Molecular Biology and Genomics Practice Questions

Q: Sigma (σ) subunit is present in which of the following?

RNA polymerase. **Explanation:** The **Sigma (σ) subunit** is a critical component of the **Prokaryotic RNA polymerase holoenzyme**. In bacteria, the core enzyme (consisting of subunits α₂ββ'ω) can synthesize RNA but lacks the ability to recognize specific promoter sequences. The addition of the sigma factor converts the core enzyme into the **holoenzyme**, which specifically recognizes and binds to the promoter region (Pribnow box/-10 and -35 sequences), thereby initiating transcription. Once the RNA chain reaches about 10 nucleotides in length, the sigma factor dissociates, and the core enzyme continues elongation. **Analysis of Options:** * **80S Ribosomes (Option A):** These are eukaryotic ribosomal complexes involved in translation. They consist of 40S and 60S subunits, not sigma factors. * **Initiation of DNA replication (Option C):** This process involves proteins like DnaA (recognition), Helicase (unwinding), and Primase, but not the sigma subunit. * **DNA Polymerase (Option D):** This enzyme is responsible for DNA synthesis. While it requires a primer and various accessory proteins (like the sliding clamp), it does not utilize a sigma subunit for initiation. **High-Yield Clinical Pearls for NEET-PG:** * **Rifampicin:** This key antitubercular drug acts by inhibiting the **β-subunit** of bacterial DNA-dependent RNA polymerase, preventing transcription initiation. * **Promoter Recognition:** In eukaryotes, the function of the sigma factor is analogous to **General Transcription Factors (GTFs)** like TFIID. * **Alpha-amanitin:** Found in *Amanita phalloides* mushrooms, it specifically inhibits **RNA Polymerase II**, leading to severe hepatotoxicity.

Q: What constitutes the driving force for the transport of proteins into and out of the nucleus?

GTP hydrolysis within the cytosol. ### Explanation The transport of proteins across the nuclear envelope (through Nuclear Pore Complexes) is an active process regulated by the **Ran-GTPase cycle**. This cycle provides the directionality and energy required for both nuclear import and export. **1. Why Option C is Correct:** The driving force for nuclear transport is the **hydrolysis of GTP to GDP**, which occurs specifically in the **cytosol**. * **The Mechanism:** The enzyme **Ran-GAP** (GTPase Activating Protein) is localized in the cytosol. It triggers Ran-GTP to hydrolyze its bound GTP into GDP. * **The Result:** This hydrolysis maintains a high concentration of Ran-GDP in the cytosol and Ran-GTP in the nucleus. This steep **Ran-GTP gradient** is what powers the release of cargo into the nucleus (import) or the movement of cargo out of the nucleus (export). **2. Why Other Options are Incorrect:** * **Options A & B (ATP):** While many cellular transport mechanisms use ATP, nuclear transport specifically utilizes the energy from **GTP**. * **Option D (GTP hydrolysis in the nucleus):** The nucleus contains **Ran-GEF** (Guanine Nucleotide Exchange Factor), which replaces GDP with GTP. Therefore, the nucleus is a site of **GTP loading**, not hydrolysis. Hydrolysis in the nucleus would destroy the gradient necessary for transport. **3. High-Yield NEET-PG Pearls:** * **Ran-GTP Gradient:** High in the Nucleus; Low in the Cytosol. * **Importins:** Bind cargo in the cytosol (where Ran-GTP is low) and release it in the nucleus (where Ran-GTP is high). * **Exportins:** Bind cargo in the nucleus only when Ran-GTP is present, forming a ternary complex. * **NLS & NES:** Proteins destined for the nucleus have a **Nuclear Localization Signal** (rich in basic amino acids like Lysine and Arginine), while those leaving have a **Nuclear Export Signal**.

Q: Biotechnology is used for which of the following?

Viral vaccine production. **Explanation:** Biotechnology is the broad application of biological systems, living organisms, or derivatives thereof to make or modify products for specific use. In the context of this question, **Viral vaccine production** is a primary and well-established application of biotechnology in medical science. **Why Option A is Correct:** Modern vaccinology relies heavily on recombinant DNA technology. For example, the **Hepatitis B vaccine** is produced by inserting the gene for the Hepatitis B surface antigen (HBsAg) into yeast cells (*Saccharomyces cerevisiae*). Similarly, viral vector vaccines (like those for COVID-19) and mRNA vaccines are direct products of biotechnological engineering. **Analysis of Incorrect Options:** * **Option B (Curing genetic disorders):** While biotechnology is used in **Gene Therapy** to *treat* disorders (e.g., SCID or Spinal Muscular Atrophy), "curing" implies a permanent, universal resolution which is currently experimental and not yet a standard clinical outcome for most genetic diseases. * **Option C (Developing GM crops):** This is an application of **Agricultural Biotechnology**, not typically the focus of medical biochemistry in a clinical NEET-PG context. * **Option D (Gene synthesis):** This is a **laboratory technique** or a tool used *within* biotechnology, rather than an end-goal application like vaccine production. **High-Yield Facts for NEET-PG:** * **First Recombinant Vaccine:** Hepatitis B vaccine (HBsAg). * **Humulin:** The first biosynthetic human insulin developed using *E. coli* (1982). * **PCR (Polymerase Chain Reaction):** The cornerstone of biotechnology, developed by Kary Mullis, used for amplifying DNA. * **Vectors:** Plasmids and bacteriophages are the most commonly used vectors in recombinant DNA technology. * **Restriction Endonucleases:** Known as "molecular scissors," they are essential for creating recombinant molecules.

Q: Which pattern of inheritance is characteristic of mitochondrial DNA?

Mother to both son and daughter transmission. **Explanation:** **1. Why the correct answer is right:** Mitochondrial DNA (mtDNA) follows **Maternal Inheritance** (also known as Non-Mendelian or Extranuclear inheritance). During fertilization, the zygote receives almost all its cytoplasm and organelles from the **ovum**, as the sperm contributes only its nuclear genetic material. Since mitochondria are located in the cytoplasm, they are inherited exclusively from the mother. Therefore, an affected mother will transmit the mitochondrial trait or disease to **all her children** (both sons and daughters). **2. Why the incorrect options are wrong:** * **Options A & B:** These are incorrect because **paternal transmission** of mitochondrial DNA does not occur in humans. A father cannot pass a mitochondrial disease to his offspring because sperm mitochondria are generally degraded or excluded during fertilization. * **Option C:** This is incomplete. While a mother does pass mtDNA to her daughters, she also passes it to her sons. The inheritance is not sex-limited. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Heteroplasmy:** This is a key concept where a cell contains a mixture of both normal and mutated mtDNA. The severity of the disease depends on the ratio of mutant to normal mitochondria (Threshold effect). * **Replication:** mtDNA is circular, double-stranded, and lacks histones. It replicates independently of the cell cycle. * **Common Mitochondrial Diseases:** * **LHON** (Leber’s Hereditary Optic Neuropathy): Sudden painless loss of vision. * **MELAS** (Mitochondrial Encephalopathy, Lactic Acidosis, and Stroke-like episodes). * **MERRF** (Myoclonic Epilepsy with Ragged Red Fibers). * **Muscle Biopsy:** Often shows "Ragged Red Fibers" on Gomori trichrome stain due to compensatory proliferation of mitochondria.

Q: The uvr ABC endonuclease is involved in which one of the following processes?

DNA repair. **Explanation:** The **uvr ABC endonuclease** is a multi-enzyme complex in prokaryotes (like *E. coli*) that plays a central role in **Nucleotide Excision Repair (NER)**. This pathway is the primary mechanism for identifying and removing bulky DNA lesions, most notably **pyrimidine dimers** (thymine dimers) caused by Ultraviolet (UV) radiation. * **Why C is Correct:** The uvr ABC complex acts as an "excision nuclease" (exinuclease). **UvrA and UvrB** scan the DNA to detect distortions in the double helix. Once a lesion is found, **UvrC** (the endonuclease) cuts the damaged strand on both sides of the lesion. The damaged segment is then removed, and the gap is filled by DNA Polymerase I and sealed by DNA Ligase. * **Why A is Incorrect:** DNA replication involves enzymes like DNA Polymerases, Helicase, and Primase. While repair occurs on DNA, uvr ABC is specifically for damage correction, not genomic duplication. * **Why B is Incorrect:** RNA splicing is a post-transcriptional modification involving the removal of introns from pre-mRNA, mediated by the spliceosome (snRNPs). * **Why D is Incorrect:** DNA recombination (e.g., VDJ recombination or crossing over) involves the exchange of genetic material between strands, typically mediated by enzymes like RecA in bacteria. **High-Yield Clinical Pearls for NEET-PG:** * **Human Homolog:** In humans, the NER pathway is more complex but functionally similar. A genetic defect in the human equivalent of this repair system leads to **Xeroderma Pigmentosum (XP)**. * **Clinical Presentation of XP:** Patients exhibit extreme photosensitivity, severe sunburns, and a 1000-fold increased risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma) due to the inability to repair UV-induced damage. * **Key Step:** Remember that NER removes a **patch** of nucleotides (an oligonucleotide), whereas Base Excision Repair (BER) removes only a single damaged base.

Q: Which of the following enzymes transcribes 5S rRNA?

RNA Polymerase III. In eukaryotic cells, three distinct types of RNA polymerases (I, II, and III) are responsible for transcribing different classes of genes. **Correct Answer: RNA Polymerase III** RNA Polymerase III is responsible for transcribing small, stable RNAs. Its primary products include **5S rRNA**, **tRNA**, and **U6 snRNA**. It is unique because it is the only polymerase that transcribes a ribosomal RNA component (5S) outside of the nucleolus. **Analysis of Incorrect Options:** * **RNA Polymerase I:** This enzyme is located in the nucleolus and transcribes a single precursor (45S pre-rRNA), which is then processed into the **28S, 18S, and 5.8S rRNAs**. It does *not* transcribe 5S rRNA. * **RNA Polymerase II:** This is the most versatile polymerase. It transcribes **mRNA** (protein-coding genes), most **snRNA** (involved in splicing), and **microRNA** (miRNA). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic (RMT):** Remember the order of products for Pol I, II, and III as **R-M-T** (**R**ibosomal, **M**essenger, **T**ransfer). * **Alpha-Amanitin Sensitivity:** This toxin (from *Amanita phalloides* mushrooms) inhibits RNA Polymerase II most strongly, followed by Pol III. Pol I is generally resistant. * **Mitochondrial RNA Polymerase:** Mitochondria have their own single RNA polymerase that resembles bacterial RNA polymerase. * **Location:** Pol I is the only one active in the **nucleolus**; Pol II and III function in the nucleoplasm.

Q: Nonsense codons are all of the following EXCEPT:

UCA. **Explanation:** In molecular biology, the genetic code consists of 64 codons. Of these, 61 are **sense codons** (coding for amino acids), and 3 are **nonsense codons** (also known as **stop codons** or termination codons). **1. Why UCA is the correct answer:** **UCA** is a sense codon that codes for the amino acid **Serine**. It does not signal the termination of protein synthesis. Therefore, it is not a nonsense codon. **2. Why the other options are wrong (The Stop Codons):** The three nonsense codons do not code for any amino acid. Instead, they signal the ribosome to terminate translation and release the newly synthesized polypeptide chain. * **UAA (Ochre):** One of the three standard stop codons. * **UAG (Amber):** One of the three standard stop codons. * **UGA (Opal):** One of the three standard stop codons. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Mnemonic to remember stop codons:** * **U** **A**re **A**way (UAA) * **U** **A**re **G**one (UAG) * **U** **G**o **A**way (UGA) * **Nonsense Mutation:** A point mutation that changes a sense codon into a nonsense codon, leading to premature termination of the protein and often resulting in a non-functional product (e.g., certain forms of β-thalassemia). * **Exceptions to the Universal Code:** In **mitochondria**, the genetic code differs slightly; for example, **UGA** codes for Tryptophan rather than acting as a stop codon. * **Initiation Codon:** **AUG** is the universal start codon (coding for Methionine in eukaryotes and Formyl-methionine in prokaryotes).

Question 1

Sigma (σ) subunit is present in which of the following?

Accepted Answer

RNA polymerase

Answer

80S ribosomes

Answer

Initiation of DNA replication

Answer

DNA polymerase

Question 2

What constitutes the driving force for the transport of proteins into and out of the nucleus?

Accepted Answer

GTP hydrolysis within the cytosol

Answer

ATP hydrolysis within the cytosol

Answer

ATP hydrolysis within the nucleus

Answer

GTP hydrolysis within the nucleus

Question 3

Which of the following is NOT true about the genetic code?

Accepted Answer

Overlapping

Answer

Degenerate

Answer

Ambiguous

Answer

Universal

Question 4

A decrease in the level of heme leads to a reduction in globin synthesis in reticulocytes. Which of the following best explains this phenomenon?

Accepted Answer

The initiation factor eIF-2 becomes phosphorylated, reducing its level of activity

Answer

RNA polymerase activity is decreased in reticulocytes by low heme

Answer

Heme normally activates peptidyl transferase in reticulocytes

Answer

A tRNA degrading enzyme is active in the absence of heme

Question 5

Biotechnology is used for which of the following?

Accepted Answer

Viral vaccine production

Answer

Curing genetic disorders

Answer

Developing genetically modified crops

Answer

Gene synthesis

Question 6

Which pattern of inheritance is characteristic of mitochondrial DNA?

Accepted Answer

Mother to both son and daughter transmission

Answer

Father to son transmission

Answer

Father to both son and daughter transmission

Answer

Mother to daughter transmission

Question 7

The uvr ABC endonuclease is involved in which one of the following processes?

Accepted Answer

DNA repair

Answer

DNA replication

Answer

RNA splicing

Answer

DNA recombination

Question 8

Which cellular organelle is primarily responsible for protein synthesis?

Accepted Answer

Both nucleus and rough endoplasmic reticulum

Answer

Nucleus

Answer

Rough endoplasmic reticulum

Answer

None of the above

Question 9

Which of the following enzymes transcribes 5S rRNA?

Accepted Answer

RNA Polymerase III

Answer

RNA Polymerase I

Answer

RNA Polymerase II

Answer

None of the above

Question 10

Nonsense codons are all of the following EXCEPT:

Accepted Answer

UCA

Answer

UAA

Answer

UAG

Answer

UGA

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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