Biology:2015:CBSE:[All India]:Set-I

Drones of honey bees are haploid and possess 16 chromosomes.
Drones of honey bees donate 16 chromosomes (one pair) to the progeny; thus, mitosis is the cell division that is involved in the production of sperms by drones.

Cistron is a section of gene that codes for a certain polypeptide or protein.

After attacking the host cell, retrovirus enters into macrophages (as in case of HIV) where RNA genome of the virus replicates to form viral DNA using reverse transcriptase enzyme. This viral DNA gets incorporated into the host's cell DNA and directs the infected cells to produce more viruses. Hence, the infected host cell possesses viral DNA.

Children cured by enzyme-replacement therapy for adenosine deaminase (ADA) deficiency need periodic treatment because it is not a completely curative method. In this method, a functional ADA cDNA is injected into a patient, by introducing it into the patient's lymphocytes using a retroviral vector. Since the lymphocytes have a definite life cycle, there is a need for periodic transfusion of the genetically engineered lymphocytes (having ADA) into the patient.

Following are the two advantages of using unleaded petrol as fuel in automobiles:

• Unleaded petrol does not release lead compounds from exhaust fumes into the atmosphere and causes less pollution.
• As unleaded petrol does not emit harmful compounds, it helps in preventing health diseases like bronchitis, asthma and lung diseases.

Mosses are bryophytes and they need water for fertilization. They lay their flagellated male gametes that swim across the water to reach the female gamete. During this process, many of the male gametes are destroyed or lost. Thus, moss plants produce very large number of male gametes so that even if some of the gametes get destroyed, the remaining can fertilize the female gamete.

These male gametes are called antherozoids.

(a) Homologous organs or structures are similar in anatomy or structure but perform different functions.

From the given options, following are homologous structures:

• Forelimbs of whales and bats are similar in structure but perform different functions of swimming and flying, respectively.
• Thorns of Bougainvillea and tendrils of Cucurbita are both modifications of a stem arising from axillary bud but perform different functions of protection and climbing, respectively.

(b) The evolution represented by homologous organs or structures is divergent evolution as they have common origin but have diverged (become dissimilar) with evolution.

(a) The plants obtained by micropropagation are called somaclones because they are genetically identical to each other as well as to the parent plant.

(b) The advantages of micropropagation are as follows:
(i) It helps in the propagation of a large number of plants in a short span of time.
(ii) It helps in the production of healthier plants that are disease and pest resistant.

The primary phase of sewage treatment involves physical removal of particles by filtration and sedimentation.

Different steps involved during the primary treatment are as follows:

• Initially, floating debris of sewage water is removed by sequential filtration.
• Then, grit (soil + small pebbles) is removed by sedimentation. The remaining solid particles which have settled down from the sludge while the supernatant forms the effluent.
• Effluent is then taken for secondary treatment.

Mutualism is a relationship between the organisms of two species in which both organisms are benefited.

Examples of the organisms involved that are commercially exploited in agriculture are as follows:
(i) Commercial exploitation of Rhizobium in agriculture
Continuous growth of crops leads to the nutrient deficiency in soil. Farmers, then, grow leguminous crops containing Rhizobium in its roots to replenish the lost nutrients (especially nitrogen) in the soil.

(ii) Commercial exploitation of Mycorrhiza in agriculture
Mycorrhiza is an association of the soil fungus with the roots of higher plants. Farmers use Mycorrhiza commercially in agriculture as it improves the soil quality and reduces soil erosion by improving plant rooting capacity. The fungal hyphae spread into the root tissues and help the plants to optimally use the soil's water and minerals.
Thus, to increase the yield of plants and to replenish the soil nutrients, Mycorrhiza is commercially exploited in agriculture.