Section B
[1 Marks]
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(A) Incomplete dominance
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(B) Polygenic inheritance
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(C) Multiple allelism
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(D) Pleiotropism
Explanation: The correct option is Pleiotropism. Pleiotropism refers to a situation where one gene influences multiple phenotypic traits. This contrasts with other options like incomplete dominance, polygenic inheritance, and multiple allelism, which involve different genetic mechanisms.
[1 Marks]
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(A) Rudimentary ovaries
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(B) Gynaecomastia
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(C) Characteristic palm crease
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(D) Furrowed tongue
Explanation: The correct option is 'Characteristic palm crease'. This is because trisomy 21, also known as Down syndrome, is associated with distinct phenotypic traits, including a single transverse palmar crease. Other options like rudimentary ovaries, gynaecomastia, and furrowed tongue are not characteristically associated specifically with trisomy 21.
[1 Marks]
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(A) ICSI
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(B) IUI
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(C) IVF
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(D) ZIFT
Explanation: The technique depicted is ICSI (Intracytoplasmic Sperm Injection), where a single sperm is directly injected into the cytoplasm of an egg to facilitate fertilization, which is clearly represented by the needle injecting sperm into the egg.
[1 Marks]
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(A) Bacteria infected cell
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(B) WBC
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(C) RBC
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(D) Virus infected cell
Explanation: The correct answer is 'Virus infected cell'. Interferons are signaling proteins produced by host cells in response to the presence of pathogens, such as viruses. They play a crucial role in the immune response by interfering with viral replication and activating immune cells.
[1 Marks]
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(A) Anaerobic sludge
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(B) Primary sludge
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(C) Activated sludge
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(D) Flocs
Explanation: The correct answer is 'Flocs'. In the biological treatment of sewage, flocs are aggregates of bacteria and other microorganisms that are bound together by extracellular polysaccharides produced by the bacteria. This mesh-like structure allows for effective treatment as it facilitates the settling process and enhances the removal of contaminants from the sewage.
[1 Marks]
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(A) Ethidium bromide stained DNA can be seen under UV light.
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(B) DNA can be seen in visible light.
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(C) DNA can be seen without staining in visible light.
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(D) Ethidium bromide stained DNA can be seen in visible light.
Explanation: Ethidium bromide stained DNA can be seen under UV light because ethidium bromide intercalates between the bases of DNA and fluoresces when exposed to ultraviolet light, making the DNA bands visible.
[1 Marks]
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(A) Pseudofertilisation
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(B) Pseudopollination
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(C) Pseudoparthenocarpy
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(D) Pseudocopulation
Explanation: The correct answer is Pseudocopulation. This term is used to describe the behavior where male insects are deceived by the mimicry of orchid flowers resembling female insects, leading them to attempt to mate with the flower, which inadvertently results in the transfer of pollen and pollination of the flower.
[1 Marks]
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(A) Option B
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(B) Option C
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(C) Option A
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(D) Option D
Explanation: The correct answer is based on the functions of the genes within the lac operon. Each gene has a specific product that contributes to lactose metabolism in bacteria, such as β-galactosidase, which breaks down lactose into glucose and galactose. Matching the genes to their respective products is essential for understanding gene regulation and metabolic pathways.
[1 Marks]
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(A) 100%
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(B) 75%
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(C) 25%
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(D) 50%
Explanation: When both parents are carriers for thalassaemia (genotype Tt, where T = normal and t = thalassaemia gene), the probability of their child being affected (tt) is 25%. This is determined using a Punnett square, which shows that out of four possible genotype combinations (TT, Tt, Tt, tt), only one results in the child being afflicted with thalassaemia.
[1 Marks]
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(A) 5' - CAUUCAU - 3'
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(B) 5' - GUAAGUA - 3'
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(C) 5' - UACUUAC - 3'
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(D) 5'-AUGAAUG-3'
Explanation: The correct RNA transcript is 5' - AUGAAUG - 3'. During transcription, the DNA coding strand is used as a template to synthesize RNA, replacing thymine (T) with uracil (U). The DNA sequence 5' - ATGAATG - 3' corresponds to the RNA sequence 5' - AUGAAUG - 3' due to base pairing (A with U and T with A).
[1 Marks]
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(A) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
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(B) (A) is true, but (R) is false.
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(C) Both (A) and (R) are true and (R) is the correct explanation of (A).
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(D) (A) is false, but (R) is true.
Explanation: Both (A) and (R) are true and (R) is the correct explanation of (A) because AIDS is indeed caused by the HIV virus, which specifically attacks and weakens the immune system, leading to the development of AIDS. The statement about HIV being a virus with DNA as its genetic material is also accurate.
[1 Marks]
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(A) Both (A) and (R) are true and (R) is the correct explanation of (A).
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(B) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
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(C) (A) is false, but (R) is true.
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(D) (A) is true, but (R) is false.
Explanation: Both (A) and (R) are true and (R) is the correct explanation of (A). The assertion states that communities with more species are more stable, which is true because greater biodiversity can buffer against environmental changes. The reason given specifies that this stability is due to reduced year-to-year variation in total biomass, highlighting a mechanism that supports the assertion.
Section C
[2 Marks]
Answer: Farmers prefer apomictic seeds due to their ability to produce seeds that are genetically identical to the parent plant, ensuring uniformity in crop characteristics. This trait allows farmers to maintain desirable traits in their crops consistently. Additionally, apomictic seeds can be harvested and replanted without losing viability, minimizing the need to purchase new seeds each planting season, which can reduce costs and enhance sustainability in farming practices.
[2 Marks]
Answer: One advantage of amniocentesis is that it provides accurate genetic information about the fetus, allowing for the early detection of chromosomal abnormalities such as Down syndrome. However, one disadvantage is that it carries a small risk of miscarriage, estimated to be around 1 in 300 to 1 in 500 procedures, which can lead to a loss of the pregnancy. Thus, it is important to weigh the benefits against the risks.
[2 Marks]
Answer: The restriction enzyme that recognizes the specific sequence of bases is EcoRI, which recognizes the GAATTC sequence. Such specific sequences are referred to as restriction sites or recognition sequences. The arrows in the figure typically indicate the cleavage sites where the restriction enzyme acts, resulting in the DNA being cut into fragments. These fragments can then be analyzed or used in various molecular biology techniques, such as cloning or genetic engineering.
[2 Marks]
Answer: Growth curve 'A' represents exponential growth, which occurs under ideal conditions with unlimited resources, such as food and space, typical of newly introduced species. Growth curve 'B' shows logistic growth, indicating a population that grows rapidly at first but slows down as it reaches carrying capacity 'K', where resource limitations begin to affect growth. 'K' represents the carrying capacity of the environment, the maximum sustainable population size that can be supported by available resources.
[2 Marks]
Answer: Plants benefit from their association with Glomus species, which are arbuscular mycorrhizal fungi. These fungi enhance nutrient absorption, particularly phosphorus, by extending the root surface area through their hyphal network. This symbiotic relationship improves plant growth, increases drought resistance, and enhances overall soil health. Furthermore, Glomus species aid in protecting plants from pathogens and help in nutrient cycling, significantly contributing to plant vitality and ecosystem stability.
[2 Marks]
Answer: In DNA, adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G). Since adenine makes up 31% of the bases, thymine also accounts for 31%. Therefore, adenine and thymine together constitute 62% of the DNA. The remaining percentage, which is 38%, is shared between cytosine and guanine. Thus, cytosine and guanine each account for 19% of the bases in the DNA fragment. Therefore, the expected percentage of cytosine is 19%.
Section D
[3 Marks]
Answer: To identify the variables a, b, c, d, e, and f in the given table, we must first analyze the data presented. A close look at the rows and columns will reveal patterns or correlations that define these variables. For instance, if a is a dependent variable influenced by b, and if c, d, e represent various independent variables affecting a, we should utilize mathematical relationships such as those in linear equations or statistical methods to derive their values. It is important to also use any given formulas or relationships provided in the context to properly deduce each variable's identity and potential numerical value. Cross-referencing with the context can ensure accurate identification.
[3 Marks]
Answer: Biologists attribute the greater biodiversity in tropical regions to several factors. One significant reason is the stable climate, which allows for year-round growth and reproduction, unlike temperate zones that experience seasonal fluctuations. Tropical areas also benefit from high levels of solar energy, leading to lush vegetation and complex ecosystems. Furthermore, mid-domain effect suggests that species richness peaks in the tropics due to geographical constraints. Lastly, historical processes, such as climatic stability over millions of years, have also allowed for greater speciation in these regions.
[3 Marks]
Answer: An ecological pyramid is a graphical representation of the distribution of biomass, numbers, or energy among the trophic levels in an ecosystem. It typically illustrates three main types: pyramid of numbers, biomass, and energy. An upright pyramid shows that each higher trophic level has less biomass or energy than the one below it, which is typical in ecosystems like a forest. Conversely, an inverted pyramid, such as that of phytoplankton in a marine ecosystem, indicates that the biomass of producers can be less than that of consumers, due to rapid turnover rates. Hence, the shape conveys crucial information about the structure and function of ecological communities.
[3 Marks]
Answer: Transgenic animals are organisms that have been genetically modified to carry genes from other species. This genetic modification is accomplished using techniques like recombinant DNA technology. The transgenic animal with the largest number is the mouse, which has been widely used in research and has a high frequency of successful genetic modifications. There are several reasons for producing transgenic animals: to study genetic diseases, to develop new medical treatments, and to improve agricultural productivity through enhanced traits like disease resistance.
[3 Marks]
Answer: In maize, a diploid plant has cells with 10 chromosomes. During fertilization, one sperm cell fuses with the egg cell to form a zygote, which will also contain 10 chromosomes, as it receives one set from each parent. The endosperm, crucial for seed development, is formed when a second sperm cell fertilizes two polar nuclei in the embryo sac, resulting in a triploid cell with 15 chromosomes (2 sets from polar nuclei + 1 set from sperm). This triploid endosperm provides nutrients to the developing zygote and is essential for seed viability.
[3 Marks]
Answer: DNA replication occurs at the replication fork because it allows for the separation of the two strands of the DNA double helix, enabling the synthesis of new strands. The enzymes involved, such as DNA polymerase, need access to single-stranded DNA. The replication fork creates a localized area where the DNA unwinds and new nucleotides are added. Importantly, the leading strand is synthesized continuously in the direction of the fork movement, while the lagging strand is synthesized discontinuously, forming Okazaki fragments.
[3 Marks]
Answer: In eukaryotes, heterogeneous nuclear RNA (hnRNA) undergoes several processing steps to become mature mRNA. This processing occurs in the nucleus. Initially, the hnRNA undergoes 5' capping, where a methylguanylate cap is added to the 5' end, protecting it from degradation and aiding in ribosome binding during translation. Next, introns, which are non-coding sequences, are removed through splicing, performed by the spliceosome. Exons are then joined together. Finally, a poly-A tail is added to the 3' end of the mRNA, enhancing its stability and export to the cytoplasm. The mature mRNA is then transported out of the nucleus for translation.
[3 Marks]
Answer: Human activities play a significant role in the accelerated extinction of species around the globe. First, habitat destruction, due to urbanization and agriculture, leads to loss of natural environments, which are crucial for many species' survival. Second, pollution, including plastic waste and chemical runoff, contaminates ecosystems and poses serious threats to wildlife health. Lastly, overexploitation of species through activities such as overfishing and poaching directly reduces their populations to unsustainable levels, disrupting ecological balance and leading to extinction.
Section E
[5 Marks]
Answer: The pBR322 cloning vector is a widely used plasmid in recombinant DNA technology. In part (i), the schematic diagram includes the origin of replication ('ori'), which is essential for plasmid replication within a host cell. The Bam HI site serves as a restriction site for inserting foreign DNA. The gene for ampicillin resistance allows for the selection of transformed cells, while the 'rop' gene helps regulate the replication of the plasmid to ensure a stable yield of cloned products. In part (ii), the 'rop' gene plays a crucial role by encoding a protein that facilitates the replication process and maintains the cloning vector's stability by regulating plasmid copy number. For part (iii), if a cloning vector does not possess a selectable marker, it will be challenging to identify successfully transformed cells, as there would be no way to differentiate between cells that have taken up the plasmid and those that have not, leading to low efficiency in cloning. Finally, in part (iv), insertional inactivation is often preferred because it provides a more straightforward mechanism to confirm successful cloning; when foreign DNA is inserted, it disrupts the function of a marker gene, allowing for easy identification of positive clones. This is more reliable than solely relying on selectable markers, which may sometimes yield false positives.
[5 Marks]
Answer: The nematode that infects the roots of the tobacco plant, reducing its yield, is Meloidogyne spp., commonly known as root-knot nematodes. These parasites form galls on the roots, leading to significant yield loss in tobacco cultivation due to impaired nutrient and water absorption. To introduce nematode-specific genes into the host tobacco plant, Agrobacterium tumefaciens is commonly used as a vector. Sense and anti-sense RNAs play critical roles in gene expression regulation. Sense RNA corresponds to the coding sequence and is involved in normal protein synthesis, while anti-sense RNA is complementary to the sense RNA, inhibiting its expression by binding to it, forming double-stranded RNA which triggers RNA interference (RNAi). Transgenic tobacco plants are engineered to express anti-sense RNA specific to the nematode's genes, effectively silencing these genes. As a result, the parasites are unable to complete their life cycle and survive, leading to reduced infestation and improved yield.
[5 Marks]
Answer: A human sperm is a complex cell with several specialized structures. The main parts include the head, which contains the nucleus with genetic material; the midpiece, packed with mitochondria for energy production; and the tail, or flagellum, which propels the sperm forward. Each part has a specific function: the head aids in penetration of the ovum, the midpiece provides energy needed for motility, and the tail ensures movement towards the egg. In regard to ovum fertilization, only one sperm can fertilize an ovum due to the block mechanisms that prevent polyspermy. After the first sperm penetrates the egg, changes in the egg's membrane occur, preventing additional sperm from entering, ensuring the correct amount of genetic material is present, and thus maintaining the species' proper genetic structure.
[5 Marks]
Answer: The development of the embryo in a dicot plant occurs through several distinct stages, beginning with the fertilization of the ovule, which leads to the formation of a zygote. This zygote undergoes a series of cell divisions, transforming into a multicellular structure. The first noticeable change is the formation of a proembryo, which then develops into a globular embryo. This globular form eventually elongates to form the heart stage embryo and finally matures into a torpedo stage before transitioning into a mature embryo. The stages of development are not simply structural; they involve complex physiological processes that ensure successful plant growth.
Endosperm development is crucial as it provides the necessary nourishment to the growing embryo. The endosperm forms from the fertilization of the central cell of the ovule and is rich in starch, proteins, and oils that sustain the embryo during its initial growth. This nutrient supply allows the embryo to develop properly. Therefore, endosperm development, which occurs in parallel with zygote formation, serves the essential role of feeding the embryo, making it critical for successful plant reproduction.
[5 Marks]
Answer: Natural selection can take on different forms, notably stabilizing, directional, and disruptive selection. In the context of industrialization in England, the type of natural selection depicted in the graph is directional selection. During this period, darker winged moths increased in frequency due to the soot-covered trees offering better camouflage from predation by birds, leading to a higher survival rate compared to white-winged moths. Over time, as environmental changes continued, the lighter-colored moths faced a decline due to their visibility against the darkened bark. Additionally, anthropogenic actions such as pollution and habitat destruction can significantly enhance evolution rates. For instance, Darwin's finches have adapted their beak sizes in response to changes in available food sources due to human influence. Such changes create selective pressures, leading to rapid evolutionary adaptations within species, affirming that human impact can drive natural selection markedly.
[5 Marks]
Answer: Hershey and Chase conducted their famous experiment using radioactive isotopes of sulfur and phosphorus to trace the molecules that contain genetic information. Sulfur-35 was used to label proteins, while phosphorus-32 labeled DNA, allowing them to determine which molecule was responsible for transferring genetic material during viral infection. Blending is crucial as it separates viral coats from the infected bacteria, ensuring only the genetic material enters the cells. Centrifugation then aids in separating lighter viral proteins from the heavier bacterial cells, revealing that only the phosphorus-labeled DNA entered the bacteria. Their conclusion was groundbreaking: DNA, not protein, is the genetic material in viruses.
a) Name the restriction enzyme that recognises the given specific sequence of bases. What are such sequence of bases referred to as?