Molecular Biology and Genomics Practice Questions

Q: A change from UAC to UAG represents which type of mutation?

Nonsense mutation. **Explanation:** The correct answer is **Nonsense mutation**. **1. Why it is correct:** A nonsense mutation occurs when a single nucleotide substitution results in a **premature stop codon** (UAA, UAG, or UGA) within the mRNA sequence. In this case, the codon **UAC** (which codes for the amino acid Tyrosine) is changed to **UAG** (the "Amber" stop codon). This leads to the premature termination of protein synthesis, resulting in a truncated, usually non-functional protein. **2. Why the other options are incorrect:** * **Frameshift mutation:** This occurs due to the **insertion or deletion** of a number of nucleotides not divisible by three. This shifts the entire reading frame downstream, altering all subsequent amino acids. A single base substitution (like UAC to UAG) does not shift the reading frame. * **Deletion:** This involves the physical removal of one or more nucleotides from the DNA sequence. While a deletion can cause a frameshift, the question describes a specific base substitution (C to G), not a loss of genetic material. * **Missense mutation:** This is a point mutation where a single nucleotide change results in a codon that codes for a **different amino acid** (e.g., UAC to UAU is silent, but UAC to UCC would be missense). Since UAG is a stop codon and not an amino acid, it is specifically classified as nonsense. **High-Yield Clinical Pearls for NEET-PG:** * **Stop Codons:** Remember them as **U** **A**re **A**way (UAA), **U** **A**re **G**one (UAG), and **U** **G**o **A**way (UGA). * **Transition vs. Transversion:** UAC to UAG is a **transversion** (Pyrimidine 'C' replaced by Purine 'G'). * **Clinical Example:** Nonsense mutations are frequently seen in severe forms of **Beta-thalassemia** (β⁰) and **Duchenne Muscular Dystrophy (DMD)**. * **Nonsense-mediated decay:** The cell often recognizes and degrades mRNA containing premature stop codons to prevent the accumulation of toxic truncated proteins.

Q: Which of the following statements regarding telomerase is true?

It is a reverse transcriptase.. **Explanation:** **1. Why Option A is Correct:** Telomerase is a specialized **ribonucleoprotein enzyme** that functions as a **RNA-dependent DNA polymerase**, also known as **reverse transcriptase**. It contains an internal RNA template (hTR) which it uses to synthesize repetitive DNA sequences (TTAGGG in humans) at the 3' ends of chromosomes (telomeres). This mechanism counteracts the "end-replication problem," where DNA polymerase cannot fully replicate the lagging strand, preventing the loss of vital genetic information. **2. Why Other Options are Incorrect:** * **Option B:** In humans, telomerase is highly active in **germ cells, stem cells, and cancer cells**. However, it is **inactive or has very low activity in most differentiated somatic cells**, leading to progressive telomere shortening and cellular senescence (the Hayflick limit). * **Option C:** Telomerase is involved in **DNA replication/maintenance**, not protein translation. Translation is the process of synthesizing proteins from mRNA via ribosomes. * **Option D:** Telomerase **prevents** the shortening of DNA. It is the *absence* of telomerase activity that leads to the progressive shortening of chromosomes during each cell division. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Cancer Link:** Approximately 85–90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as double-stranded DNA breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by mutations in telomerase components, leading to premature aging and bone marrow failure. * **Sequence:** Remember the human telomeric repeat sequence: **5’-TTAGGG-3’**.

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.

Question 1

A change from UAC to UAG represents which type of mutation?

Accepted Answer

Nonsense mutation

Answer

Frameshift mutation

Answer

Deletion

Answer

Missense mutation

Question 2

Which of the following statements regarding telomerase is true?

Accepted Answer

It is a reverse transcriptase.

Answer

It is active in most somatic cells.

Answer

It is involved in protein translation.

Answer

It contributes to the shortening of DNA.

Question 3

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 4

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 5

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

Accepted Answer

Inversion

Answer

Insertion

Answer

Deletion

Answer

Robertsonian translocation

Question 6

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 7

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 8

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 9

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

Accepted Answer

Bacteriophages

Answer

Lactobacillus

Answer

Tuberculous bacteria

Answer

Mycoplasma

Question 10

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

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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