Molecular Biology and Genomics Practice Questions

Q: Which statement is TRUE regarding prophase of meiosis I?

The resultant cell is diploid. ### Explanation **Correct Option: B. The resultant cell is diploid** In the context of the cell cycle, **Prophase I** is the first stage of Meiosis I. At this stage, the cell has already undergone DNA replication during the S-phase but has not yet completed its first division. Therefore, the cell still contains the full **diploid (2n)** complement of chromosomes (46 chromosomes in humans), although each chromosome consists of two sister chromatids (4n DNA content). The reduction from diploid to haploid only occurs *after* the completion of Meiosis I (Telophase I/Cytokinesis). --- ### Analysis of Incorrect Options: * **A. Chromosomes separate:** This occurs during **Anaphase**. In Anaphase I, homologous chromosomes separate; in Anaphase II (and Mitosis), sister chromatids separate. Prophase is characterized by condensation, not separation. * **C. The resultant cell is haploid:** This is the outcome of the **entire Meiosis I division**, not Prophase I. The transition to haploidy (n) happens when homologous pairs are pulled to opposite poles and the cell divides. * **D. Sister chromatids replicate:** DNA replication occurs during the **S-phase of Interphase**, which precedes Meiosis. No DNA replication occurs during any phase of Prophase. --- ### NEET-PG High-Yield Pearls: 1. **Sub-stages of Prophase I:** Remember the mnemonic **"Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis"** (L-Z-P-D-D). 2. **Pachytene:** This is the most high-yield sub-stage where **crossing over** (genetic recombination) occurs via the enzyme **recombinase**. 3. **Diplotene:** This is where **Chiasmata** become visible. In females, primary oocytes are arrested in the Diplotene stage (Dictyotene) from birth until ovulation. 4. **DNA vs. Chromosome Count:** In Prophase I, the cell is **2n** (diploid) but **4C** (four times the haploid DNA content).

Q: Nonsense mutation is seen in which of the following conditions?

Thalassemia. ### Explanation **Correct Answer: B. Thalassemia** **1. Why Thalassemia is correct:** A **nonsense mutation** occurs when a single nucleotide substitution results in a premature stop codon (UAG, UAA, or UGA). This leads to the production of a truncated, non-functional protein. In **$\beta$-Thalassemia**, nonsense mutations are a common cause of the $\beta^0$ phenotype (total absence of $\beta$-globin chain production). For example, a mutation at codon 39 ($CAG \to UAG$) transforms a Glutamine codon into a stop codon, halting translation prematurely and leading to severe disease. **2. Analysis of Incorrect Options:** * **A. AIHA (Autoimmune Hemolytic Anemia):** This is an acquired condition caused by antibodies against RBC antigens, not a primary genetic mutation of the globin chain. * **C. Sickle Cell Anemia:** This is the classic example of a **missense mutation**. A single base substitution ($GAG \to GTG$) at the 6th position of the $\beta$-globin gene replaces Glutamic acid with Valine. * **D. Hemophilia:** While Hemophilia A and B can involve various mutations (including nonsense), the most characteristic genetic defect in severe Hemophilia A is an **intron inversion** (specifically Intron 22 inversion), not primarily a nonsense mutation in the context of standard comparative biochemistry questions. **3. High-Yield Clinical Pearls for NEET-PG:** * **Point Mutations:** * **Silent:** Same amino acid (due to degeneracy of genetic code). * **Missense:** Different amino acid (e.g., Sickle Cell, HbC). * **Nonsense:** Premature stop codon (e.g., $\beta$-Thalassemia). * **Frameshift Mutation:** Insertion or deletion of nucleotides (not in multiples of 3), seen in **Tay-Sachs disease** and certain forms of Thalassemia. * **Trinucleotide Repeat Expansion:** Seen in Huntington’s (CAG) and Fragile X (CGG).

Q: Classic human disorders related to genomic imprinting include all EXCEPT?

Duchenne muscular dystrophy. ### Explanation **Correct Answer: D. Duchenne muscular dystrophy** #### 1. Why Duchenne Muscular Dystrophy (DMD) is the Correct Answer Duchenne Muscular Dystrophy is an **X-linked recessive disorder** caused by a mutation in the *DMD* gene (encoding the protein dystrophin) on the X chromosome. It is characterized by progressive muscle degeneration. It is **not** related to genomic imprinting. Genomic imprinting involves the differential expression of genes depending on whether they are inherited from the mother or the father, usually regulated by DNA methylation. DMD follows classic Mendelian inheritance patterns. #### 2. Why the Other Options are Incorrect * **A & B. Prader-Willi (PWS) and Angelman (AS) Syndromes:** These are the classic "textbook" examples of imprinting disorders involving chromosome **15q11-q13**. * **PWS:** Loss of the *paternally* expressed gene (maternal imprinting). * **AS:** Loss of the *maternally* expressed gene (paternal imprinting). * **C. Beckwith-Wiedemann Syndrome (BWS):** This is a pediatric overgrowth disorder (macroglossia, omphalocele, hemihyperplasia) caused by alterations in imprinting at the **11p15.5** locus (involving genes like *IGF2* and *H19*). #### 3. High-Yield Clinical Pearls for NEET-PG * **Mechanism:** Imprinting occurs during gametogenesis via **DNA methylation** (specifically at CpG islands) by DNA methyltransferases. * **Uniparental Disomy (UPD):** A common cause of imprinting disorders where an individual receives two copies of a chromosome from one parent and none from the other. * **Other Imprinting Disorders:** Silver-Russell Syndrome (Chr 11/7) and Albright Hereditary Osteodystrophy (Pseudohypoparathyroidism Type 1A). * **DMD Mnemonic:** Remember "DMD is X-linked" (Dystrophin gene is the largest known human gene).

Q: Regarding the wobble hypothesis, what is the true statement about the variation?

5' end of anticodon. ### Explanation The **Wobble Hypothesis**, proposed by Francis Crick, explains why there are fewer tRNA molecules (approx. 40-50) than there are codons (61 sense codons). It states that while the first two bases of a codon pair strictly with the anticodon via standard Watson-Crick base pairing, the **third base of the codon** and the **first base of the anticodon** can undergo non-standard pairing. #### Why Option B is Correct The "wobble" occurs at the **5' end of the anticodon** (the first position of the tRNA anticodon). This flexibility allows a single tRNA to recognize and bind to multiple codons that differ only in their third base (the 3' end of the mRNA codon). For example, if the 5' position of the anticodon contains **Inosine (I)**, it can pair with U, C, or A at the 3' position of the mRNA. #### Why Other Options are Incorrect * **Option A (3' end of anticodon):** The 3' end of the anticodon corresponds to the first base of the mRNA codon. Pairing here is absolute and must follow strict Watson-Crick rules (A-U, G-C) to ensure translational fidelity. * **Option C & D (mRNA/tRNA):** These options are too broad. While the wobble effect involves both mRNA and tRNA, the specific "variation" or "wobble position" refers to a specific nucleotide location (the 5' end of the anticodon or the 3' end of the codon), not the entire molecule. #### High-Yield Clinical Pearls for NEET-PG * **Wobble Rules:** * If the 5' anticodon base is **G**, it can pair with **U or C**. * If the 5' anticodon base is **U**, it can pair with **A or G**. * If the 5' anticodon base is **I (Inosine)**, it can pair with **A, U, or C**. * **Significance:** The wobble hypothesis explains the **degeneracy of the genetic code** (multiple codons coding for one amino acid) and protects against silent mutations at the third nucleotide position. * **Directionality:** Always remember: **5' of Anticodon pairs with 3' of Codon.**

Q: In humans, the genetic code is represented by a code of three nucleotides. If one amino acid is coded by more than one triplet, then this is known as?

Degeneracy. ### Explanation **1. Why "Degeneracy" is the Correct Answer:** The genetic code consists of 64 possible codons (triplets) that encode for only 20 standard amino acids. Because there are more codons than amino acids, most amino acids are specified by multiple codons. This phenomenon is termed **Degeneracy** (or Redundancy). * **Mechanism:** Degeneracy primarily occurs at the **third position** of the codon (the 3' end), often explained by the **Wobble Hypothesis**. For example, Leucine is coded by six different codons (UUA, UUG, CUU, CUC, CUA, CUG). This provides a protective mechanism against mutations, as a change in the third nucleotide may still result in the same amino acid (Silent Mutation). **2. Why Other Options are Incorrect:** * **Ambiguity:** This would mean one codon codes for multiple different amino acids. The genetic code is **non-ambiguous**; each specific codon always codes for only one specific amino acid. * **Frame-shift Mutation:** This occurs when the addition or deletion of nucleotides (not in multiples of three) shifts the reading frame, altering all subsequent amino acids. It is a type of mutation, not a property of the code itself. * **Mutation:** This is a general term for any permanent change in the DNA sequence. While degeneracy helps mitigate the effects of mutations, it is not a synonym for the process. **3. High-Yield Clinical Pearls for NEET-PG:** * **Universal Code:** The genetic code is nearly universal across all species. **Exception:** Human mitochondrial DNA (e.g., UGA codes for Tryptophan instead of a Stop codon). * **Non-overlapping & Comma-less:** The code is read sequentially from a fixed starting point without skipping any nucleotides. * **Initiation Codon:** **AUG** (Methionine). In prokaryotes, it codes for N-formylmethionine. * **Stop Codons (Nonsense Codons):** UAA (Ochre), UAG (Amber), UGA (Opal). These do not code for any amino acid.

Q: Proteins are synthesized by:

Golgi Bodies. **Explanation:** The synthesis of proteins is a multi-step process involving different organelles. While ribosomes are the primary site of translation, the **Golgi Apparatus (Golgi Bodies)** plays a critical role in the final stages of protein synthesis, specifically **post-translational modification** and functional maturation. 1. **Why Golgi Bodies is Correct:** After polypeptide chains are formed on the ribosomes, they are transported to the Golgi apparatus. Here, they undergo essential modifications such as **glycosylation** (adding sugar moieties), sulfation, and phosphorylation. Without these modifications, proteins remain non-functional. Therefore, in the context of producing a *functional* protein ready for secretion or membrane integration, the Golgi is indispensable. 2. **Analysis of Incorrect Options:** * **Ribosomes:** These are the sites of **translation** (mRNA to polypeptide). While they assemble the amino acid chain, they do not complete the synthesis of complex, functional proteins. * **Mitochondria:** Known as the "Powerhouse of the cell," their primary role is ATP production via oxidative phosphorylation. Though they contain their own DNA and mitoribosomes, they synthesize only a tiny fraction (13 proteins) of the cell's total protein requirement. * **Nuclear Membrane:** This acts as a physical barrier protecting the genetic material and regulates nucleocytoplasmic transport; it is not a site for protein synthesis. **NEET-PG High-Yield Pearls:** * **Cis-face vs. Trans-face:** The Golgi has a 'Cis' face (entry/receiving) and a 'Trans' face (exit/shipping). * **I-Cell Disease:** A clinical correlation where a deficiency in phosphorylating enzymes in the Golgi leads to lysosomal storage issues. * **Marker Enzyme:** **Thiamine Pyrophosphatase** is the biochemical marker for the Golgi apparatus.

Q: Which of the following methods is used for sequencing a long DNA fragment?

Chromosome walking. ### Explanation **Why Chromosome Walking is Correct:** Chromosome walking is a technique used to sequence or map **long DNA fragments** (typically >50 kb) that are too large to be sequenced in a single read. The process involves "walking" along the chromosome by using the end of one known DNA clone as a primer/probe to identify the next overlapping clone in a genomic library. By repeating this process, researchers can characterize long, contiguous stretches of DNA. It is particularly useful in **positional cloning** to identify genes associated with specific diseases (e.g., the Cystic Fibrosis gene). **Analysis of Incorrect Options:** * **A & B. Sanger’s Technique / Chain Termination Method:** These are the same method. While highly accurate, Sanger sequencing is limited to **short reads** (typically 500–1000 base pairs). It cannot sequence a long DNA fragment or an entire chromosome in one go without prior fragmentation and assembly. * **D. Restriction Fragment Length Polymorphism (RFLP):** This is a tool used for **DNA profiling and linkage analysis**, not sequencing. It detects variations in DNA sequences by observing different lengths of fragments after digestion with restriction enzymes. **High-Yield Clinical Pearls for NEET-PG:** * **Chromosome Jumping:** A variation used to bypass long repetitive sequences or "unclonable" regions to reach a target site faster than walking. * **Next-Generation Sequencing (NGS):** The modern "high-throughput" standard that allows sequencing of the entire genome simultaneously by massive parallel processing. * **Sanger Method Reagents:** Remember that it requires **ddNTPs** (dideoxynucleoside triphosphates), which act as chain terminators because they lack the **3'-OH group** necessary for phosphodiester bond formation.

Q: Regarding ribosomes, all statements are true except?

Binding of aminoacyl tRNA to the ribosome requires 2 ATP.. **Explanation:** The correct answer is **D** because the binding of aminoacyl-tRNA to the A-site of the ribosome requires **1 GTP**, not 2 ATP. This process is mediated by **EF-Tu (in prokaryotes)** or **eEF-1α (in eukaryotes)**. While the overall process of protein synthesis is energy-intensive, the specific step of tRNA binding utilizes the hydrolysis of a single GTP molecule to ensure accuracy and positioning. **Analysis of other options:** * **Option A (True):** In prokaryotes, the **16S rRNA** (part of the 30S subunit) contains a sequence complementary to the **Shine-Dalgarno sequence** on mRNA. This base-pairing is essential for correct alignment of the AUG start codon. * **Option B (True):** Proteins are synthesized in two locations: **RER-bound ribosomes** (for secreted, lysosomal, or membrane proteins) and **Free cytosolic ribosomes** (for cytosolic, nuclear, mitochondrial, and **peroxisomal** proteins). * **Option C (True):** The large ribosomal subunit (23S in prokaryotes, 28S in eukaryotes) possesses **Peptidyl transferase** activity. This is a **ribozyme** (RNA enzyme) that catalyzes the formation of peptide bonds. **High-Yield NEET-PG Pearls:** * **Energy Requirements:** 2 high-energy phosphates (ATP → AMP) are used during **tRNA charging** (aminoacyl-tRNA synthetase). 1 GTP is used for **A-site binding**, and 1 GTP is used for **translocation** (EF-G/eEF-2). * **Antibiotic Targets:** Many antibiotics target these subunits (e.g., **Aminoglycosides/Tetracyclines** bind the 30S; **Macrolides/Chloramphenicol** bind the 50S). * **Eukaryotic vs. Prokaryotic:** Eukaryotes (80S: 60S + 40S); Prokaryotes (70S: 50S + 30S). Remember: "Even numbers for Eukaryotes, Odd for Prokaryotes."

Question 1

Which statement is TRUE regarding prophase of meiosis I?

Accepted Answer

The resultant cell is diploid

Answer

Chromosomes separate

Answer

The resultant cell is haploid

Answer

Sister chromatids replicate

Question 2

Nonsense mutation is seen in which of the following conditions?

Accepted Answer

Thalassemia

Answer

AIHA

Answer

Sickle cell anemia

Answer

Hemophilia

Question 3

Classic human disorders related to genomic imprinting include all EXCEPT?

Accepted Answer

Duchenne muscular dystrophy

Answer

Prader-Willi syndrome

Answer

Angelman syndrome

Answer

Beckwith-Wiedemann syndrome

Question 4

Regarding the wobble hypothesis, what is the true statement about the variation?

Accepted Answer

5' end of anticodon

Answer

3' end of anticodon

Answer

mRNA

Answer

tRNA

Question 5

In humans, the genetic code is represented by a code of three nucleotides. If one amino acid is coded by more than one triplet, then this is known as?

Accepted Answer

Degeneracy

Answer

Frame-shift mutation

Answer

Ambiguity

Answer

Mutation

Question 6

Proteins are synthesized by:

Accepted Answer

Golgi Bodies

Answer

Mitochondria

Answer

Ribosomes

Answer

Nuclear Membrane

Question 7

Anticipation is seen in which of the following genetic phenomena?

Accepted Answer

Trinucleotide repeat expansion

Answer

Translocation

Answer

Chromosome breaking

Answer

Mitochondrial mutation

Question 8

Which method is used to map genes located on a particular locus in a chromosome?

Accepted Answer

Linkage analysis

Answer

Genomic imprinting

Answer

Segregation analysis

Answer

Genome-wide analysis

Question 9

Which of the following methods is used for sequencing a long DNA fragment?

Accepted Answer

Chromosome walking

Answer

Sanger's technique

Answer

Chain termination method

Answer

Restriction fragment length polymorphism (RFLP)

Question 10

Regarding ribosomes, all statements are true except?

Accepted Answer

Binding of aminoacyl tRNA to the ribosome requires 2 ATP.

Answer

16S rRNA identifies the Shine-Dalgarno sequence.

Answer

Cytosolic ribosomes synthesize proteins destined for peroxisomes.

Answer

The large subunit catalyzes peptide bond formation.

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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