoimmunity is the disease when an immune system could not distinguish
between self and non self antigens and attack self cells. E.g., Rheumatoid arthritis, Multiple sclerosis. Type 1 diabetes mellitus
(ii) Antibodies: Vaccines reduce risks of getting a disease by triggering the natural defense system of body. It produces antibodies which fight immediately against pathogen and keeps the memory for future defense as well.
(iii) IgA: Immunoglobulin A (IgA) is found secretions of body such as saliva, tears, respiratory, gastrointestinal, and mucus.
OR
An example of opioid drug is heroin or smack or it is popularly known as morphine. It is obtained from the plant called poppy plant, scientifically known as Papaver somniferum. The drug action is that the opioid receptors bind to the central nervous system or the gastro-intestinal tract thereby slowing down the system.
Question 8.
Uncontrolled cell-division occurs when a process called contact inhibition fails. In healthy organisms, during this process, when cells come in contact with other cells, the process of cell replication ceases. How are cancerous cells different from normal cells?
Answer:
They are different from each other based on the following points:
- As compared to normal cells, cancerous cells undergo vigorous mitotic cell division.
- Contact inhibition is not seen in the cancerous cells which can be seen in normal cells.
- Cancerous cells lead to the formation of tumours.
- The cancerous cells possess short life as compared to normal cells which have long life.
- The cancerous cells show high invasiveness that is not with the normal cells.
Question 9.
Write a brief note on restriction enzymes. Explain the mechanism of action of restriction enzymes.
Answer:
The restriction enzymes act as molecular scissors that cut the DNA segment at the specific sites. The discovery of two such enzymes was made in 1963, one of which one was identified later as restriction endonuclease. The first restriction endonuclease was Hind II, the function of which is to recognise the specific sequence of six base pairs and cut the segment at that location.
The restriction enzyme belongs to a larger class named nucleases. The two types of such enzymes are endonuclease and exonucleases. The exonucleases play a role in removing the nucleotides from the ends of DNA. It also inspects the length of DNA sequence. The endonuclease makes a cut at specific positions of DNA fragment.
Once the restriction endonuclease finds a specific location, that has a capacity to recognise the palindromic nucleotide sequence, the enzyme gets bind to DNA and makes a cut at specific points in sugar-phosphate backbone of each of the two strands of the double helix.
After the cutting, the overhanging sticky portion plays its role. The sticky end allows joining of two counterparts of DNA strand with the help of another enzyme called DNA ligase. The cuts made by restriction endonuclease give similar sticky ends at both the strands. After cutting, separation and getting attached to the host DNA, the recombinant DNA is formed.
Question 10.
Biodiversity refers to variety of life that can be studied on many levels. Give three hypothesis for explaining why tropics show greatest levels of species richness.
Answer:
There are three different hypothesis proposed by ecologists for explaining species richness in the tropics, they are:
- As we come towards the tropical latitude, there is high species diversity as compared to temperate region because of receiving maximum solar energy.
- Tropical latitude faces less seasonal variations and have more or less a constant environment. This results in niche specialisation giving rich species diversity.
- The temperate regions undergo the process of glaciations, while the tropical regions have no such processes, which give rise to species richness in tropics.
Question 11.
(i) Mention the specific geographical region where these organisms are found.
(ii) Name and explain the phenomenon that has resulted in the evolution of such diverse species in the region.
(iii) Explain giving reasons the existence of Placental wolf and Tasmanian wolf sharing the same habitat.
Answer:
(i) Australia
(ii) Adaptive radiation (Divergent evolution). The process of evolution of different species in a given geographical area starting ,from a point and literally radiating to other areas of geography (habitats) is called adaptive radiation. It is the development of different function structures from a common ancestral form.
(iii) Placental wolf and Tasmanian Wolf share similar habitat due to convergent evolution and evolved into unrelated groups of organisms.
Question 12.
A cloning vector is a small piece of DNA that can be stably maintained in an organism, and into which a
foreign DNA fragment can be inserted for cloning purposes. Describe the characteristics a cloning vector must possess. Why the DNA cannot pass through the cell membrane? Explain. How a bacterial cell is made ‘competent’ to take up recombinant DNA from the medium?
Answer:
The cloning vector must possess following characteristic features:
(i) Origin of replication (ori): It is the initiation point from where the replication process starts and any piece of DNA when linked to this sequence can be made to replicate the DNA within the host cell. The ori is also responsible for controlling the number of copies of the linked DNA.
(ii) A selectable marker: The vector must have a selectable marker which will help it in identifying and eliminating the non-transformants and selectively permitting the growth of transformants only.
(iii) It must possess the cloning site or recognition site for a restriction enzyme to recognise.
As DNA is a hydrophilic molecule, it cannot pass through the cell membrane. The bacterial cell is made competent through treating it with the specific concentration of Ca2+ ions. Further incubation of the cells on the ice and providing short heat shocks can make the bacterial cell competent to take up the recombinant DNA available in the medium.
Section – C
Question 13.
Study the given figure showing production of Humulin and answer the questions that follows:
(i) Identify the steps A, B, C, D and E.
(ii) Active insulin consists of how many polypeptide chains?
(iii) The Insulin A vector does not contain …………………..
(iv) What are the shortcomings of insulin obtained from slaughtered animals? Write down its disadvantages.
OR
DNA amplification is a powerful technique that has had an immense impact on scientific research in the past 2 decades. While polymerase chain reaction (PCR) is still the most popular method, alternative methods of DNA amplification are constantly being developed. In addition, the extraordinary versatility of PCR has led to its use in novel ways that have opened new avenues of research. These novel methods for DNA amplification and the versatility of PCR are highlighted in DNA Amplification: Current Technologies and Applications.
(i) Expand VNTR and describe its role in DNA fingerprinting.
(ii) List any two applications of DNA fingerprinting technique.
(iii) How are DNA fragments visualised during gel-electrophoresis? What is elution?
Answer:
(i) A-Isolation of human insulin gene, B-Insertion of human insulin gene into plasmid, C-Formation of rDNA, D-Production of multiple copies of rDNA, E- Multiplication of transformed cells.
(ii) Active insulin consists of two polypeptide chains, the A chain and the B chain. The A chain consists of 21 amino acids and the B chain consists of 30 amino acids. The chains are joined together by two disulphide linkages.
(iii) The Insulin A vector does not contain B-chain, it contains the A chain. A separate pBR322 vector is needed for the production of the B-chain known as the insulin B vector.
(iv) Insufficient in quantity and contain antigens which can cause allergy.
Following are the disadvantages of insulin from slaughtered animals:
- Demand for insulin is very high which necessitates slaughtering of a large number of animals.
- Slaughtering of animals amounts to cruelty against animals.
- Insulin from animal origin can result in allergic reactions because of body’s tendency to encounter foreign substance.
- The slaughtered animal can be infected with some dangerous microbes. This can contaminate the
insulin.
OR
(i) VNTR – Variable Number of Tandem Repeats. It is used as a probe because of its high degree of polymorphism.
(ii) (a) DNA fingerprinting is used for paternity testing through the use of PCR technique which
produces the genetic fingerprint and is highly specific for each individual.
(b) It is used in the data security where DNA regions used for individual identification are specific isolated genetic loci in the non-coding regions of the genomic DNA.
(iii) To visualise the DNA, the gel is stained with a fluorescent dye that binds to the DNA, and is placed on an ultraviolet transilluminator that shows up the stained DNA as bright bands. The dye can also be mixed with the gel before it is poured. The DNA fragments glow, allowing to see the DNA present at different locations along the length of the gel.
In gel-electrophoresis, the separated bands of DNA are cut out from the agarose gel and extracted from the gel piece. This step is called elution.
