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UAE TLS Biology Test study materials
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UAE TLS Biology Test practice questions

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Question 1Cell Biology and Biochemistry

Which feature of phospholipid molecules allows them to arrange into a bilayer in an aqueous environment?

  1. They each contain three fatty acid chains attached to glycerol
  2. They are hydrophobic along their entire length
  3. They are joined to one another by strong covalent bonds
  4. They have a hydrophilic head and hydrophobic tails Correct answer.

Explanation

Phospholipids are amphipathic: the phosphate-containing head is polar and interacts with water, while the two fatty acid tails are non-polar and avoid water. In an aqueous environment the tails turn inwards and the heads face outwards, producing a stable bilayer. The molecules are held together mainly by weak hydrophobic interactions and van der Waals forces, not by covalent bonds.

Question 2Cell Biology and Biochemistry

A cell specialised for secreting digestive enzymes would contain an unusually large number of which organelle?

  1. Centriole
  2. Smooth endoplasmic reticulum
  3. Lysosome
  4. Rough endoplasmic reticulum Correct answer.

Explanation

Secreted enzymes are proteins, and proteins destined for export are synthesised by ribosomes attached to the rough endoplasmic reticulum. The rough ER therefore proliferates in cells that secrete large amounts of protein, such as pancreatic acinar cells. The protein then passes through the Golgi apparatus before release.

Question 3Cell Biology and Biochemistry

Which statement best describes the induced fit model of enzyme action?

  1. The substrate permanently changes shape to match the enzyme
  2. The active site is a rigid shape that only one substrate can enter
  3. The active site adjusts slightly as the substrate binds Correct answer.
  4. Enzymes lower activation energy by binding to the reaction products

Explanation

In the induced fit model the active site is not rigid; it moulds itself around the substrate as binding occurs, which brings reactive groups into the correct position. This closer fit strains bonds in the substrate and lowers the activation energy. The change in the enzyme is temporary and reverses when the products leave.

Question 4Cell Biology and Biochemistry

Which structure is found in a prokaryotic cell?

  1. A Golgi apparatus
  2. A membrane-bound nucleus containing DNA
  3. A circular DNA molecule Correct answer.
  4. A mitochondrion

Explanation

Prokaryotes keep their genetic material as a single circular DNA molecule in the cytoplasm, called the nucleoid, with no surrounding nuclear envelope. They also lack membrane-bound organelles such as mitochondria and Golgi apparatus. Their ribosomes are present but are smaller than eukaryotic ribosomes.

Question 5Cell Biology and Biochemistry

Which property of water makes it useful as a coolant in living organisms?

  1. Its high specific heat capacity Correct answer.
  2. Its low latent heat of vaporisation
  3. Its ability to dissolve non-polar substances readily
  4. Its low boiling point at atmospheric pressure

Explanation

Water has a high specific heat capacity because many hydrogen bonds must be broken to raise its temperature, so it absorbs a lot of heat with only a small temperature change. This buffers organisms against sudden temperature swings and makes water an effective coolant. Its high latent heat of vaporisation also allows sweating and transpiration to remove large amounts of heat.

Question 6Cell Biology and Biochemistry

Two monosaccharides are joined to form a disaccharide. Which type of reaction is involved?

  1. Neutralisation
  2. Hydrolysis
  3. Oxidation
  4. Condensation Correct answer.

Explanation

A condensation reaction joins two monosaccharides with the loss of one water molecule, forming a glycosidic bond. Hydrolysis is the reverse process, breaking the bond with the addition of water. Oxidation and neutralisation do not form glycosidic bonds.

Question 7Cell Biology and Biochemistry

Which statement correctly describes osmosis?

  1. Movement of water vapour from a region of high pressure
  2. Movement of ions through channel proteins against a gradient
  3. Movement of water across a partially permeable membrane Correct answer.
  4. Movement of solute particles down their concentration gradient through a membrane

Explanation

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. It is a passive process and requires no energy input. Solute movement and ion movement against a gradient are separate processes.

Question 8Cell Biology and Biochemistry

Which statement is part of the cell theory?

  1. Cells can arise spontaneously from non-living material
  2. All cells contain a membrane-bound nucleus
  3. All cells are visible using a light microscope
  4. All living organisms are made of cells Correct answer.

Explanation

Cell theory states that all living organisms are composed of one or more cells, that the cell is the basic unit of life, and that new cells arise from existing cells. Prokaryotic cells have no nucleus, and some cells are too small to see with a light microscope. Spontaneous generation of cells was disproved.

Question 9Cell Biology and Biochemistry

A substance enters a cell down its concentration gradient through a carrier protein, without any input of ATP. Which process is this?

  1. Facilitated diffusion Correct answer.
  2. Phagocytosis of the substance by the membrane
  3. Active transport using ATP
  4. Simple diffusion directly through the bilayer

Explanation

Facilitated diffusion uses membrane proteins to move substances down their concentration gradient without energy from ATP. It differs from simple diffusion because the substance cannot cross the lipid bilayer on its own. Active transport moves substances against a gradient and requires energy.

Question 10Cell Biology and Biochemistry

An inhibitor reduces an enzyme's rate, but the inhibition becomes less effective when substrate concentration is increased. Which type of inhibitor is this?

  1. An irreversible inhibitor
  2. A feedback inhibitor that binds an allosteric site
  3. A non-competitive inhibitor
  4. A competitive inhibitor Correct answer.

Explanation

A competitive inhibitor binds to the active site, so it competes directly with the substrate. Raising the substrate concentration increases the chance that substrate, rather than inhibitor, occupies the active site, so inhibition is reduced. Non-competitive inhibitors bind elsewhere and their effect is not overcome by more substrate.

Question 11Cell Biology and Biochemistry

How is energy released from ATP?

  1. The ribose sugar is completely oxidised to carbon dioxide
  2. A phosphate group is added to ADP
  3. The adenine base is removed from the molecule
  4. The terminal phosphate group is removed by hydrolysis Correct answer.

Explanation

ATP is hydrolysed to ADP and inorganic phosphate, and the breaking of the terminal phosphoanhydride bond releases a small, usable amount of energy. This reaction is catalysed by ATPase and is reversible during respiration. Adding phosphate to ADP is the energy-requiring synthesis reaction, not release.

Question 12Cell Biology and Biochemistry

Which level of protein structure is determined by the sequence of amino acids in the polypeptide chain?

  1. Primary structure Correct answer.
  2. Tertiary structure
  3. Quaternary structure
  4. Secondary structure

Explanation

The primary structure is the order of amino acids joined by peptide bonds, and it is determined by the gene. This sequence dictates how the chain folds into secondary structures such as alpha helices and beta pleated sheets, and then into the tertiary shape. Quaternary structure refers to the arrangement of more than one polypeptide chain.

Question 13Cell Biology and Biochemistry

Where does glycolysis take place in a eukaryotic cell?

  1. In the cytoplasm Correct answer.
  2. In the nucleus
  3. In the mitochondrial matrix
  4. On the inner mitochondrial membrane

Explanation

Glycolysis occurs in the cytoplasm because its enzymes are dissolved there, and it does not require membrane-bound organelles. It converts glucose to pyruvate with a net gain of two ATP and two reduced NAD. The mitochondrial reactions that follow use pyruvate and yield much more ATP.

Question 14Cell Biology and Biochemistry

A plant cell with a water potential of -400 kPa is placed in a solution with a water potential of -200 kPa. What is the most likely result?

  1. The cell loses water and becomes plasmolysed
  2. There is no net movement of water
  3. The cell gains water and becomes turgid Correct answer.
  4. Water moves out until both reach -400 kPa

Explanation

Water moves by osmosis from the solution with the higher water potential (-200 kPa) into the cell with the lower water potential (-400 kPa). As water enters, the cell's water potential rises until it equals that of the surrounding solution. In a plant cell the incoming water presses the membrane against the cell wall, producing a turgid state.

Question 15Cell Biology and Biochemistry

How does the structure of a triglyceride differ from that of a phospholipid?

  1. It has three fatty acid chains rather than two Correct answer.
  2. It contains glycerol, whereas a phospholipid does not
  3. It contains a phosphate group attached to glycerol
  4. It is formed by hydrolysis of glycerol and fatty acids

Explanation

A triglyceride consists of glycerol esterified to three fatty acid chains, whereas a phospholipid has two fatty acids plus a phosphate-containing group. The phosphate group makes the phospholipid amphipathic, so it can form bilayers. Both molecules are built by condensation reactions, not hydrolysis.

Question 16Cell Biology and Biochemistry

The sodium-potassium pump moves ions against their concentration gradients. Which feature makes this process active transport?

  1. It uses channel proteins
  2. It requires an existing concentration gradient
  3. It moves ions in both directions at once
  4. It requires hydrolysis of ATP Correct answer.

Explanation

Active transport is defined by the movement of substances against their concentration gradient using energy, which in this case comes from ATP hydrolysis. The pump is a carrier protein that changes shape when phosphorylated. Moving ions in two directions is not what makes transport active, and channel proteins are not used by this pump.

Question 17Cell Biology and Biochemistry

The rate of an enzyme-catalysed reaction rises with temperature up to about 40 degrees Celsius, then falls sharply. Which explanation best accounts for the fall?

  1. The enzyme is used up during the reaction
  2. The enzyme becomes denatured Correct answer.
  3. Substrate molecules move too slowly to collide
  4. The activation energy of the reaction increases

Explanation

Above the optimum temperature, increased vibration breaks the hydrogen and ionic bonds that maintain the enzyme's tertiary shape. The active site loses its complementary shape, so the substrate can no longer bind and the rate falls sharply. This denaturation is usually irreversible.

Question 18Cell Biology and Biochemistry

Which component is found in RNA but not in DNA?

  1. A sugar-phosphate backbone
  2. Ribose sugar Correct answer.
  3. Phosphodiester bonds
  4. A phosphate group

Explanation

RNA nucleotides contain ribose, which has a hydroxyl group at the 2-prime position, whereas DNA contains deoxyribose. Both molecules have phosphate groups, a sugar-phosphate backbone and phosphodiester bonds. RNA also contains uracil instead of thymine.

Question 19Cell Biology and Biochemistry

After a cell homogenate is centrifuged at low speed, which organelle is most likely to be found in the pellet?

  1. Nuclei Correct answer.
  2. Fragments of endoplasmic reticulum
  3. Mitochondria
  4. Ribosomes

Explanation

Cell fractionation separates organelles by mass and density, so the largest and densest structures sediment first at low speed. Nuclei form the first pellet; mitochondria and chloroplasts sediment at higher speeds, and ribosomes require very high speeds. This principle underlies the differential centrifugation used to isolate organelles.

Question 20Cell Biology and Biochemistry

A small cell exchanges substances with its surroundings more efficiently than a large cell mainly because it has which feature?

  1. A lower rate of metabolism
  2. A thicker cell membrane
  3. A greater number of organelles per unit volume of cytoplasm
  4. A larger surface area to volume ratio Correct answer.

Explanation

As a cell grows, its volume increases faster than its surface area, so the surface area to volume ratio falls. A small cell therefore has relatively more membrane surface available for each unit of cytoplasm, allowing faster diffusion in and out. This limits the maximum size a cell can reach before dividing.

Question 21Cell Biology and Biochemistry

During oxidative phosphorylation, what drives ATP synthase to produce ATP?

  1. Reduction of NADH at the inner mitochondrial membrane
  2. Electrons passing through ATP synthase to oxygen
  3. Phosphorylation of ADP by a kinase in the matrix
  4. Protons flowing down their electrochemical gradient Correct answer.

Explanation

Electron transport pumps protons from the matrix into the intermembrane space, creating an electrochemical gradient. Protons then flow back through ATP synthase, and the flow causes the enzyme to rotate and join ADP to inorganic phosphate. Electrons pass along the electron transport chain, not through ATP synthase.

Question 22Cell Biology and Biochemistry

An inhibitor increases the Km of an enzyme but leaves Vmax unchanged. Which type of inhibition does this indicate?

  1. Irreversible inhibition
  2. Feedback inhibition at an allosteric site
  3. Non-competitive inhibition
  4. Competitive inhibition Correct answer.

Explanation

Competitive inhibitors raise the apparent Km because more substrate is needed to reach half maximum velocity, but a high substrate concentration can still displace the inhibitor so Vmax is unchanged. Non-competitive inhibitors reduce Vmax without changing Km. Irreversible inhibitors lower the amount of functional enzyme and so also reduce Vmax.

Question 23Cell Biology and Biochemistry

Glucose is absorbed from the gut lumen into epithelial cells even when the glucose concentration inside the cells is higher. Which explanation fits these observations?

  1. Simple diffusion of glucose directly through the phospholipid bilayer
  2. Facilitated diffusion through a glucose carrier protein down its gradient
  3. Co-transport with sodium ions moving down their gradient Correct answer.
  4. Osmosis of water, which carries dissolved glucose along with it

Explanation

Sodium ions are actively pumped out of the epithelial cell into the blood, keeping the sodium concentration low inside the cell. Sodium therefore diffuses from the lumen into the cell through a co-transporter that carries glucose with it, even against the glucose gradient. The glucose then leaves the cell by facilitated diffusion into the blood.

Question 24Cell Biology and Biochemistry

A mutation prevents the signal peptide from being made on a protein that is normally secreted. What is the most likely outcome?

  1. The protein will be synthesised in the smooth endoplasmic reticulum instead
  2. The protein will remain in the cytosol Correct answer.
  3. The protein will be glycosylated in the Golgi apparatus as usual
  4. The protein will be secreted more rapidly than normal

Explanation

The signal peptide directs the ribosome to the rough endoplasmic reticulum, where the polypeptide enters the lumen for folding and modification. Without it, the ribosome stays free in the cytosol and the protein is released there. Such a protein cannot pass through the Golgi apparatus and is not secreted.

Question 25Cell Biology and Biochemistry

A buffer solution resists a fall in pH when a small amount of acid is added because the buffer does which of the following?

  1. Raises the activation energy of dissociation reactions
  2. Removes hydrogen ions by forming strong covalent bonds with them
  3. Accepts the added hydrogen ions Correct answer.
  4. Dilutes the acid to a harmless concentration

Explanation

A buffer contains a weak acid and its conjugate base, and the base accepts the extra hydrogen ions added by the acid. This removes most of the free hydrogen ions from solution, so the pH changes only slightly. Buffers work by reversible proton binding, not by dilution or by covalent bonding.

Question 26Cell Biology and Biochemistry

Which bond is formed when the amino group of one amino acid reacts with the carboxyl group of another amino acid?

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

Explanation

A condensation reaction between an amino group and a carboxyl group joins two amino acids and releases water. The covalent bond formed between the carbon and nitrogen atoms is a peptide bond. Glycosidic and ester bonds link other types of biological molecules.

Question 27Cell Biology and Biochemistry

A protein destined for secretion is labelled with a radioactive trace. Through which sequence of structures will the label pass before leaving the cell?

  1. Rough ER → Golgi apparatus → secretory vesicle → plasma membrane Correct answer.
  2. Golgi apparatus → rough endoplasmic reticulum → nucleus → plasma membrane
  3. Cytoplasm → smooth endoplasmic reticulum → lysosome → plasma membrane
  4. Nucleus → mitochondrion → Golgi apparatus → plasma membrane

Explanation

Secretory proteins are synthesised on ribosomes attached to the rough endoplasmic reticulum. They are then transported to the Golgi apparatus for modification and packaged into secretory vesicles. Fusion of these vesicles with the plasma membrane releases the protein outside the cell.

Question 28Cell Biology and Biochemistry

A phospholipid bilayer allows small nonpolar molecules to cross readily, while ions cross only through transport proteins. Which property best explains this difference?

  1. The hydrophobic fatty acid tails form the interior of the bilayer. Correct answer.
  2. The bilayer is freely permeable to all small molecules regardless of charge.
  3. The hydrophilic phosphate heads face the interior of the bilayer.
  4. The bilayer contains cholesterol, which binds ions tightly.

Explanation

The interior of the bilayer is hydrophobic because the fatty acid tails point inward. Nonpolar molecules can dissolve in and diffuse through this hydrophobic region. Charged ions are repelled by the hydrophobic core and therefore require transport proteins.

Question 29Cell Biology and Biochemistry

Which feature of mitochondria most directly supports the idea that they originated from engulfed bacteria?

  1. They contain their own circular DNA and 70S ribosomes. Correct answer.
  2. They are inherited only from the mother in many species.
  3. They produce ATP by oxidative phosphorylation.
  4. They have a double membrane with folded cristae.

Explanation

Circular DNA and 70S ribosomes are characteristic of bacteria and are not typical of the eukaryotic nucleus and cytosol. Their presence inside mitochondria supports a bacterial ancestry. The other listed features are consistent with mitochondrial function or inheritance but do not point as directly to a bacterial origin.

Question 30Cell Biology and Biochemistry

A drug prevents the degradation of cyclin B during the cell cycle. What is the most likely consequence for a cell that has already entered mitosis?

  1. It cannot enter S phase.
  2. It completes a second round of DNA replication.
  3. Its chromosomes fail to condense.
  4. It cannot exit mitosis. Correct answer.

Explanation

Cyclin B binds CDK1 and helps drive entry into mitosis. For the cell to complete mitosis and exit, cyclin B must be degraded by the APC/C ubiquitin ligase. If that degradation is blocked, CDK1 activity remains high and the cell arrests in M phase.

Question 31Genetics and Heredity

In DNA, which nitrogenous base pairs with adenine?

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

Explanation

In DNA, adenine forms two hydrogen bonds with thymine, while guanine pairs with cytosine. This complementary base pairing keeps the two strands of the double helix matched. Uracil is found in RNA, where it replaces thymine.

Question 32Genetics and Heredity

How many chromosomes are present in a typical human somatic cell?

  1. 92
  2. 23
  3. 46 Correct answer.
  4. 44

Explanation

Human body (somatic) cells are diploid and contain 46 chromosomes arranged as 23 homologous pairs. Gametes are haploid and contain 23 chromosomes after meiosis. The diploid number is restored when gametes fuse at fertilisation.

Question 33Genetics and Heredity

Which term describes the genetic makeup of an organism, such as Aa?

  1. Phenotype
  2. Karyotype
  3. Genotype Correct answer.
  4. Allele

Explanation

Genotype refers to the particular alleles an organism carries, written as letters such as Aa or BB. Phenotype is the observable trait that results from the genotype interacting with the environment. A karyotype is a picture of the chromosome set, and an allele is one version of a gene.

Question 34Genetics and Heredity

An organism carrying two identical alleles for a gene, such as BB, is described as:

  1. Homozygous Correct answer.
  2. Hemizygous
  3. Codominant
  4. Heterozygous

Explanation

Homozygous means both copies of a gene carry the same allele, as in BB or bb. Heterozygous means the two alleles differ, as in Bb. Hemizygous describes having only one copy of a gene, such as X-linked genes in human males.

Question 35Genetics and Heredity

In a monohybrid cross between two heterozygous pea plants (Tt × Tt), what phenotypic ratio is expected among the offspring?

  1. 3:1 Correct answer.
  2. 9:3:3:1
  3. 1:2:1
  4. 1:1

Explanation

A Tt × Tt cross produces 1 TT : 2 Tt : 1 tt genotypes. TT and Tt both show the dominant phenotype, so three-quarters of the offspring are dominant and one-quarter recessive, giving 3:1. The 1:2:1 ratio describes genotypes rather than phenotypes.

Question 36Genetics and Heredity

A person with AB blood type expresses both A and B antigens on the surface of red blood cells. This pattern is an example of:

  1. Pleiotropy
  2. Epistasis
  3. Codominance Correct answer.
  4. Incomplete dominance

Explanation

In codominance both alleles are fully expressed in the heterozygote, as when AB blood shows both A and B antigens. Incomplete dominance would produce an intermediate phenotype instead. Epistasis and pleiotropy describe interactions between different genes or traits.

Question 37Genetics and Heredity

Meiosis produces daughter cells that are best described as:

  1. Diploid and genetically varied
  2. Haploid and genetically varied Correct answer.
  3. Haploid and genetically identical
  4. Diploid and genetically identical

Explanation

Meiosis halves the chromosome number, producing haploid cells. Crossing over and independent assortment of homologous chromosomes make the products genetically varied. Mitosis, by contrast, produces diploid cells that are genetically identical to the parent cell.

Question 38Genetics and Heredity

During transcription, one strand of a gene is used as a template to produce:

  1. A polypeptide
  2. Transfer RNA
  3. A new DNA strand
  4. Messenger RNA Correct answer.

Explanation

Transcription builds a complementary RNA molecule from a DNA template, and for protein-coding genes this RNA is messenger RNA. Translation then uses mRNA to assemble a polypeptide. DNA replication, by contrast, produces a new DNA strand.

Question 39Genetics and Heredity

In a dihybrid cross between two individuals heterozygous for both genes (AaBb × AaBb), assuming independent assortment, what phenotypic ratio is expected?

  1. 9:3:3:1 Correct answer.
  2. 1:1:1:1
  3. 1:2:1
  4. 3:1

Explanation

Independent assortment makes each gene behave as a separate 3:1 cross, and multiplying the two ratios gives 9:3:3:1. This represents 9 dominant-dominant, 3 dominant-recessive, 3 recessive-dominant and 1 recessive-recessive. A 1:1:1:1 ratio would require a test cross with a double recessive partner.

Question 40Genetics and Heredity

A test cross is carried out to determine:

  1. The number of chromosomes in a cell
  2. The rate at which a protein is synthesised
  3. Whether a dominant phenotype is homozygous or heterozygous Correct answer.
  4. The sequence of bases within a gene

Explanation

A test cross mates an individual showing a dominant trait with a homozygous recessive partner. If any recessive offspring appear, the tested parent must be heterozygous; if none appear, it is probably homozygous. The cross reveals allele composition, not chromosome number or gene sequence.

Question 41Genetics and Heredity

In snapdragons, a red-flowered plant (RR) crossed with a white-flowered plant (WW) produces pink-flowered offspring (RW). This pattern of inheritance is:

  1. Epistasis
  2. Codominance
  3. Sex-linked inheritance
  4. Incomplete dominance Correct answer.

Explanation

In incomplete dominance the heterozygote shows a phenotype intermediate between the two homozygotes, as with pink flowers from red and white parents. Neither allele is fully expressed, which differs from codominance where both phenotypes appear together. Epistasis and sex linkage do not explain an intermediate flower colour.

Question 42Genetics and Heredity

A colour-blind man (X^c Y) marries a woman who is homozygous for normal colour vision (X^C X^C). What fraction of their sons is expected to be colour-blind?

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

Explanation

A son inherits his Y chromosome from his father and his X chromosome from his mother. Because the mother carries only normal alleles, every son receives X^C and has normal colour vision. The father's X^c chromosome can only be passed to daughters, who become carriers.

Question 43Genetics and Heredity

Two unaffected parents are both carriers of an autosomal recessive condition. What is the probability that their next child will be affected?

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

Explanation

Two carriers (Aa × Aa) have a one-in-four chance of producing an affected homozygous recessive child (aa) at each pregnancy. Each child's genotype is independent of previous children. Unaffected children still have a two-in-three chance of being carriers.

Question 44Genetics and Heredity

A base substitution changes a codon from GAA to GAG, and both codons specify glutamic acid. This mutation is classified as:

  1. Silent Correct answer.
  2. Nonsense
  3. Missense
  4. Frameshift

Explanation

A silent mutation changes a DNA base but not the amino acid, because the genetic code is degenerate and several codons can specify the same amino acid. Here GAA and GAG both code for glutamic acid. A missense mutation would change the amino acid, and a nonsense mutation would create a stop codon.

Question 45Genetics and Heredity

DNA replication is described as semi-conservative because:

  1. The original DNA molecule remains completely unchanged
  2. Each new molecule contains one original and one new strand Correct answer.
  3. Only one strand of the original molecule is copied
  4. Both strands of each new molecule are newly made

Explanation

Semi-conservative replication means each daughter DNA molecule keeps one parental strand and gains one newly synthesised strand. The parental strands act as templates, so the original base sequence is preserved. The conservative and dispersive models would give different distributions of old and new DNA.

Question 46Genetics and Heredity

A tRNA molecule carries the anticodon UAC. Which amino acid does the matching mRNA codon AUG specify?

  1. Leucine
  2. Tyrosine
  3. Methionine Correct answer.
  4. Tryptophan

Explanation

The codon AUG is the start codon and specifies methionine. The tRNA carrying methionine has the complementary anticodon UAC, which pairs antiparallel with AUG. Tyrosine, leucine and tryptophan are specified by other codons.

Question 47Genetics and Heredity

A karyotype shows three copies of chromosome 21 instead of the normal two. Which condition does this indicate?

  1. Down syndrome Correct answer.
  2. Turner syndrome
  3. Edwards syndrome
  4. Klinefelter syndrome

Explanation

Down syndrome results from trisomy of chromosome 21, giving a total of 47 chromosomes. Turner syndrome is 45,X and Klinefelter syndrome is 47,XXY. Edwards syndrome involves trisomy of chromosome 18.

Question 48Genetics and Heredity

In a population at Hardy–Weinberg equilibrium, the frequency of the dominant allele A is 0.7. What is the expected frequency of heterozygotes?

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

Explanation

If p = 0.7, then q = 1 − p = 0.3. Heterozygote frequency is 2pq = 2 × 0.7 × 0.3 = 0.42. By comparison, p² = 0.49 is the frequency of homozygous dominants and q² = 0.09 is the frequency of homozygous recessives.

Question 49Genetics and Heredity

During which stage of meiosis does crossing over occur?

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

Explanation

Crossing over takes place in prophase I, when homologous chromosomes pair as tetrads and exchange equivalent segments. This recombination creates new combinations of alleles on the chromatids. Later meiotic stages separate the chromosomes rather than exchanging material.

Question 50Genetics and Heredity

A man with blood type AB has children with a woman who has blood type O. Which blood types are possible among their children?

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

Explanation

The father's genotype is I^A I^B and the mother's is ii. Each child receives either I^A or I^B from the father and i from the mother, giving type A (I^A i) or type B (I^B i). Type AB or type O children cannot be produced by this pair.