Genetics — Set 5

Correct Answer: D. Deoxyribose

• **Deoxyribose** = Deoxyribose is a 5-carbon pentose sugar that forms the backbone of the DNA molecule along with phosphate groups. • **Lacks one oxygen atom** — at the 2' carbon position, deoxyribose has an H instead of an OH, which is what distinguishes DNA from RNA. • This structural difference makes DNA more chemically stable than RNA, fitting its role as the long-term repository of genetic information. • 💡 Option A (Glucose) is wrong because glucose is a 6-carbon energy sugar and is not a structural component of any nucleic acid; Option B (Ribose) is wrong because ribose is the sugar found in RNA — it has an extra -OH group at the 2' position, making RNA less stable; Option C (Fructose) is wrong because fructose is a fruit sugar involved in carbohydrate metabolism and is completely unrelated to nucleic acid structure.

Correct Answer: B. Hugo de Vries

• **Hugo de Vries** = De Vries observed sudden, heritable changes in the Evening Primrose (Oenothera lamarckiana) and coined the term 'mutation' for these discontinuous jumps in 1901. • **Challenged gradual evolution** — he argued that new species could arise in a single generation through mutation, not just through Darwin's slow selection. • His Mutation Theory was published in 'Die Mutationstheorie' (1901–1903) and remains a cornerstone of modern genetics. • 💡 Option A (Mendel) is wrong because Mendel's work focused on the laws of inheritance and segregation — he did not study or theorise about spontaneous changes in traits; Option C (Charles Darwin) is wrong because Darwin proposed Natural Selection in 'On the Origin of Species' — he had no knowledge of genetics or mutations at the molecular level; Option D (Lamarck) is wrong because Lamarck proposed the inheritance of acquired characteristics (use and disuse), a concept now disproven and unrelated to mutation theory.

Correct Answer: A. Turner Syndrome

• **Turner Syndrome** = Turner Syndrome (45,X) is caused by the absence of one sex chromosome, giving 45 total chromosomes instead of 46 — a classic aneuploidy. • **Aneuploidy = abnormal chromosome number** — it arises from non-disjunction during meiosis, resulting in monosomy or trisomy. • Turner females typically show short stature, lack of secondary sexual characteristics, and infertility. • 💡 Option B (Hemophilia) is wrong because hemophilia is a sex-linked recessive gene mutation on the X chromosome — the total chromosome count is normal (46); Option C (Sickle cell anemia) is wrong because sickle cell anemia results from a point mutation in the beta-globin gene on chromosome 11 — no change in chromosome number; Option D (Phenylketonuria) is wrong because PKU is an autosomal recessive metabolic disorder caused by a defective enzyme gene, not an abnormality in chromosome number.

Correct Answer: A. Karyotype

• **Karyotype** = A karyotype is a standardised image of all chromosomes in a cell, arranged by size and shape in homologous pairs from largest to smallest. • **Used to detect chromosomal abnormalities** — conditions like Down Syndrome, Turner Syndrome, and Klinefelter Syndrome are diagnosed by examining the karyotype. • Chromosomes are photographed during metaphase when they are most condensed and clearly visible. • 💡 Option B (Genogram) is wrong because a genogram is a multi-generational family tree used in psychology and medicine to track inherited diseases across generations, not a chromosome image; Option C (Punnett Square) is wrong because a Punnett Square is a grid used to predict the probable genotypes of offspring from two parents — it is a probability tool, not a chromosome diagram; Option D (Phenotype) is wrong because phenotype refers to the observable physical traits of an organism, not a diagrammatic representation of chromosomes.

Correct Answer: B. G-C

• **G-C (Guanine–Cytosine)** = Guanine pairs exclusively with Cytosine through three hydrogen bonds, following Chargaff's base-pairing rules. • **A pairs with T via two H-bonds** — the other canonical pair; Purines (A, G) always pair with Pyrimidines (T, C) to maintain a uniform width of the double helix. • G-C pairs are stronger than A-T pairs, which is why GC-rich DNA regions have higher melting temperatures. • 💡 Option A (A-G) is wrong because Adenine and Guanine are both purines — two purines cannot pair as it would make the helix too wide and structurally unstable; Option C (C-T) is wrong because Cytosine and Thymine are both pyrimidines — two pyrimidines cannot pair as the helix would become too narrow; Option D (A-C) is wrong because Adenine is a purine and Cytosine is a pyrimidine but they are not complementary to each other — Adenine's hydrogen bond donors and acceptors only match Thymine.

Correct Answer: B. European royalty

• **European royalty** = Queen Victoria of England was a carrier of the hemophilia B gene (factor IX deficiency) and passed it to multiple European royal families through her descendants. • **Spread to Russia, Spain, and Germany** — Tsarevich Alexei of Russia (Victoria's great-grandson) was the most famous hemophilia patient affected. • Hemophilia is an X-linked recessive disorder, meaning carrier females pass it to affected sons — explaining how Victoria silently spread it across royal lines through marriages. • 💡 Option A (Indian kings) is wrong because there is no documented prevalence of hemophilia in Indian royal lineages — the 'Royal Disease' label is specifically tied to Queen Victoria's European descendants; Option C (Ancient Rome) is wrong because Queen Victoria lived in the 19th century and is the traced origin — Roman royalty predates any documented royal hemophilia lineage; Option D (Egyptian pharaohs) is wrong because Egyptian pharaonic dynasties are separated from Victorian-era European royalty by both geography and millennia — no hemophilia link has been established there.

Correct Answer: C. Codon

• **Codon** = A codon is a triplet of nucleotides in mRNA that specifies which amino acid will be incorporated into a growing polypeptide chain. • **64 possible codons for 20 amino acids** — the genetic code is degenerate (multiple codons can code for the same amino acid) but unambiguous (one codon codes for only one amino acid). • The codon is read by the anticodon of tRNA during translation at the ribosome. • 💡 Option A (Exon) is wrong because an exon is a coding segment of the pre-mRNA gene that remains after splicing — it is a region of DNA/RNA, not the three-nucleotide reading unit itself; Option B (Cistron) is wrong because a cistron is the entire functional gene unit coding for one polypeptide, not the three-nucleotide sub-unit within it; Option D (Operon) is wrong because an operon is a cluster of functionally related genes in prokaryotes controlled by a single promoter — it is a regulatory unit, not a coding triplet.

Correct Answer: D. Phosphodiester bond

• **Phosphodiester bond** = A phosphodiester bond links the 3' carbon of one nucleotide's deoxyribose sugar to the 5' carbon of the next nucleotide via a phosphate group. • **Forms the sugar-phosphate backbone** — this covalent bond gives the DNA strand its structural rigidity and directionality (5'→3'). • DNA polymerase forms phosphodiester bonds during replication, always synthesising in the 5' to 3' direction. • 💡 Option A (Peptide bond) is wrong because peptide bonds connect amino acids to form proteins — they involve the carbonyl carbon of one amino acid and the amino group of the next, with no nucleotides involved; Option B (Ionic bond) is wrong because ionic bonds involve electrostatic attraction between oppositely charged ions — no such bond exists within the DNA backbone; Option C (Hydrogen bond) is wrong because hydrogen bonds connect the two strands of the double helix by pairing complementary bases — they do not link nucleotides within a single strand.

Correct Answer: B. O-

• **O-** = O negative blood carries no A, B, or Rh antigens on the red blood cell surface, so it cannot trigger an immune reaction in recipients of any blood type. • **Essential in emergencies** — when a patient's blood group is unknown and immediate transfusion is needed, O- blood is given to avoid fatal transfusion reactions. • Only about 7% of the population has O- blood, making it a scarce and high-priority donation. • 💡 Option A (A-) is wrong because A- blood carries the A antigen, which would cause a severe reaction in recipients with blood type B or O; Option C (AB+) is wrong because AB+ is the Universal Recipient — it carries A, B, and Rh antigens that would cause reactions in non-AB recipients; Option D (B+) is wrong because B+ blood carries both the B antigen and the Rh antigen, making it incompatible with A, O, and Rh-negative recipients.

Correct Answer: D. Ribosome

• **Ribosome** = Ribosomes are the molecular machines that read mRNA codons and catalyse the formation of peptide bonds between amino acids to build proteins. • **Found in all living cells** — ribosomes occur free in the cytoplasm (for cytosolic proteins) or attached to the rough endoplasmic reticulum (for secretory proteins). • Each ribosome is made of two subunits (large and small) composed of rRNA and ribosomal proteins. • 💡 Option A (Lysosome) is wrong because lysosomes are membrane-bound organelles that contain digestive enzymes for breaking down waste materials — they have no role in protein synthesis; Option B (Mitochondria) is wrong because mitochondria have their own ribosomes and synthesise a few of their own proteins, but the primary site of cellular translation is the cytoplasmic ribosome, not the mitochondrion; Option C (Nucleus) is wrong because the nucleus is the site of transcription (DNA → mRNA) — mRNA then exits the nucleus through nuclear pores to reach ribosomes in the cytoplasm for translation.