Molecular Biology and Genomics Practice Questions

Q: Which of the following is coding RNA?

mRNA. **Explanation:** The classification of RNA is based on its function in protein synthesis. RNA is broadly divided into **Coding RNA** and **Non-coding RNA (ncRNA)**. **1. Why mRNA is the correct answer:** **mRNA (messenger RNA)** is the only type of RNA that carries the genetic blueprint from DNA to the ribosomes to be translated into a polypeptide chain. It contains **codons** that dictate the specific amino acid sequence of a protein. Therefore, it is the only "coding" RNA. **2. Why other options are incorrect:** * **snRNA (Small nuclear RNA):** These are non-coding RNAs found in the nucleus. They combine with proteins to form **snRNPs ("snurps")**, which are essential components of the **spliceosome**. Their role is structural and catalytic (removing introns), not coding. * **miRNA (microRNA):** These are short, non-coding RNA molecules (approx. 22 nucleotides) that function in **RNA interference (RNAi)**. They regulate gene expression post-transcriptionally by binding to mRNA and inhibiting translation or promoting degradation. * **tRNA and rRNA (Other non-coding RNAs):** Though not in the options, remember that Transfer RNA and Ribosomal RNA are also non-coding; they facilitate translation but do not provide the code themselves. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **RNA Polymerase II** is responsible for the synthesis of mRNA (the coding RNA). * **Post-transcriptional modifications** of mRNA include 5' capping (7-methylguanosine), 3' polyadenylation (Poly-A tail), and splicing. * **snRNA Deficiency:** Antibodies against snRNPs (specifically Anti-Smith antibodies) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **miRNA Clinical Link:** Dysregulation of miRNAs is frequently linked to oncogenesis (oncomiRs) and is a target for future cancer therapies.

Q: On the ribosome, mRNA binds?

to the small subunit. ### Explanation **1. Why the Correct Answer is Right:** In both prokaryotes and eukaryotes, the initiation of translation begins with the binding of mRNA to the **small ribosomal subunit** (30S in prokaryotes, 40S in eukaryotes). * In **prokaryotes**, the 16S rRNA of the 30S subunit recognizes the **Shine-Dalgarno sequence** on the mRNA. * In **eukaryotes**, the 40S subunit (along with initiation factors) binds to the **5' methylguanosine cap** and scans for the AUG start codon. The small subunit acts as the platform for mRNA decoding, ensuring the correct alignment of the mRNA codons with the tRNA anticodons. **2. Why the Other Options are Wrong:** * **Option A (Between the subunits):** While the mRNA is physically positioned between the two subunits once the full ribosome (70S or 80S) assembles, the initial and primary binding site for the mRNA strand is specifically on the small subunit. * **Option B (To the large subunit):** The large ribosomal subunit (50S or 60S) is primarily responsible for **peptidyl transferase activity** (catalyzing peptide bond formation) and contains the A, P, and E sites for tRNA binding. It does not have a high-affinity binding site for mRNA. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Aminoglycosides (e.g., Streptomycin):** These antibiotics act by binding to the **30S (small) subunit**, causing mRNA misreading and inhibiting the initiation complex. * **Tetracyclines:** Also bind to the **30S subunit**, preventing the attachment of aminoacyl-tRNA to the A-site. * **Ribozyme Activity:** The 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes) in the **large subunit** acts as the peptidyl transferase; this is a classic example of a non-protein enzyme. * **Kozak Sequence:** In eukaryotes, the mRNA sequence (ACCAUGG) helps the small subunit identify the correct start codon.

Q: Which of the following is true about the function of Restriction Endonucleases?

All of the above.. **Explanation:** Restriction Endonucleases (REs), often called "molecular scissors," are enzymes derived primarily from bacteria that recognize and cut specific palindromic sequences in double-stranded DNA (dsDNA). **Why Option D is correct:** Restriction enzymes function by cleaving the phosphodiester backbone of **both strands** of the DNA helix (Option A). Depending on the specific enzyme and its cleavage site, the resulting DNA fragments can have two types of ends: 1. **Sticky (Cohesive) ends:** These have short, single-stranded overhanging sequences (e.g., *EcoRI*). These are highly useful in recombinant DNA technology as they can easily base-pair with complementary sequences (Option B). 2. **Blunt ends:** These occur when the enzyme cuts both strands at the same base pair position, leaving no overhangs (e.g., *SmaI* or *HaeIII*) (Option C). Since all three statements accurately describe the potential actions and outcomes of restriction endonuclease activity, **Option D** is the correct choice. **High-Yield Clinical Pearls for NEET-PG:** * **Nomenclature:** The first letter is the Genus, the next two are the species, and the Roman numeral denotes the order of discovery (e.g., *EcoRI*: *E. coli*, strain R, 1st enzyme). * **Type II REs:** These are the most commonly used in genetic engineering because they cut within or at fixed pointers near the recognition site and do not require ATP. * **Methylation:** Bacteria protect their own DNA from these enzymes by methylating their recognition sequences (Restriction-Modification System). * **Applications:** Essential for Restriction Fragment Length Polymorphism (RFLP) analysis, gene cloning, and DNA fingerprinting.

Q: Which of the following is NOT true about Polymerase Chain Reaction?

Uses thermolabile enzymes. **Explanation:** Polymerase Chain Reaction (PCR) is an *in vitro* technique used to amplify specific DNA sequences. The process relies on repeated cycles of high-temperature heating and cooling. **Why Option D is the Correct Answer:** PCR requires **thermostable** (heat-stable) enzymes, most notably **Taq Polymerase** (derived from the bacterium *Thermus aquaticus*). This enzyme can withstand the high temperatures (approx. 95°C) required for DNA denaturation without losing its catalytic activity. **Thermolabile** enzymes (heat-sensitive) would denature and become inactive during the first heating step, making them unsuitable for the PCR process. **Analysis of Other Options:** * **Option A:** True. As mentioned, a thermostable enzyme is essential to survive the denaturation phase. * **Option B:** True. Each PCR cycle consists of three distinct steps: **Denaturation** (separation of strands at ~95°C), **Annealing** (primers bind to target sequences at ~55°C), and **Extension** (DNA synthesis at ~72°C). * **Option C:** True. PCR requires two synthetic oligonucleotide **primers** that are complementary to the sequences flanking the target DNA region to initiate synthesis. **High-Yield Clinical Pearls for NEET-PG:** * **RT-PCR:** Used for RNA viruses (like SARS-CoV-2); it uses **Reverse Transcriptase** to convert RNA to cDNA before amplification. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green). * **Applications:** Diagnosis of genetic mutations (e.g., Sickle Cell Anemia), detection of infectious agents (TB, HIV), and forensic medicine (DNA fingerprinting).

Q: Which of the following activities increases in permissive chromatin?

Acetylation of histones. **Explanation:** Chromatin exists in two functional states: **Heterochromatin** (condensed, transcriptionally inactive) and **Euchromatin** (relaxed, transcriptionally active/permissive). The transition between these states is regulated by post-translational modifications of histone tails. **Why Acetylation is Correct:** Histone Acetylation is mediated by **Histone Acetyltransferases (HATs)**. Histones are positively charged (rich in Lysine and Arginine), which allows them to bind tightly to the negatively charged DNA backbone. Acetylation neutralizes the positive charge on lysine residues, weakening the histone-DNA interaction. This results in a "relaxed" or **permissive chromatin** structure, allowing transcription factors and RNA polymerase access to the DNA. **Analysis of Incorrect Options:** * **A. Methylation of CpG islands:** DNA methylation (specifically at Cytosine-Guanine dinucleotides) is a hallmark of **transcriptional silencing**. It leads to chromatin condensation and is associated with genomic imprinting and X-inactivation. * **B. Phosphorylation:** While histone phosphorylation (e.g., H3) is involved in DNA repair and transcriptional activation, it is most characteristically associated with **chromosome condensation** during mitosis/meiosis. * **C. Sumoylation:** Histone sumoylation is generally a mark of **transcriptional repression**, acting as an antagonist to acetylation and promoting a more closed chromatin state. **NEET-PG High-Yield Pearls:** * **Mnemonic:** **A**cetylation **A**ctivates transcription; **M**ethylation **M**utes transcription (usually). * **HDACs:** Histone Deacetylases (HDACs) remove acetyl groups, leading to gene silencing. HDAC inhibitors (e.g., Valproic acid, Vorinostat) are used in psychiatry and oncology to re-activate silenced genes. * **CpG Islands:** Found in promoter regions; their hypermethylation is a common epigenetic mechanism in silencing tumor suppressor genes in cancer.

Q: In Eukaryotes, what produces mRNA?

RNA Polymerase II. ### Explanation In eukaryotes, gene transcription is specialized across three primary nuclear RNA polymerases. **RNA Polymerase II** is the enzyme responsible for synthesizing **messenger RNA (mRNA)**, which carries genetic information from DNA to ribosomes for protein synthesis. It also produces most microRNA (miRNA) and small nuclear RNA (snRNA). #### Analysis of Options: * **RNA Polymerase I (Option A):** Located in the nucleolus, it synthesizes the precursor of **ribosomal RNA (rRNA)**, specifically the 28S, 18S, and 5.8S subunits. (Memory aid: **R**NA Pol **I** = **R**ibosomal). * **RNA Polymerase II (Option B):** The correct answer. It transcribes all protein-coding genes into mRNA. It is uniquely sensitive to **$\alpha$-amanitin** (found in *Amanita phalloides* mushrooms). * **RNA Polymerase III (Option C):** Synthesizes small RNAs, primarily **transfer RNA (tRNA)** and the 5S rRNA subunit. (Memory aid: **T**RNA Pol **III** = **T**ransfer). * **RNA Polymerase IV (Option D):** Found only in plants, where it plays a role in siRNA synthesis and gene silencing; it is not present in humans. #### High-Yield Clinical Pearls for NEET-PG: 1. **Mushroom Poisoning:** $\alpha$-amanitin binds specifically to **RNA Polymerase II**, inhibiting mRNA synthesis and leading to severe hepatotoxicity and liver failure. 2. **Prokaryotes vs. Eukaryotes:** Unlike eukaryotes, prokaryotes have only **one** RNA polymerase (a multi-subunit complex) that synthesizes all types of RNA. 3. **Mitochondrial RNA Polymerase:** Mitochondria have their own distinct RNA polymerase, which resembles bacterial enzymes. 4. **Promoter Region:** RNA Pol II recognizes the **TATA box** (Hogness box) located approximately 25 base pairs upstream of the start site.

Q: The anticodon region is an important part of the:

t-RNA. **Explanation:** The **anticodon region** is a specific sequence of three nucleotides located on the **tRNA (transfer RNA)** molecule. Its primary function is to recognize and base-pair with a complementary **codon** on the mRNA strand during translation. This interaction ensures that the correct amino acid, which is covalently attached to the 3' end (CCA tail) of the tRNA, is incorporated into the growing polypeptide chain. **Why the other options are incorrect:** * **mRNA (Messenger RNA):** Contains **codons**, not anticodons. Codons are the three-letter genetic codes transcribed from DNA that dictate the amino acid sequence. * **rRNA (Ribosomal RNA):** Forms the structural and catalytic core of the ribosome (e.g., peptidyl transferase activity). It does not contain anticodons. * **hnRNA (Heterogeneous nuclear RNA):** This is the primary transcript (pre-mRNA) found in the nucleus before splicing and processing. It contains introns and exons but lacks an anticodon region. **High-Yield Clinical Pearls for NEET-PG:** * **Wobble Hypothesis:** Proposed by Francis Crick, it states that the 3rd base of the mRNA codon and the 1st base of the tRNA anticodon can have non-traditional base pairing, allowing one tRNA to recognize multiple codons. * **Aminoacyl-tRNA Synthetase:** The enzyme responsible for "charging" tRNA with its specific amino acid. It is the true "translator" of the genetic code. * **Structure:** tRNA has a **Cloverleaf** secondary structure and an **L-shaped** tertiary structure. * **Inosine:** Often found in the first position of the anticodon, it can pair with A, U, or C, facilitating the "wobble" effect.

Question 1

Which of the following is coding RNA?

Accepted Answer

mRNA

Answer

snRNA

Answer

Both snRNA and mRNA

Answer

miRNA

Question 2

On the ribosome, mRNA binds?

Accepted Answer

to the small subunit

Answer

between the subunits

Answer

to the large subunit

Answer

none of these

Question 3

Which of the following is true about the function of Restriction Endonucleases?

Accepted Answer

All of the above.

Answer

They cut both strands of double-stranded DNA.

Answer

The cut ends produced are sticky.

Answer

The cut ends produced are blunt.

Question 4

Which of the following is NOT true about Polymerase Chain Reaction?

Accepted Answer

Uses thermolabile enzymes

Answer

Requires a thermostable enzyme

Answer

Involves DNA denaturation, followed by primer annealing

Answer

Requires specific primers

Question 5

What does transcription refer to?

Accepted Answer

The process where a DNA sequence is copied into RNA for the purpose of gene expression.

Answer

The process where an mRNA is used as a template for protein production.

Answer

The process where DNA wraps around histones to form a nucleosome.

Answer

The process of replication of DNA prior to cell division.

Question 6

In a hybrid RNA-DNA molecule, the DNA sequence is 5'CTTAAG3'. What is the complementary RNA sequence?

Accepted Answer

5'CUUAAG3'

Answer

5'CTTAAG3'

Answer

5'GAATTC3'

Answer

5'GAAUUC3'

Question 7

Which of the following activities increases in permissive chromatin?

Accepted Answer

Acetylation of histones

Answer

Methylation of CpG islands

Answer

Phosphorylation

Answer

Sumoylation

Question 8

In Eukaryotes, what produces mRNA?

Accepted Answer

RNA Polymerase II

Answer

RNA Polymerase I

Answer

RNA Polymerase III

Answer

RNA Polymerase IV

Question 9

Introns are not found in which type of DNA?

Accepted Answer

Mitochondrial DNA

Answer

Nuclear DNA

Answer

B DNA

Answer

Z DNA

Question 10

The anticodon region is an important part of the:

Accepted Answer

t-RNA

Answer

r-RNA

Answer

mRNA

Answer

hn-RNA

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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