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NEB Class 12 Biology study materials
NEB Class 12 Biology study resources

NEB Class 12 Biology practice questions

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Question 1Biology (Grade 12) - content domains not enumerated in source

Which process directly produces the ATP used by the contractile proteins during muscle contraction?

  1. Substrate-level phosphorylation in the sarcoplasm
  2. Breakdown of ATP bound to the myosin head Correct answer.
  3. Hydrolysis of creatine phosphate
  4. Oxidative phosphorylation in the mitochondrial cristae

Explanation

The immediate energy for the power stroke comes from ATP already bound to the myosin head, which is hydrolysed to ADP and inorganic phosphate. Creatine phosphate and cellular respiration only regenerate ATP; they do not supply the ATP that is split at the cross-bridge.

Question 2Biology (Grade 12) - content domains not enumerated in source

In a dihybrid cross between two heterozygous individuals (AaBb × AaBb) with independent assortment, what fraction of the offspring is expected to show both recessive phenotypes?

  1. 1/4
  2. 3/16
  3. 1/16 Correct answer.
  4. 9/16

Explanation

Each recessive phenotype appears with probability 1/4 in a monohybrid cross of heterozygotes. Because the two genes assort independently, the joint probability is 1/4 × 1/4 = 1/16.

Question 3Biology (Grade 12) - content domains not enumerated in source

Which structure in the human nephron is the principal site of glucose reabsorption from the filtrate?

  1. Distal convoluted tubule
  2. Collecting duct
  3. Proximal convoluted tubule Correct answer.
  4. Descending limb of the loop of Henle

Explanation

The proximal convoluted tubule has a brush border rich in cotransporters that reclaim nearly all filtered glucose by secondary active transport with sodium. Later segments normally handle little or no glucose because it has already been removed.

Question 4Biology (Grade 12) - content domains not enumerated in source

A plant is kept in continuous darkness for several days. Which immediate consequence is expected in its leaves?

  1. Chlorophyll synthesis increases to capture residual light
  2. Stored starch is mobilised and respiration continues, releasing carbon dioxide Correct answer.
  3. The rate of carbon dioxide release by respiration falls sharply
  4. Stomata remain open because photosynthesis continues

Explanation

Without light, the light-dependent reactions stop and photosynthesis ceases, but respiration continues using stored carbohydrate such as starch. The plant therefore behaves as a net producer of carbon dioxide until reserves are depleted.

Question 5Biology (Grade 12) - content domains not enumerated in source

Which event occurs during the anaphase stage of mitosis?

  1. Sister chromatids separate and move toward opposite poles Correct answer.
  2. Homologous chromosomes pair to form bivalents
  3. Chromosomes condense and the nuclear envelope breaks down
  4. The nuclear envelope re-forms around each chromosome set

Explanation

Anaphase begins when cohesion holding sister chromatids is cleaved, allowing each chromatid to be pulled poleward by spindle fibres. Condensation and envelope breakdown occur in prophase, and envelope re-formation occurs in telophase.

Question 6Biology (Grade 12) - content domains not enumerated in source

Which property of the genetic code allows a single base substitution to leave the encoded amino acid unchanged?

  1. Degeneracy Correct answer.
  2. Universality
  3. Non-overlapping nature
  4. Triplet nature

Explanation

Degeneracy means several codons can specify the same amino acid, so a change in the third base often has no effect on the protein. Universality, non-overlap and the triplet reading frame do not provide this tolerance.

Question 7Biology (Grade 12) - content domains not enumerated in source

Which type of immunity is conferred on a newborn by antibodies received through colostrum?

  1. Artificial passive immunity
  2. Artificial active immunity
  3. Natural active immunity
  4. Natural passive immunity Correct answer.

Explanation

Antibodies transferred naturally from mother to infant, whether across the placenta or in colostrum, give immediate but temporary protection without the infant making its own response. This is therefore natural passive immunity.

Question 8Biology (Grade 12) - content domains not enumerated in source

In a population at Hardy–Weinberg equilibrium, the frequency of a recessive allele is 0.3. What proportion of the population is expected to be heterozygous?

  1. 0.21
  2. 0.49
  3. 0.42 Correct answer.
  4. 0.09

Explanation

If q = 0.3, then p = 0.7, and the heterozygote frequency is 2pq = 2 × 0.7 × 0.3 = 0.42. The homozygous recessive frequency q² would be 0.09 and the homozygous dominant p² would be 0.49.

Question 9Biology (Grade 12) - content domains not enumerated in source

Which enzyme relieves the torsional stress that builds ahead of a replication fork as the DNA helix unwinds?

  1. DNA polymerase III
  2. Primase
  3. DNA topoisomerase Correct answer.
  4. DNA ligase

Explanation

Topoisomerases cut and rejoin DNA strands to release supercoiling generated by unwinding, allowing the fork to advance. Ligase joins nicks, polymerase extends chains, and primase lays down RNA primers.

Question 10Biology (Grade 12) - content domains not enumerated in source

Which feature distinguishes a monocot stem from a dicot stem in transverse section?

  1. Scattered vascular bundles throughout the ground tissue Correct answer.
  2. Vascular bundles with a cambium layer
  3. Vascular bundles arranged in a ring
  4. Presence of a distinct pith at the centre

Explanation

Monocot stems show scattered vascular bundles distributed through the ground tissue, whereas dicot stems arrange bundles in a ring around a central pith. Many monocot bundles also lack cambium, limiting secondary growth.

Question 11Biology (Grade 12) - content domains not enumerated in source

A person living at high altitude shows an increased red blood cell count. Which mechanism best explains this adaptation?

  1. Reduced oxygen affinity of haemoglobin in the lungs
  2. Increased erythropoietin secretion stimulated by low oxygen tension Correct answer.
  3. Greater diffusion of oxygen through the skin
  4. Increased carbon dioxide retention in the plasma

Explanation

Low partial pressure of oxygen in the air reduces blood oxygen saturation, and the kidneys respond by secreting erythropoietin. This hormone stimulates red bone marrow to produce more red blood cells, improving oxygen-carrying capacity.

Question 12Biology (Grade 12) - content domains not enumerated in source

Which hormone promotes the conversion of glycogen to glucose in the liver during fasting?

  1. Thyroxine
  2. Glucagon Correct answer.
  3. Aldosterone
  4. Insulin

Explanation

Glucagon, released by pancreatic alpha cells when blood glucose falls, stimulates hepatic glycogenolysis and gluconeogenesis to raise blood glucose. Insulin has the opposite effect, promoting glucose uptake and storage.

Question 13Biology (Grade 12) - content domains not enumerated in source

In a food chain, only about 10% of the energy at one trophic level is transferred to the next. Which statement best explains this low efficiency?

  1. Energy transfer between trophic levels is governed solely by biomass
  2. Decomposers return most energy directly to the primary producers
  3. Predators store almost all the energy they consume as new biomass
  4. Energy is lost mainly as heat during respiration and in undigested material Correct answer.

Explanation

Most assimilated energy is dissipated as heat through respiration, and further energy remains in unconsumed or undigested parts. Only the small fraction retained as new biomass is available to the next trophic level.

Question 14Biology (Grade 12) - content domains not enumerated in source

Which component of a nucleotide carries the negative charge that gives DNA its acidic character?

  1. The nitrogenous base
  2. The deoxyribose sugar
  3. The phosphate group Correct answer.
  4. The hydrogen bonds between strands

Explanation

The phosphate group is ionised at cellular pH, bearing a negative charge on its oxygen atoms, which is why nucleic acids are acidic. The sugar and base carry no comparable charge, and hydrogen bonds are not part of the backbone.

Question 15Biology (Grade 12) - content domains not enumerated in source

A mutation changes a codon from UAC to UAA in the middle of a coding sequence. What is the most likely effect on the polypeptide?

  1. A single amino acid is substituted
  2. Translation terminates prematurely Correct answer.
  3. The reading frame shifts downstream
  4. No change occurs in the protein

Explanation

UAA is a stop codon, so the ribosome will terminate translation at that point, producing a truncated polypeptide. This is a nonsense mutation, distinct from a missense or frameshift change.

Question 16Biology (Grade 12) - content domains not enumerated in source

Which sequence correctly describes the pathway of a nerve impulse across a chemical synapse?

  1. Vesicle fusion → calcium influx → receptor binding → neurotransmitter release → postsynaptic potential
  2. Sodium influx → vesicle fusion → neurotransmitter release → receptor binding → calcium influx
  3. Neurotransmitter release → calcium influx → receptor binding → vesicle fusion → postsynaptic potential
  4. Calcium influx → vesicle fusion → neurotransmitter release → receptor binding → postsynaptic potential Correct answer.

Explanation

Arrival of an action potential opens voltage-gated calcium channels in the presynaptic terminal, and calcium entry triggers vesicle fusion and exocytosis of neurotransmitter. The transmitter then binds postsynaptic receptors, generating a graded potential.

Question 17Biology (Grade 12) - content domains not enumerated in source

Which of the following is a characteristic of the phylum Arthropoda?

  1. Soft unsegmented body with a calcareous shell
  2. Joint appendages and a chitinous exoskeleton Correct answer.
  3. Radial symmetry with a water vascular system
  4. Presence of a notochord at some stage

Explanation

Arthropods are segmented animals with jointed appendages and a chitinous exoskeleton that is shed during moulting. Radial symmetry and water vascular systems belong to echinoderms, while a notochord characterises chordates.

Question 18Biology (Grade 12) - content domains not enumerated in source

In a cross between a heterozygous tall pea plant (Tt) and a dwarf plant (tt), what proportion of offspring is expected to be dwarf?

  1. 1/2 Correct answer.
  2. All offspring
  3. 1/4
  4. 3/4

Explanation

The heterozygote produces T and t gametes in equal numbers, while the dwarf parent produces only t gametes. Half the offspring are tt and therefore dwarf, and half are Tt and tall.

Question 19Biology (Grade 12) - content domains not enumerated in source

Which structure of the human eye is responsible for the greatest refraction of incoming light?

  1. Cornea Correct answer.
  2. Iris
  3. Retina
  4. Lens

Explanation

Most of the bending of light occurs at the air–cornea interface because of the large difference in refractive index. The lens provides fine focusing through accommodation rather than the bulk of the refraction.

Question 20Biology (Grade 12) - content domains not enumerated in source

Which of the following best describes the role of restriction endonucleases in recombinant DNA technology?

  1. Amplifying DNA through repeated thermal cycling
  2. Joining DNA fragments by forming phosphodiester bonds
  3. Cutting DNA at specific palindromic recognition sites Correct answer.
  4. Synthesising cDNA from an mRNA template

Explanation

Restriction endonucleases recognise specific palindromic sequences and cleave the DNA, often producing sticky ends useful for ligation. Ligation, reverse transcription and PCR are performed by different enzymes.

Question 21Biology (Grade 12) - content domains not enumerated in source

Why is the loss of a keystone species expected to have a disproportionate effect on an ecosystem?

  1. It occupies the lowest trophic level in every food web
  2. It is the only species capable of photosynthesis in the habitat
  3. It is always the most abundant species in the community
  4. It has a large influence on community structure relative to its abundance Correct answer.

Explanation

A keystone species exerts influence on community structure far beyond what its abundance would suggest, for example by controlling prey or competitor populations. Its removal can therefore trigger cascading changes throughout the food web.

Question 22Biology (Grade 12) - content domains not enumerated in source

In the light-dependent reactions of photosynthesis, what is the immediate fate of the electrons released when water is split at photosystem II?

  1. They replace electrons lost by the reaction centre of photosystem II Correct answer.
  2. They reduce NADP+ to NADPH
  3. They are used to phosphorylate ADP directly
  4. They are transferred to oxygen to form water

Explanation

Electrons from the photolysis of water replace those lost by the reaction centre of photosystem II, P680. Transport toward photosystem I follows light-driven excitation and transfer of electrons from the reaction centre to an acceptor, so it is a later step.

Question 23Biology (Grade 12) - content domains not enumerated in source

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What is the probability that a son born to them will be affected?

  1. 0
  2. 1/2 Correct answer.
  3. 1
  4. 1/4

Explanation

A son inherits his X chromosome from his mother, and a carrier mother passes the affected allele to half of her sons. The father contributes a Y chromosome, so his genotype does not protect the son.

Question 24Biology (Grade 12) - content domains not enumerated in source

Which feature of the mammalian alveolus most directly facilitates efficient gas exchange?

  1. Ciliated epithelium lining the alveolar cavity
  2. A large surface area with a thin, moist epithelial lining Correct answer.
  3. A thick, muscular wall for active transport of gases
  4. Cartilage rings supporting the alveolar wall

Explanation

Alveoli provide an enormous total surface area with walls only one cell thick and a moist film, so oxygen and carbon dioxide diffuse rapidly. Surrounding capillaries maintain a steep concentration gradient.

Question 25Biology (Grade 12) - content domains not enumerated in source

Which statement correctly describes the role of tRNA in translation?

  1. It delivers specific amino acids to the ribosome by matching its anticodon to the mRNA codon Correct answer.
  2. It forms the structural core of the ribosome
  3. It catalyses the formation of peptide bonds between amino acids
  4. It carries the genetic message from DNA to the ribosome

Explanation

Each tRNA is charged with a particular amino acid and bears an anticodon that base-pairs with the complementary codon on mRNA. This ensures the correct amino acid is positioned for peptide bond formation.

Question 26Biology (Grade 12) - content domains not enumerated in source

Which plant tissue is chiefly responsible for the transport of water and dissolved minerals from roots to leaves in a flowering plant?

  1. Epidermis
  2. Phloem
  3. Cambium
  4. Xylem Correct answer.

Explanation

Xylem consists of dead tracheary elements whose lignified walls form continuous tubes, allowing water and dissolved minerals to move upward under tension from transpiration. Phloem carries organic solutes, not the main water stream, and cambium is a meristem.

Question 27Biology (Grade 12) - content domains not enumerated in source

During which stage of meiosis does crossing over between homologous chromosomes occur?

  1. Metaphase I
  2. Anaphase II
  3. Prophase I Correct answer.
  4. Telophase II

Explanation

Crossing over takes place during prophase I, when homologous chromosomes are paired as bivalents and non-sister chromatids exchange segments at chiasmata. Later stages separate the already recombined chromosomes.

Question 28Biology (Grade 12) - content domains not enumerated in source

Which blood vessel carries deoxygenated blood from the right ventricle of the mammalian heart to the lungs?

  1. Pulmonary artery Correct answer.
  2. Coronary artery
  3. Pulmonary vein
  4. Aorta

Explanation

The pulmonary artery leaves the right ventricle and carries deoxygenated blood to the lungs, making it the artery that does not carry oxygenated blood. The pulmonary vein returns oxygenated blood to the left atrium.

Question 29Biology (Grade 12) - content domains not enumerated in source

Which nitrogenous base is found in RNA but not in DNA?

  1. Guanine
  2. Adenine
  3. Thymine
  4. Uracil Correct answer.

Explanation

RNA contains uracil in place of the thymine found in DNA, while adenine, guanine and cytosine occur in both nucleic acids. Uracil pairs with adenine during transcription and translation.

Question 30Biology (Grade 12) - content domains not enumerated in source

Which group of organisms is characterised by a notochord, a dorsal hollow nerve cord and pharyngeal gill slits at some stage of life?

  1. Arthropoda
  2. Mollusca
  3. Chordata Correct answer.
  4. Annelida

Explanation

These three features, together with a post-anal tail, are the defining hallmarks of phylum Chordata at some developmental stage. Arthropods, molluscs and annelids are invertebrates lacking a notochord.

Question 31Biology (Grade 12) - content domains not enumerated in source

Which type of plastid is responsible for storing starch in storage organs such as potato tubers?

  1. Chloroplast
  2. Ribosome
  3. Chromoplast
  4. Leucoplast Correct answer.

Explanation

Leucoplasts are colourless plastids that store starch, oil or protein, and the starch-storing form is called an amyloplast. Chloroplasts carry out photosynthesis and chromoplasts contain pigments.

Question 32Biology (Grade 12) - content domains not enumerated in source

Which vitamin deficiency in humans is most directly associated with impaired blood clotting?

  1. Vitamin B12
  2. Vitamin C
  3. Vitamin A
  4. Vitamin K Correct answer.

Explanation

Vitamin K is required for the synthesis of several clotting factors, including prothrombin, in the liver, so its deficiency prolongs clotting time. Vitamin C relates to collagen formation and scurvy.

Question 33Biology (Grade 12) - content domains not enumerated in source

Which process in the nitrogen cycle converts atmospheric nitrogen gas into ammonia?

  1. Nitrogen fixation Correct answer.
  2. Nitrification
  3. Denitrification
  4. Ammonification

Explanation

Nitrogen fixation reduces inert atmospheric nitrogen to ammonia, carried out by free-living bacteria such as Azotobacter and by symbiotic Rhizobium in root nodules. Nitrification instead oxidises ammonia to nitrite and nitrate.

Question 34Biology (Grade 12) - content domains not enumerated in source

A cell with 2n = 24 chromosomes undergoes meiosis. What is the chromosome number of each gamete produced, assuming normal division?

  1. 6
  2. 24
  3. 48
  4. 12 Correct answer.

Explanation

Meiosis halves the diploid chromosome number, so a cell with 2n = 24 produces haploid gametes containing n = 12 chromosomes. The two meiotic divisions reduce chromosome number while keeping the DNA content of each gamete constant.

Question 35Biology (Grade 12) - content domains not enumerated in source

In a dihybrid cross between two plants heterozygous for both genes (RrYy x RrYy), assuming independent assortment, what fraction of offspring is expected to show the recessive phenotype for both traits?

  1. 1/4
  2. 1/16 Correct answer.
  3. 9/16
  4. 3/16

Explanation

Each recessive phenotype appears with probability 1/4, and independent assortment allows the probabilities to be multiplied, giving 1/4 x 1/4 = 1/16 for the double recessive class. This is the smallest of the four phenotypic classes.

Question 36Biology (Grade 12) - content domains not enumerated in source

A solution of pH 3 contains how many times more hydrogen ion concentration than a solution of pH 5?

  1. 1000 times
  2. 100 times Correct answer.
  3. 2 times
  4. 10 times

Explanation

The pH scale is logarithmic, so each unit represents a tenfold change in hydrogen ion concentration. A difference of two pH units therefore corresponds to 10 x 10 = 100 times greater concentration.

Question 37Biology (Grade 12) - content domains not enumerated in source

Which feature of an enzyme explains why its activity falls sharply at temperatures well above the optimum?

  1. Reduced activation energy
  2. Increased collision frequency
  3. Denaturation of the active site Correct answer.
  4. Increased substrate concentration

Explanation

High temperature disrupts the hydrogen and ionic bonds maintaining tertiary structure, so the active site loses its complementary shape and substrate binding fails. The fall in activity reflects loss of conformation, not a change in substrate supply.

Question 38Biology (Grade 12) - content domains not enumerated in source

In the human kidney, in which region does the bulk of glucose reabsorption from the filtrate take place?

  1. Collecting duct
  2. Proximal convoluted tubule Correct answer.
  3. Descending limb of the loop of Henle
  4. Distal convoluted tubule

Explanation

The proximal convoluted tubule has a brush border rich in transport proteins and reabsorbs essentially all filtered glucose by active transport under normal conditions. Later segments handle water, ions and fine adjustment of urine composition.

Question 39Biology (Grade 12) - content domains not enumerated in source

Which change occurs in the respiratory system during strenuous exercise?

  1. Increased depth and rate of breathing Correct answer.
  2. Reduced tidal volume
  3. Decreased cardiac output
  4. Lowered body temperature

Explanation

Exercise raises carbon dioxide and hydrogen ion levels in blood, stimulating chemoreceptors to increase both the rate and depth of breathing so that gas exchange matches the higher metabolic demand. Cardiac output rises rather than falls.

Question 40Biology (Grade 12) - content domains not enumerated in source

In a test cross, an organism showing the dominant phenotype is crossed with a homozygous recessive individual. What is the purpose of this cross?

  1. To increase the number of dominant offspring
  2. To produce pure-breeding lines directly
  3. To establish linkage between genes
  4. To determine whether the dominant individual is homozygous or heterozygous Correct answer.

Explanation

A test cross reveals the genotype of a dominant-phenotype individual: homozygous dominant gives all dominant offspring, whereas heterozygous gives a 1:1 ratio. It is a diagnostic cross rather than a breeding method for increasing dominants.

Question 41Biology (Grade 12) - content domains not enumerated in source

Which statement best explains why the light-independent reactions of photosynthesis stop soon after light is withdrawn?

  1. Carbon dioxide can no longer diffuse into the leaf
  2. Rubisco is denatured in the absence of light
  3. The supply of ATP and NADPH from the light reactions ceases Correct answer.
  4. Chlorophyll is permanently destroyed by darkness

Explanation

The Calvin cycle consumes ATP and NADPH produced by the light-dependent reactions, so without illumination these supplies are exhausted and carbon fixation soon halts. The enzymes themselves remain intact and resume activity when light returns.

Question 42Biology (Grade 12) - content domains not enumerated in source

A bacterial population is exposed to an antibiotic, and a few cells survive because they already carry a resistance gene. Which statement best describes this situation?

  1. The surviving cells learned to resist the drug
  2. The antibiotic selected pre-existing resistant variants Correct answer.
  3. The antibiotic induced a resistance mutation in each cell
  4. Resistance arose because the bacteria needed to survive

Explanation

Resistance mutations arise randomly and independently of the drug; the antibiotic merely kills susceptible cells and allows already-resistant variants to reproduce. This is directional selection acting on existing variation.

Question 43Biology (Grade 12) - content domains not enumerated in source

In a community, a fungus obtains nutrients from a living plant while causing measurable harm to that plant. Which term correctly describes this relationship?

  1. Amensalism
  2. Parasitism Correct answer.
  3. Mutualism
  4. Commensalism

Explanation

Parasitism is a relationship in which one partner benefits at the expense of the other, exactly as described here. Mutualism benefits both partners and commensalism benefits one without affecting the other.

Question 44Biology (Grade 12) - content domains not enumerated in source

A man of blood group AB marries a woman of blood group O. Which blood groups are possible among their children?

  1. O only
  2. AB and O only
  3. A and B only Correct answer.
  4. A, B and AB

Explanation

The father contributes either the IA or the IB allele, while the mother can contribute only the i allele. Offspring are therefore IAi (group A) or IBi (group B), and neither AB nor O is possible.

Question 45Biology (Grade 12) - content domains not enumerated in source

Which structural feature of the small intestine most increases the surface area available for absorption?

  1. Circular muscle layer
  2. Mesentery
  3. Villi and microvilli Correct answer.
  4. Serosa

Explanation

Finger-like villi project into the lumen, and their epithelial cells bear microvilli, multiplying the absorptive surface many times over. The muscle layers and serosa serve movement and protection rather than absorption.

Question 46Biology (Grade 12) - content domains not enumerated in source

A cell is placed in a hypertonic solution. What is the most likely immediate consequence for an animal cell?

  1. Water enters and the cell bursts
  2. The cell wall prevents any change in volume
  3. Solute moves into the cell by osmosis
  4. Water leaves the cell and it shrinks Correct answer.

Explanation

In a hypertonic solution the external solute concentration is higher, so water moves out of the cell by osmosis and the cell shrinks. Animal cells lack a rigid wall to resist this loss of volume.

Question 47Biology (Grade 12) - content domains not enumerated in source

A patient's ECG shows no distinct P wave before each QRS complex, although ventricular beats occur at a steady rate. Which structure is most likely failing to conduct normally?

  1. Purkinje fibres
  2. Atrioventricular valve
  3. Semilunar valve
  4. Sinoatrial node Correct answer.

Explanation

The P wave represents atrial depolarisation initiated by the sinoatrial node, so its absence suggests the SA node is not generating normal atrial excitation. The ventricles may still beat through a slower subsidiary pacemaker, giving a steady but abnormal rhythm.

Question 48Biology (Grade 12) - content domains not enumerated in source

In a pedigree, an unaffected couple has an affected son and an affected daughter, and the trait appears in every generation. Which mode of inheritance is most consistent with this pattern?

  1. Autosomal dominant Correct answer.
  2. Y-linked
  3. X-linked recessive
  4. Mitochondrial

Explanation

Affected children of both sexes from an affected parent, appearing in every generation, fit autosomal dominant inheritance, since each child of an affected heterozygote has a 1/2 chance of inheriting the allele. An unaffected couple producing affected children would instead point to a recessive trait.

Question 49Biology (Grade 12) - content domains not enumerated in source

A stretch of template DNA reads 3'-TAC GCA ATG-5'. Assuming standard genetic code and transcription from left to right, what is the sequence of the mRNA produced?

  1. 5'-UAC GCA AUG-3'
  2. 5'-ATG CGT TAC-3'
  3. 5'-AUG CGU UAC-3' Correct answer.
  4. 5'-AUG CGU AAC-3'

Explanation

mRNA is antiparallel and complementary to the template strand, so each base is paired (A with U, T with A, G with C) and the polarity is reversed. Reading the template 3' to 5' gives the mRNA 5'-AUG CGU UAC-3', which begins with a start codon.

Question 50Biology (Grade 12) - content domains not enumerated in source

A lake receives heavy runoff of nitrate and phosphate from farmland. Which sequence of events is most likely to follow?

  1. Algal bloom, then oxygen depletion as decomposers act Correct answer.
  2. Rapid decline of algae, then clearer water
  3. Immediate oxygen increase, then fish population rise
  4. Increase in dissolved oxygen, then fall in nitrate

Explanation

Nutrient enrichment causes eutrophication: algae multiply into a bloom, then die and are decomposed by aerobic bacteria that consume dissolved oxygen. The resulting hypoxia can kill fish and other aerobic organisms.