Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 291: Which of the following is a function of proteins containing a zinc finger motif?

A. DNA binding (Correct Answer)
B. Histone binding
C. Phosphotyrosine binding
D. Phosphoinositide binding

Explanation: **Explanation:** **1. Why DNA Binding is Correct:** The **zinc finger motif** is one of the most common structural motifs found in eukaryotic **transcription factors**. It consists of a protein fold stabilized by the coordination of a zinc ion ($Zn^{2+}$) between specific amino acid residues (usually Cysteine and Histidine). This structure creates a "finger-like" projection that fits precisely into the **major groove of the DNA double helix**. By binding to specific DNA sequences, these proteins regulate gene expression. Classic examples include the Steroid Hormone Receptors (e.g., Estrogen and Glucocorticoid receptors). **2. Why Other Options are Incorrect:** * **B. Histone binding:** Proteins that bind to histones typically contain motifs like **Bromodomains** (which bind acetylated lysines) or **Chromodomains** (which bind methylated lysines), rather than zinc fingers. * **C. Phosphotyrosine binding:** This is the characteristic function of **SH2 (Src Homology 2)** and **PTB (Phosphotyrosine Binding)** domains, which are crucial in intracellular signal transduction (e.g., Insulin signaling). * **D. Phosphoinositide binding:** This is performed by **PH (Pleckstrin Homology)** domains, which recruit proteins to the cell membrane by binding to phosphorylated lipids like $PIP_3$. **Clinical Pearls & High-Yield Facts:** * **Common Motifs:** Other DNA-binding motifs high-yield for NEET-PG include the **Leucine Zipper** (e.g., c-fos, c-jun), **Helix-Turn-Helix** (Homeodomain proteins), and **Helix-Loop-Helix**. * **Steroid Receptors:** Remember that all steroid hormone receptors are zinc-finger proteins. * **Vitamin D:** The Vitamin D receptor (VDR) is a zinc-finger protein; mutations here can lead to Vitamin D-resistant rickets (Type II).

Question 292: Which of the following is not an example of epigenetic change?

A. Histone acetylation
B. Poly A tailing
C. siRNA interference (Correct Answer)
D. DNA methylation

Explanation: **Explanation:** **Epigenetics** refers to heritable changes in gene expression that occur without altering the underlying DNA sequence. These changes typically involve modifications to the chromatin structure or the DNA molecule itself, influencing how genes are "read" by the cell. **Why siRNA interference is the correct answer:** **siRNA (Small Interfering RNA) interference** is a mechanism of **post-transcriptional gene silencing**. It involves the degradation of specific mRNA molecules after they have been transcribed. Since it acts on the RNA product in the cytoplasm rather than modifying the chromatin or DNA template to regulate expression levels at the source, it is generally not classified as a classical epigenetic modification. **Analysis of incorrect options:** * **DNA Methylation:** This is a hallmark of epigenetics. Addition of a methyl group to cytosine (usually at CpG islands) by DNA methyltransferases typically leads to **gene silencing** (e.g., X-inactivation, genomic imprinting). * **Histone Acetylation:** This involves the addition of acetyl groups to lysine residues on histone tails by Histone Acetyltransferases (HATs). It reduces the positive charge of histones, relaxing the chromatin (euchromatin) and **increasing transcription**. * **Poly A tailing:** While primarily a post-transcriptional modification, it is often considered part of the broader epigenetic regulatory landscape in some contexts; however, in the hierarchy of this question, siRNA is the "most" correct answer as it is a transient interference mechanism rather than a structural modification of the genetic apparatus. **High-Yield Clinical Pearls for NEET-PG:** * **Genomic Imprinting:** A classic epigenetic phenomenon (e.g., Prader-Willi and Angelman syndromes) involving differential DNA methylation based on parental origin. * **Writer vs. Eraser:** Enzymes that add marks (like HATs) are "writers," while those that remove them (like Histone Deacetylases - HDACs) are "erasers." * **Drug Link:** **5-Azacytidine** is a DNA methyltransferase inhibitor used in treating myelodysplastic syndrome.

Question 293: Which codon is recognized by tRNA-Met?

A. AUG (Correct Answer)
B. UGC
C. GUG
D. GCU

Explanation: **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.

Question 294: Selective suppression of a functional gene by a functional allele is called?

A. Transgene
B. Pseudogene
C. Inclusion
D. Knockout (Correct Answer)

Explanation: **Explanation:** The question refers to the process of **Gene Knockout**, a genetic engineering technique used to study gene function. **1. Why "Knockout" is correct:** A **Gene Knockout** involves the selective suppression or "turning off" of a specific functional gene. This is typically achieved through **homologous recombination**, where a functional gene is replaced or disrupted by an engineered allele (often a mutated or non-functional version). By observing the phenotypic changes in the organism (the "knockout mouse"), researchers can determine the original gene's physiological role. **2. Why other options are incorrect:** * **Transgene:** This refers to a gene that has been transferred naturally, or by any of a number of genetic engineering techniques from one organism to another. It involves *adding* genetic material rather than selectively suppressing an existing gene. * **Pseudogene:** These are non-functional segments of DNA that resemble functional genes but have lost their protein-coding ability due to accumulated mutations (e.g., premature stop codons). They are "evolutionary relics" rather than a process of selective suppression. * **Inclusion:** In biochemistry/pathology, inclusions are typically abnormal aggregations of proteins or substances within a cell (e.g., Negri bodies in Rabies or Lewy bodies in Parkinson’s). They are unrelated to gene suppression. **High-Yield Clinical Pearls for NEET-PG:** * **Knock-in:** A related technique where a specific gene is *inserted* or substituted with a variant (e.g., replacing a mouse gene with a human gene). * **RNA Interference (RNAi):** Another method of gene suppression (Gene Silencing) that acts at the post-transcriptional level using siRNA or miRNA. * **Mario Capecchi, Martin Evans, and Oliver Smithies** won the Nobel Prize (2007) for their work on gene modifications using embryonic stem cells to create knockout mice.

Question 295: Which gene present in theca cells and absent in granulosa cells enables theca cells to produce androgens?

A. CYP17 gene (Correct Answer)
B. CYP12 gene
C. p53
D. KRAS

Explanation: This question tests the understanding of the **Two-Cell, Two-Gonadotropin Theory** of ovarian steroidogenesis, a high-yield concept in both Biochemistry and Physiology. ### **Explanation of the Correct Answer** The production of estrogen requires the cooperation of **Theca cells** and **Granulosa cells**. * **Theca Cells:** Under the influence of LH, these cells take up cholesterol and convert it into androgens (androstenedione and testosterone). This process requires the enzyme **17α-hydroxylase/17,20-lyase**, which is encoded by the **CYP17 gene**. * **Granulosa Cells:** These cells lack the CYP17 gene and therefore cannot produce androgens from cholesterol. Instead, they take up the androgens produced by theca cells and convert them into estrogens (estradiol) using the enzyme **Aromatase (CYP19A1)** under the influence of FSH. Thus, the presence of the **CYP17 gene** is the defining biochemical difference that enables theca cells to synthesize the androgenic precursors necessary for estrogen production. ### **Analysis of Incorrect Options** * **B. CYP12 gene:** This gene is involved in Vitamin D metabolism (specifically 1α-hydroxylase in the kidney is CYP27B1; CYP12 is not a primary steroidogenic enzyme in the ovary). * **C. p53:** This is a classic tumor suppressor gene ("guardian of the genome") involved in cell cycle regulation and apoptosis, not steroidogenesis. * **D. KRAS:** This is a proto-oncogene involved in signal transduction. Mutations in KRAS are associated with various malignancies, including ovarian mucinous tumors, but it does not regulate androgen synthesis. ### **NEET-PG High-Yield Pearls** * **Theca Cells:** LH stimulated → cAMP pathway → **CYP17** expression → Androgen synthesis. * **Granulosa Cells:** FSH stimulated → cAMP pathway → **CYP19 (Aromatase)** expression → Estrogen synthesis. * **Clinical Correlation:** In **Polycystic Ovary Syndrome (PCOS)**, there is often hyperresponsiveness of the CYP17 enzyme in theca cells, leading to the characteristic hyperandrogenism.

Question 296: What is the best method for assessing protein binding regions on a DNA molecule?

A. DNA footprinting (Correct Answer)
B. PCR
C. Microarray
D. Western blotting

Explanation: ### 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.

Question 297: All of the following are DNA binding domains found in transcription factors, EXCEPT?

A. Zinc finger
B. Helix-turn-Helix
C. Helix-loop-helix (Correct Answer)
D. Basic domains

Explanation: ### Explanation The core of this question lies in distinguishing between **DNA-binding domains** (which physically interact with the DNA major groove) and **Protein-protein interaction domains** (which facilitate the formation of dimers). **Why Option C is the Correct Answer:** The **Helix-loop-helix (HLH)** motif is primarily a **dimerization domain**, not a DNA-binding domain. While HLH proteins are transcription factors, the HLH motif itself allows two proteins to stick together. To actually bind to DNA, these proteins must also possess a separate **Basic Domain** (forming a bHLH motif). Without the basic region, the HLH motif cannot bind DNA. **Analysis of Incorrect Options:** * **A. Zinc Finger:** This is the most common DNA-binding motif in humans (e.g., Steroid hormone receptors). It uses a zinc ion to stabilize a "finger-like" projection that inserts into the major groove of DNA. * **B. Helix-turn-Helix (HTH):** A classic DNA-binding motif frequently found in prokaryotic repressors (like the Lac repressor) and eukaryotic homeodomain proteins. One helix acts as the "recognition helix" that fits into the DNA major groove. * **D. Basic Domains:** These are regions rich in Arginine and Lysine (positively charged) that interact with the negatively charged phosphate backbone of DNA. They are often paired with Leucine Zippers (bZIP) or HLH motifs to enable DNA binding. **High-Yield NEET-PG Pearls:** * **Leucine Zipper:** Like HLH, this is a **dimerization motif**, not a direct DNA-binding motif. It requires an adjacent **Basic Domain** to bind DNA (bZIP). * **Steroid Receptors:** These utilize **Zinc Finger** motifs to bind to Hormone Response Elements (HREs). * **Homeobox (HOX) Genes:** These encode proteins containing the **Helix-turn-Helix** (Homeodomain) motif, crucial for embryonic body patterning. * **Transcription Factor Hierarchy:** DNA binding usually occurs at the **Major Groove** because it offers more sequence-specific information than the minor groove.

Question 298: Which of the following statements are true about telomeres?

A. They are made up of the repeated sequence TTAGGG. (Correct Answer)
B. Telomerase shortens them.
C. They are shortened in cancer cells.
D. They are responsible for cellular aging.

Explanation: **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.

Question 299: What is translocation necessary for?

A. Initiation of codon
B. Binding of mRNA to ribosomes
C. Folding of proteins
D. Elongation of proteins (Correct Answer)

Explanation: **Explanation:** **Translocation** is a critical step in the **elongation phase** of protein synthesis (translation). It involves the movement of the ribosome along the mRNA template by exactly one codon (three nucleotides) in the 5' to 3' direction. **Why the correct answer is right:** During elongation, after a peptide bond is formed, the deacylated tRNA sits in the P-site and the peptidyl-tRNA (carrying the growing chain) sits in the A-site. Translocation, mediated by **Elongation Factor-G (EF-G)** in prokaryotes or **eEF-2** in eukaryotes, shifts the peptidyl-tRNA from the A-site to the P-site. This clears the A-site, allowing the next aminoacyl-tRNA to enter and continue the elongation of the protein chain. **Why the other options are incorrect:** * **A & B (Initiation/Binding):** Initiation involves the assembly of the ribosomal subunits, mRNA, and the initiator methionyl-tRNA at the P-site. Translocation only occurs *after* the initiation complex is fully formed and the first peptide bond is made. * **C (Folding):** Protein folding is a post-translational or co-translational process mediated by **chaperones** (e.g., Heat Shock Proteins) to achieve a functional 3D conformation; it is not driven by the translocation movement of the ribosome. **High-Yield Clinical Pearls for NEET-PG:** * **Diphtheria Toxin & Pseudomonas Exotoxin A:** Both inhibit protein synthesis by catalyzing the ADP-ribosylation of **eEF-2**, effectively blocking translocation. * **Macrolides (e.g., Erythromycin):** These antibiotics bind to the 50S ribosomal subunit and specifically inhibit the translocation step in bacteria. * **Energy Requirement:** Translocation is an energy-intensive process requiring the hydrolysis of **GTP**.

Question 300: Which of the following is NOT a component of Polymerase Chain Reaction (PCR)?

A. Primer
B. Taq polymerase
C. DNA polymerase
D. Restriction enzyme (Correct Answer)

Explanation: **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* enzymatic method used to amplify specific DNA sequences. The process mimics natural DNA replication but occurs in a thermal cycler through three repeating steps: Denaturation, Annealing, and Extension. **Why Restriction Enzymes are NOT a component:** Restriction enzymes (Restriction Endonucleases) are "molecular scissors" used to cut DNA at specific palindromic sequences. While they are essential for **Recombinant DNA technology** and **RFLP (Restriction Fragment Length Polymorphism)**, they play no role in the PCR amplification cycle. In fact, cutting the target sequence would prevent the polymerase from synthesizing a continuous new strand. **Analysis of Incorrect Options:** * **Primer (A):** These are short, synthetic oligonucleotides (usually 18–25 bp) that provide a free 3'-OH group, allowing the DNA polymerase to initiate synthesis. Two primers (forward and reverse) are required. * **Taq Polymerase (B):** A specific, heat-stable DNA polymerase derived from *Thermus aquaticus*. It is crucial because it does not denature at the high temperatures (94–96°C) required to separate DNA strands. * **DNA Polymerase (C):** This is the general class of enzyme required for PCR. While Taq is the most common, others like *Pfu* polymerase (for high fidelity) are also used. **High-Yield Clinical Pearls for NEET-PG:** * **Components of PCR Mix:** Template DNA, Primers, dNTPs (Deoxynucleotide triphosphates), Taq Polymerase, and **Magnesium ions ($Mg^{2+}$)** which act as a necessary cofactor. * **Reverse Transcriptase PCR (RT-PCR):** Used for RNA viruses (like SARS-CoV-2) where RNA is first converted to cDNA. * **Real-Time PCR (qPCR):** Uses fluorescent probes (e.g., SYBR Green) to quantify DNA in real-time.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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