Solved Board Paper For CBSE Class 12 Biology

a) Prolactin

Explanation: Prolactin is a hormone produced by the pituitary gland. It stimulates breast development and milk production in women.

a) 1^st December
Explanation: World AIDS Day is celebrated on 1st December every year since 1988.

b) IgE
Explanation: Our immune system reacts to an allergen by producing antibodies known as Immunoglobulin E (IgE). These antibodies travel to cells that release chemicals, causing an allergic reaction.

b) Lymphocytes
Explanation: Lymphocytes are one of the main types of immune cells. Lymphocytes are of two types namely B and T cells. B lymphocytes produce antibodies.

b) 3’-GTTAAC-5’
Explanation: A palindromic sequence is a sequence of nucleic acids in a double stranded DNA and/or RNA wherein reading from 5' to 3' of one strand matches to 5' to 3' on the other complementary strand.

a) 0.88x10⁹ nm

Explanation: Increase in the distance between two consecutive bases=0.44 nm.

Total Base pairs in DNA double helix =2x10⁹

The length of DNA double helix in the presence of saturating amount of the compound X=0.44 nm x 2x10⁹ = 0.88 x10⁹ nm

a) Biosphere reserves
Explanation: The conservation of plants and animals outside their natural habitat is known as ex-situ conservation. Biosphere reserve is an example of in-situ conservation.

The advantages of micropropagation are as follows:
i. It facilitates the production of a large number of plants in a short duration.
ii. The plants produced through this technique are genetically identical.

An ideal contraceptive should possess following characteristics:
i. It should be user-friendly.
ii. It should be easily available.
iii. It should be effective with least side-effects.
iv.It should not interfere with sexual drive.

Thus, there are 50% chances of their son to be colourblind.

According to Chargaff’s rule, the DNA molecule should have an equal ratio of pyrimidine and purine. i.e.,

A=T and C=G

% A = % T

% G = % C

The given organism contains 18% of cytosine.

Then according to the law, it would have 18% of guanine.

Thus, percentage of G + C content = 36%

The remaining 64% represents both A + T molecule.

Since adenine and guanine are always present in equal numbers, the percentage of adenine molecule is 32%.

Agrobacterium tumefaciens is a pathogen of various dicot plants. It is able to deliver a piece of DNA known as “TDNA” to transform normal cells into tumour cells. This tumour-inducing (Ti) plasmid is used to deliver the gene of interest into a plant, where it multiplies.
For example: The nematode-resistant gene is introduced into the tobacco plant by using Agrobacterium vectors.

i. This is a pyramid of biomass.

ii. The amount of energy reduces at each trophic level in an ecological pyramid. Due to which, they often have three or four levels but seldom five.

Fertilisation refers to the process of fusion of gametes to form a diploid zygote. In angiosperms, two male gametes of a single male gametophyte fuse with two different cells/nuclei of the same embryo sac to produce two different structures. One male gamete fuses with the egg to form diploid zygote which gives rise to diploid embryo and another male gamete fuses with two haploid polar nuclei or diploid secondary nucleus to form triploid primary endosperm nucleus (3n) which gives rise to triploid endosperm. Thus, due to these two fusions that occur in an embryo sac of an angiosperm, the phenomenon is called double fertilisation.

Karyotype: It refers to the standard arrangement of the chromosome complement prepared for chromosome analysis.

Humans possess 46 chromosomes in total, which are arranged into 23 pairs. In this image, trisomy of chromosome 21 is shown. It leads to Down's syndrome, which is also called trisomy 21. It is caused by the presence of three copies of chromosome 21.

a)

b) Function of promoter gene:

The promoter gene is the region for binding of RNA polymerase. It acts by identifying and binding sites for RNA polymerase.

Inbreeding depression: Continued inbreeding especially close inbreeding reduces fertility and productivity. This is termed as inbreeding depression.

Following are the advantages of inbreeding:
i. It helps in the development of a pure line in any animal.
ii. It exposes harmful recessive genes which are eliminated by selection.

i. UAA is the 'stop' codon. Thus, polypeptide chain will not grow after 24th amino acid so, rest of codons will not be able to translate in the absence of new initiating codon.

ii. The number of distinct proteins containing ‘n’ amino acids = 20ⁿ . Hence, total number of proteins in this case = 20¹⁰⁰

iii. In DNA, 5400 nucleotides will form 2400/3=800 codons.

An average protein contains =100 amino acids

i.e. 100 codons form one protein

Therefore, 800/100=8 Proteins can be formed.

A virus-free healthy plant can be obtained from a diseased sugarcane plant through meristem culture. Although the plant is virus infected, the apical and axillary meristem is devoid of virus. In this technique, the meristem is removed from the plant and grown in-vitro by micropropagation. The medium rich in nutrients such as sucrose, vitamins and growth regulators like auxins, cytokinin is provided to the developing meristem.
A large number of virus-free plants are produced in a very short duration by employing this technique. These genetically identical new plants are called somaclones.

Bacillus thuringiensis is the source of Cry genes. CryIAc, CryIAb, and CryIIAb are isolated from Bacillus thuringiensis. These genes are beneficial for crops in the following ways:

• CryIAc and CryIIAb gene control the cotton bollworms while CryIAb controls corn borer.
• Cry gene codes for certain crystal protein (Bt toxin) that kills certain lepidopterans and dipterans.
• Bt toxin is an insecticidal protein. It exists in inactive state in bacteria but it becomes active upon entering into the body of insects due to the alkaline pH of the gut of insect. The activated toxin causes lysis and eventually death of the insect.

Biodiversity refers to the diversity of biological organisation ranging from cellular macromolecules to biomes.

Genetic level: It refers to the diversity shown by a single species at genetic level. E.g., Mango

Specific level: It refers to the diversity at species level. E.g., amphibian species

Ecological levels: It refers to the diversity at ecosystem level. E.g., Estuaries

Two types of desirable approaches to conserve biodiversity are:
i. In-situ conservation
ii. Ex-situ conservation
iii. In-situ conservation: It refers to maintaining species of wild animals and plants in their natural habitats. The examples of In-situ conservation include national parks and biosphere reserves.
iv. Ex-situ conservation: It refers to maintaining species and population diversity outside the natural habitats. The examples of ex-situ conservation include seed banks, botanical gardens, zoological gardens, aquaria etc.

Recombinant DNA technology involves following steps:

• Isolation of DNA
• Fragmentation of DNA by restriction endonucleases
• Isolation of the desired DNA fragment
• Amplification of the gene of interest
• Ligation of the DNA fragment into a vector using DNA ligase
• Transfer of recombinant DNA into the host
• Culturing of the host cells on a suitable medium on a large scale
• Extraction of the desired product
• Downstream processing of the product

i. Spermatogenesis occurs in seminiferous tubules in human testis.

ii. Process of spermatogenesis

iii. Path of spermatozoa:
Seminiferous tubules - rete testis - Vasa efferentia - Epididymis - Vas deferens - ejaculatory duct

i. In the aerobic digestion tank, microorganisms such as bacteria and fungi convert organic compounds to simple inorganic forms.

ii. Chlorine is commonly used as a disinfectant in most sewage treatment plants. It is used to sterilise the treated sewage before it is discharged out from the treatment plant. It kills microorganisms during water treatment and thus, maintains the quality of the water.

iii. Following are the steps of sewage treatment:

When untreated sewage is added into the water body, the nutrients present in it, dissolve into the water. Untreated sewage contains nitrates and phosphates, which allow the microscopic flora in water bodies to grow very rapidly. It results in a serious oxygen shortage in the water. Thus, the concentration of oxygen decreases in the water.

In the given image:

At point 1: BOD is high due to the respiration carried out by certain bacteria and fungi that feed on organic matter.

At point 2: BOD is less than at the point of entry of the pollutants. It is because, by this point, the amount of organic matter has decreased due to dispersal and break-down.

At point 3: BOD is just above normal. It is because pollutants are now much reduced.

At point 4: BOD back at normal point as all organic matter has been broken down.

(a) Parent: RR X RR

(Red coat) (Red coat)

Gamete: R R

Hence, all (100%) the offspring will bear red coat as both the parents are homozygous for the dominant allele (RR).

(b) Parent: RR X Rr

(Red coat) (Roan coat)

Gamete: R R, r

Hence, half (50%) of the offspring will bear red coat and the other half will bear roan coat, as one of the parents is homozygous for the dominant allele (RR) and other one is heterozygous for the trait (Rr).

(c) Parent: RR X rr

(Red coat) (White coat)

Gamete: R r

Hence, all (100%) the offspring will bear roan coat, as one of the parents is homozygous for the dominant allele (RR) and the other one is homozygous for the recessive allele (rr).

An operon is a part of DNA which functions as a gene regulatory unit in the process of transcription.

1. Regulatory gene: It controls the operator gene and produces a protein known as repressor. This repressor combines with the operator gene and stops its function.
2. Operator gene: It regulates the functioning of structural genes. Structural genes are expressed when operator gene is turned on by inducer while structural genes are not expressed when operator gene is turned off by repressor.
3. Promoter gene: It refers to the site at which the RNA polymerase binds and reaches the structural genes to initiate the transcription process.
4. Structural genes: These genes synthesise specific proteins.

If lactose is withdrawn from the culture medium the operon is not expressed. The repressor combines with the operator gene and turns it off. Due to this, structural genes are inactivated and protein synthesis or transcription stops.

a) Plant breeding refers to the purposeful manipulation of plant species to create desired plants that are better suited for cultivation, give better yields and are disease resistant. Classical plant breeding involves following two steps:

1. Hybridisation of pure lines
2. Artificial selection

Following are the limitations of traditional breeding technique:

1. It fails to keep pace with demand
2. It fails to provide fast and efficient system of crop improvement

b) Biofortification refers to breeding crops with higher levels of vitamins, minerals, fats or proteins to improve public health. For example, wheat variety Atlas 66 possesses high protein content and has been used as a donor for improving the quality of wheat.

Following events occur in human body to cause immunodeficiency when HIV gains entry into the body:

1. The virus enters the macrophages.
2. The RNA replicates and DNA is formed by reverse transcription.
3. The viral DNA gets incorporated into the host cell DNA and directs the infected cell to produce virus particles.
4. The macrophages continue to produce virus particles.
5. The virus then enters the helper T-lymphocytes, replicates and forms progeny viruses, which are released in the blood and attack other helper T-cells. It causes a progressive decline in the number of T-lymphocytes.
6. The decline in the number of T-lymphocytes causes immunodeficiency in the body of the affected person. The person is unable to protect himself/herself against any infection.

a) When untreated sewage and artificial fertilisers are added into the river water, the nutrients present in it, dissolve into the water. It causes an increase in the growth of green algae. In this table, green algae are showing their abundance at the points D and E. While, at the point “A”, it is scarce due to very clean water.

b) Growth of green algae in the river water, results in an increase in bacterial growth along with a decrease in the oxygen concentration in the water. It leads to the death of aquatic animals. This is the reason behind the total absence of fish at points, “C”, “D” and “E”. While, at the point “A”, there are many fish present in the river due to clean water.

i. Following is the population growth curve of the lynx and snowshoe hare.

ii. Lynx is the predator and snowshoe hare is the prey.
iii. When hares are abundant, lynx populations expand.
iv. When lynx are abundant, hare populations decline.
v. The factors, which may be responsible for the unusual number of hare in the year 1895 include exposure to stress, whether in the form of predation, disease or scarcity of food.