CBSE previous Year Solved Papers Class 12 Biology Delhi 2011
Time allowed : 3 hours Maximum Marks: 70
General Instructions :
- There are a total of 26 questions and five sections in the question paper, All questions are compulsory.
- Section A contains question number 1 to 5, Very Short Answer type questions of one mark each.
- Section B contains question number 6 to 10, Short Answer type I questions of two marks each.
- Section C contains question number 11 to 22, Short Answer type II questions of three marks each.
- Section D contains question number 23, Value Based Question of four marks.
- Section E contains question number 24 to 26, Long Answer type questions of five marks each.
- There is no overall choice in the question paper, however, an internal choice is provided in one question of two marks, one question of three marks and all three questions of five marks. An examined is to attempt any one of the questions out of two given in the question paper with the same question number.
Question.1. Name the type of cell division that takes place in the zygote of an organism exhibiting haplontic life cycle.
Answer : Meiosis occurring in the zygote of an organism exhibiting haplontic life Cycle.
Question.2. Write the scientific name of the microbe used for fermenting malted cereals and fruit juices.
Answer: Saccharomyces cerevisiae.
Question.3. Write the unit used for measuring ozone thickness.
Answer : Dobson units (DU).
Question.4. Name the event during cell division cycle that results in the gain or loss of chromosome.
Answer: Aneuploidy results in the loss or gain of chromosome during cell division cycle.
Question.5. How can bacterial DNA be released from the bacterial cell for biotechnology experiments?
Answer : To release the bacterial DNA from the bacterial cell for biotechnology experiments the cell wall need to be open using enzyme lysozyme and DNA is isolated along with RNA and protein.
Question.6. Write the importance of cryopreservation in conservation of biodiversity.
Answer : Cryopreservation is used in storing the gametes of endangered species that can be fertilized in-vitro followed by propagation through tissue culture methods, seeds can be preserved in seed banks, preserving vegetatively propagated crops.
Question.7. Mention the role of the codons AUG and UGA during protein synthesis.
Answer : AUG is the start codon that initiates the protein synthesis. UGA is the stop codon that signals for the termination of protein synthesis.
Question. 8. Normally one embryo develops in one seed but when an orange seed is squeezed many embryos of different shapes and sizes are seen. Mention how it has happened.
Answer : The nucellus cells start dividing and protrude into the embryo sac to develop into embryos. Hence, each ovule may contain several embryos. The presence of many embryos of different shapes and sizes in a seed is called Polyembryony.
Question.9. How do histones acquire positive charge ?
Answer : Histones acquire a positive charge due to presence of abundance of basic amino acid residues such as lysine and arginine carry positive charges in their side chains.
Question.10. Why is CuT considered a good contraceptive device to space children ?
Answer : CuT is a method of Intrauterine device (IUD) considered to be a good contraceptive device because it releases copper ions which increase phagocytosis of sperms and decrease sperm motility and fertilizing capacity of the sperm. Hence, the sperms lose their motility before reaching the ova.
Question.11. Differentiate between albuminous and non-albuminous seeds, giving one example of each.
Answer : The differences between albuminous and non-albuminous seeds are as under :
Question.12. Explain the process of RNA interference.
Answer: RNA interference is the cellular defense mechanism in all eukaryotic organisms. This method involves silencing of a specific mRNA of the parasite due to complementary dsRNA molecule that binds and prevents translation of the mRNA(silencing). RNA interference is a method adopted to prevent infestation of roots of tobacco plants by a nematode Meloidogyne incognita.
Question.13. List the key tools used in recombinant DNA technology.
Answer: The key tools used in recombinant DNA technology are restriction enzymes, Polymerase Chain Reaction, competent host and cloning vectors, polymerase enzymes, ligases and host organisms.
Question.14. Name the two types of immune systems in a human body. Why are cell mediated and humoral immunities so called ?
Answer : The two types of immunity found in humans are Humoral and Cell-mediated immunity. Humoral immunity is called so because, B lymphocytes produce antibodies against pathogens into our blood stream to fight with them. Cell-mediated immunity is called so because it is performed by specialised cells in the body known as T-lymphocytes. T-cell also help B cells to produce antibodies.
Write the scientific names of the causal organisms of elephantiasis and ringworm in humans. Mention the body parts affected by them.
The Scientific name of the causal organism of Elephantiasis in human is Wuchereria bancrofti. The body part affected by them is Lymphatic vessel of lower Limbs.
Ringworm in human is, Microsporum, Trichophyton and Epidermophyton. The body part affected is skin.
Question.15. Justify with the help of an example where a deliberate attempt by humans has led to the extinction of a particular species.
Answer: The deliberate attempt by human has led to the extinction of a particular species. For example, the introduction of Nile perch in Lake Victoria that led to the extinction of more than 200 species of cichlid fish in the lake. Abingdon tortoise in Galapagos islands became extinct, after goats were introduced.
Question.16. Identify A, D, E and F in the diagram of an antibody molecule given below:
Answer : A- Antigen binding site.
D- Light chain
E – Heavy chain (constant region)
F – disulphide bond
Question.17. Study the graph given below. Explain how is oxygen ‘ concentration affected jn the river when sewage is discharged into it.
Answer : The introduction of sewage caused a sudden and sharp decline of dissolved oxygen at the point of sewage discharge in the river. This is because oxygen is used up by microorganisms involved in biodegradation resulting in an increase in the biological oxygen demand (BOD). This in turn leads to the death of fishes and other aquatic life. As the sewage gets diluted due the flow of the water, the river slowly regains some of its oxygen concentration.
Question.18. Explain how a hereditary disease can be corrected. Give an example of first successful attempt made towards correction of such diseases.
Answer : Gene therapy is the correction of a genetic defect by introduction of normal gene into an individual or embryo to take over or compensate the function for a non-functional gene. The first disease to have a gene therapy is ADA (Adenosine deaminase) deficiency. In this, the gene coding for enzyme ADA gets deleted leading to deficiency of ADA and problems in immune system.
Gene therapy for ADA deficiency includes :
- In in-vitro culturing of isolated lymphocytes from the patients blood.
- Then introduced fuctional ADA cDNA into the cluttered lymphocytes.
- These lymphocytes are returned back to the patients body.
- Permanent care of this problem is the introduction of isolated gene from bone marrow cells producing ADA into cells at early embryonic stages.
SECTION – C
Question.19. Draw a diagram of a, male gametophyte of an angiosperm. Label any four parts. Why is sporopollenin considered the most resistant organic material ?
Answer: Following is the labelled diagram of a male gametophyte of an angiosperm.
Sporopollenin is chemically very stable and is usually well preserved in soils and sediments. It can withstand high temperatures, acidic and alkaline conditions, and enzymes.
Question.20. How are dominance, codominance and incomplete dominance patterns of inheritance different from each other ?
Answer: Dominance: An allele expresses itself in the presence of the hybrid heterozygous condition. For example, tall plant, round seed, violet flower, etc. are dominant characters in a pea plant.
Co-dominance : Co-dominance is the phenomenon in which both the alleles of a trait are expressed in heterozygous condition. Both the alleles of a gqne are equally dominant. Example: ABO blood group system. .
Incomplete dominance : Neither of the two alleles of a gene is completely dominant over the other in heterozygous condition, the hybrid is intermediate. For example, a monohybrid cross between the plants having red flowers and white flowers in Antirrhinum species will result in all pink flower plants in Fj generation.
Question.21. The base sequence in one of the strands of DNA is TAGCATGAT.
(i) Give the base sequence of its complementary strand.
(ii) How are these base pairs held together in a DNA molecule?
(iii) Explain the base complementarity rules. Name the scientist who framed this rule.
Answer : (i) The base sequence of the complementary strand will be ATCGTACTA.
(ii) The base pairs in the DNA molecules are held with hydrogen bonds, between A and T and C and G on the two strands.
(iii) Base Complementarity Rule: A purine will always pair with a pyrimidine in a DNA molecule i.e. A will pair with T and G will pair with C. The ratio of A and T or C and G will always he 1. The base complementarity rule was framed by Erwin Chargaff.
Question.22. (a) Sickle cell anaemia in humans is a result of point mutation. Explain.
(b) Write the genotypes of both the parents who have produced a sickle celled anaemic offspring.
Answer : (a) Sickle celled anaemia in humans is a result of point mutation beacuse mutation of a single base change (substitution) in the gene leads to the replacement of GAG
by GUG. This leads to the substitution of the amino acid Glutamine (Glu) by Valine (Val) at sixth position of beta globin chain of haemoglobin.
(b) Genotype of parents who have produced a sickle cell anaemic offspring. Father – HbA HbS, Mother – HbA HbS.
Question.23. What is inbreeding depression and how is it caused in organisms ? Write any two advantages of inbreeding.
Answer : Inbreeding depression is the reduction in the fertility and productivity of an organism due to continuous inbreeding.
Advantages : (i) Production of pure lines
(ii) Elimination inferior genes,
(iii) Accumulation of superior genes.
Question.24. (a) Identify (A) and (B) illustrations in the following:
(b) Write the term given to (A) and (C) and why ?
(c) Expand PCR. Mention its importance in biotechnology.
Answer : (a) The part labelled A-AATTC is the sticky end. The part labelled B is the foreign DNA insert.
(b) The term used For A and C are called the palindrofmc nucleotide sequence, because the sequence of base pairs reads same on the two strands when the orientation of the reading is kept the same.
(c) PCR stands for Polymerase Chain Reaction.
PCR is a technique in molecular biology, used to amplify a gene or a piece of DNA to .obtain its several copies. Biotechnological Importance – amplification of gene of interest in vitro.
The diagram above is that of a typical biogas plant. Explain the sequence of events occurring in a biogas plant. Identify a, b and c.
Answer: The’sequences of events occurring in a biogas plant are:
- The complex molecules are broken down into simple sugars, amino acids & fatty acids.
- Further breakdown of remaining , components by fermentative bacteria.
- Simple molecules obtained by acidogenesis phase further digested by acetogens to produce large amount of acetic acid, CO2 and Ify.
- Methanogens utilise the intermediate products of the previous steps and convert them into methane, carbondioxide and water.
(a) Sludge loader.
(b) Gas holder CH4 and GCK
(c) Inlet to feed slurry.
Question.26. How can crop varieties be made disease resistant to overcome food crisis in India ? Explain.
Name one disease resistant variety in India of:
(a) Wheat to leaf and stripe rust
(b) Brassica to white rust
Answer: A wide range of fungal, bacterial and viral pathogens affect the yield of cultivated crops. Disease resistant can be provided by conventional breeding, mutational breeding or genetic engineering.
- Conventional breeding: It includes the basic steps of screening, germplasm, hybridisation, selection, testing and release.
- Mutational breeding: In this method, genetic variations
are created, which then result in the creation of traits not found in the parental type.
- Genetic engineering: Certain wild varieties have disease- resistant characteristics, but they are low yielding, Disease- resistant genes from such varieties are introduce in high- yielding varieties through recombinant DNA technology. One disease resistant variey in India of:
(a) Himgiri (b) Pusa swarnim
Write the source and the effect on the human body of the following drugs:
(i) Morphine (ii) Cocaine (iii) Marijuana The source and effect of the following drugs are :
Answer: (i) Morphine :
Source—Latex of poppy plants (Papaver somniferum)
Effect—It is a depressant; slows down body functions
Source—Coca plant Erytjfroxylum coca
Effect—Stimulates the CNS, producing a sense of euphoria
and increased energy and hallucination.
Source—Inflorescences of the plant Cannabis sativa
Effect—Effects the cardiovascular system.
Question.27. Name the type of interaction seen in each of the following examples:
(1) Ascaris worms living in the intestine of human
(2) Wasp pollinating fig inflorescence
(3) Clown fish living among the tentacles of sea-anemone
(4) Myeorrhizae living on the roots of higher plants
(5) Orchid growing on ai branch of a mango tree
(6) Disappearance of smaller barnacles when Balanus dominated in the Coast of Scotland.
- Ascaris worms living in the intestine of human- Parasitism
- Wasp pollinating fig inflorescence – Mutualism
- Clown fish living among the tentacles of sea-anemone Commensalism
- Myeorrhizae living on the roots of higher plants – Mutualism
- Orchid growing on a branch of a mango tree – Commensalism
- Disappearance of Smaller barnacles when Balanus dominated in the Coast of Scodand – Competition
Question.28. (a) Draw a labelled diagram of the human female repro-ductive system.
(b) Enumerate the events in the ovary of a human female during:
(i) Follicular phase (ii) Luteal phase of menstrual cycle
Answer: (a) Female reproductive system :
(b) (i) Follicular phase : The growth of the primary ovarian follicle and also causes maturation of the primary oocyte in this follicle. The follicular cells of the Graafian follicle secrete estrogen. Due to which the uterine endometrium becomes thick, more vascular and more glandular.
(ii) Luteal phase : Development of corpus luteum continues to release the hormone, progesterone. It lasts for about 12-14 days and extends from the 16th – 28th day of the menstrual cycle.
(a) Write the specific location and the functions of the following cells in human males:
(i) Leydig cells (ii) Sertoli cells (iii) Primary spermatocyte
(b) Explain the role of any two accessory glands in human male reproductive system.
Answer : (a) (i) Leydig cell are found in interstitial spaces between seminiferous tubules in the testes. They synthesise and secrete testicular hormones called androgens (testosterone).
(ii) Sertoli cells are found in between spermatocyte inside seminiferous tubules. They act as nurse cell and provide nutrition to the male germ cells.
(iii) Primary spermatocytes are found in between spermatocyte inside seminiferous tubules. They are diploid” cells that are derived from the spermatogonia. They undergo meiotic division to give rise to secondary spermatocyte and thereby male gamete-sperm.
(b) The male accessory glands include the seminal vesicles, prostrate gland, and the bulbourethral gland.
Prostate Gland: It stores and secretes an alkaline, milky fluid known as seminal plasma, which provide alkalinity of the semen is to neutralize the acidity of the vaginal tract. Bulbourethral glands: This gland secretes a viscous secretion during sexual arousal which helps to lubricate the urethra for easy passage of spermatozoa to pass through, and to help ‘ flush out any residual urine or foreign matter and also help in lubrication of penis.
Question.29. Explain the salient features of Hugo de Vries theory of mutation. How is Darwin’s theory of natural selection different from it? Explain.
Answer : Salient features of theory of Hugo de Vries :
- Mutations cause evolution.
- New species originate due to large mutations
- Evolution is a discontinuous process and not gradual
- Mutations are directionless
- Mutations appear suddenly
- Mutations exhibit their effect immediately
(a) Name the primates that lived about 15 million years ago. List their characteristic features.
(b) (i). Where was the first man-like animal found?
(ii) Write the order in which Neanderthals, Homo habilis and Homo erectus appeared on earth. State the brain capacity of each one of them.
(iii) When did modern Homo sapiens appear on this planet?
Answer: (a) Dryopithecus (ape-like) and Ramapithecus (man¬like). These primates were hairy and their walk was similar to that of chimpanzees.
(b) (i) The first man-like animal was found in Africa.
(ii) Order = Homo habilis , Homo erectus , Neanderthals Cranial capacity =Homo habilis = 650 – 800 cc,
Homo erectus = 900 cc, Neanderthals = 1400 cc
(iii) During ice age / 75000 – 10000 years ago
Question.30. (a) Explain primary productivity and the factors that influence it.
(b) Describe how do oxygen and chemical composition of detritus control decomposition ?
Answer : (a) The amount of biomass produced per unit area over a time period by plants during photosynthesis is defined as the primary productivity. It is expressed as weight (g-2) or energy (Kcal m-2).
Factors : Availability of nutrients, quality of light available, availability of water, temperature of the given place, type of plant species of the area, photosynthetic capacity of the plants; (b) Decomposition is an oxygen consuming process thus it is directly proportional to the concentration of the oxygen in the environment. It is controlled by the chemical composition of detritus and climatic conditions. The decomposition rate decreases when the detritus is rich in lignin and chitin.
(a) What is El Nino effect? Explain how it accounts for biodiversity loss.
(b) Explain any three measures that you as an individual would take, to reduce environmental pollution.
Answer : (a) Rise in temperature leading to deleterious changes in the environment and resulting in odd climatic changes is El Nino effect. An El Nino event is a temporary change in the temperature, surface air pressure and currents of the Pacific Ocean in the region surrounding the equator. El Nino causes global warming and the melting of polar ice cap. This leads to the submerging of coastal regions and loss of species endemic to that area.
(b) The measures that we as an individual can take in order to reduce environmental pollution can be :
- Reducing our garbage generation and dumping of organic waste in places far from residential areas.
- Less use of fossil fuel and using car pool so as to reduce pollution and fuel consumption.
- Turning off air conditioners when not in use.
- Planting more and more trees and deducing the deforestation.
Question.1. How does Penicillium reproduce asexually ?
Answer : Penicillium reproduces asexually by means of spores called conidia which are non-motile and developed on conidiospores.
Question.5. Mention the uses of cloning vector in biotechnology.
Answer : The uses of cloning vector in biotechnology:
- Help in linking the foreign DNA with the host.
- Help in the selection of recombinants from the non-recombinants.
Question.8. Mention the function of trophoblast in human embryo.
Answer: Functions of Trophoblast in human embryo :
- During pre embryonic development stage (6 or 7 Day), the trophoblast cells contact with the endometrial lining and secrete enzyme that digest endometrial cells.
- The trophoblast releases the hormone human chorionic gonadotrophin, it keeps corpus luteum alive and is useful for detection of pregnancy.
Question.10. Name the oral pills used as a contraceptive by human females. Explain how does it prevent pregnancy.
Answer: The oral pill used as a contraceptive by human females is ‘Saheli’. Saheli inhibits ovulation and implantation. It alters the quality of cervical mucus and prevents the entry of sperms into the cervix.
Question.14. Explain the advantage of crossbreeding of the two species of sugarcane in India.
Answer : Saccharum barberi, grown in North India, had poor sugar content and yield, whereas saccharum officinarum, grown in South India, had thicker stem and higher sugar content, but did not grow well in North India. The hybrid produces by cross breeding of these two species has the following desirable traits :
- High Yield
- Thick stem
- High sugar content
- Ability to grow in North Indian sugarcane fields.
How do cellular barriers and cytokine barriers provide innate immunity in humans ?
Answer : Cellular Barriers : Leukocytes and monocytes are natural killers in the blood and they destroy microbes. Macrophages are present in the tissues. They phagocytose and destroy microbes.
Cytokine Barriers : Virus infected cells secrete proteins called interferons and protect non-infected cells from the viral infection.
Question.19. Differentiate between geitonogamy and xenogamy in plants. Which one between the two will lead to inbreeding depression and why ?
Question.22. A pea plant with purple flowers was crossed with white flowers producing 50 plants with only purple flowers. On selfing, these plants produced 482 plants with purple flowers and 162 with white flowers. What genetic mechanism accounts for these results ?
(i) Factors segregate from each other that remained together in a hybrid during gamete formation.
(ii) A homozygous parent produces all gamete that are similar while heterozygous parent produces two kinds of gametes in equal ratio.
Question.23. Mention the property of plant cells that has helped them to grow into a new plant in in-vitro conditions. Explain the advantages of micropropagation.
Answer : The property of plant cells that has helped them to grow into a new plant in in-vitro condition is totipotency. Advantages of micropropagation are :
- Many important food plants like tomato, banana, apple etc. can be produced on a commercial scale.
- It helps in recovery of healthy plants from diseased plants because of the presence of virus free meristem in tb,e plant.
One can remove the meristem and grow it in vitro to obtain virus free plant. Scientists have succeeded in culturing meristem of banana, sugarcane, potato etc.
(a) (i) Name the biogeochemical (nutrient) cycle shown above.
(ii) Name an activity of the living organisms not depicted in the cycle by which this nutrient is returned to the atmosphere.
(b) How would the flow of the nutrient in the cycle be affected due to large scale deforestation ? Explain giving reasons.
(c) Describe the effect of an increased level of this nutrient in the atmosphere on our environment.
Answer : (a) (i) The biogeochemical (nutrient cycle) shown is carbon cycle.
(ii) Decomposition of organic wastesby microbes is an activity of the living organisms is not depicted in the cycle.
(b) Deforestation leads to increase in carbon dioxide levels in the air. Because the CO2 present is not being utilized for photosynthesis in the absence of plants.
(c) The effect of an increased level of this nutrient in the atmosphere on our environment is due to human activity. Decomposers also contribute substantially to CO2 pool by their processing of waste materials and dead organic matter of land or oceans. The increased level of this gas produced higher global temperature and changes in precipitation patterns.
(a) Healthy ecosystems are the base of wide range of (ecosystem) services. Justify.
(b) Explain the differences and the similarities between hydrarch and xerarch succession of plants.
Answer : (a)Healthy ecosystem provides following ecological services:
- Purification of air and maintenance of gaseous composition.
- Mitigation of drought and floods.
- Cycling of nutrients.
- Storehouse of carbon.
- Maintainance of biodiversity.
- Habitat for a number of wildlife.
- Influence of hydrological cycle.
(b) Differences between hydrarch and xerarch succession of plants :
Similarities: Both hydrarch and xerarch succession lead towards mesic conditions, e.g., with moderate water conditions.
Question.3. Why green plants are not found beyond a certain depth in the ocean ?
Answer : The green plants are not found beyond a certain . depth in the ocean because sunlight cannot penetrate beyond a certain depth.
Question.4. Write the scientific name of the microbe used for fermenting malted cereals and fruit juices.
Answer : Saccharomyces cervisiae is the scientific name of the microbe used for fermenting malted cereals ahd fruit juices.
Question.9. State a dual role of deoxyribonucleoside triphosphate during DNA replication.
Answer: Deoxyribonucleoside triphosphate serves dual purpose
- It acts as substrate.
- It also provides energy for polymerization reaction.
Question.13. Explain the role of Ti plasmids in biotechnology.
Answer : Ti Plasmid (tumor-inducing plasmid) refers to the plasmid of Agrobacterium tumefacims. A tumefaceiens is a plant pathogen and is known to produce tumours in the plants it infects.
Role of Ti plasmids in biotechnology; Ti plasmid can be modified into a cloning vector by removing the genes responsible for pathogenicity.
Question.16. State the functions of primary and secondary lymphoid organs in humans.
Answer : Role of primary and secondary lymphoid organs in humans are:
Primary Lymphoid Organs: Here immature lymphocytes are differentiated into antigen sensitive lymphocytes.
Secondary Lymphoid Organs: Where lymphocytes interact with antigen accessory cells and proliferate to become effector cells, resulting in the initiation of the antigen specific response.
Question.21. Why do you see two different types of replicating strands in the given DNA replication fork ? Explain Name these strands.
- The DNA dependent DNA polymerase catalyse polymerization of the nucleotides only in 5’—>3’ direction. Since the two strands of DNA run in opposite direction, one is in 5’—>3’ and the other is in 3’—>5’ direction.
- Replication proceeds in two opposite directions. DNA synthesis is continuous on the 3’—>5’ template strand while on the other template strand with polarity 5’—>3’ direction, the synthesis of DNA is discontinuous i.e short stretches of DNA are formed.
5’—>y Continuous sysnthesis 3’—>5’ Discontinuous sysnthesis
Question.25. (a) Why do farmer prefer biofertilizers to chemical fertilizer these days ? Explain.
(b) How do Anabaena and mycorrhiza acts as biofertilisers ?
Answer : (a) Farmer prefer biofertilizers to chemical fertilizer because:
- Biofertilisers would be better in the long run because they contain organic materials while fertilizers contain chemical substanes that when used in excess become harmful to the consumers.
- Continuous application of fertilizers to the soil, decreases its fertility and also leads to increase in the acidity of the soil.
- Fertilizer run off from the fields also causes water pollution,
(b) Anabaena and mycorrhiza acts as biofertiliser, fungi sucii as Glomus form symbiotic associations with plants. This relationship is known as Mycorrhiza. They absorb phosphorus from the soil and pass it to plants.
Cyanobacteria such as Anabaena and Nostocalsoftx atmospheric nitrogen and act as bio – fertilisers especially in paddy fields.
Question.26. (a) Name the stage of plasmodium that gains entry into the human body.
(b) Trace the stages of plasmodium in the body of female Anopheles after its entry.
(c) Explain the cause of periodic recurrence of chill and high fever during malarial attack in humans.
Answer: (a) The stage of the plasmodium is Sporozoites.
(b) Plasmodium requires two hosts to complete its life cycle. When the Anopheles mosquito bites a diseased person, gametocytes of Plasmodium are introduced into the mosquito.
Gametocytes fertilises and developed inside the intestine of mosquito to form sporozoites. Sporozoites are stored in the salivary glands of mosquito and are released into a healthy person who is bitten by the mosquito.
(c) Parasites initially multiply within the liver and then attack the red blood cells resulting in their rupture. The rupture of RBCs is associated with the release of a toxic substance called haemozoin. Haemozoin is responsible for the chill and high fever recurring every three to four days.
Trace the events that occur in human body to cause immunodeficiency when HIV gains entry into the body.
Answer: Events occurring in the human host after the entry of HIV:
- After entering in the human body, viruses enter into the macrophages.
- Macrophages becomes a virtual HIV factory.
- Thereafter HIV enters helper T-Lymphocytes, replicates and produces progenies.
- As the progenies are released they attack other T-Lymphocytes.
- Therefore, T-lymphocytes start decreasing in number and immune response of the person become weak.
- Even infection that Could be overcome easily starts aggravating.
Question.28. (a) Describe the stages of oogenesis in human females,
(b) Draw a labelled diagram of human ovum released after ovulation?
Answer: (a) Oogenesis is the formation of the egg (ovum) in the female.
- During fetal development females have oogonia which . are diploid sex cells.
(i) While still in the womb the oogonia. divide by mitosis to form 14-1 million primary oocytes.
(ii) These primary oocytes will begin the first meiotic division but stall during prophase I.
(iii) The female is born with these primary oocytes.
- By the time the female reaches puberty approximately 40,000 of the primary oocytes will remain.
- Beginning during puberty, each month hormones from the anterior pituitary stimulate a primary oocyte to complete the first meiotic division generating two secondary oocytes of unequal size.
(i) The smaller secondary oocyte is called a polar body, containing one set of chromosomes.
(ii) The larger secondary oocyte is the ovum (egg) that will be released from the ovary for fertilization by the spermatozoa.
(a) Only if the ovum is fertilized will it continue the second meiotic division.
(b) If fertilized the ovum divides again to produce a second polar body, with the fertilized ovum forming the diploid zygote.
(c) If the ovum is not fertilized within 24 hours after release it will be broken down.
(a) When and where does spermatogenesis occur in human male ?
(b) Draw a diagram of a mature human male gamete. Label the following parts: acrosome, nucleus, middle piece and tail.
(c) Mention the function of acrosome and middle piece.
Answer: (a) Spermatogenesis occur in testis at the time of puberty.
(c) Functions of acrosome : Acrosome contains hydrolytic enzymes that help in dissolving membranes of the ovum fo fertilization.
Middle piece: It contains a number of mitochondria that provide energy for the movement of the tail and provides motility to sperm.