Molecular Biology and Genomics Practice Questions

Q: What is true about Polymerase Chain Reaction (PCR)?

Amplification of a target sequence of DNA. **Explanation:** **Polymerase Chain Reaction (PCR)** is a fundamental molecular biology technique used to make millions of copies of a specific DNA segment. It is essentially "in vitro cloning." 1. **Why Option A is Correct:** The primary objective of PCR is the **exponential amplification** of a specific target DNA sequence. This is achieved through repeated cycles of denaturation, annealing, and extension, allowing clinicians to detect even minute quantities of genetic material. 2. **Why Other Options are Incorrect:** * **Option B:** PCR requires a **thermostable** enzyme (not thermolabile), most commonly **Taq Polymerase** (derived from *Thermus aquaticus*). This enzyme must survive the high temperatures (94–96°C) used during the denaturation step. * **Option C:** PCR is an **extremely sensitive** technique. It can amplify DNA from a single cell or a tiny drop of blood, making it superior to many traditional culture or staining methods. * **Option D:** PCR requires **two primers** (forward and reverse). These are short synthetic oligonucleotides that are complementary to the sequences flanking the target DNA region on both strands. **High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** 1. Denaturation (~95°C), 2. Annealing (~55°C), 3. Extension (~72°C). * **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification (Gold standard for **COVID-19** and HIV viral load). * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes. * **Applications:** Diagnosis of genetic mutations (e.g., Sickle cell anemia), forensic medicine (DNA fingerprinting), and detecting infectious agents (TB, Hepatitis).

Q: Which of the following statements about microRNAs (miRNAs) is false?

They can encode proteins.. **Explanation:** **1. Why Option A is the Correct Answer (The False Statement):** MicroRNAs (miRNAs) are a class of **non-coding RNAs**. Their primary characteristic is that they do **not** serve as templates for translation into proteins. Instead, they function as regulatory molecules that modulate the expression of other protein-coding genes. In the central dogma, while mRNA carries the code for proteins, miRNA acts as a "silencer" of that code. **2. Analysis of Other Options:** * **Option B (Length):** This is true. Mature miRNAs are small, single-stranded RNA molecules, typically ranging from **21 to 25 nucleotides** (often generalized to 30 in broader contexts). * **Option C (Discovery):** This is true. **Andrew Fire and Craig Mello** were awarded the Nobel Prize in 2006 for their discovery of RNA interference (RNAi), specifically demonstrating how double-stranded RNA can silence gene expression. * **Option D (Function):** This is true. miRNAs inhibit gene expression post-transcriptionally. They bind to the 3' untranslated region (UTR) of target mRNAs, leading to either **mRNA degradation** or **inhibition of translation**. **3. High-Yield Clinical Pearls for NEET-PG:** * **Biogenesis:** miRNAs are transcribed by **RNA Polymerase II**. Key enzymes involved in their processing include **Drosha** (in the nucleus) and **Dicer** (in the cytoplasm). * **RISC Complex:** miRNAs function by being loaded into the **RNA-induced silencing complex (RISC)**. * **OncomiRs:** miRNAs that are dysregulated in cancer. Some act as tumor suppressors, while others act as oncogenes. * **Therapeutic Potential:** Synthetic miRNA mimics or "antagomirs" (inhibitors) are being researched as targeted therapies for genetic disorders and malignancies.

Q: Which one of the following is the complementary sequence of 5' TTAAGCTAC 3'?

5' GTAGCTTAA 3'. **Explanation:** In molecular biology, DNA strands are **antiparallel** and follow **Chargaff’s rule** of base pairing. To find the complementary sequence, two rules must be applied: 1. **Base Pairing:** Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). 2. **Directionality:** The complementary strand runs in the opposite direction (3' to 5') relative to the template (5' to 3'). **Step-by-step derivation:** * **Template:** 5' T T A A G C T A C 3' * **Complement (3' to 5'):** 3' A A T T C G A T G 5' * **Reverse for standard notation (5' to 3'):** 5' G T A G C T T A A 3' **Analysis of Options:** * **Option A (Correct):** This is the 3' to 5' complement written in the standard 5' to 3' direction. * **Option B:** This is a "direct" complement (A for T, T for A) but fails to account for the antiparallel nature; it incorrectly keeps the 5' end matching the 5' end of the template. * **Option C:** This is a random sequence that does not follow base-pairing rules for the given template. * **Option D:** This is nearly identical to the original sequence with minor changes, representing a common distractor for students who do not reverse the direction. **Clinical Pearls for NEET-PG:** * **Chargaff’s Rule:** In double-stranded DNA, A+G (purines) = T+C (pyrimidines). This rule does not apply to single-stranded RNA or DNA. * **Bonding:** A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3 hydrogen bonds**. High G-C content increases the **Melting Temperature (Tm)** of DNA. * **Directionality:** DNA polymerase always synthesizes DNA in the **5' to 3' direction**, adding nucleotides to the 3' hydroxyl (-OH) group.

Q: Which of the following statements about nucleotide excision repair is FALSE?

Only the damaged nucleotides are removed.. **Explanation:** **1. Why Option D is the Correct (False) Statement:** In Nucleotide Excision Repair (NER), the repair mechanism does not just remove the single damaged nucleotide. Instead, an **exinuclease** (excision endonuclease) makes two incisions on the damaged strand—one on the 5' side and one on the 3' side of the lesion. This results in the removal of an **oligonucleotide fragment** (a short stretch of about 12–13 nucleotides in prokaryotes and 24–32 in eukaryotes). Therefore, the statement that "only the damaged nucleotides are removed" is incorrect. **2. Analysis of Incorrect Options:** * **Option A:** NER is highly specific for bulky lesions on a **single strand**. The complementary undamaged strand serves as a template for DNA polymerase to fill the gap. * **Option B:** This is the classic function of NER. It identifies and repairs bulky, helix-distorting lesions, most notably **pyrimidine dimers (thymine dimers)** caused by UV radiation. * **Option C:** After the oligonucleotide fragment is removed, **DNA Polymerase (δ/ε)** fills the gap using the opposite strand as a template, and **DNA Ligase** seals the final phosphodiester bond. **3. Clinical Pearls & High-Yield Facts:** * **Xeroderma Pigmentosum (XP):** A classic NEET-PG topic. It is caused by a genetic deficiency in NER proteins (e.g., XP-A to XP-G). Patients present with extreme photosensitivity and a 1000-fold increased risk of skin cancers. * **Cockayne Syndrome:** Another disorder linked to a subtype of NER (Transcription-Coupled Repair), characterized by growth retardation and neurological degeneration. * **Contrast with BER:** Base Excision Repair (BER) uses **glycosylases** to remove a single "wrong" base, whereas NER removes a "patch" of nucleotides.

Q: Examine the provided pedigree carefully. What type of transmission does it depict?

Holandric inheritance. ***Holandric inheritance*** - **Y-linked inheritance** shows **male-to-male transmission** where every son of an affected father is affected, with no skipped generations among males. - **No females are affected** and there are **no female carriers** since the Y chromosome is only present in males. *Autosomal dominant inheritance* - Would show **both males and females affected** in roughly equal proportions across generations. - Typically shows **vertical transmission** with affected individuals in each generation, but would include affected females. *Autosomal recessive inheritance* - Characteristically **skips generations** and often shows **horizontal transmission** (affected siblings with unaffected parents). - Would require **consanguineous marriages** or **carrier parents** to manifest, and both sexes would be equally affected. *X-linked recessive inheritance* - Shows **no male-to-male transmission** since fathers pass the Y chromosome to sons, not the X chromosome. - Would display **carrier females** and **affected males through maternal transmission**, with some sons unaffected even from carrier mothers.

Q: The Hershey-Chase experiment was performed on which of the following?

Bacteriophages. ### Explanation **Correct Answer: C. Bacteriophages** The **Hershey-Chase experiment (1952)**, also known as the "blender experiment," provided definitive proof that **DNA, not protein, is the genetic material**. **Underlying Concept:** Alfred Hershey and Martha Chase utilized **T2 bacteriophages** (viruses that infect bacteria) for their study. They exploited the chemical differences between DNA and proteins: * **DNA** contains Phosphorus but no Sulfur. They labeled it with radioactive **$^{32}P$**. * **Proteins** contain Sulfur (in cysteine/methionine) but no Phosphorus. They labeled it with radioactive **$^{35}S$**. When the labeled phages infected *E. coli*, only the $^{32}P$ (DNA) was found inside the bacterial cells, while the $^{35}S$ (protein coat) remained outside. This proved that DNA is the molecule responsible for carrying genetic information into the host. **Why Incorrect Options are Wrong:** * **A & B (Lactobacillus & Tuberculous bacteria):** While bacteria were used as the *host* (*E. coli*), the experiment was specifically designed to observe the material injected *by* a virus into a cell. * **D (Mycoplasma):** These are the smallest free-living organisms and lack a cell wall, but they were not involved in this landmark molecular biology study. **High-Yield Clinical Pearls for NEET-PG:** * **Griffith’s Experiment (1928):** Discovered the "Transforming Principle" using *Streptococcus pneumoniae* in mice. * **Avery, MacLeod, and McCarty (1944):** First to suggest DNA was the transforming principle (preceded Hershey-Chase). * **Bacteriophage Structure:** Consists of a nucleic acid core (DNA/RNA) and a protein coat called a **capsid**. * **Transduction:** The process by which a bacteriophage transfers genetic material from one bacterium to another (a key mechanism of antibiotic resistance transfer).

Q: Which of the following activities is associated with 5' to 3' exonuclease activity?

Repair of damaged DNA. **Explanation:** In prokaryotes, **DNA Polymerase I** is unique because it possesses three distinct enzymatic activities: 5'→3' polymerase, 3'→5' exonuclease, and **5'→3' exonuclease**. **1. Why "Repair of damaged DNA" is correct:** The 5'→3' exonuclease activity is specifically designed to remove nucleotides in the same direction as DNA synthesis. This is essential for: * **Primer Removal:** It excises RNA primers during lagging strand synthesis. * **DNA Repair (Nick Translation):** It removes damaged DNA segments or mismatched bases ahead of the polymerase, allowing DNA Polymerase I to simultaneously fill the gap with correct nucleotides. This dual action is vital for Base Excision Repair (BER) and Nucleotide Excision Repair (NER). **2. Why other options are incorrect:** * **A. Proofreading:** This is the function of **3'→5' exonuclease activity**. It allows the enzyme to "backspace" and remove an incorrectly incorporated base at the 3' end. * **C. DNA synthesis:** This refers to the **5'→3' polymerase activity**, which adds dNTPs to the growing chain. * **D. DNA polymerase:** This is a general term for the enzyme. While it *has* the activity, the question asks which specific *biological process* is associated with the 5'→3' exonuclease function. **High-Yield Clinical Pearls for NEET-PG:** * **DNA Polymerase III:** The primary enzyme for elongation; it lacks 5'→3' exonuclease activity. * **Klenow Fragment:** A proteolytic fragment of DNA Pol I that retains polymerase and 3'→5' exonuclease (proofreading) but **lacks** 5'→3' exonuclease activity. * **Xeroderma Pigmentosum:** A clinical condition caused by a defect in Nucleotide Excision Repair (NER), highlighting the importance of DNA repair mechanisms.

Q: DNA Gyrase is:

Prokaryotic DNA Topoisomerase II. ### Explanation **DNA Gyrase** is a specialized enzyme found in **prokaryotes** that belongs to the **Type II Topoisomerase** family. Its primary function is to introduce **negative supercoils** into DNA, which helps relieve the positive superhelical tension that builds up ahead of the replication fork. Unlike Type I topoisomerases, Type II enzymes (like DNA Gyrase) catalyze the breakage of both strands of the DNA double helix and require ATP for their activity. #### Analysis of Options: * **Option D (Correct):** DNA Gyrase is the bacterial version of Topoisomerase II. It is unique because it is the only enzyme capable of introducing negative supercoils, which is essential for compacting the bacterial chromosome and facilitating replication. * **Option A:** Eukaryotic Topoisomerase I relaxes supercoils by cutting a single strand of DNA and does not require ATP. * **Option B:** Prokaryotic Topoisomerase III is a Type I topoisomerase involved primarily in recombination and resolving "knots" in DNA. * **Option C:** Prokaryotic Topoisomerase I (also known as Omega protein) relaxes negative supercoils but cannot introduce them. #### Clinical Pearls for NEET-PG: * **Pharmacological Target:** DNA Gyrase is the primary target of **Quinolones and Fluoroquinolones** (e.g., Ciprofloxacin, Nalidixic acid). These drugs inhibit the A-subunit of the enzyme, leading to bacterial cell death. * **Subunit Structure:** It is a tetramer composed of two **GyrA** subunits (involved in DNA breakage/rejoining) and two **GyrB** subunits (possessing ATPase activity). * **Eukaryotic Counterpart:** Humans possess Topoisomerase II, but it lacks the ability to introduce negative supercoils; it only relaxes them. This structural difference allows for selective toxicity of antibiotics.

Q: Which of the following is NOT a characteristic of the genetic code?

Contains punctuation signals. The genetic code is the set of rules by which information encoded in genetic material is translated into proteins. Understanding its properties is fundamental for NEET-PG biochemistry. ### **Explanation of the Correct Answer** **C. Contains punctuation signals:** This is the correct answer because the genetic code is **commaless**. Once translation begins at the start codon (AUG), the mRNA is read continuously, three nucleotides at a time, without skipping any bases or using "punctuation" marks between codons. There are no spacers between the triplets. While there are "Stop" codons (UAA, UAG, UGA) that signal the end of translation, they do not act as internal punctuation between individual amino acids. ### **Analysis of Incorrect Options** * **A. Degenerate:** This is a true characteristic. Most amino acids are coded by more than one codon (e.g., Leucine has six). This provides a buffer against mutations (often at the 3rd "wobble" position). * **B. Universal:** The code is consistent across almost all organisms, from bacteria to humans. *Exception:* Mitochondrial DNA (e.g., UGA codes for Tryptophan instead of Stop). * **D. Non-overlapping:** Each nucleotide is part of only one codon. In a sequence ABCDEF, the codons are ABC and DEF, never BCD. ### **High-Yield Clinical Pearls for NEET-PG** * **Wobble Hypothesis:** Proposed by Francis Crick; explains why the 3rd base of a codon can have non-traditional pairing, allowing one tRNA to recognize multiple codons. * **Initiation Codon:** **AUG** (codes for Methionine in eukaryotes and N-formylmethionine in prokaryotes). * **Nonsense Mutations:** Occur when a point mutation changes an amino acid codon into a **Stop codon** (UAA, UAG, UGA), leading to premature protein termination. * **Frameshift Mutations:** Result from the insertion or deletion of nucleotides (not in multiples of three), which alters the entire reading frame because the code is commaless and non-overlapping.

Question 1

What is true about Polymerase Chain Reaction (PCR)?

Accepted Answer

Amplification of a target sequence of DNA

Answer

Uses a thermolabile enzyme

Answer

A less sensitive technique

Answer

Uses one primer

Question 2

Which of the following statements about microRNAs (miRNAs) is false?

Accepted Answer

They can encode proteins.

Answer

They are typically 21-30 nucleotides in length.

Answer

They were discovered by Andrew Fire and Craig Mello.

Answer

They function by inhibiting gene expression.

Question 3

All of the following are associated with changes in genetic material, except?

Accepted Answer

Inversion

Answer

Insertion

Answer

Deletion

Answer

Robertsonian translocation

Question 4

Which one of the following is the complementary sequence of 5' TTAAGCTAC 3'?

Accepted Answer

5' GTAGCTTAA 3'

Answer

5' AATTCGCATG 3'

Answer

5' CATGCGAATT 3'

Answer

5' TTAAGCGTAC 3'

Question 5

Which of the following statements about nucleotide excision repair is FALSE?

Accepted Answer

Only the damaged nucleotides are removed.

Answer

Removal of damaged bases occurs only on one strand of the DNA.

Answer

It removes thymine dimers generated by UV light.

Answer

Requires ligase and polymerase.

Question 6

Examine the provided pedigree carefully. What type of transmission does it depict?

Accepted Answer

Holandric inheritance

Answer

Autosomal dominant inheritance

Answer

Autosomal recessive inheritance

Answer

X-linked recessive inheritance

Question 7

The Hershey-Chase experiment was performed on which of the following?

Accepted Answer

Bacteriophages

Answer

Lactobacillus

Answer

Tuberculous bacteria

Answer

Mycoplasma

Question 8

Which of the following activities is associated with 5' to 3' exonuclease activity?

Accepted Answer

Repair of damaged DNA

Answer

Proofreading

Answer

DNA synthesis

Answer

DNA polymerase

Question 9

DNA Gyrase is:

Accepted Answer

Prokaryotic DNA Topoisomerase II

Answer

Eukaryotic DNA Topoisomerase I

Answer

Prokaryotic DNA Topoisomerase III

Answer

Prokaryotic DNA Topoisomerase I

Question 10

Which of the following is NOT a characteristic of the genetic code?

Accepted Answer

Contains punctuation signals

Answer

Degenerate

Answer

Universal

Answer

Non-overlapping

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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