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GCE A/L Biology study materials
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GCE A/L Biology practice questions

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Question 1Cell Biology and Basic Biochemistry

Which organelle is the principal site of ATP synthesis by oxidative phosphorylation in a eukaryotic cell?

  1. Mitochondrion Correct answer.
  2. Golgi apparatus
  3. Lysosome
  4. Ribosome

Explanation

The inner mitochondrial membrane carries the electron transport chain and ATP synthase, so oxidative phosphorylation occurs there. Ribosomes build polypeptides, the Golgi modifies and sorts proteins, and lysosomes carry out hydrolysis, none of which generate ATP on this scale.

Question 2Cell Biology and Basic Biochemistry

A student views a cell with a light microscope and identifies a rigid outer layer lying outside the plasma membrane. This layer is composed mainly of cellulose microfibrils. Which conclusion is best supported?

  1. The cell is a fungal cell
  2. The cell is a plant cell Correct answer.
  3. The cell is an animal cell
  4. The cell is a prokaryotic cell

Explanation

Cellulose is the characteristic structural polysaccharide of plant cell walls, deposited outside the plasma membrane. Animal cells lack a wall, fungal walls are mainly chitin, and bacterial walls are built from peptidoglycan, so none of those fit the observation.

Question 3Cell Biology and Basic Biochemistry

Which statement correctly describes the behaviour of the phospholipid bilayer of a cell membrane?

  1. Phospholipids can move laterally within the same layer Correct answer.
  2. Phospholipids are covalently bonded to one another
  3. Phospholipids are held in place and cannot move within the bilayer
  4. Phospholipid heads are hydrophobic and tails are hydrophilic

Explanation

The fluid mosaic model describes phospholipids as free to diffuse laterally within their own monolayer, which is why membranes are fluid. They are not covalently cross-linked, and the amphipathic arrangement places hydrophilic heads outward and hydrophobic tails inward.

Question 4Cell Biology and Basic Biochemistry

Which property of water is most directly responsible for its ability to act as a coolant in sweating?

  1. Its high specific heat capacity
  2. Its high dielectric constant
  3. Its high surface tension
  4. Its high latent heat of vaporisation Correct answer.

Explanation

Evaporating water requires breaking many hydrogen bonds, so a large amount of heat is absorbed as latent heat of vaporisation, removing heat from the skin. High specific heat explains thermal buffering of body fluids, surface tension relates to cohesion at surfaces, and dielectric constant concerns solvent behaviour toward ions.

Question 5Cell Biology and Basic Biochemistry

Which bond links two adjacent amino acids in a polypeptide chain?

  1. Peptide bond Correct answer.
  2. Ester bond
  3. Glycosidic bond
  4. Phosphodiester bond

Explanation

A condensation reaction between the carboxyl group of one amino acid and the amino group of the next forms a peptide bond, with loss of water. Glycosidic bonds join sugars, phosphodiester bonds join nucleotides, and ester bonds link glycerol to fatty acids.

Question 6Cell Biology and Basic Biochemistry

An enzyme-catalysed reaction is studied while substrate concentration is increased, with enzyme concentration and temperature kept constant. Which change is expected as substrate concentration rises towards saturation?

  1. The rate remains constant from the start of the experiment
  2. The rate falls steadily because substrate inhibits the enzyme
  3. The rate increases indefinitely in direct proportion to substrate concentration
  4. The rate rises and then plateaus at a maximum value Correct answer.

Explanation

At low substrate concentration most active sites are empty, so rate rises with substrate. As active sites become saturated, the enzyme works at its maximum rate and further substrate has little effect, giving a plateau.

Question 7Cell Biology and Basic Biochemistry

A competitive inhibitor is added to an enzyme-catalysed reaction. Which combination of effects is expected?

  1. Maximum rate is unchanged and more substrate cannot restore the original rate
  2. Maximum rate is unchanged and more substrate can restore the original rate Correct answer.
  3. Maximum rate is reduced and more substrate cannot restore the original rate
  4. Maximum rate is reduced and more substrate restores the original rate

Explanation

A competitive inhibitor binds the active site reversibly, so it competes with substrate; raising substrate concentration outcompetes it and the original maximum rate is regained. The apparent substrate affinity falls, but the maximum rate is unchanged because the inhibitor is displaced at high substrate concentration.

Question 8Cell Biology and Basic Biochemistry

Which feature is shared by facilitated diffusion and active transport across a cell membrane?

  1. Both move solute down its concentration gradient
  2. Both transport only non-polar molecules
  3. Both require hydrolysis of ATP
  4. Both require a membrane transport protein Correct answer.

Explanation

Both processes depend on specific integral membrane proteins, such as channels, carriers or pumps, to move polar solutes across the bilayer. Only active transport consumes ATP and can move solute against the gradient, and both can move charged or polar substances.

Question 9Cell Biology and Basic Biochemistry

In an experiment, a plant tissue is placed in a sucrose solution and loses mass. Which explanation is correct?

  1. The external solution is isotonic and there is no net water movement
  2. The external solution is hypertonic to the cell sap and water leaves the cells Correct answer.
  3. Sucrose is actively transported into the cells, increasing their mass
  4. The external solution is hypotonic to the cell sap and water enters the cells

Explanation

Loss of mass indicates net water movement out of the cells by osmosis. This occurs when the external solution has a lower water potential than the cell sap, that is, it is hypertonic, causing plasmolysis in plant tissue.

Question 10Cell Biology and Basic Biochemistry

Which statement about the induced fit model of enzyme action is correct?

  1. The active site changes shape slightly as the substrate binds, improving the fit Correct answer.
  2. The substrate changes shape permanently to match a rigid active site
  3. The active site is already complementary in shape to the substrate before binding
  4. Enzymes lower the activation energy by raising the substrate concentration

Explanation

The induced fit model proposes that the active site is flexible and adjusts its conformation on binding the substrate, straining bonds and lowering activation energy. The lock-and-key model instead assumes a pre-existing rigid complementary shape.

Question 11Cell Biology and Basic Biochemistry

During which stage of aerobic respiration is carbon dioxide released?

  1. Glycolysis
  2. Link reaction and Krebs cycle Correct answer.
  3. Chemiosmosis
  4. Electron transport chain

Explanation

Decarboxylation occurs when pyruvate is converted to acetyl coenzyme A in the link reaction and during the Krebs cycle, releasing carbon dioxide. Glycolysis releases no carbon dioxide, and the electron transport chain and chemiosmosis involve electron transfer and proton movement only.

Question 12Cell Biology and Basic Biochemistry

Which statement best explains why the Krebs cycle stops when oxygen is unavailable?

  1. Oxygen is required to convert glucose into pyruvate
  2. Without oxygen, NAD and FAD cannot be reoxidised, so the cycle lacks electron acceptors Correct answer.
  3. Oxygen is a substrate that is carboxylated in the cycle
  4. Oxygen directly donates electrons to the Krebs cycle enzymes

Explanation

The Krebs cycle needs NAD and FAD to accept hydrogen, and these are regenerated only when the electron transport chain passes electrons to oxygen. Without oxygen the reduced coenzymes accumulate, the cycle is starved of oxidised coenzymes, and it halts.

Question 13Cell Biology and Basic Biochemistry

Which statement about the light-dependent reactions of photosynthesis is correct?

  1. Photolysis of water supplies electrons to photosystem II and releases oxygen Correct answer.
  2. Photolysis of water supplies electrons to photosystem I and releases carbon dioxide
  3. Carbon dioxide is fixed onto ribulose bisphosphate in this stage
  4. ATP and reduced NADP are consumed to reduce glycerate-3-phosphate

Explanation

In the light-dependent stage, absorbed light energy drives photolysis of water, which replaces electrons lost from photosystem II and releases oxygen as a by-product. Carbon dioxide fixation and reduction of glycerate-3-phosphate belong to the light-independent stage.

Question 14Cell Biology and Basic Biochemistry

Which description of the fluid mosaic model is accurate?

  1. A double layer of protein with lipid molecules embedded within it
  2. A static bilayer in which cholesterol replaces phospholipids
  3. A fluid phospholipid bilayer with proteins dispersed within and across it Correct answer.
  4. A rigid lipid layer with proteins fixed on the outer surface only

Explanation

The fluid mosaic model describes a phospholipid bilayer that is fluid, with proteins arranged in a mosaic pattern, some spanning the bilayer and others on either surface or partly embedded. Cholesterol is present in animal membranes as a modifier of fluidity, not as a replacement for phospholipids.

Question 15Cell Biology and Basic Biochemistry

Which statement about the structure of starch and cellulose is correct?

  1. Starch is a polymer of glucose while cellulose is a polymer of fructose
  2. Both are branched polymers with the same degree of branching
  3. Both are polymers of glucose but differ in the orientation of glycosidic linkages Correct answer.
  4. Both are polymers of glucose joined by identical glycosidic linkages

Explanation

Starch and cellulose are both glucose polymers, but starch uses alpha-glucose with alpha-1,4 linkages while cellulose uses beta-glucose with beta-1,4 linkages. This difference in linkage orientation gives cellulose straight chains able to form hydrogen-bonded microfibrils, whereas starch forms coiled or branched molecules.

Question 16Cell Biology and Basic Biochemistry

A polypeptide is heated strongly and then cooled. Which level of protein structure is most likely to be irreversibly lost?

  1. Secondary and tertiary structure Correct answer.
  2. No structure is affected by heating
  3. Only the primary structure
  4. Primary structure

Explanation

Heating disrupts the hydrogen bonds, ionic interactions and hydrophobic interactions that maintain secondary and tertiary structure, causing denaturation that may be irreversible. The primary structure, the sequence of peptide bonds, is not broken by heating alone.

Question 17Cell Biology and Basic Biochemistry

Which observation would provide direct evidence that a substance enters a cell by active transport rather than facilitated diffusion?

  1. Uptake stops when the membrane is cooled to 0 degrees Celsius
  2. Uptake is faster at higher external concentrations
  3. Uptake is saturable at high external concentrations
  4. Uptake continues when the external concentration is lower than the internal concentration Correct answer.

Explanation

Movement of a solute from a region of lower concentration to one of higher concentration is against the gradient and cannot occur by passive facilitated diffusion, so it requires active transport. Saturability and temperature sensitivity occur in both carrier-mediated processes and do not distinguish them.

Question 18Cell Biology and Basic Biochemistry

A non-competitive inhibitor binds to a site on an enzyme other than the active site. Which explanation of its effect is correct?

  1. It increases the activation energy of the reaction
  2. It permanently destroys the peptide bonds of the enzyme
  3. It changes the tertiary shape of the active site so substrate can no longer bind efficiently Correct answer.
  4. It competes directly with the substrate for the active site

Explanation

Binding at an allosteric site alters the conformation of the enzyme, distorting the active site so the substrate fits poorly, and increasing substrate concentration does not overcome this. The maximum rate therefore falls while the apparent affinity for substrate may be unchanged.

Question 19Cell Biology and Basic Biochemistry

In the light-independent stage of photosynthesis, which statement correctly describes the fate of the carbon atoms entering the Calvin cycle?

  1. Carbon dioxide combines with ribulose bisphosphate to form two molecules of glycerate-3-phosphate Correct answer.
  2. Carbon dioxide is reduced directly by reduced NADP to form glucose
  3. Carbon dioxide is fixed onto pyruvate to form acetyl coenzyme A
  4. Carbon dioxide combines with glycerate-3-phosphate to form ribulose bisphosphate

Explanation

The enzyme rubisco catalyses fixation of carbon dioxide onto the five-carbon ribulose bisphosphate, which immediately splits into two molecules of the three-carbon glycerate-3-phosphate. These are then reduced using ATP and reduced NADP, and ribulose bisphosphate is regenerated.

Question 20Cell Biology and Basic Biochemistry

A cell is placed in a solution of a permeant solute that enters the cell slowly. Which sequence of water movement is expected?

  1. No water movement occurs at any stage
  2. Water leaves first, then the solute enters and water follows, restoring volume Correct answer.
  3. Water enters first, then the solute leaves and water follows, shrinking the cell
  4. Water leaves and the cell shrinks permanently with no recovery

Explanation

Initially the external solution has lower water potential, so water leaves by osmosis and the cell shrinks. As the permeant solute gradually enters, the internal solute concentration rises and water potential falls, so water re-enters and the volume recovers.

Question 21Cell Biology and Basic Biochemistry

Which statement about the role of the electron transport chain in respiration is correct?

  1. It generates a proton gradient across the inner mitochondrial membrane that drives ATP synthase Correct answer.
  2. It phosphorylates ADP by substrate-level phosphorylation in the matrix
  3. It converts pyruvate to lactate in the absence of oxygen
  4. It transfers electrons directly to glucose, producing carbon dioxide

Explanation

Electrons passed along carriers release energy used to pump protons into the intermembrane space, creating a gradient whose return through ATP synthase drives ATP production by chemiosmosis. Substrate-level phosphorylation occurs in glycolysis and the Krebs cycle instead.

Question 22Cell Biology and Basic Biochemistry

Which structural feature of the chloroplast is directly related to its function in absorbing light?

  1. The stroma contains enzymes for photolysis of water
  2. The outer membrane contains ribosomes for protein synthesis
  3. The intermembrane space contains ATP synthase for the light-independent stage
  4. The thylakoid membranes contain stacks of pigment-protein complexes Correct answer.

Explanation

Thylakoid membranes are folded into grana and carry photosystems containing chlorophyll and accessory pigments, providing a large surface area for light absorption. The stroma houses the enzymes of the Calvin cycle, while photolysis occurs on the thylakoid membranes.

Question 23Cell Biology and Basic Biochemistry

Which statement correctly compares the structure of DNA and RNA?

  1. Both contain ribose but DNA contains thymine instead of uracil
  2. DNA is single-stranded while RNA is double-stranded
  3. DNA contains deoxyribose and thymine, while RNA contains ribose and uracil Correct answer.
  4. Both contain deoxyribose but RNA contains uracil instead of thymine

Explanation

DNA has deoxyribose sugar and the base thymine, whereas RNA has ribose and uracil in place of thymine. DNA is normally double-stranded with complementary base pairing, while RNA is usually single-stranded.

Question 24Cell Biology and Basic Biochemistry

Which statement about the role of ATP in cells is correct?

  1. ATP stores large amounts of energy in its adenine base for long-term use
  2. ATP is a permanent energy store comparable to glycogen
  3. ATP can be synthesised only in the presence of light
  4. ATP releases energy when its terminal phosphate group is hydrolysed Correct answer.

Explanation

Hydrolysis of the terminal phosphoanhydride bond of ATP releases energy and produces ADP and inorganic phosphate, which can be used to drive endergonic reactions. ATP is a short-term carrier, not a long-term store, and is regenerated in respiration in the dark as well as the light.

Question 25Cell Division, Genetics and Molecular Biology

During which stage of mitosis do sister chromatids separate and move towards opposite poles of the cell?

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

Explanation

Anaphase begins when the cohesin complexes holding sister chromatids together are cleaved, allowing the spindle fibres to pull each chromatid, now an individual chromosome, towards the pole it faces. Metaphase is the preceding stage where chromosomes align at the equator, and telophase follows anaphase.

Question 26Cell Division, Genetics and Molecular Biology

A cell with the genotype AaBb undergoes meiosis. Assuming the genes are on different chromosome pairs and no crossing over affects these loci, which gamete genotypes are possible?

  1. AB and ab only
  2. AB, Ab, aB and ab in equal proportions Correct answer.
  3. AA, BB, aa and bb only
  4. AaBb and aabb only

Explanation

Independent assortment during meiosis I means the A/a and B/b alleles are distributed independently into gametes, producing four equally frequent types: AB, Ab, aB and ab. This is the basis of the 1:1:1:1 gamete ratio for two unlinked heterozygous loci.

Question 27Cell Division, Genetics and Molecular Biology

Which statement correctly describes the behaviour of homologous chromosomes during meiosis I?

  1. They replicate during prophase I and separate at anaphase II
  2. They align individually on the metaphase plate and separate at anaphase II
  3. They pair as bivalents and separate to opposite poles at anaphase I Correct answer.
  4. They fuse permanently during prophase I to form a single chromosome

Explanation

Homologous chromosomes synapse during prophase I to form bivalents, and at anaphase I the homologues are pulled to opposite poles, halving the chromosome number. Separation of sister chromatids is deferred until anaphase II.

Question 28Cell Division, Genetics and Molecular Biology

In a monohybrid cross between two heterozygous individuals (Aa x Aa) for a completely dominant allele, what is the expected phenotypic ratio in the offspring?

  1. 9 dominant : 3 intermediate : 3 recessive : 1 novel
  2. 1 dominant : 2 intermediate : 1 recessive
  3. 3 dominant : 1 recessive Correct answer.
  4. 1 dominant : 1 recessive

Explanation

A cross of two heterozygotes gives a genotypic ratio of 1 AA : 2 Aa : 1 aa, and because the dominant allele masks expression in the heterozygote, this collapses to 3 dominant : 1 recessive phenotypically.

Question 29Cell Division, Genetics and Molecular Biology

A woman who is a carrier of an X-linked recessive disorder marries an unaffected man. What proportion of their sons is expected to be affected?

  1. One quarter
  2. None
  3. All
  4. One half Correct answer.

Explanation

Sons inherit their single X chromosome from their mother, so half of the sons receive the mutant X allele and are affected because they are hemizygous. Daughters receive a normal X from the father and are therefore unaffected, though half are carriers.

Question 30Cell Division, Genetics and Molecular Biology

In a DNA molecule, adenine pairs with thymine using how many hydrogen bonds, and guanine pairs with cytosine using how many?

  1. Three between A and T; two between G and C
  2. Two between A and T; three between G and C Correct answer.
  3. Two between A and T; two between G and C
  4. Three between A and T; three between G and C

Explanation

A-T base pairs are held by two hydrogen bonds, whereas G-C pairs form three, which is why DNA rich in G and C requires a higher temperature to denature. This complementary pairing underlies accurate replication and transcription.

Question 31Cell Division, Genetics and Molecular Biology

Which enzyme is responsible for unwinding the DNA double helix ahead of the replication fork during DNA replication?

  1. DNA ligase
  2. DNA helicase Correct answer.
  3. DNA polymerase III
  4. RNA primase

Explanation

DNA helicase breaks the hydrogen bonds between complementary bases, separating the strands so that each can act as a template. The other enzymes act later in the process, either synthesising or joining DNA.

Question 32Cell Division, Genetics and Molecular Biology

Which of the following best describes the role of tRNA in translation?

  1. It carries specific amino acids to the ribosome and pairs its anticodon with mRNA codons Correct answer.
  2. It carries the genetic message from DNA in the nucleus to the ribosome
  3. It forms the structural core of the ribosome and catalyses peptide bond formation
  4. It removes introns from pre-mRNA before translation begins

Explanation

Each tRNA is charged with a particular amino acid and has an anticodon that base-pairs with the complementary codon on mRNA, ensuring the correct amino acid is added to the growing polypeptide. This adaptor function is central to translation.

Question 33Cell Division, Genetics and Molecular Biology

A mutation changes a single DNA base so that a codon for glutamic acid becomes a codon for valine, while the reading frame is preserved. What type of mutation is this?

  1. A frameshift mutation
  2. A missense mutation Correct answer.
  3. A silent mutation
  4. A nonsense mutation

Explanation

A missense mutation is a base substitution that changes one amino acid in the polypeptide, as in the glutamic acid to valine change. The protein may still function, though its properties can be altered.

Question 34Cell Division, Genetics and Molecular Biology

Which of the following is a correct description of the structure of a nucleotide in DNA?

  1. A ribose sugar bonded to two phosphate groups and an amino acid
  2. A deoxyribose sugar bonded to a phosphate group and an amino acid
  3. A glucose molecule bonded to a phosphate group and a nitrogenous base
  4. A deoxyribose sugar bonded to a phosphate group and a nitrogenous base Correct answer.

Explanation

A DNA nucleotide consists of a deoxyribose sugar, a phosphate group and one of the four nitrogenous bases, adenine, thymine, guanine or cytosine. Nucleotides are joined into a polynucleotide chain by phosphodiester bonds between the sugar and phosphate.

Question 35Cell Division, Genetics and Molecular Biology

A couple, both with normal colour vision, have a colour-blind son. Which statement best explains this observation?

  1. The trait is autosomal recessive, so both parents must be homozygous recessive
  2. The father carries the colour-blind allele on his Y chromosome and passed it to his son
  3. A new mutation must have arisen in the son, since neither parent is colour-blind
  4. The mother is a carrier of the X-linked recessive allele, and the son inherited her affected X chromosome Correct answer.

Explanation

Colour blindness is X-linked recessive, so an affected son inherits his single X from his mother; she can be phenotypically normal if she carries one normal allele. The father contributes a Y chromosome to his son and so cannot transmit an X-linked allele to him.

Question 36Cell Division, Genetics and Molecular Biology

In a dihybrid cross between two individuals heterozygous at both loci (AaBb x AaBb), what proportion of offspring is expected to show both recessive phenotypes?

  1. Three sixteenths
  2. One quarter
  3. One sixteenth Correct answer.
  4. Nine sixteenths

Explanation

For each locus, the probability of the recessive phenotype is one quarter, and independent assortment means the joint probability is the product, one quarter times one quarter, giving one sixteenth. This corresponds to the aabb class in the 9:3:3:1 ratio.

Question 37Cell Division, Genetics and Molecular Biology

Which of the following correctly describes the outcome of crossing over during prophase I of meiosis?

  1. Reciprocal exchange of segments between non-sister chromatids of homologous chromosomes produces recombinant chromatids Correct answer.
  2. Chromatids lose their centromeres and become independent chromosomes
  3. Sister chromatids exchange identical segments, producing no new allele combinations
  4. Homologous chromosomes exchange whole chromosomes, doubling the chromosome number

Explanation

Crossing over occurs at chiasmata, where non-sister chromatids of a homologous pair break and rejoin reciprocally, creating new combinations of alleles on the recombinant chromatids. This increases genetic variation among the gametes produced.

Question 38Cell Division, Genetics and Molecular Biology

A gene is described as showing codominance. Which observation would support this?

  1. Heterozygotes show an intermediate phenotype between the two homozygotes
  2. Heterozygotes produce only one type of gamete
  3. Heterozygotes are identical in phenotype to one of the homozygotes
  4. Heterozygotes express both alleles fully and simultaneously, as in AB blood group individuals Correct answer.

Explanation

In codominance both alleles are expressed in the heterozygote, so both gene products are detectable, as with the A and B antigens in AB blood. This differs from incomplete dominance, where a blended intermediate phenotype appears.

Question 39Cell Division, Genetics and Molecular Biology

Which of the following statements about the genetic code is correct?

  1. It is degenerate, meaning that most amino acids are specified by more than one codon Correct answer.
  2. Each amino acid is specified by exactly one codon
  3. It contains codons that specify amino acids but no stop signals
  4. It is overlapping, so each base is read as part of three codons

Explanation

The code is degenerate because 64 codons specify only 20 amino acids, so most amino acids correspond to several codons. This redundancy reduces the harmful effects of some point mutations.

Question 40Cell Division, Genetics and Molecular Biology

During PCR, what is the function of the step that heats the reaction mixture to about 95 degrees Celsius?

  1. It denatures the double-stranded DNA template into single strands Correct answer.
  2. It activates DNA polymerase to extend the primers
  3. It degrades unwanted RNA contaminants in the sample
  4. It allows primers to anneal to complementary sequences

Explanation

The high-temperature step breaks the hydrogen bonds between complementary bases, separating the template into single strands so primers can bind in the next step. This denaturation is the first stage of each PCR cycle.

Question 41Cell Division, Genetics and Molecular Biology

Which of the following best describes the difference between a chromosome mutation and a gene mutation?

  1. Chromosome mutations affect only somatic cells, while gene mutations affect only gametes
  2. Chromosome mutations alter the number or structure of whole chromosomes, while gene mutations alter the base sequence of a single gene Correct answer.
  3. Chromosome mutations change DNA to RNA, while gene mutations change RNA to protein
  4. Chromosome mutations are always inherited, while gene mutations are never inherited

Explanation

Chromosome mutations involve changes in chromosome number, such as aneuploidy, or in chromosome structure, such as deletion, inversion or translocation. Gene mutations are changes at the nucleotide level within a single gene.

Question 42Cell Division, Genetics and Molecular Biology

A cell in G1 phase has a DNA content of 2C. After the S phase, what is the DNA content of the cell, and why?

  1. 4C, because each chromosome has been replicated into two sister chromatids Correct answer.
  2. 3C, because the DNA content increases by half during replication
  3. 2C, because the DNA content remains constant throughout interphase
  4. 1C, because the DNA is halved in preparation for division

Explanation

During S phase each chromosome is replicated, so the cell temporarily contains twice the G1 DNA content, 4C, arranged as pairs of sister chromatids. The chromosome number itself is unchanged; only the amount of DNA doubles.

Question 43Cell Division, Genetics and Molecular Biology

In the lac operon of E. coli, what is the role of the repressor protein when lactose is absent?

  1. It binds to the promoter and enhances transcription of the structural genes
  2. It binds to the operator and prevents RNA polymerase from transcribing the structural genes Correct answer.
  3. It acts as an inducer by binding to lactose and activating the operon
  4. It codes for the enzyme beta-galactosidase directly

Explanation

In the absence of lactose, the repressor protein binds to the operator sequence and blocks RNA polymerase from transcribing the lacZ, lacY and lacA genes. When lactose is present, it binds the repressor and causes it to release the operator, allowing transcription.

Question 44Cell Division, Genetics and Molecular Biology

A mutation deletes a single base pair in the middle of a coding sequence. Which consequence is most likely?

  1. Only one amino acid is changed and the rest of the protein is unaffected
  2. The reading frame shifts, so all subsequent codons are altered and the protein is usually non-functional Correct answer.
  3. The resulting protein is always longer because a stop codon is removed
  4. The mRNA is not produced because transcription is blocked

Explanation

A deletion of one base changes the triplet grouping downstream of the mutation, so every subsequent codon is read differently; this frameshift usually produces a truncated, non-functional protein. Such mutations are often more damaging than substitutions.

Question 45Cell Division, Genetics and Molecular Biology

Which of the following correctly describes the relationship between the processes of transcription and translation in a eukaryotic cell?

  1. Transcription and translation both occur simultaneously on the same ribosome in the nucleus
  2. Transcription produces mRNA in the nucleus, and translation occurs on ribosomes in the cytoplasm Correct answer.
  3. Transcription produces tRNA in the cytoplasm, and translation produces mRNA in the nucleus
  4. Transcription produces protein in the nucleus, and translation produces mRNA in the cytoplasm

Explanation

In eukaryotes, DNA is transcribed into pre-mRNA in the nucleus, the transcript is processed and exported, and translation of the mature mRNA into protein occurs on ribosomes in the cytoplasm. The spatial separation is a key difference from prokaryotes.

Question 46Cell Division, Genetics and Molecular Biology

In a population, the frequency of the recessive allele for a particular condition is 0.2. Assuming Hardy-Weinberg equilibrium, what proportion of the population is expected to be heterozygous?

  1. 0.16
  2. 0.64
  3. 0.32 Correct answer.
  4. 0.04

Explanation

With q equal to 0.2, p is 0.8, and the heterozygote frequency is 2pq, which is 2 times 0.8 times 0.2, giving 0.32. The Hardy-Weinberg model assumes random mating, a large population and no selection, mutation or migration.

Question 47Cell Division, Genetics and Molecular Biology

Which of the following best explains why meiosis is described as a reduction division?

  1. The number of genes is reduced by half in each gamete
  2. The chromosome number is halved from diploid to haploid during the first meiotic division Correct answer.
  3. The cell divides twice, so four haploid cells are produced from one diploid cell
  4. The DNA content is halved during interphase before meiosis begins

Explanation

Meiosis I separates homologous chromosomes into two daughter cells, each with half the original chromosome number, so the diploid cell becomes haploid. The second division separates sister chromatids without further reducing the chromosome number.

Question 48Cell Division, Genetics and Molecular Biology

A woman with blood group AB and a man with blood group O have children. Which blood groups are possible in their offspring?

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

Explanation

The woman produces gametes carrying either the A or the B allele, and the man, genotype OO, produces only O gametes. The offspring are therefore either AO, blood group A, or BO, blood group B, with neither AB nor O possible.

Question 49Cell Division, Genetics and Molecular Biology

Which of the following describes the function of the spindle fibres during mitosis?

  1. They attach to kinetochores and move chromosomes to the poles of the cell Correct answer.
  2. They synthesise new DNA to replace damaged strands during cell division
  3. They form the nuclear envelope around the daughter chromosomes
  4. They digest the cell wall to allow the cell to divide

Explanation

Spindle fibres are microtubules that attach to kinetochores on the centromeres and pull chromosomes to opposite poles during anaphase. Their coordinated action ensures each daughter cell receives an identical set of chromosomes.

Question 50Cell Division, Genetics and Molecular Biology

A DNA sample contains 30% of its bases as adenine. What percentage of the bases in this double-stranded molecule is guanine?

  1. 20% Correct answer.
  2. 30%
  3. 40%
  4. 35%

Explanation

By complementary base pairing, the proportion of thymine equals that of adenine, so A + T = 30% + 30% = 60%. The remaining 40% is shared equally between guanine and cytosine, giving 20% guanine.