Molecular Biology and Genomics Practice Questions

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.

Q: What is the function of mitochondrial DNA?

Encodes proteins of the respiratory chain. ### Explanation **Correct Answer: B. Encodes proteins of the respiratory chain** Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule (approx. 16.5 kb) that follows maternal inheritance. Its primary function is to encode essential components of the **Oxidative Phosphorylation (OXPHOS)** system. Specifically, human mtDNA encodes **13 polypeptides**, all of which are subunits of the respiratory chain complexes (Complex I, III, IV, and V). It also encodes 2 rRNAs and 22 tRNAs required for their translation within the mitochondria. **Why other options are incorrect:** * **Option A:** Proteins of the cell membrane are encoded by **nuclear DNA** and synthesized by ribosomes in the cytosol or on the rough endoplasmic reticulum. * **Option C:** While mitochondria replicate independently of the cell cycle, the overall process of cell replication (mitosis/meiosis) is governed by nuclear DNA and regulatory proteins like cyclins and CDKs. * **Option D:** While mtDNA does form its own specific mitochondrial rRNAs (12S and 16S), the vast majority of cellular rRNA (which forms the cytoplasmic ribosomes) is synthesized in the **nucleolus** by RNA Polymerase I. **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother; thus, mitochondrial diseases (e.g., **MELAS, LHON, MERRF**) affect both sons and daughters, but only daughters can pass the trait to the next generation. * **Heteroplasmy:** The presence of a mixture of more than one type of organellar genome (normal and mutated mtDNA) within a cell. This explains the variable clinical severity of mitochondrial diseases. * **High Mutation Rate:** mtDNA lacks histones and has less efficient repair mechanisms compared to nuclear DNA, making it 10 times more prone to mutations.

Q: Which of the following statements describing restriction endonucleases is true?

They cleave both strands in duplex DNA. **Explanation:** **Restriction Endonucleases (REs)**, often called "molecular scissors," are bacterial enzymes used in recombinant DNA technology. The correct answer is **D** because these enzymes function by recognizing specific palindromic sequences and catalyzing the hydrolysis of phosphodiester bonds on **both strands** of the DNA duplex. **Why the other options are incorrect:** * **Option A:** While many REs (like *EcoRI*) create "sticky" or overhanging ends, others (like *SmaI*) cut straight across the duplex to produce **blunt ends**. Therefore, they do not *always* yield overhanging ends. * **Option B:** REs generally **do not** recognize methylated DNA. In fact, bacteria use **DNA methylation** (via methyltransferases) to protect their own genome from being cleaved by their own restriction enzymes. This is known as the Restriction-Modification System. * **Option C:** REs recognize specific **palindromic sequences** (typically 4–8 base pairs long that read the same 5'→3' on both strands), not triplet repeats. Triplet repeats are associated with genetic disorders like Huntington’s disease. **High-Yield Clinical Pearls for NEET-PG:** * **Type II REs** are the most commonly used in labs because they cut DNA at specific, predictable sites and do not require ATP. * **Restriction Fragment Length Polymorphism (RFLP):** A technique using REs to detect genetic variations (e.g., in sickle cell anemia, where a mutation abolishes a specific *MstII* recognition site). * **Palindromes:** Remember that REs look for symmetry. Example: 5'-GAATTC-3' (the complementary strand is also 5'-GAATTC-3').

Question 1

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 2

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 3

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

Accepted Answer

CCA sequence

Answer

Poly 'A' tail

Answer

Anticodon

Answer

D arm

Question 4

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 5

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 6

Mitochondrial DNA is:

Accepted Answer

Circular double stranded

Answer

Circular single stranded

Answer

Linear double helix

Answer

None of these

Question 7

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 8

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

Question 9

What is the function of mitochondrial DNA?

Accepted Answer

Encodes proteins of the respiratory chain

Answer

Encodes proteins of the cell membrane

Answer

Helps in cell replication

Answer

Formation of rRNA

Question 10

Which of the following statements describing restriction endonucleases is true?

Accepted Answer

They cleave both strands in duplex DNA

Answer

They always yield overhanging single-stranded ends

Answer

They recognize methylated DNA sequences

Answer

They recognize triplet repeats

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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