Molecular Biology and Genomics Practice Questions

Q: Which of the following enzymes converts primary microRNA (pri-miRNA) to precursor microRNA (pre-miRNA)?

Drosha. ### Explanation **MicroRNA (miRNA)** biogenesis is a highly regulated process essential for post-transcriptional gene silencing. The correct answer is **Drosha** because it is the primary nuclear enzyme responsible for the initial processing of miRNA. #### Why Drosha is Correct: The synthesis of miRNA begins in the nucleus, where RNA Polymerase II transcribes DNA into long **primary miRNA (pri-miRNA)**. * **Drosha**, an RNase III enzyme, works within a protein complex (Microprocessor complex) to cleave the pri-miRNA into a ~70-nucleotide hairpin structure called **precursor miRNA (pre-miRNA)**. * This pre-miRNA is then exported from the nucleus to the cytoplasm via Exportin-5. #### Why Other Options are Incorrect: * **A. Dicer:** This RNase III enzyme acts in the **cytoplasm**. It cleaves the pre-miRNA into a short, double-stranded miRNA duplex (~22 nucleotides). * **C. RISC (RNA-Induced Silencing Complex):** This is a multi-protein complex (containing Argonaute proteins) that incorporates the mature single-stranded miRNA to target and degrade specific mRNA or inhibit its translation. * **D. Exonuclease:** These enzymes degrade nucleic acids from the ends; they are involved in RNA turnover but not in the specific maturation steps of miRNA. #### High-Yield Clinical Pearls for NEET-PG: * **Mechanism of Action:** miRNAs regulate gene expression by binding to the **3' Untranslated Region (3' UTR)** of target mRNA. * **OncomiRs:** miRNAs that are overexpressed in cancer (e.g., miR-21) act as oncogenes by silencing tumor suppressor genes. * **Key Difference:** Unlike siRNA (which requires perfect complementarity), miRNA can bind with **imperfect complementarity**, allowing one miRNA to regulate multiple different mRNAs.

Q: Which of the following is true about the genetic code?

AUA codes for methionine in mitochondria. The genetic code is nearly universal, but specific exceptions exist, particularly within the **mitochondrial genome**, which follows its own set of rules. **Explanation of the Correct Option:** * **Option A:** In human mitochondrial DNA, the codon **AUA** codes for **Methionine** (instead of Isoleucine, as it does in the standard nuclear code). Additionally, **UGA** codes for Tryptophan rather than acting as a stop codon in mitochondria. **Analysis of Incorrect Options:** * **Option B:** While **UGA** is typically a stop codon, it can code for **Selenocysteine** (the 21st amino acid) only when a specific mRNA sequence called the **SECIS element** is present. However, in the context of general mitochondrial exceptions, Option A is the more definitive "textbook" rule change. * **Option C:** This is a distractor. While **AUG** is indeed the start codon, it codes for **Methionine** in eukaryotes (mammals) and **N-formylmethionine (fMet)** in prokaryotes and mitochondria. The statement is partially true but less specific than the mitochondrial variation mentioned in Option A. * **Option D:** In the standard genetic code, **AGA and AGG** code for **Arginine**. In human **mitochondria**, however, they act as **Stop codons** (chain terminators). The option incorrectly attributes this function to "mammals" (implying the general nuclear code) rather than specifying mitochondria. **High-Yield NEET-PG Pearls:** 1. **Non-overlapping & Commaless:** The genetic code is read continuously without punctuation. 2. **Degeneracy:** One amino acid can be coded by multiple codons (except Methionine and Tryptophan). 3. **Wobble Hypothesis:** Explains why multiple codons can be recognized by a single tRNA (flexibility at the 3rd base of the codon). 4. **Mitochondrial Exceptions:** * **UGA:** Tryptophan (Stop in nuclear) * **AUA:** Methionine (Isoleucine in nuclear) * **AGA/AGG:** Stop (Arginine in nuclear)

Q: What is the function of a Type II restriction enzyme after binding to a unique recognition site?

Double-stranded cut at a palindromic DNA sequence. **Explanation:** **1. Why Option C is Correct:** Type II restriction endonucleases are essential tools in recombinant DNA technology. Unlike Type I or III enzymes, Type II enzymes are highly specific: they recognize a **unique palindromic sequence** (a sequence that reads the same 5’ to 3’ on both strands) and perform a precise **double-stranded cut** within or immediately adjacent to that recognition site. This predictable cleavage produces either "sticky ends" (overhangs) or "blunt ends," which are fundamental for gene cloning and DNA mapping. **2. Analysis of Incorrect Options:** * **Option A:** Protein folding is managed by molecular chaperones (e.g., Heat Shock Proteins), not restriction enzymes. * **Option B:** Restriction enzymes do not "remove" DNA; they cleave the phosphodiester backbone at specific internal sites. The removal of nucleotides from the ends of DNA is the function of exonucleases. * **Option D:** Many Type II enzymes (like *HaeIII* or *AluI*) actually **create** blunt ends. Therefore, preventing their formation is not a characteristic function; rather, the type of end produced depends on the specific enzyme used. **3. High-Yield Clinical Pearls for NEET-PG:** * **Nomenclature:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., *EcoRI* from *E. coli*). * **Cofactor Requirement:** Type II enzymes typically require **Magnesium (Mg²⁺)** for their catalytic activity but do not require ATP (unlike Type I and III). * **RFLP:** Restriction Fragment Length Polymorphism (RFLP) utilizes these enzymes to detect genetic variations, such as the mutation in **Sickle Cell Anemia** (where the loss of an *MstII* recognition site is diagnostic). * **Methylation:** Bacteria protect their own DNA from these enzymes through **DNA methylation**, a process part of the "Restriction-Modification System."

Q: Submicroscopic deletions of any size can be detected by:

Multiplex ligation-dependent probe amplification (MLPA). **Explanation:** The correct answer is **Multiplex ligation-dependent probe amplification (MLPA)**. **Why MLPA is correct:** MLPA is a high-throughput variation of PCR that permits the detection of copy number variations (CNVs) such as **submicroscopic deletions or duplications** of specific gene sequences. Unlike standard PCR, MLPA does not amplify the target DNA itself; instead, it amplifies pairs of probes that hybridize to the target. It is highly sensitive and can detect changes in a single exon, making it the gold standard for diagnosing conditions caused by gene dosage imbalances, such as Duchenne Muscular Dystrophy (DMD) and Spinal Muscular Atrophy (SMA). **Why other options are incorrect:** * **Southern Blotting:** While it can detect large deletions, it is labor-intensive, requires large amounts of DNA, and lacks the resolution to detect very small, submicroscopic deletions across multiple exons simultaneously. * **Cytogenomic Array Technology (CMA):** Although CMA (like CGH) is excellent for detecting submicroscopic deletions across the whole genome, it generally has a lower resolution (typically >20-50kb) compared to MLPA, which can detect deletions at the single-exon level. * **Chromosome Painting (FISH):** This uses fluorescent probes to visualize chromosomes. It is limited by the resolution of light microscopy and cannot detect very small (submicroscopic) deletions unless they are larger than the probe size (usually >100kb). **High-Yield Clinical Pearls for NEET-PG:** * **MLPA** is the investigation of choice for **DMD/BMD** (detecting exon deletions/duplications). * **Karyotyping** resolution is ~5-10 Mb; **FISH** resolution is ~100 kb; **MLPA** can detect changes at the **single nucleotide/exon level**. * MLPA is also used for detecting **methylation status** (e.g., Prader-Willi and Angelman syndromes).

Q: Which type of RNA is NOT directly involved in the process of translation?

Small nuclear RNA (snRNA). **Explanation:** The correct answer is **Small nuclear RNA (snRNA)** because its primary function is localized within the nucleus, specifically in **splicing**, rather than the cytoplasmic process of translation. 1. **Why snRNA is the correct answer:** snRNAs complex with specific proteins to form **snRNPs** (Small Nuclear Ribonucleoprotein particles), which are the core components of the **spliceosome**. Their role is to identify and remove introns from pre-mRNA and ligate exons together. Since translation occurs in the cytoplasm after mRNA processing is complete, snRNA is not directly involved in protein synthesis. 2. **Why the other options are incorrect:** * **mRNA (Messenger RNA):** Acts as the template for translation. It carries the genetic code from DNA to the ribosome. * **tRNA (Transfer RNA):** Acts as the "adapter" molecule. It carries specific amino acids to the ribosome and matches them to the mRNA codons via its anticodon loop. * **rRNA (Ribosomal RNA):** The structural and catalytic component of ribosomes. It ensures proper alignment of mRNA and catalyzes peptide bond formation (peptidyl transferase activity). **High-Yield NEET-PG Pearls:** * **Splicing Pathology:** Antibodies against snRNPs (specifically **Anti-Smith antibodies**) are highly specific for **Systemic Lupus Erythematosus (SLE)**. * **Ribozyme:** rRNA is a classic example of a ribozyme (an RNA with enzymatic activity). * **RNA Polymerases:** Remember the "RMT" mnemonic for eukaryotes: Pol I → **r**RNA, Pol II → **m**RNA (and snRNA), Pol III → **t**RNA.

Q: Which phase of cell division is most appropriate for chromosomal studies?

Metaphase. **Explanation:** **Metaphase** is the most appropriate phase for chromosomal studies (Karyotyping) because chromosomes reach their **maximum state of condensation** during this stage. At metaphase, the sister chromatids are clearly visible, aligned at the equatorial plate, and possess distinct morphologies (size, shape, and centromere position). This makes them easiest to visualize under a light microscope after staining (e.g., Giemsa staining). **Analysis of Incorrect Options:** * **Prophase (A):** Chromatin begins to condense, but the chromosomes are still elongated, entangled, and not yet distinct enough for detailed structural analysis. * **Anaphase (D):** Sister chromatids begin to pull apart toward opposite poles. Because they are moving and no longer paired, it is difficult to assess their full morphology or count them accurately. * **Telophase (C):** Chromosomes begin to de-condense back into chromatin to form the daughter nuclei, making them invisible as distinct structures. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Colchicine/Colcemid:** In cytogenetic laboratories, cell division is intentionally arrested in metaphase using **Colchicine**, which inhibits spindle fiber formation by binding to tubulin. * **Karyotyping Indications:** Used to detect numerical aberrations (e.g., Trisomy 21) or structural aberrations (e.g., Translocations like t(9;22) in CML). * **Best Sample:** Peripheral blood lymphocytes are most commonly used for postnatal karyotyping; they are stimulated to divide using **Phytohemagglutinin (PHA)**.

Q: Which of the following is NOT a location where circular DNA is found in eukaryotic cells?

Nucleus. **Explanation:** In eukaryotic cells, the organization of genetic material differs significantly based on the organelle. The **Nucleus** (Option A) contains **linear DNA** organized into complex structures called chromosomes, wrapped around histone proteins. This linear arrangement is a hallmark of eukaryotic nuclear genomes, distinguishing them from the circular genomes of prokaryotes. **Analysis of Options:** * **Mitochondrion (Option C):** According to the endosymbiotic theory, mitochondria originated from ancient bacteria. Consequently, they possess their own **double-stranded circular DNA (mtDNA)**, which is inherited maternally and lacks histones. * **Yeast (Option D):** While yeast is a eukaryote, it is unique because many strains contain the **2-micron circle**, a specialized extrachromosomal circular DNA element. * **Plasmid (Option B):** Plasmids are small, circular, extrachromosomal DNA molecules. While primarily associated with bacteria, they are found in certain lower eukaryotes like yeast and fungi. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial DNA (mtDNA):** It is circular, 16.6 kb in size, and encodes 13 polypeptides of the respiratory chain, 22 tRNAs, and 2 rRNAs. Mutations here lead to "Maternal Inheritance" patterns (e.g., LHON, MELAS). * **Exceptions:** While nuclear DNA is linear, some specialized processes (like the formation of extrachromosomal circular DNA or eccDNA) can occur in cancer cells, but the standard physiological form remains linear. * **Prokaryotes:** Almost always possess a single, circular chromosome located in the nucleoid.

Q: The BRCA-1 gene, associated with breast and ovarian cancer, is located on which chromosome arm?

17q. **Explanation:** The **BRCA-1 (Breast Cancer 1)** gene is a critical tumor suppressor gene involved in the repair of double-stranded DNA breaks via homologous recombination. It is located on the **long arm (q)** of **Chromosome 17**, specifically at the locus **17q21**. Mutations in this gene significantly increase the lifetime risk of developing breast, ovarian, and fallopian tube cancers. **Analysis of Options:** * **17q (Correct):** This is the specific location of BRCA-1. A helpful mnemonic is "BRCA-**1** is on **17**q" (both have the digit '1'). * **17p:** This is the location of the **TP53** gene (17p13.1), which encodes the p53 protein, the "guardian of the genome." Mutations here lead to Li-Fraumeni Syndrome. * **22q:** This is the location of the **NF2** (Neurofibromatosis type 2) gene and the **BCR** gene (involved in the Philadelphia chromosome t(9;22)). * **18q:** This is the location of the **DCC** (Deleted in Colorectal Cancer) gene and the **SMAD4** gene. **High-Yield Clinical Pearls for NEET-PG:** * **BRCA-2 Location:** Located on **13q12.3**. (Mnemonic: BRCA-**2** is on **13**; 2+1=3). * **Inheritance:** Both BRCA-1 and BRCA-2 follow an **Autosomal Dominant** pattern with variable penetrance. * **Associated Cancers:** BRCA-1 is more strongly associated with triple-negative breast cancer; BRCA-2 is more strongly associated with **male breast cancer** and pancreatic cancer. * **Treatment:** Tumors with BRCA mutations are highly sensitive to **PARP inhibitors** (e.g., Olaparib) due to the principle of synthetic lethality.

Q: Which amino acid is coded for by the initiator codon?

Methionine. **Explanation:** In molecular biology, the translation of mRNA into a protein begins at a specific sequence known as the **initiator codon**, which is almost universally **AUG**. This codon specifies the amino acid **Methionine**. 1. **Why Methionine is Correct:** The genetic code is read in triplets (codons). The AUG codon serves a dual function: it acts as the "Start" signal for the ribosome to begin protein synthesis and simultaneously codes for Methionine. In eukaryotes, the initiator tRNA carries unmodified methionine, whereas in prokaryotes (and mitochondria), it carries **N-formylmethionine (fMet)**. 2. **Why Other Options are Incorrect:** * **Cystine (A):** This is a dimer of two Cysteine molecules. Cysteine is coded by UGU and UGC. * **Arginine (B):** A basic amino acid coded by six different codons (CGU, CGC, CGA, CGG, AGA, AGG), none of which function as start signals. * **Lysine (D):** A basic amino acid coded by AAA and AAG. **High-Yield Clinical Pearls for NEET-PG:** * **Kozak Consensus Sequence:** In eukaryotes, the efficiency of translation initiation is increased if the AUG is embedded within a specific sequence (ACCAUGG). * **Shine-Dalgarno Sequence:** In prokaryotes, this purine-rich sequence helps the 16S rRNA of the small ribosomal subunit align correctly with the AUG start codon. * **Non-Standard Start Codons:** Occasionally, GUG or UUG can act as initiator codons in prokaryotes, but they still recruit fMet-tRNA. * **Post-translational Modification:** The initial methionine is often enzymatically removed from the N-terminus of the mature protein.

Question 1

Which of the following enzymes converts primary microRNA (pri-miRNA) to precursor microRNA (pre-miRNA)?

Accepted Answer

Drosha

Answer

Dicer

Answer

RISC

Answer

Exonuclease

Question 2

Which of the following is true about the genetic code?

Accepted Answer

AUA codes for methionine in mitochondria

Answer

UGA codes for selenocysteine

Answer

AUG codes for the initiator codon in mammalian cells

Answer

AGA and AGG act as chain terminators in mammals

Question 3

What is the function of a Type II restriction enzyme after binding to a unique recognition site?

Accepted Answer

Double-stranded cut at a palindromic DNA sequence

Answer

Prevention of protein folding

Answer

Removing formed DNA

Answer

Preventing blunt end formation

Question 4

Submicroscopic deletions of any size can be detected by:

Accepted Answer

Multiplex ligation-dependent probe amplification (MLPA)

Answer

Southern blotting

Answer

Cytogenomic array technology

Answer

Chromosome painting

Question 5

Which type of RNA is NOT directly involved in the process of translation?

Accepted Answer

Small nuclear RNA (snRNA)

Answer

Messenger RNA (mRNA)

Answer

Transfer RNA (tRNA)

Answer

Ribosomal RNA (rRNA)

Question 6

Which phase of cell division is most appropriate for chromosomal studies?

Accepted Answer

Metaphase

Answer

Prophase

Answer

Telophase

Answer

Anaphase

Question 7

Which of the following is NOT a location where circular DNA is found in eukaryotic cells?

Accepted Answer

Nucleus

Answer

Plasmid

Answer

Mitochondrion

Answer

Yeast

Question 8

The BRCA-1 gene, associated with breast and ovarian cancer, is located on which chromosome arm?

Accepted Answer

17q

Answer

17p

Answer

22q

Answer

18q

Question 9

Which amino acid is coded for by the initiator codon?

Accepted Answer

Methionine

Answer

Cystine

Answer

Arginine

Answer

Lysine

Question 10

In a patient with non-Hodgkin B cell lymphoma, a nuclear gene is actively transcribed to mRNA and transported to the cytoplasm, where a protein is translated. This process is associated with up-regulation of BCL2. In a control group without lymphoma, translation of this mRNA does not occur. How is the silencing of this active gene's mRNA most likely to occur?

Accepted Answer

Binding to microRNA (miRNA)

Answer

Absence of transfer RNA (tRNA)

Answer

Methylation of DNA

Answer

Mutation of messenger RNA (mRNA)

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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