Molecular Biology and Genomics Practice Questions

Q: What are the small DNA segments termed that are produced during discontinuous DNA replication?

Okazaki fragments. **Explanation:** DNA replication is **semi-discontinuous** because DNA polymerase can only synthesize DNA in the **5' to 3' direction**. While the leading strand is synthesized continuously toward the replication fork, the lagging strand must be synthesized in the opposite direction (away from the fork). **1. Why Okazaki fragments is correct:** To accommodate the 5' to 3' synthesis requirement on the lagging strand, DNA is synthesized in short, discrete segments. These segments are called **Okazaki fragments** (named after Reiji and Tsuneko Okazaki). Each fragment begins with an RNA primer synthesized by **Primase**, which is later removed and replaced with DNA by DNA Polymerase I, and the nicks are sealed by **DNA Ligase**. **2. Why other options are incorrect:** * **Crick strands / Watson fragments:** These are distractors named after Francis Crick and James Watson, who discovered the double-helix structure of DNA. There are no specific "fragments" or "strands" named after them in the context of replication. * **Tsuneko strands:** While Tsuneko Okazaki was the co-discoverer of these fragments, the term "Tsuneko strands" is not standard biological nomenclature. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **DNA Ligase:** The enzyme responsible for joining Okazaki fragments. A deficiency in DNA ligase can lead to impaired DNA repair and replication (e.g., Bloom Syndrome). * **Directionality:** Remember that both strands are *read* 3'→5' but *synthesized* 5'→3'. * **Length:** In eukaryotes, Okazaki fragments are typically 100–200 nucleotides long, whereas in prokaryotes, they are much longer (1000–2000 nucleotides). * **Topoisomerase:** Relieves torsional strain (supercoiling) ahead of the replication fork; targeted by drugs like **Fluoroquinolones** (DNA Gyrase) and **Etoposide/Irinotecan**.

Q: Angelman syndrome is due to what genetic mechanism?

Uniparental disomy. **Explanation:** Angelman syndrome (AS) is a classic example of **Genomic Imprinting**, where the expression of a gene depends on whether it is inherited from the mother or the father. It is caused by the loss of function of the **UBE3A gene** on chromosome 15 (15q11-q13). While the most common cause is a maternal deletion, **Paternal Uniparental Disomy (UPD)**—where a child inherits two copies of chromosome 15 from the father and none from the mother—accounts for approximately 3–7% of cases. Since the paternal UBE3A gene is normally silenced (imprinted), UPD results in a total lack of active UBE3A expression in the brain. **Analysis of Options:** * **Mitochondrial inheritance (A):** This involves genes located in the mitochondrial DNA, passed exclusively from the mother (e.g., MELAS, LHON). AS involves nuclear DNA on chromosome 15. * **Chimerism (B):** This refers to an individual composed of cells from two different zygotes. It is not the mechanism for imprinting disorders. * **Mosaicism (C):** This is the presence of two or more populations of cells with different genotypes in one individual (e.g., some cells having Trisomy 21 and others being normal). While rare mosaic forms of AS exist, UPD is a primary recognized mechanism. **High-Yield Clinical Pearls for NEET-PG:** * **Prader-Willi Syndrome (PWS):** The "sister" condition caused by **Maternal UPD** (or paternal deletion) of the same 15q11-q13 region. * **Mnemonic:** **M**aternal deletion/Paternal UPD = **A**ngelman (**M**appy Child: **M**aternal **A**ngelman, **P**uppet-like gait, **P**aroxysms of laughter). * **Clinical Features of AS:** "Happy Puppet" syndrome—inappropriate laughter, seizures, ataxia, and severe intellectual disability.

Q: Secretory proteins are synthesized in which cellular organelle?

Endoplasmic reticulum. **Explanation:** The synthesis of proteins destined for secretion, membrane integration, or lysosomal enzymes occurs specifically on the **Rough Endoplasmic Reticulum (RER)**. **1. Why the Endoplasmic Reticulum is correct:** The process begins with the "Signal Hypothesis." While translation initiates on free ribosomes in the cytoplasm, proteins destined for secretion possess a specific **N-terminal signal sequence**. This sequence is recognized by the **Signal Recognition Particle (SRP)**, which halts translation and docks the ribosome-protein complex onto the RER membrane at the **Sec61 translocon**. The protein is then synthesized directly into the RER lumen (co-translational translocation), where it undergoes folding and post-translational modifications (like N-linked glycosylation). **2. Analysis of Incorrect Options:** * **A. Cytoplasm:** Free ribosomes in the cytoplasm synthesize proteins that remain within the cell (e.g., hemoglobin, mitochondrial proteins, or nuclear proteins). * **C & D. Sequential Options:** These are distractors. While the *instruction* starts in the cytoplasm, the actual synthesis of the secretory protein bulk occurs exclusively across the RER membrane into the lumen. **High-Yield Clinical Pearls for NEET-PG:** * **Nissl Bodies:** These are large aggregations of RER found in neurons; they are responsible for synthesizing neurotransmitters (secretory proteins). * **Golgi Apparatus:** After the RER, secretory proteins move to the Golgi for "sorting and packaging" into secretory vesicles. * **I-Cell Disease:** A clinical correlation where a defect in protein tagging (mannose-6-phosphate) in the Golgi leads to the failure of secretory enzymes reaching lysosomes, causing them to be secreted extracellularly instead.

Q: Which of the following amino acid substitutions is considered a conservative mutation?

Alanine-leucine. ### Explanation **Concept Overview:** A **conservative mutation** is a type of missense mutation where one amino acid is replaced by another with **similar physicochemical properties** (e.g., charge, hydrophobicity, or size). Because the new amino acid "mimics" the original, the overall tertiary structure and function of the protein often remain largely unaffected. **Why Option C is Correct:** * **Alanine and Leucine** are both **non-polar, hydrophobic, branched-chain aliphatic amino acids**. * Substituting Alanine with Leucine preserves the hydrophobic nature of the protein core. While the side chain size differs slightly, their chemical behavior is identical, making this a classic example of a conservative substitution. **Analysis of Incorrect Options:** * **A. Glutamic acid (Acidic/Negative) to Glutamine (Neutral/Polar):** This changes a charged residue to an uncharged one, potentially disrupting ionic bonds (salt bridges). * **B. Histidine (Basic/Positive) to Glycine (Smallest/Neutral):** This is a non-conservative change. Glycine provides extreme conformational flexibility and lacks the bulky, charged imidazole ring of Histidine. * **D. Arginine (Basic/Positive) to Aspartic acid (Acidic/Negative):** This is a **radical substitution**. Reversing the charge from positive to negative drastically alters protein folding and enzymatic activity. **NEET-PG High-Yield Pearls:** * **Sickle Cell Anemia:** A classic **non-conservative** mutation where Glutamic acid (polar) is replaced by Valine (non-polar) at the 6th position of the $\beta$-globin chain. * **Transition vs. Transversion:** Transitions (Purine to Purine) are more likely to result in conservative mutations than Transversions (Purine to Pyrimidine). * **Silent Mutation:** A point mutation that changes the codon but results in the *same* amino acid (due to degeneracy of the genetic code).

Q: What sequence is present at the 3' end of tRNA?

CCA sequence. **Explanation:** The correct answer is **B. CCA sequence**. Transfer RNA (tRNA) is a cloverleaf-shaped molecule essential for protein synthesis. The **3' end** of all mature tRNA molecules terminates in the highly conserved sequence **5'-CCA-3'**. This is known as the **acceptor stem**. The amino acid is covalently attached to the hydroxyl (-OH) group of the adenosine (A) residue in this CCA sequence by the enzyme *aminoacyl-tRNA synthetase*. **Analysis of Incorrect Options:** * **A. Poly 'A' tail:** This is a long chain of adenine nucleotides added to the **3' end of mRNA** (not tRNA) to enhance stability and facilitate nuclear export. * **C. Anticodon:** This is a triplet of nucleotides located in the **Anticodon Loop** (at the opposite end of the acceptor stem). It base-pairs with the complementary codon on mRNA to ensure the correct amino acid is incorporated. * **D. D arm:** This is a lateral loop of the tRNA containing **Dihydrouridine**. Its primary function is the recognition of the specific aminoacyl-tRNA synthetase enzyme. **High-Yield NEET-PG Pearls:** 1. **Post-transcriptional modification:** In eukaryotes, the CCA sequence is not coded by the DNA but is added enzymatically by **tRNA nucleotidyltransferase**. 2. **TψC Arm:** Contains Pseudouridine and is responsible for binding the tRNA to the **ribosome** (specifically the 5S rRNA). 3. **Smallest RNA:** tRNA is the smallest (73–93 nucleotides) and is also known as **Soluble RNA (sRNA)**. 4. **Charging:** The process of adding an amino acid to the CCA end is called "charging" or aminoacylation, which requires ATP.

Q: Which of the following conditions is NOT associated with a defect in DNA repair mechanisms?

Huntington's disease. **Explanation:** The correct answer is **Huntington’s disease** because it is a **trinucleotide repeat expansion disorder** (CAG repeats on chromosome 4), not a primary defect in DNA repair. It is characterized by the toxic gain-of-function of the huntingtin protein, leading to neurodegeneration in the striatum (caudate nucleus). **Analysis of Options:** * **Xeroderma Pigmentosum (XP):** A classic DNA repair defect caused by a deficiency in **Nucleotide Excision Repair (NER)**. Patients cannot repair pyrimidine dimers formed by UV light, leading to extreme photosensitivity and early-onset skin cancers. * **Fanconi Anemia:** Caused by defects in a cluster of proteins responsible for repairing **DNA interstrand cross-links**. It presents with bone marrow failure, physical anomalies (e.g., absent radii/thumbs), and a high risk of AML. * **Ataxia-telangiectasia:** Caused by a mutation in the **ATM gene**, which is essential for detecting **DNA double-strand breaks** and activating the p53 pathway. It presents with cerebellar ataxia, telangiectasias, and immunodeficiency. **Clinical Pearls for NEET-PG:** 1. **DNA Repair Mnemonic:** * **NER** defect $\rightarrow$ Xeroderma Pigmentosum. * **Mismatch Repair (MMR)** defect $\rightarrow$ Lynch Syndrome (HNPCC). * **Double-strand break** defect $\rightarrow$ Ataxia-telangiectasia & BRCA1/2 mutations. 2. **Huntington’s Disease** shows **Anticipation** (earlier onset in successive generations), typically during paternal transmission. 3. **Diagnostic Test for Fanconi Anemia:** Chromosomal breakage study using Diepoxybutane (DEB) or Mitomycin C.

Q: Mitochondrial DNA is:

Circular double stranded. **Explanation:** Human mitochondrial DNA (mtDNA) is a unique genetic material distinct from nuclear DNA. It consists of a **circular, double-stranded** molecule (Option A). This structure is reminiscent of bacterial DNA, supporting the **Endosymbiotic Theory**, which suggests mitochondria evolved from ancient aerobic prokaryotes. Each mitochondrion contains multiple copies of this circular genome. It is approximately 16.5 kb in size and encodes 37 genes: 13 for oxidative phosphorylation proteins, 22 for tRNA, and 2 for rRNA. Unlike nuclear DNA, mtDNA lacks introns and is not packaged with histones. **Analysis of Incorrect Options:** * **Option B (Circular single stranded):** While some viruses possess single-stranded circular DNA, human mtDNA is always double-stranded, consisting of a "Heavy" (H) strand and a "Light" (L) strand. * **Option C (Linear double helix):** This describes **nuclear DNA**. Nuclear DNA is organized into linear chromosomes associated with histone proteins, whereas mtDNA remains circular and "naked." **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother because the sperm's mitochondria are degraded upon fertilization. * **Heteroplasmy:** This refers to the presence of a mixture of more than one type of organellar genome (mutated vs. wild-type) within a cell. It explains the variable clinical severity of mitochondrial diseases. * **High Mutation Rate:** mtDNA has a mutation rate 10 times higher than nuclear DNA due to the lack of robust repair mechanisms and proximity to free radicals generated by the electron transport chain. * **Leber’s Hereditary Optic Neuropathy (LHON)** and **MELAS** are classic examples of mitochondrial inheritance disorders.

Q: All of the following disorders are due to defective nucleotide excision repair, EXCEPT?

Hereditary nonpolyposis colon cancer. **Explanation:** The correct answer is **Hereditary Nonpolyposis Colon Cancer (HNPCC)**, also known as Lynch Syndrome. **1. Why HNPCC is the correct answer:** HNPCC is caused by a defect in **Mismatch Repair (MMR)**, not Nucleotide Excision Repair (NER). Mutations typically occur in the *MSH2* or *MLH1* genes. This defect leads to **microsatellite instability (MSI)**, characterized by the accumulation of errors in short, repetitive DNA sequences, significantly increasing the risk of colorectal, endometrial, and ovarian cancers. **2. Analysis of incorrect options (NER defects):** Nucleotide Excision Repair (NER) is responsible for removing "bulky" DNA lesions, such as pyrimidine dimers caused by UV radiation. * **Xeroderma Pigmentosum (XP):** The classic example of an NER defect. Patients have extreme photosensitivity and a 2000-fold increased risk of skin cancer due to the inability to repair UV-induced damage. * **Cockayne Syndrome:** A rare autosomal recessive disorder caused by a defect in transcription-coupled NER. It presents with growth failure, microcephaly, and "progeroid" (premature aging) features, but notably *no* increased risk of skin cancer. * **Trichothiodystrophy (TTD):** Another NER-related disorder characterized by sulfur-deficient brittle hair, developmental delay, and ichthyosis. **3. High-Yield Clinical Pearls for NEET-PG:** * **NER Defect:** Xeroderma Pigmentosum, Cockayne Syndrome, Trichothiodystrophy. * **Mismatch Repair (MMR) Defect:** HNPCC (Lynch Syndrome). * **Base Excision Repair (BER) Defect:** MUTYH-associated polyposis. * **Homologous Recombination Defect:** BRCA 1 & 2 (Breast/Ovarian cancer), Fanconi Anemia, Bloom Syndrome. * **Non-homologous End Joining (NHEJ) Defect:** SCID (Severe Combined Immunodeficiency), Ataxia-telangiectasia.

Question 1

What are the small DNA segments termed that are produced during discontinuous DNA replication?

Accepted Answer

Okazaki fragments

Answer

Crick strands

Answer

Watson fragments

Answer

Tsuneko strands

Question 2

Angelman syndrome is due to what genetic mechanism?

Accepted Answer

Uniparental disomy

Answer

Mitochondrial inheritance

Answer

Chimerism

Answer

Mosaicism

Question 3

Secretory proteins are synthesized in which cellular organelle?

Accepted Answer

Endoplasmic reticulum

Answer

Cytoplasm

Answer

Cytoplasm and then Endoplasmic Reticulum

Answer

Endoplasmic Reticulum and then cytoplasm

Question 4

Which of the following amino acid substitutions is considered a conservative mutation?

Accepted Answer

Alanine-leucine

Answer

Glutamic acid-glutamine

Answer

Histidine-glycine

Answer

Arginine-aspartic acid

Question 5

What sequence is present at the 3' end of tRNA?

Accepted Answer

CCA sequence

Answer

Poly 'A' tail

Answer

Anticodon

Answer

D arm

Question 6

Which of the following conditions is NOT associated with a defect in DNA repair mechanisms?

Accepted Answer

Huntington's disease

Answer

Xeroderma Pigmentosa

Answer

Fanconi anemia

Answer

Ataxia-telangiectasia

Question 7

Radio-labeled DNA was allowed to replicate twice in a non-radioactive environment. Which of the following is true?

Accepted Answer

Half of the DNA will have no radioactivity

Answer

All the strands will have radioactivity

Answer

No strands will have radioactivity

Answer

Three-fourth of the DNA replicated will have radioactivity

Question 8

Mitochondrial DNA is:

Accepted Answer

Circular double stranded

Answer

Circular single stranded

Answer

Linear double helix

Answer

None of these

Question 9

The process which occurs over cytidine residues and often results in gene inactivation is?

Accepted Answer

DNA Methylation

Answer

Gene rearrangement

Answer

Pseudogene formation

Answer

Histone acetylation

Question 10

All of the following disorders are due to defective nucleotide excision repair, EXCEPT?

Accepted Answer

Hereditary nonpolyposis colon cancer

Answer

Xeroderma pigmentosum

Answer

Cockayne syndrome

Answer

Trichothiodystrophy

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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