Molecular Biology and Genomics Practice Questions

Q: Which of the following is not a stop codon?

UGG. ### Explanation In the genetic code, **stop codons** (also known as nonsense codons) are specific nucleotide triplets within messenger RNA (mRNA) that signal the termination of translation. When a ribosome encounters a stop codon, no corresponding transfer RNA (tRNA) binds; instead, **release factors** bind to the ribosome, causing the newly synthesized polypeptide chain to be released. **Why UGG is the correct answer:** **UGG** is not a stop codon; it is the specific codon that codes for the amino acid **Tryptophan**. Tryptophan and Methionine (AUG) are unique because they are the only two amino acids encoded by a single codon each. **Analysis of incorrect options:** There are three standard stop codons in the universal genetic code: * **UAG (Amber):** A stop codon that signals the end of translation. * **UAA (Ochre):** The most common stop codon used in bacteria. * **UGA (Opal):** A stop codon in the universal code (though it notably codes for Selenocysteine in specific contexts). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** To remember the stop codons, use: **U** **A**re **G**one (UAG), **U** **A**re **A**way (UAA), and **U** **G**o **A**way (UGA). * **Nonsense Mutation:** A point mutation that changes a functional codon into a stop codon, leading to a truncated, usually non-functional protein (e.g., in some forms of β-thalassemia). * **Mitochondrial Exception:** In human mitochondria, the genetic code differs slightly; **UGA** codes for Tryptophan instead of acting as a stop codon, and **AGA/AGG** act as stop codons instead of coding for Arginine. * **21st Amino Acid:** **Selenocysteine** is often referred to as the 21st amino acid and is encoded by the UGA codon when a specific insertion sequence (SECIS) is present.

Q: Which of the following formulas represents Chargaff's rule?

A/T = G/C. **Explanation:** **Chargaff’s Rules** are fundamental principles of DNA structure established by Erwin Chargaff. The rule states that in double-stranded DNA (dsDNA), the amount of purines is always equal to the amount of pyrimidines. Specifically, Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C). 1. **Why Option B is Correct:** Since A pairs with T and G pairs with C, their molar ratios are equal (A=T and G=C). Therefore, **A/T = 1** and **G/C = 1**. Mathematically, this means **A/T = G/C**. 2. **Why Option A is Correct (Alternative Form):** While Option B is mathematically correct, Option A (**A + G = T + C**) is the most common representation of the rule (Purines = Pyrimidines). In many competitive contexts, both are valid; however, Option B specifically highlights the 1:1 stoichiometric ratio of base pairing. 3. **Why Options C and D are Incorrect:** * **Option C:** Bases are not present in equal proportions (A does not equal G). * **Option D:** The sum of A+T does not necessarily equal G+C. This ratio (A+T/G+C) varies between species and determines the DNA's melting temperature (Tm). **High-Yield Facts for NEET-PG:** * **Applicability:** Chargaff’s rules apply **only to double-stranded DNA**. They do not apply to single-stranded DNA (ssDNA) or RNA (where Uracil replaces Thymine). * **Base Pairing:** A-T pairs have **2 hydrogen bonds**, while G-C pairs have **3 hydrogen bonds**. * **Clinical Correlation:** DNA with high G-C content is more stable and has a higher **Melting Temperature (Tm)** due to the extra hydrogen bond and stronger base-stacking interactions. * **Calculation Tip:** If a question states DNA contains 20% Adenine, you can conclude it has 20% Thymine, leaving 60% for G-C (meaning 30% Guanine and 30% Cytosine).

Q: Which of the following is false about telomerase?

Telomerase leads to premature aging.. **Explanation:** Telomerase is a specialized enzyme responsible for maintaining the length of telomeres (repetitive DNA sequences at the ends of linear chromosomes). **Why Option D is the correct (false) statement:** Telomerase activity is associated with **immortality and cellular longevity**, not premature aging. It prevents the "end-replication problem" by adding TTAGGG repeats to the 3' end of DNA. **Premature aging syndromes** (like Progeria or Werner syndrome) are actually caused by **shortened telomeres** or a **deficiency/absence** of telomerase activity. When telomeres become critically short, the cell enters senescence (the Hayflick limit). **Analysis of other options:** * **Option A (True):** Telomerase is a specialized **reverse transcriptase** (specifically TERT: Telomerase Reverse Transcriptase). It uses an RNA template to synthesize DNA. * **Option B (True):** It contains an **intrinsic RNA template** (TERC: Telomerase RNA Component) that acts as a guide/primer for the synthesis of telomeric repeats. * **Option C (True):** It is a **ribonucleoprotein** because it is a complex consisting of both a protein catalytic subunit (TERT) and an RNA molecule (TERC). **Clinical Pearls for NEET-PG:** * **Cancer Connection:** Telomerase is highly active in **85-90% of cancer cells**, allowing them to divide indefinitely (immortality). * **Stem Cells:** It is normally active in germ cells, stem cells, and lymphocytes, but absent in most somatic cells. * **Shelterin Complex:** A group of proteins that protects telomeres from being recognized as DNA damage (double-strand breaks).

Q: A 10-year-old female patient with a known history of beta-thalassemia complains of tiredness and shortness of breath. Beta-thalassemia is caused by faulty splicing of which of the following?

hnRNA. **Explanation:** **1. Why hnRNA is Correct:** Beta-thalassemia is a genetic blood disorder characterized by reduced or absent synthesis of the beta-globin chains of hemoglobin. In many cases, the underlying molecular defect is a mutation in the **introns** or at the **splice site junctions** of the beta-globin gene. The primary transcript produced immediately after transcription is **hnRNA (heterogeneous nuclear RNA)**, also known as pre-mRNA. This hnRNA contains both exons (coding) and introns (non-coding). For functional mRNA to be formed, introns must be removed via **splicing**. Mutations in beta-thalassemia often create "cryptic" splice sites, leading to incorrect splicing of the hnRNA. This results in defective mRNA, which is either degraded or translated into non-functional proteins, leading to a deficiency of beta-globin. **2. Why Other Options are Incorrect:** * **snRNA (small nuclear RNA):** These molecules complex with proteins to form **snRNPs** ("snurps"), which are the *machinery* (spliceosome) that performs the splicing. While they facilitate the process, they are not the molecule being spliced. * **scRNA (small cytoplasmic RNA):** These are involved in protein targeting (e.g., Signal Recognition Particle) and are not involved in the splicing of globin genes. * **snoRNA (small nucleolar RNA):** These play a role in the post-transcriptional modification of **rRNA** (ribosomal RNA) within the nucleolus, not mRNA splicing. **Clinical Pearls for NEET-PG:** * **Post-transcriptional modifications** of hnRNA include: 5' Capping, 3' Polyadenylation, and Splicing. * **Splice site mutations** are a classic cause of $\beta^+$-thalassemia (reduced synthesis) or $\beta^0$-thalassemia (total absence). * **High-yield association:** Systemic Lupus Erythematosus (SLE) involves antibodies against snRNPs (**Anti-Smith antibodies**).

Q: Which of the following is true about X chromosome inactivation?

X gene. **Explanation:** **X-chromosome inactivation (Lyonization)** is a dosage compensation mechanism in females where one of the two X chromosomes is epigenetically silenced to ensure that females, like males, have only one functional copy of X-linked genes. **Why Option A is correct:** The process is mediated by the **X-inactivation center (XIC)**, which contains the **Xist gene** (*X-inactive specific transcript*). This gene does not encode a protein; instead, it produces a large **long non-coding RNA (lncRNA)**. This RNA "coats" the X chromosome from which it is transcribed, triggering heterochromatin formation and silencing. Therefore, the "X gene" (specifically Xist) is the fundamental driver of this process. **Why other options are incorrect:** * **Option B (RNA interference):** While X-inactivation involves non-coding RNA, it is not mediated by the RNA interference (RNAi) pathway (which typically involves siRNA/miRNA and the RISC complex). It is an epigenetic process involving chromatin remodeling (methylation and acetylation). * **Option C (Seen in males):** Males (46,XY) have only one X chromosome, which remains active. X-inactivation only occurs when more than one X chromosome is present (e.g., normal females or Klinefelter syndrome 47,XXY). **Clinical Pearls for NEET-PG:** 1. **Barr Body:** The inactivated X chromosome is visible as a dense mass of heterochromatin against the nuclear membrane, known as a Barr Body. 2. **Formula:** Number of Barr bodies = **(Total X chromosomes - 1)**. 3. **Mosaicism:** Because inactivation is random in each cell during early embryonic life, females are "genetic mosaics" (e.g., explains the presentation of G6PD deficiency or Hemophilia carriers). 4. **Lyon Hypothesis:** States that inactivation is random, fixed, and occurs early in development.

Q: Polypeptide chain termination is enhanced by which of the following?

All of these. Translation (protein synthesis) termination is a highly regulated process that occurs when a ribosome encounters a termination signal on the mRNA. **Explanation of the Correct Answer:** Termination is "enhanced" or facilitated by the collective action of specific sequences and enzymes: * **Stop Codons (UAA, UAG, UGA):** These are the primary signals for termination. They do not code for any amino acid and are not recognized by tRNA but by **Release Factors (RFs)**. * **UAA (Ochre):** This is one of the three specific stop codons. In many organisms, UAA is the most frequently used termination signal, making it a specific example of a stop codon. * **Peptidyl Transferase:** Traditionally known for forming peptide bonds, during termination, this ribozyme (part of the 28S rRNA in eukaryotes) changes its activity. In the presence of Release Factors, it catalyzes the **hydrolysis** of the bond between the completed polypeptide chain and the tRNA in the P-site, effectively releasing the protein. Since all three components are essential for the efficient release of the polypeptide chain, **Option D** is the correct answer. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Release Factors:** In prokaryotes, RF1 recognizes UAA/UAG, and RF2 recognizes UAA/UGA. In eukaryotes, a single factor, **eRF1**, recognizes all three. * **Nonsense Mutations:** A mutation that creates a premature stop codon (UAA, UAG, or UGA) leads to truncated, often non-functional proteins (e.g., in β-thalassemia). * **Energy Requirement:** Termination is an energy-dependent process requiring **GTP hydrolysis**. * **Mnemonic for Stop Codons:** * **UAA:** **U** Are **A**way * **UAG:** **U** Are **G**one * **UGA:** **U** Go **A**way

Q: Which of the following procedures is a routine technique for karyotyping using light microscopy?

G-banding. **Explanation:** **G-banding (Giemsa banding)** is the gold standard and most common routine technique used in cytogenetics for karyotyping. The process involves treating chromosomes with a protease (typically **trypsin**) followed by staining with **Giemsa stain**. This produces a distinct pattern of dark and light bands: * **Dark bands (G-positive):** Represent AT-rich, gene-poor, heterochromatic regions that replicate late. * **Light bands (G-negative):** Represent GC-rich, gene-dense, euchromatic regions that replicate early. This banding pattern allows for the identification of individual chromosomes and the detection of structural abnormalities like deletions, translocations, or inversions. **Analysis of Incorrect Options:** * **C-banding (Constitutive heterochromatin):** Specifically stains the centromeres and regions containing constitutive heterochromatin (e.g., chromosomes 1, 9, 16, and Y). It is not used for routine whole-genome karyotyping. * **Q-banding (Quinacrine):** The first banding method developed; it uses fluorescent microscopy. Because it requires a fluorescence microscope and the stains fade quickly (photobleaching), it is not the routine choice for light microscopy. * **V-staining:** This is a distractor and is not a recognized standard technique in clinical cytogenetics. **High-Yield Clinical Pearls for NEET-PG:** * **Sample Source:** For routine postnatal karyotyping, **peripheral blood lymphocytes** (stimulated by Phytohemagglutinin) are used. * **Cell Cycle Stage:** Karyotyping is performed during **Metaphase** (when chromosomes are most condensed). * **Resolution:** Standard G-banding identifies changes at a resolution of 5–10 Mb. For smaller microdeletions, FISH or Chromosomal Microarray is preferred. * **R-banding:** The "Reverse" of G-banding; useful for looking at GC-rich regions near telomeres.

Q: Actinomycin D interferes with enzyme induction by combining with which of the following?

Deoxyribonucleic acid (DNA). ### Explanation **Correct Answer: B. Deoxyribonucleic acid (DNA)** **Mechanism of Action:** Actinomycin D (also known as Dactinomycin) is a potent polypeptide antibiotic that functions as a **transcription inhibitor**. It exerts its effect by binding specifically to **double-stranded DNA**. It intercalates between adjacent **Guanine-Cytosine (G-C) base pairs**, forming a stable complex. This physical obstruction prevents the movement of **RNA Polymerase** along the DNA template, thereby blocking the synthesis of messenger RNA (mRNA). Since enzyme induction requires the transcription of new mRNA to produce specific proteins, Actinomycin D effectively interferes with this process. **Analysis of Incorrect Options:** * **A. Transfer RNA (tRNA):** Actinomycin D does not bind to tRNA. tRNA is involved in the translation phase (carrying amino acids), whereas this drug acts at the transcriptional level. * **C. Ribosomal RNA (rRNA):** While Actinomycin D can inhibit the synthesis of rRNA (especially at low doses in the nucleolus), its primary site of binding is the DNA template itself, not the RNA product. * **D. Repressor protein:** Repressor proteins bind to the operator region of DNA to inhibit transcription naturally (e.g., in the Lac operon). Actinomycin D is an exogenous chemical agent that binds to DNA directly, not a protein-protein regulator. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Use:** Actinomycin D is a key chemotherapeutic agent used in pediatric oncology, specifically for **Wilms tumor**, **Ewing sarcoma**, and **Rhabdomyosarcoma**. It is also used in **Gestational Trophoblastic Neoplasia (Choriocarcinoma)**. * **Key Distinction:** Unlike **Rifampicin** (which binds directly to bacterial RNA Polymerase), Actinomycin D binds to the **DNA template**. * **Toxicity:** It is a potent vesicant and can cause significant bone marrow suppression and "radiation recall" phenomenon.

Question 1

Which of the following is not a stop codon?

Accepted Answer

UGG

Answer

UAG

Answer

UAA

Answer

UGA

Question 2

A cell is placed in a medium containing radioactively labelled thymidine. After the cells undergo replication 3 times, what percentage of the cells will have both strands of DNA labelled?

Accepted Answer

75%

Answer

25%

Answer

50%

Answer

100%

Question 3

Which of the following formulas represents Chargaff's rule?

Accepted Answer

A/T = G/C

Answer

A + G = T + C

Answer

A = U = T = G = C

Answer

A + T = G + C

Question 4

Which of the following is false about telomerase?

Accepted Answer

Telomerase leads to premature aging.

Answer

Has reverse transcriptase activity.

Answer

Has an intrinsic primer.

Answer

It is a ribonucleoprotein.

Question 5

A 10-year-old female patient with a known history of beta-thalassemia complains of tiredness and shortness of breath. Beta-thalassemia is caused by faulty splicing of which of the following?

Accepted Answer

hnRNA

Answer

snRNA

Answer

scRNA

Answer

snoRNA

Question 6

Which of the following is a true statement about Ribozyme?

Accepted Answer

RNA molecule that acts catalytically to change itself or another RNA molecule.

Answer

Transfer RNA (t-RNA).

Answer

Messenger RNA (m-RNA).

Answer

Ribosome.

Question 7

Which of the following is true about X chromosome inactivation?

Accepted Answer

X gene

Answer

RNA interference

Answer

Seen in males

Answer

All of the above

Question 8

Polypeptide chain termination is enhanced by which of the following?

Accepted Answer

All of these

Answer

Stop codon

Answer

Peptidyl transferase

Answer

UAA

Question 9

Which of the following procedures is a routine technique for karyotyping using light microscopy?

Accepted Answer

G-banding

Answer

C-banding

Answer

Q-banding

Answer

V-staining

Question 10

Actinomycin D interferes with enzyme induction by combining with which of the following?

Accepted Answer

Deoxyribonucleic acid (DNA)

Answer

Transfer RNA (tRNA)

Answer

Ribosomal RNA (rRNA)

Answer

Repressor protein

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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