Molecular Biology and Genomics Practice Questions

Q: What banding technique is used for studying translocations involving the centromere?

C banding. **Explanation:** **C-banding (Constitutive Heterochromatin banding)** is the specific technique used to stain the **centromeres** and other regions containing constitutive heterochromatin (such as the distal portion of the Y chromosome). It involves pre-treating chromosomes with an alkali (like barium hydroxide) followed by Giemsa staining. This process denatures euchromatin while preserving the highly repetitive DNA at the centromere, making it the gold standard for identifying **dicentric chromosomes** and **centromeric translocations**. **Analysis of Incorrect Options:** * **Q-banding (Quinacrine):** Uses fluorescent dyes to produce bright and dull bands. It was the first banding method but requires a fluorescence microscope and the signal fades quickly. It is useful for identifying the Y chromosome and polymorphisms. * **G-banding (Giemsa):** The most common clinical technique. Chromosomes are treated with **Trypsin** followed by Giemsa. It produces dark (AT-rich, gene-poor) and light (GC-rich, gene-active) bands, used for general karyotyping. * **R-banding (Reverse):** The "reverse" of G-banding. It stains GC-rich regions (euchromatin) darkly. It is particularly useful for studying the **ends of chromosomes (telomeres)**, which often stain poorly in G-banding. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for G-banding:** **G**iemsa + **T**rypsin (**G**reat **T**eacher). * **Centromere = C-banding:** Always associate "C" with Centromere and Constitutive heterochromatin. * **Telomeres = R-banding:** Best for distal deletions or translocations. * **Karyotyping Phase:** All these techniques are performed on cells arrested in **Metaphase** using Colchicine (which inhibits spindle formation).

Q: The graph below depicts a polymerase chain reaction. What type of PCR is shown?

Real-time PCR. ***Real-time PCR*** - The **sigmoidal amplification curve** showing fluorescence intensity vs cycle number with a **threshold (Ct) value** is characteristic of real-time PCR. - This technique allows **continuous monitoring** of DNA amplification during the reaction using fluorescent reporters like **SYBR Green** or **TaqMan probes**. *DNA PCR* - This is a general term that encompasses all PCR types that amplify DNA, not a specific technique. - It doesn't describe the **real-time monitoring capability** shown in the graph with fluorescence detection. *Standard PCR* - Uses **gel electrophoresis** for end-point detection after all cycles are complete, not during the reaction. - Does not produce a **real-time amplification curve** or measure fluorescence during thermal cycling. *Nested PCR* - Involves **two rounds of amplification** with different primer sets to increase specificity and sensitivity. - Uses **gel electrophoresis** for detection and does not generate real-time fluorescence curves during amplification.

Q: Which codon is recognized by tRNA-Met?

AUG. **Explanation:** The correct answer is **A. AUG**. In molecular biology, **AUG** is the universal **start codon** (initiation codon) that signals the beginning of translation in both prokaryotes and eukaryotes. It codes for the amino acid **Methionine**. During the initiation of protein synthesis, the initiator tRNA (tRNA-Met) recognizes and binds to the AUG sequence on the mRNA through its anticodon (UAC). **Analysis of Incorrect Options:** * **B. UGC:** This codon codes for **Cysteine**. * **C. GUG:** This typically codes for **Valine**. While GUG can occasionally act as an alternative start codon in some prokaryotes, it still codes for Valine in internal positions and is not the standard recognition site for tRNA-Met. * **D. GCU:** This codon codes for **Alanine**. **High-Yield Clinical Pearls for NEET-PG:** * **N-formylmethionine (fMet):** In prokaryotes and mitochondria, the initiator tRNA carries fMet, whereas in eukaryotes, it carries unmodified Methionine. * **Kozak Consensus Sequence:** In eukaryotes, the efficiency of translation initiation is increased if the AUG is embedded within a specific sequence (ACCAUGG). * **Non-Degeneracy:** While most amino acids are coded by multiple codons (degeneracy), Methionine (AUG) and Tryptophan (UGG) are unique because they are each coded by only **one** codon. * **Stop Codons (Nonsense Codons):** Remember **UAA** (U Are Away), **UAG** (U Are Gone), and **UGA** (U Go Away) do not code for any amino acids and signal the termination of translation.

Q: What is the best method for assessing protein binding regions on a DNA molecule?

DNA footprinting. ### Explanation **DNA Footprinting** is the gold-standard technique for identifying the specific site where a protein (such as a transcription factor or RNA polymerase) binds to a DNA molecule. **Why it is correct:** The principle relies on the fact that a protein bound to a specific DNA sequence "protects" that segment from enzymatic cleavage. In this method, DNA is labeled at one end and treated with a cleavage agent (like DNase I) that cuts randomly. When the DNA fragments are separated by electrophoresis, a "gap" or "footprint" appears in the ladder of bands. This gap corresponds exactly to the region where the protein was bound, as the protein physically blocked the enzyme from cutting the DNA at those specific nucleotides. **Why the other options are incorrect:** * **PCR (Polymerase Chain Reaction):** Used for the amplification of specific DNA sequences, not for mapping protein-DNA interactions. * **Microarray:** Used for high-throughput analysis of gene expression (mRNA levels) or detecting genetic variations (SNPs) across the entire genome. * **Western Blotting:** A technique used to detect and quantify specific **proteins** in a sample using antibodies; it does not involve DNA binding analysis. **High-Yield Clinical Pearls for NEET-PG:** * **Southwestern Blotting:** A related technique used specifically to identify proteins that have the ability to bind to a specific DNA probe. * **ChIP (Chromatin Immunoprecipitation):** Another high-yield method used to identify DNA-protein interactions *in vivo* (within the living cell). * **Electrophoretic Mobility Shift Assay (EMSA):** Also known as a "gel shift" assay; it determines *if* a protein binds to DNA, but unlike footprinting, it does not identify the exact sequence/region of binding.

Q: Which of the following is NOT a property of the Signal Recognition Particle (SRP)?

Contains a single peptidase activity. ### Explanation The **Signal Recognition Particle (SRP)** is a cytosolic ribonucleoprotein complex essential for the co-translational targeting of proteins to the Rough Endoplasmic Reticulum (RER). **Why Option B is the correct answer:** The SRP does **not** possess peptidase activity. Its primary role is recognition and transport, not cleavage. The removal of the signal peptide is performed by a separate enzyme called **Signal Peptidase**, which is located on the luminal surface of the ER membrane, not within the SRP itself. **Analysis of Incorrect Options:** * **Option A:** The SRP is a complex consisting of one **7S RNA** molecule and **six different polypeptides** (P9, P14, P54, P68, P72, and P19). * **Option C:** Upon binding to the ribosome-nascent chain complex, the SRP induces a **translation arrest**. This "pause" prevents the protein from being released into the cytoplasm and ensures it doesn't fold prematurely before reaching the ER translocon. * **Option D:** The SRP specifically recognizes and binds to the **N-terminal hydrophobic signal sequence** of nascent secretory or membrane proteins as they emerge from the ribosome. --- ### High-Yield Clinical Pearls for NEET-PG: * **GTPase Activity:** The SRP and its receptor (SR) are both GTP-binding proteins. Hydrolysis of GTP triggers the release of the SRP from the ribosome, allowing translation to resume. * **I-Cell Disease (Mucolipidosis II):** A high-yield related pathology where a defect in the Golgi enzyme (phosphotransferase) prevents the tagging of lysosomal enzymes with **Mannose-6-Phosphate**, leading to their secretion outside the cell rather than targeting to lysosomes. * **Zellweger Syndrome:** Contrast SRP (ER targeting) with **PEX genes/Peroxins**, which are required for protein targeting to **Peroxisomes**.

Q: Which of the following statements are true about telomeres?

They are made up of the repeated sequence TTAGGG.. **Explanation:** **1. Why Option A is Correct:** Telomeres are specialized nucleoprotein structures located at the ends of linear eukaryotic chromosomes. In humans, they consist of thousands of tandem repeats of the hexanucleotide sequence **5'-TTAGGG-3'**. Their primary function is to act as a "protective cap," preventing the DNA repair machinery from recognizing chromosome ends as double-stranded breaks, thus maintaining genomic stability. **2. Analysis of Incorrect Options:** * **Option B:** Telomerase is a ribonucleoprotein (a reverse transcriptase) that **lengthens** telomeres by adding TTAGGG repeats, not shortens them. * **Option C:** In most **cancer cells**, telomerase is **upregulated**. This allows cancer cells to maintain their telomere length despite repeated divisions, granting them "replicative immortality." Telomeres shorten in normal somatic cells, not typically in active cancer cells. * **Option D:** While telomere shortening is a *marker* and a *mechanism* of cellular senescence (the Hayflick limit), the statement is technically incomplete compared to the structural fact in Option A. However, in the context of NEET-PG, Option A is the definitive biochemical characteristic. **3. High-Yield Clinical Pearls for NEET-PG:** * **The End Replication Problem:** DNA polymerase cannot replicate the extreme 3' end of linear chromosomes, leading to progressive shortening with each cell division. * **Telomerase Composition:** It contains an RNA template (**TERC**) and a catalytic protein subunit (**TERT**). * **Shelterin Complex:** A group of six proteins that binds to telomeres to protect them from DNA damage responses. * **Clinical Correlation:** **Dyskeratosis Congenita** is a classic "telomere syndrome" caused by mutations in telomerase components, leading to premature aging, bone marrow failure, and mucosal leukoplakia.

Q: Introduction of DNA into cells with the help of electricity is known as?

Electroporation. **Explanation:** **Electroporation** (also known as electropermeabilization) is a molecular biology technique that uses an **external electric field** to increase the permeability of the cell membrane. When high-voltage pulses are applied, they temporarily disrupt the phospholipid bilayer, creating microscopic pores (nanopores). This allows hydrophilic molecules like DNA, which normally cannot cross the hydrophobic membrane, to enter the cell. Once the electric field is removed, the pores reseal, trapping the DNA inside. **Analysis of Options:** * **Electrotransfer:** While often used interchangeably with "electrophoretic transfer" (like in Western Blotting), it refers to the movement of molecules from a gel to a membrane, not specifically the introduction of DNA into living cells. * **Electrofusion:** This is the use of electric currents to induce the fusion of two different cells (e.g., creating hybridomas for monoclonal antibody production). * **Electrolysis:** This is a chemical process that uses direct electric current to drive a non-spontaneous chemical reaction (e.g., splitting water into hydrogen and oxygen); it is not a gene transfer technique. **High-Yield Clinical Pearls for NEET-PG:** * **Transformation vs. Transfection:** Electroporation is a method of **transfection** (introducing nucleic acids into eukaryotic cells) or **transformation** (into bacteria). * **Applications:** It is widely used in creating **transgenic animals**, gene therapy, and producing recombinant proteins (like Insulin). * **Other Gene Transfer Methods:** * *Chemical:* Calcium Phosphate precipitation. * *Physical:* Microinjection, Biolistics (Gene gun). * *Biological:* Viral vectors (Retrovirus, Adenovirus).

Question 1

What banding technique is used for studying translocations involving the centromere?

Accepted Answer

C banding

Answer

Q banding

Answer

G banding

Answer

R banding

Question 2

Xeroderma pigmentosum is produced as a result of a defect in:

Accepted Answer

DNA polymerase I and DNA ligase

Answer

DNA polymerase III

Answer

DNA exonuclease

Answer

DNA ligase

Question 3

Which of the following is not an example of epigenetic change?

Accepted Answer

siRNA interference

Answer

Histone acetylation

Answer

Poly A tailing

Answer

DNA methylation

Question 4

The graph below depicts a polymerase chain reaction. What type of PCR is shown?

Accepted Answer

Real-time PCR

Answer

DNA PCR

Answer

Standard PCR

Answer

Nested PCR

Question 5

Which codon is recognized by tRNA-Met?

Accepted Answer

AUG

Answer

UGC

Answer

GUG

Answer

GCU

Question 6

What is the best method for assessing protein binding regions on a DNA molecule?

Accepted Answer

DNA footprinting

Answer

PCR

Answer

Microarray

Answer

Western blotting

Question 7

All of the following are DNA binding domains found in transcription factors, EXCEPT?

Accepted Answer

Helix-loop-helix

Answer

Zinc finger

Answer

Helix-turn-Helix

Answer

Basic domains

Question 8

Which of the following is NOT a property of the Signal Recognition Particle (SRP)?

Accepted Answer

Contains a single peptidase activity

Answer

Contains RNA and several polypeptides

Answer

Temporarily arrests translation

Answer

Binds to the signal sequence of secretory proteins

Question 9

Which of the following statements are true about telomeres?

Accepted Answer

They are made up of the repeated sequence TTAGGG.

Answer

Telomerase shortens them.

Answer

They are shortened in cancer cells.

Answer

They are responsible for cellular aging.

Question 10

Introduction of DNA into cells with the help of electricity is known as?

Accepted Answer

Electroporation

Answer

Electrotransfer

Answer

Electrofusion

Answer

Electrolysis

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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