Molecular Biology and Genomics Practice Questions

Q: Lac operon transcription is induced by what substance?

An inducer without glucose. The **Lac Operon** is a classic model of prokaryotic gene regulation, functioning as an inducible system that ensures bacteria only expend energy to metabolize lactose when it is present and glucose is absent. ### **Why "An inducer without glucose" is correct:** Transcription of the Lac operon requires two simultaneous conditions: 1. **Presence of an Inducer (Allolactose):** The inducer binds to the **repressor protein**, causing it to detach from the operator site. This "unlocks" the gene for transcription. 2. **Absence of Glucose (Positive Control):** When glucose levels are low, **Adenylate Cyclase** is active, increasing **cAMP** levels. cAMP binds to the **Catabolite Activator Protein (CAP)**. The cAMP-CAP complex then binds to the promoter, acting as a "gas pedal" to recruit RNA polymerase for high-level transcription. ### **Why other options are incorrect:** * **A & B (Glucose present):** Glucose is the preferred energy source. If glucose is present, cAMP levels remain low. Without the cAMP-CAP complex, RNA polymerase cannot bind efficiently to the promoter (Catabolite Repression), even if an inducer is present. * **D:** Transcription is highly specific; it is inhibited by the presence of glucose and the absence of an inducer. ### **High-Yield Clinical Pearls for NEET-PG:** * **Inducer:** The natural inducer is **allolactose** (a lactose metabolite). In laboratory settings, **IPTG** (Isopropyl β-D-1-thiogalactopyranoside) is used as a "gratuitous inducer" because it isn't metabolized. * **Diauxic Growth:** If both glucose and lactose are provided, the bacteria show a biphasic growth curve—using glucose first, followed by a lag phase, then using lactose. * **Constitutive Mutations:** Mutations in the **i gene** (repressor) or the **operator** can lead to "constitutive expression," where the operon is always "on" regardless of inducer presence.

Q: In DNA, the coding region reads 5'-CGT-3'. This would code in the RNA as:

5'-CGU-3'. ### Explanation **1. Why Option A is Correct:** In molecular biology, DNA consists of two strands: the **coding (sense) strand** and the **template (antisense) strand**. * The **coding strand** (5' to 3') has the same sequence and polarity as the resulting mRNA, with the sole exception that **Thymine (T)** in DNA is replaced by **Uracil (U)** in RNA. * The **template strand** (3' to 5') is the one actually read by RNA polymerase to synthesize mRNA via complementary base pairing. Since the question provides the **coding region** as 5'-CGT-3', the mRNA will be identical in sequence and direction, replacing T with U. Thus, 5'-CGT-3' (DNA) becomes **5'-CGU-3' (RNA)**. **2. Why Other Options are Incorrect:** * **Option B (5'-GCA-3'):** This is the sequence of the **template strand** (complementary to the coding strand). While RNA polymerase uses this as a template, the resulting RNA is complementary to it, not identical to it. * **Option C (5'-ACG-3'):** This represents the sequence if read backward or incorrectly transcribed without maintaining polarity. * **Option D (5'-UGC-3'):** This is the **anticodon** sequence (found on tRNA) that would pair with the mRNA codon 5'-CGU-3'. **3. NEET-PG High-Yield Pearls:** * **The "Golden Rule":** mRNA sequence = Coding strand sequence (T $\rightarrow$ U). * **Directionality:** RNA synthesis always occurs in the **5' $\rightarrow$ 3' direction**. * **Template vs. Coding:** The template strand is also called the **non-coding** or **antisense** strand. The coding strand is also called the **sense** strand. * **Clinical Correlation:** Many antibiotics (like Rifampicin) and toxins (like $\alpha$-amanitin) act by inhibiting RNA polymerase, preventing this transcription process.

Q: Which enzyme is used for cDNA synthesis?

RNA dependent DNA polymerase. **Explanation:** **1. Why the Correct Answer is Right:** The synthesis of **cDNA (complementary DNA)** involves creating a DNA strand from a mature mRNA template. This process is known as **Reverse Transcription**. The enzyme responsible for this is **RNA-dependent DNA polymerase**, commonly referred to as **Reverse Transcriptase**. It "reads" an RNA sequence and "writes" a complementary DNA sequence. This is a crucial step in molecular biology techniques like RT-PCR and in the life cycle of retroviruses. **2. Analysis of Incorrect Options:** * **A. DNA-dependent RNA polymerase:** This enzyme is responsible for **Transcription** (synthesizing RNA from a DNA template), such as the production of mRNA, tRNA, and rRNA in the nucleus. * **B. DNA-dependent DNA polymerase:** This is the primary enzyme for **DNA Replication** (e.g., DNA Polymerase III or δ/ε). It synthesizes a new DNA strand using an existing DNA template. * **C. RNA-dependent RNA polymerase:** Also known as **RNA Replicase**, this enzyme is used by certain RNA viruses (like Poliovirus or SARS-CoV-2) to replicate their RNA genome directly without a DNA intermediate. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Source:** Reverse transcriptase was originally isolated from retroviruses like **HIV**. * **cDNA Characteristics:** Unlike genomic DNA, cDNA **lacks introns** because it is synthesized from processed mRNA. This makes it essential for cloning eukaryotic genes into prokaryotic vectors. * **Diagnostic Use:** **RT-PCR** (Reverse Transcription Polymerase Chain Reaction) is the gold standard for detecting RNA viruses (e.g., HIV viral load, COVID-19). * **Telomerase:** A specialized human version of RNA-dependent DNA polymerase that maintains chromosomal ends (telomeres) using its own internal RNA template.

Q: Which of the following enzymatic activities is possessed by the ribosome?

Peptidyl transferase. **Explanation:** The ribosome is a complex molecular machine composed of ribosomal RNA (rRNA) and proteins. Its primary enzymatic function is **Peptidyl transferase** activity, which catalyzes the formation of peptide bonds between adjacent amino acids during translation. 1. **Why Peptidyl Transferase is correct:** In eukaryotes, this activity is located in the **28S rRNA** of the large (60S) ribosomal subunit; in prokaryotes, it is in the **23S rRNA** of the 50S subunit. Because the catalytic activity resides in the RNA rather than a protein, the ribosome is classified as a **Ribozyme**. It facilitates the transfer of the growing polypeptide chain from the P-site tRNA to the amino acid on the A-site tRNA. 2. **Why other options are incorrect:** * **Peptidase:** These enzymes break peptide bonds (proteolysis). The ribosome builds them. * **Aminoacyl tRNA synthetase:** These are separate cytoplasmic enzymes that "charge" tRNA by attaching the correct amino acid to its 3' end. This occurs *before* the tRNA reaches the ribosome. * **GTPase:** While translation factors (like EF-Tu and EF-G) possess GTPase activity to provide energy, this is not an intrinsic enzymatic property of the ribosome itself. **Clinical Pearls for NEET-PG:** * **Antibiotic Target:** Several antibiotics inhibit peptidyl transferase. **Chloramphenicol** specifically binds to the 50S subunit and inhibits this enzyme in bacteria. * **Ribozyme Concept:** Remember that the ribosome is the most prominent example of a ribozyme. Another high-yield ribozyme is **RNase P**, involved in tRNA processing. * **Shine-Dalgarno Sequence:** In prokaryotes, the 16S rRNA (small subunit) recognizes the mRNA to initiate translation, while the 23S (large subunit) handles the peptidyl transferase activity.

Q: Which of the following genetic elements does not undergo recombination during gametogenesis?

Mitochondrial chromosome. **Explanation:** The correct answer is **Mitochondrial chromosome (Option D)**. **Why it is correct:** Recombination (crossing over) occurs during **Prophase I of Meiosis**, where homologous chromosomes pair up and exchange genetic material. Mitochondrial DNA (mtDNA) is unique because it is **inherited exclusively from the mother** (maternal inheritance). Since mitochondria are not part of the nuclear meiotic process and do not have a homologous partner to pair with during gametogenesis, they do not undergo meiotic recombination. They replicate via binary fission and are passed directly through the ooplasm to the zygote. **Why the other options are incorrect:** * **Autosome 21 (Option C):** All autosomes (1–22) exist as homologous pairs. During meiosis, they undergo synapsis and crossing over to ensure genetic diversity. * **X and Y chromosomes (Options A & B):** Although they are heterologous, the X and Y chromosomes contain **Pseudoautosomal Regions (PAR)** at their tips. These regions are homologous and undergo obligatory recombination during male meiosis to ensure proper segregation. **High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** All offspring of an affected mother will inherit a mitochondrial disease, but an affected father will never pass it on. * **Heteroplasmy:** The presence of a mixture of wild-type and mutant mtDNA within a single cell, which explains the variable clinical severity of mitochondrial diseases (e.g., MELAS, LHON). * **Recombination Hotspots:** These are specific regions on nuclear chromosomes where recombination occurs more frequently, often mediated by the protein **PRDM9**.

Q: Which RNA polymerase transcribes micro RNA?

RNA polymerase II. **Explanation:** The transcription of microRNA (miRNA) is primarily mediated by **RNA polymerase II**. Most miRNAs are transcribed from DNA sequences into long primary transcripts called **pri-miRNAs**. These transcripts possess a 5' cap and a 3' poly-A tail, which are characteristic features of RNA polymerase II products (similar to mRNA). While a small subset of miRNAs associated with repetitive elements can be transcribed by RNA polymerase III, the standard consensus for medical examinations is RNA polymerase II. **Analysis of Options:** * **Option A (RNA Pol I):** Located in the nucleolus, it exclusively transcribes the **45S pre-rRNA**, which is processed into 5.8S, 18S, and 28S ribosomal RNA. * **Option B (RNA Pol II):** Correct. It transcribes all protein-coding genes (**mRNA**), most **snRNA**, and **miRNA**. * **Option C (RNA Pol III):** Transcribes small "housekeeping" RNAs, including **tRNA**, **5S rRNA**, and U6 snRNA. * **Option D (DNA Polymerase):** This enzyme is involved in DNA replication and repair, not transcription (RNA synthesis). **High-Yield Clinical Pearls for NEET-PG:** 1. **Amanita phalloides (Death Cap Mushroom):** Contains **α-amanitin**, which potently inhibits RNA Polymerase II, leading to severe hepatotoxicity. 2. **miRNA Function:** They regulate gene expression post-transcriptionally by binding to the 3' UTR of target mRNA, leading to mRNA degradation or translational repression. 3. **Processing Enzymes:** Remember the "MicroRNA Pathway": **Drosha** (nuclear processing) → **Dicer** (cytoplasmic processing) → **RISC complex** (silencing). 4. **Mnemonic for RNA Pol I, II, III:** **R-M-T** (1-rRNA, 2-mRNA/miRNA, 3-tRNA).

Q: What is the approximate total number of base pairs in the human haploid set of chromosomes?

3 billion. **Explanation:** The human genome is organized into chromosomes within the cell nucleus. The term **haploid genome** refers to a single set of 23 chromosomes (22 autosomes and 1 sex chromosome), typically found in germ cells (sperm and egg). **1. Why Option B is Correct:** According to the Human Genome Project, the human haploid genome consists of approximately **3.2 billion base pairs (3.2 × 10⁹ bp)**. In a standard diploid somatic cell (containing 46 chromosomes), this number doubles to approximately 6.4 billion base pairs. For examination purposes, "3 billion" is the standard high-yield figure used to represent the haploid set. **2. Why the Other Options are Incorrect:** * **Option A (3 million):** This is far too small. For context, the bacterium *E. coli* has a genome size of about 4.6 million base pairs. * **Option C (33 billion):** This is an overestimation by a factor of ten. While some plants and amphibians have genomes of this size (C-value paradox), it does not apply to humans. * **Option D (5 million):** This is roughly the size of a typical bacterial genome, not a complex eukaryotic genome. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Coding vs. Non-coding:** Only about **1.5% to 2%** of the human genome actually codes for proteins (exons). * **Mitochondrial DNA (mtDNA):** Unlike the nuclear genome, mtDNA is circular, double-stranded, and contains only **16,569 base pairs** coding for 37 genes. * **Chargaff’s Rule:** In any double-stranded DNA, the number of Adenine (A) equals Thymine (T), and Guanine (G) equals Cytosine (C). * **Packaging:** This 3 billion bp DNA string is approximately 1 meter long per haploid set, necessitating highly organized packaging into **nucleosomes** (DNA wrapped around histone octamers).

Q: What banding technique is used for studying translocations involving the centromere?

C banding. **Explanation:** **C-banding (Constitutive Heterochromatin banding)** is the specific technique used to stain the **centromeres** and other regions containing constitutive heterochromatin (such as the distal portion of the Y chromosome). It involves pre-treating chromosomes with an alkali (like barium hydroxide) followed by Giemsa staining. This process denatures euchromatin while preserving the highly repetitive DNA at the centromere, making it the gold standard for identifying **dicentric chromosomes** and **centromeric translocations**. **Analysis of Incorrect Options:** * **Q-banding (Quinacrine):** Uses fluorescent dyes to produce bright and dull bands. It was the first banding method but requires a fluorescence microscope and the signal fades quickly. It is useful for identifying the Y chromosome and polymorphisms. * **G-banding (Giemsa):** The most common clinical technique. Chromosomes are treated with **Trypsin** followed by Giemsa. It produces dark (AT-rich, gene-poor) and light (GC-rich, gene-active) bands, used for general karyotyping. * **R-banding (Reverse):** The "reverse" of G-banding. It stains GC-rich regions (euchromatin) darkly. It is particularly useful for studying the **ends of chromosomes (telomeres)**, which often stain poorly in G-banding. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for G-banding:** **G**iemsa + **T**rypsin (**G**reat **T**eacher). * **Centromere = C-banding:** Always associate "C" with Centromere and Constitutive heterochromatin. * **Telomeres = R-banding:** Best for distal deletions or translocations. * **Karyotyping Phase:** All these techniques are performed on cells arrested in **Metaphase** using Colchicine (which inhibits spindle formation).

Question 1

Lac operon transcription is induced by what substance?

Accepted Answer

An inducer without glucose

Answer

Glucose

Answer

Glucose with an inducer

Answer

All of the above

Question 2

In DNA, the coding region reads 5'-CGT-3'. This would code in the RNA as:

Accepted Answer

5'-CGU-3'

Answer

5'-GCA-3'

Answer

5'-ACG-3'

Answer

5'-UGC-3'

Question 3

Which statement regarding nucleotide repair mechanisms is FALSE?

Accepted Answer

Mismatch repair primarily occurs during the G1 phase of the cell cycle.

Answer

UV radiation exposure leads to the formation of thymine-thymine dimers.

Answer

MUTYH-associated polyposis is associated with base excision repair.

Answer

Base excision repair mainly occurs in the G1 phase.

Question 4

Which enzyme is used for cDNA synthesis?

Accepted Answer

RNA dependent DNA polymerase

Answer

DNA dependent RNA polymerase

Answer

DNA dependent DNA polymerase

Answer

RNA dependent RNA polymerase

Question 5

Which of the following enzymatic activities is possessed by the ribosome?

Accepted Answer

Peptidyl transferase

Answer

Peptidase

Answer

Aminoacyl tRNA synthetase

Answer

GTPase

Question 6

Which of the following genetic elements does not undergo recombination during gametogenesis?

Accepted Answer

Mitochondrial chromosome

Answer

X chromosome

Answer

Y chromosome

Answer

Autosome 21

Question 7

Which RNA polymerase transcribes micro RNA?

Accepted Answer

RNA polymerase II

Answer

RNA polymerase I

Answer

RNA polymerase III

Answer

DNA polymerase

Question 8

What is the approximate total number of base pairs in the human haploid set of chromosomes?

Accepted Answer

3 billion

Answer

3 million

Answer

33 billion

Answer

5 million

Question 9

What banding technique is used for studying translocations involving the centromere?

Accepted Answer

C banding

Answer

Q banding

Answer

G banding

Answer

R banding

Question 10

Xeroderma pigmentosum is produced as a result of a defect in:

Accepted Answer

DNA polymerase I and DNA ligase

Answer

DNA polymerase III

Answer

DNA exonuclease

Answer

DNA ligase

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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