Molecular Biology and Genomics Practice Questions

Q: Which RNA polymerase transcribes microRNA?

RNA polymerase II. **Explanation:** The synthesis of microRNA (miRNA) is a multi-step process that begins in the nucleus. The majority of miRNAs are transcribed by **RNA polymerase II**. These transcripts, known as primary miRNAs (pri-miRNAs), are characterized by a 5' cap and a 3' poly-A tail, similar to messenger RNA (mRNA). RNA polymerase II is the primary enzyme responsible for transcribing all protein-coding genes and several non-coding RNAs, including miRNA and most snRNA. **Analysis of Options:** * **RNA Polymerase I:** This enzyme is localized in the nucleolus and is exclusively responsible for transcribing the **pre-rRNA (45S)**, which is processed into 18S, 5.8S, and 28S ribosomal RNAs. * **RNA Polymerase III:** This enzyme transcribes small, stable RNAs, most notably **tRNA** and the **5S rRNA**. While a very small subset of miRNAs can be transcribed by Pol III, Pol II is the standard answer for medical examinations. * **DNA Polymerase:** This enzyme is involved in **DNA replication** and repair, not the transcription of RNA from a DNA template. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for RNA Pol I, II, III:** **R-M-T** (1-2-3) * Pol **I**: **R**ibosomal RNA * Pol **II**: **M**essenger RNA (and miRNA) * Pol **III**: **T**ransfer RNA * **Amanita phalloides (Death Cap Mushroom):** Contains **$\alpha$-amanitin**, which potently inhibits **RNA Polymerase II**, leading to severe hepatotoxicity. * **miRNA Function:** They regulate gene expression post-transcriptionally by binding to the 3' UTR of target mRNA, leading to translational repression or mRNA degradation.

Q: What is required for the initiation of DNA synthesis?

A short RNA molecule. **Explanation:** **1. Why the correct answer is right:** DNA synthesis is catalyzed by the enzyme **DNA polymerase**, which has a specific limitation: it cannot initiate the synthesis of a new DNA strand *de novo*. It requires a pre-existing **free 3'-hydroxyl (-OH) group** to attach the first deoxynucleotide. In biological systems, this is provided by **Primase** (a specialized RNA polymerase), which synthesizes a **short RNA molecule** (typically 10–12 nucleotides long) known as a **Primer**. This RNA primer provides the necessary 3'-OH terminus that DNA polymerase III (in prokaryotes) or Pol $\alpha/\delta/\epsilon$ (in eukaryotes) uses to begin elongation. **2. Why the incorrect options are wrong:** * **Options A & B:** While a five-carbon sugar (specifically deoxyribose) is a structural component of DNA nucleotides, their mere presence is insufficient to trigger synthesis. DNA polymerase cannot link these sugars together without a template and a primer. * **Option D:** While proteins are essential for the replication complex (e.g., Helicase, SSBPs), they do not provide the free hydroxyl group required for DNA chain initiation. The 3'-OH must come from a nucleic acid (RNA or, in laboratory PCR, a DNA primer). **3. High-Yield Clinical Pearls for NEET-PG:** * **Primase** is an RNA polymerase; unlike DNA polymerase, it does *not* require a primer to start. * **Removal:** In eukaryotes, RNA primers are removed by **RNase H** and **FEN1**; in prokaryotes, they are removed by the 5'→3' exonuclease activity of **DNA Polymerase I**. * **Clinical Correlation:** Certain antiviral drugs (like **Zidovudine/AZT**) work as chain terminators because they lack the 3'-OH group, preventing further DNA synthesis. * **Telomerase:** This is a specialized reverse transcriptase that carries its own internal RNA template to synthesize the ends of linear chromosomes.

Q: Which protein motif is characteristic of steroid receptor regulatory proteins?

Zinc finger. **Explanation:** The correct answer is **Zinc finger**. This question tests the understanding of DNA-binding motifs in transcription factors, a high-yield topic in both Biochemistry and Immunology. **1. Why Zinc Finger is Correct:** Steroid hormone receptors (e.g., glucocorticoid, estrogen, and progesterone receptors) belong to a superfamily of ligand-activated transcription factors. These receptors contain a specific DNA-binding domain (DBD) characterized by the **Zinc finger motif**. In this motif, a zinc ion is coordinated by cysteine and histidine residues, stabilizing a "finger-like" loop of amino acids that fits precisely into the major groove of the DNA double helix. This allows the steroid-receptor complex to bind to specific **Hormone Response Elements (HREs)** and regulate gene expression. **2. Why the other options are incorrect:** * **Helix-turn-helix:** This is the simplest and most common motif found primarily in **prokaryotic** DNA-binding proteins (e.g., the *lac* repressor) and homeodomain proteins involved in development. * **Leucine zipper:** This motif is characterized by a leucine residue at every seventh position, forming an amphipathic helix. It is typical of transcription factors like **c-Jun and c-Fos** (AP-1 complex), which function as dimers. * **RNA:** RNA is a nucleic acid, not a protein structural motif. **Clinical Pearls for NEET-PG:** * **Zinc Finger:** Associated with Steroid receptors, Thyroid hormone receptors, and Vitamin D receptors. * **Leucine Zipper:** Associated with proto-oncogenes (Jun, Fos, Myc). * **Mnemonic:** "Zinc fingers grip the Steroids" — helps remember the association between Zinc motifs and steroid/thyroid receptors.

Q: Which of the following is NOT a core histone protein?

H1. **Explanation:** The fundamental unit of chromatin is the **nucleosome**, which consists of a protein core wrapped by DNA. Histones are small, basic proteins rich in lysine and arginine that facilitate this compact packaging. **1. Why H1 is the correct answer:** Histone **H1** is known as the **linker histone**. It is not part of the octameric core. Instead, it binds to the "linker DNA" (the DNA between nucleosome beads) and the site where DNA enters and exits the core. Its primary role is to stabilize the nucleosome structure and facilitate the folding of the "beads-on-a-string" fiber into more complex 30-nm chromatin fibers. **2. Why the other options are incorrect:** The **nucleosome core** is an octamer consisting of two molecules each of the following four histones: * **H2A & H2B:** These form two heterodimers (H2A-H2B) that bind to the central tetramer. * **H3 & H4:** These form a stable tetramer (H3₂-H4₂) that serves as the initial scaffold for DNA wrapping. Since H2A, H2B, and H3 are integral components of this central octamer, they are classified as **core histones**. **High-Yield NEET-PG Clinical Pearls:** * **Charge:** Histones are **positively charged** (due to Arginine/Lysine), allowing them to bind tightly to the **negatively charged** phosphate backbone of DNA. * **Epigenetics:** Histone tails undergo post-translational modifications (Acetylation, Methylation, Phosphorylation) which regulate gene expression. * **Acetylation:** Usually "relaxes" chromatin (euchromatin), increasing transcription (neutralizes the positive charge). * **Drug Link:** **Sodium Valproate** (anti-epileptic) acts as a Histone Deacetylase (HDAC) inhibitor.

Q: After digestion by restriction endonucleases, DNA strands can be joined again by which enzyme?

DNA ligase. **Explanation:** **DNA ligase** is the correct answer because it acts as the "molecular glue" of the cell. After restriction endonucleases cut DNA (producing either "sticky" or "blunt" ends), DNA ligase facilitates the joining of these strands by catalyzing the formation of a **phosphodiester bond** between the 3'-hydroxyl group of one nucleotide and the 5'-phosphate group of another. This process requires energy, typically in the form of **ATP** (in eukaryotes and T4 phage) or **NAD+** (in some bacteria). **Analysis of Incorrect Options:** * **DNA polymerase:** Its primary role is the synthesis of new DNA strands by adding deoxynucleotides to a pre-existing primer during replication or repair; it cannot join two independent double-stranded fragments. * **DNA topoisomerase:** These enzymes regulate the overwinding or underwinding of DNA. They relieve torsional strain by making transient single-strand (Type I) or double-strand (Type II) breaks, but they do not function to permanently join recombinant DNA fragments. * **DNA gyrase:** A specific type of bacterial Topoisomerase II that introduces negative supercoils. It is a target for fluoroquinolone antibiotics (e.g., Ciprofloxacin) but is not used for joining DNA strands in cloning. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Genetic Engineering:** DNA ligase is indispensable in recombinant DNA technology for inserting a gene of interest into a plasmid vector. * **Okazaki Fragments:** In vivo, DNA ligase is essential for joining Okazaki fragments on the lagging strand during DNA replication. * **Clinical Correlation:** Mutations in the *LIG4* gene (DNA Ligase IV) lead to **LIG4 syndrome**, characterized by immunodeficiency, microcephaly, and sensitivity to ionizing radiation due to defective double-strand break repair.

Q: If codon no 302 UAG is replaced by UAA, then this mutation is:

Silent. **Explanation:** The correct answer is **Silent mutation** because both **UAG** and **UAA** are **Stop Codons** (Nonsense codons). 1. **Why it is Silent:** In the genetic code, there are three termination codons: **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These codons do not code for any amino acid; instead, they signal the termination of translation. Since the mutation changes one stop codon (UAG) to another stop codon (UAA), the resulting polypeptide chain remains identical in length and composition. Because there is no change in the final protein product, it is classified as a silent mutation. 2. **Why other options are incorrect:** * **Missense:** This occurs when a point mutation changes a codon such that it codes for a *different* amino acid (e.g., GAA to GUA). * **Nonsense:** This occurs when a codon coding for an amino acid is changed into a stop codon, leading to premature termination. Here, the site was *already* a stop codon. * **Insufficient Information:** The identity of all three stop codons is a fundamental fact in molecular biology, making the information provided sufficient. **High-Yield NEET-PG Pearls:** * **Stop Codons:** UAA (Ochre), UAG (Amber), UGA (Opal). *Mnemonic: **U** **A**re **A**way, **U** **A**re **G**one, **U** **G**o **A**way.* * **Degeneracy of Genetic Code:** Multiple codons coding for the same amino acid (usually differing at the 3rd base/Wobble position) is the most common cause of silent mutations. * **Transition vs. Transversion:** UAG to UAA is a **transversion** (G is a purine, A is a purine—wait, G to A is actually a **transition**). *Correction:* Purine to Purine (A↔G) or Pyrimidine to Pyrimidine (C↔U) is a **Transition**.

Q: The ends of chromosomes are replicated by which enzyme?

Telomerase. ### Explanation **1. Why Telomerase is Correct:** DNA polymerase requires a RNA primer to initiate synthesis and can only add nucleotides in a 5' to 3' direction. During lagging strand synthesis, once the terminal RNA primer is removed, there is no upstream 3'-OH group available for DNA polymerase to fill the gap. This is known as the **"End Replication Problem."** To prevent the progressive shortening of chromosomes, the enzyme **Telomerase** (a specialized **Reverse Transcriptase**) adds repetitive DNA sequences (TTAGGG in humans) to the 3' end. It carries its own internal RNA template, making it a **ribonucleoprotein**. **2. Why Other Options are Incorrect:** * **Centromere:** This is a structural region of the chromosome (not an enzyme) that joins sister chromatids and serves as the attachment site for spindle fibers during cell division. * **Restriction Endonuclease:** These are bacterial enzymes used in recombinant DNA technology to cut DNA at specific palindromic sequences. They are not involved in normal cellular DNA replication. * **Exonuclease:** These enzymes remove nucleotides one at a time from the ends of a DNA molecule (e.g., 3'→5' proofreading activity of DNA Polymerase). They degrade or trim DNA rather than replicating chromosome ends. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cellular Aging:** Telomerase activity is high in germ cells and stem cells but low or absent in somatic cells, leading to "replicative senescence" (Hayflick limit). * **Cancer:** Approximately 85-90% of cancer cells upregulate telomerase to achieve immortality. * **Shelterin Complex:** A group of proteins that protect telomeres from being recognized as DNA double-strand breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by mutations in telomerase components, leading to premature aging and bone marrow failure.

Q: What are the consensus sequences for the initiation and termination of a gene sequence's introns?

GU at initiation, AG at termination. ### Explanation **1. The Concept: The GT-AG Rule (GU-AG in RNA)** In eukaryotic gene expression, pre-mRNA undergoes splicing to remove non-coding regions (introns) and join coding regions (exons). This process is governed by highly conserved consensus sequences at the intron-exon boundaries. * **5' Splice Site (Donor Site):** The intron begins with the dinucleotide **GU** (DNA equivalent: GT). * **3' Splice Site (Acceptor Site):** The intron ends with the dinucleotide **AG**. This is known as the **Chambon’s Rule** or the **GU-AG rule**. These sequences are recognized by the spliceosome (snRNPs) to ensure precise cleavage and ligation. **2. Analysis of Incorrect Options** * **Option B (AG at initiation, GU at termination):** This is a reversal of the actual sequence. AG is the signal for the end (acceptor site), not the start. * **Option C (GA at initiation, GU at termination):** GA is not a conserved consensus sequence for the 5' donor site. * **Option D (UG at initiation, AG at termination):** While "U" and "T" are interchangeable in RNA/DNA contexts, the standard biological convention for the donor site is GU, not UG. **3. High-Yield Clinical Pearls for NEET-PG** * **Splice Site Mutations:** Mutations in these consensus sequences (e.g., a G → A transition at the 5' site) lead to **aberrant splicing**. This can result in "exon skipping" or the retention of introns, leading to non-functional proteins. * **Clinical Example:** **β-Thalassemia** is frequently caused by mutations in the GU-AG splice sites of the β-globin gene. * **The Lariat Loop:** During splicing, the 5' GU end attaches to an internal **Adenine (Branch point)**, forming a lariat-shaped structure before the 3' AG site is cleaved. * **Autoimmunity:** Antibodies against snRNPs (e.g., **Anti-Smith antibodies**) are highly specific diagnostic markers for **Systemic Lupus Erythematosus (SLE)**.

Question 1

All of the following statements about Ribozymes are false, EXCEPT:

Accepted Answer

They play a key role in post-transcriptional conversion of pre-mRNA to mature mRNA

Answer

They are DNA molecules

Answer

They are not present in ribosomes

Answer

They play a key role in RNA synthesis

Question 2

Which RNA polymerase transcribes microRNA?

Accepted Answer

RNA polymerase II

Answer

RNA polymerase I

Answer

RNA polymerase III

Answer

DNA polymerase

Question 3

What is required for the initiation of DNA synthesis?

Accepted Answer

A short RNA molecule

Answer

Five carbon sugar

Answer

Deoxyribose alone

Answer

Proteins with free hydroxyl groups

Question 4

Which protein motif is characteristic of steroid receptor regulatory proteins?

Accepted Answer

Zinc finger

Answer

Helix-turn-helix

Answer

Leucine zipper

Answer

RNA

Question 5

Which of the following is NOT a core histone protein?

Accepted Answer

H1

Answer

H2A

Answer

H2B

Answer

H3

Question 6

After digestion by restriction endonucleases, DNA strands can be joined again by which enzyme?

Accepted Answer

DNA ligase

Answer

DNA polymerase

Answer

DNA topoisomerase

Answer

DNA gyrase

Question 7

Proteins destined for secretion from eukaryotic cells have which of the following in common?

Accepted Answer

A signal peptide located at the amino terminus; synthesized on rough ribosomes; not embedded within the ER membrane.

Answer

A signal peptide located at the carboxy terminus; synthesized on rough ribosomes; embedded within the ER membrane.

Answer

A signal peptide located at the amino terminus; synthesized on cytoplasmic ribosomes; not embedded within the ER membrane.

Answer

A signal peptide located at the carboxy terminus; synthesized on rough ribosomes; embedded within the ER membrane.

Question 8

If codon no 302 UAG is replaced by UAA, then this mutation is:

Accepted Answer

Silent

Answer

Missense

Answer

Nonsense

Answer

Given information is not sufficient to identify

Question 9

The ends of chromosomes are replicated by which enzyme?

Accepted Answer

Telomerase

Answer

Centromere

Answer

Restriction endonuclease

Answer

Exonuclease

Question 10

What are the consensus sequences for the initiation and termination of a gene sequence's introns?

Accepted Answer

GU at initiation, AG at termination

Answer

AG at initiation, GU at termination

Answer

GA at initiation, GU at termination

Answer

UG at initiation, AG at termination

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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