Molecular Biology and Genomics Practice Questions

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

DNA binding. **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).

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

Knockout. **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.

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 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: What is translocation necessary for?

Elongation of proteins. **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**.

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

Restriction enzyme. **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.

Question 1

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

Accepted Answer

DNA binding

Answer

Histone binding

Answer

Phosphotyrosine binding

Answer

Phosphoinositide binding

Question 2

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 3

Which codon is recognized by tRNA-Met?

Accepted Answer

AUG

Answer

UGC

Answer

GUG

Answer

GCU

Question 4

Selective suppression of a functional gene by a functional allele is called?

Accepted Answer

Knockout

Answer

Transgene

Answer

Pseudogene

Answer

Inclusion

Question 5

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

Accepted Answer

CYP17 gene

Answer

CYP12 gene

Answer

p53

Answer

KRAS

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 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 9

What is translocation necessary for?

Accepted Answer

Elongation of proteins

Answer

Initiation of codon

Answer

Binding of mRNA to ribosomes

Answer

Folding of proteins

Question 10

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

Accepted Answer

Restriction enzyme

Answer

Primer

Answer

Taq polymerase

Answer

DNA polymerase

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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