Molecular Biology and Genomics Practice Questions

Q: What determines the function of a gene?

Inserting or knocking out a gene. **Explanation:** The function of a gene is best determined by observing the phenotypic consequences of its presence or absence within a living system. This is the principle of **Reverse Genetics**. * **Why Option D is Correct:** * **Gene Knockout:** By "knocking out" (inactivating) a specific gene, researchers can observe what biological processes fail or what pathologies develop. If removing Gene X leads to insulin deficiency, the function of Gene X is determined to be related to insulin production. * **Gene Insertion (Transgenesis/Knock-in):** Adding a gene to an organism to see if it gains a specific function further confirms the gene's role. These methods provide a direct causal link between a DNA sequence and its biological function. * **Why Other Options are Incorrect:** * **Southern Blot (A):** Used for the detection of a specific **DNA sequence** in a sample. It tells you if a gene is present, but not what it does. * **Western Blot (B):** Used for the detection and quantification of specific **proteins**. While it confirms gene expression (translation), it does not define the functional role of that protein in the organism. * **Inoculating in Transgenic Mice (C):** This is a distractor. Transgenic mice are the *tools* used, but the *process* that determines function is the actual insertion or deletion of the gene within them. **High-Yield NEET-PG Pearls:** * **Knockout Mice:** Created using homologous recombination in embryonic stem cells. * **CRISPR-Cas9:** The most modern and efficient tool for gene editing (knocking out or inserting genes). * **North-Western Blot:** Used to detect DNA-binding proteins. * **South-Western Blot:** Used to detect protein-DNA interactions. * **Reporter Genes (e.g., GFP):** Used to study the *expression pattern* and localization of a gene rather than its physiological function.

Q: Transfer RNA (tRNA) primarily interacts with which of the following molecules during protein synthesis?

Messenger ribonucleic acid (mRNA). ### Explanation **Why mRNA is the Correct Answer:** Protein synthesis (translation) occurs on the ribosome, where **tRNA** acts as an "adapter molecule." Each tRNA molecule carries a specific amino acid and possesses an **anticodon** loop. This anticodon recognizes and binds to a complementary **codon** on the **messenger RNA (mRNA)** through base pairing. This interaction ensures that the genetic code carried by the mRNA is accurately translated into a specific sequence of amino acids to form a polypeptide chain. **Analysis of Incorrect Options:** * **A. DNA:** tRNA does not interact with DNA during translation. DNA serves as the template for transcription (forming mRNA, tRNA, and rRNA) in the nucleus, but the functional role of tRNA is restricted to the cytoplasm/ribosomes. * **C & D. miRNA and siRNA:** These are small non-coding RNAs involved in **gene silencing** and RNA interference (RNAi). They typically interact with mRNA to degrade it or inhibit its translation, but they do not directly interact with tRNA during the assembly of proteins. **High-Yield NEET-PG Pearls:** * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why the 3rd base of the mRNA codon can have non-traditional pairing with the 1st base of the tRNA anticodon, allowing one tRNA to recognize multiple codons. * **Aminoacyl-tRNA Synthetase:** The enzyme responsible for "charging" tRNA by attaching the correct amino acid (requires ATP). This is the true "translator" of the genetic code. * **CCA Tail:** All tRNA molecules have a **CCA sequence at the 3' end**, which is the attachment site for the amino acid. * **DHU Arm:** Responsible for recognition by the aminoacyl-tRNA synthetase enzyme. * **TψC Arm:** Involved in binding the tRNA to the ribosomal surface.

Q: Which of the following is NOT required for Polymerase Chain Reaction (PCR)?

Dideoxyribonucleotides. ### Explanation **1. Why Dideoxyribonucleotides (ddNTPs) is the correct answer:** Polymerase Chain Reaction (PCR) is an *in vitro* method used to amplify specific DNA sequences. It requires **Deoxyribonucleotides (dNTPs)**—dATP, dCTP, dGTP, and dTTP—to build the new DNA strands. **Dideoxyribonucleotides (ddNTPs)**, however, lack a 3'-OH group. If incorporated, they cause **chain termination** because no further phosphodiester bonds can be formed. Therefore, ddNTPs are the hallmark of **Sanger Sequencing**, not standard PCR. **2. Why the other options are incorrect:** * **Deoxyribonucleotides (dNTPs):** These are the essential "building blocks" required by the DNA polymerase to synthesize the complementary strand. * **Taq polymerase:** This is a heat-stable DNA polymerase (isolated from *Thermus aquaticus*). It is vital because it remains functional during the high-temperature denaturation step (94-96°C) of PCR. * **Template DNA:** This is the target sample containing the specific region of interest that needs to be amplified. Without a template, there is no sequence for the primers to bind to. **3. High-Yield Clinical Pearls for NEET-PG:** * **Steps of PCR:** Denaturation (95°C) → Annealing (55-65°C) → Extension (72°C). * **RT-PCR:** Uses Reverse Transcriptase to convert RNA into cDNA before amplification; it is the gold standard for diagnosing **COVID-19 (SARS-CoV-2)**. * **Real-Time PCR (qPCR):** Allows for the quantification of DNA in real-time using fluorescent dyes (e.g., SYBR Green) or probes (TaqMan). * **Components of PCR Mix:** Template, Primers (forward and reverse), dNTPs, Taq Polymerase, and $Mg^{2+}$ (cofactor).

Q: What is the result of a transition mutation in the DNA sequence GATCCT?

GGTCCT. ### Explanation **1. Understanding the Correct Answer (Option A: GGTCCT)** A **transition mutation** is a point mutation where a **purine is replaced by another purine** (A ↔ G) or a **pyrimidine is replaced by another pyrimidine** (C ↔ T). In the original sequence **GATCCT**, the second base is Adenine (A), which is a purine. In Option A (**GGTCCT**), the Adenine has been replaced by Guanine (G), which is also a purine. Since a purine is replaced by a purine, this is a classic transition mutation. **2. Analysis of Incorrect Options** * **Option B (GTTCCT):** Here, Adenine (Purine) is replaced by Thymine (Pyrimidine). This is a **transversion mutation** (Purine ↔ Pyrimidine). * **Option C (GABCCT):** "B" is not a standard nitrogenous base in DNA. This option is biologically invalid. * **Option D (GrUGGT):** This sequence contains "U" (Uracil), which is found in RNA, not DNA. Furthermore, it involves multiple base changes, whereas the question implies a single point mutation. **3. High-Yield Clinical Pearls for NEET-PG** * **Transition vs. Transversion:** Transitions are more common in the genome than transversions (ratio approx. 2:1), despite there being more possible transversion pathways. * **Silent Mutations:** Often occur at the 3rd position of a codon (wobble hypothesis) and do not change the amino acid. * **Missense Mutation:** Results in a different amino acid (e.g., Sickle Cell Anemia: Glutamate → Valine). * **Nonsense Mutation:** Results in a premature stop codon (UAA, UAG, UGA), leading to a truncated protein. * **Frameshift Mutation:** Caused by insertions or deletions (indels) not in multiples of three; these are usually more deleterious than point mutations.

Q: What does a Poly(A) tail translate into?

Polysine. ### Explanation The correct answer is **Polylysine**. **The Underlying Concept:** In eukaryotic mRNA, the **Poly(A) tail** consists of a long chain of adenine nucleotides added to the 3' end during post-transcriptional modification. In the genetic code, the triplet codon for the amino acid **Lysine is AAA**. Therefore, if a ribosome were to translate a continuous sequence of adenines (Poly-A), it would result in a polypeptide chain consisting entirely of lysine residues (**Polylysine**). **Analysis of Options:** * **B. Polylysine (Correct):** As established, the codon **AAA** codes for Lysine. This is a high-yield fact often tested in the context of translation and mRNA processing. * **A. Polyproline:** Proline is encoded by codons starting with CC (e.g., **CCC**). A Poly(C) tail would translate into Polyproline. * **C. Polyalanine:** Alanine is encoded by codons starting with GC (e.g., **GCC**). * **D. Polyglycine:** Glycine is encoded by codons starting with GG (e.g., **GGG**). A Poly(G) tail would translate into Polyglycine. **High-Yield Clinical Pearls for NEET-PG:** * **Polyadenylation:** Occurs in the nucleus; it is catalyzed by the enzyme **Poly(A) Polymerase**, which does *not* require a DNA template. * **Function:** The Poly(A) tail increases mRNA stability, facilitates nuclear export, and enhances translation efficiency. * **Non-stop Decay (NSD):** In vivo, if a ribosome translates into the Poly(A) tail (due to a missing stop codon), the resulting polylysine stretch acts as a signal for the **Exosome complex** to degrade the faulty mRNA and the stalled ribosome. * **Nirenberg’s Experiment:** Marshall Nirenberg used synthetic homopolymers (like Poly-U, Poly-A, and Poly-C) to decipher the genetic code. He discovered Poly-U translates to Polyphenenylalanine, Poly-C to Polyproline, and **Poly-A to Polylysine**.

Q: Okazaki segments are required for:

DNA synthesis. **Explanation:** **Why DNA Synthesis is Correct:** DNA replication is **semidiscontinuous**. The DNA polymerase enzyme can only synthesize new strands in the **5' to 3' direction**. During replication, the "Leading Strand" is synthesized continuously toward the replication fork. However, the "Lagging Strand" runs in the opposite direction (3' to 5' relative to the fork). To overcome this, the cell synthesizes short fragments of DNA in a 5' to 3' direction away from the fork. These short sequences (100–200 nucleotides in eukaryotes) are called **Okazaki fragments**. They are eventually joined together by the enzyme **DNA Ligase** to form a continuous strand. **Why Other Options are Incorrect:** * **RNA Synthesis (Transcription):** This process involves RNA polymerase synthesizing a single-stranded RNA molecule from a DNA template. It does not involve discontinuous fragments or Okazaki segments. * **Protein Synthesis (Translation):** This occurs in the ribosomes where mRNA is decoded into amino acids. It involves tRNA and rRNA, not DNA fragments. **High-Yield NEET-PG Pearls:** * **Directionality:** DNA synthesis always occurs in the **5' → 3'** direction. * **Enzymology:** Okazaki fragments are initiated by **RNA Primers** (synthesized by DNA Primase). * **The "Glue":** **DNA Ligase** is the enzyme responsible for joining Okazaki fragments by forming phosphodiester bonds. * **Clinical Correlation:** Deficiencies in DNA ligase or proteins involved in lagging strand synthesis (like the FEN1 endonuclease) can lead to genomic instability and are linked to conditions like **Bloom Syndrome** or increased cancer predisposition.

Q: What is the estimated total number of genes in the human genome?

19,000-20,000. **Explanation:** The human genome consists of approximately 3.2 billion base pairs. While initial estimates from the Human Genome Project (HGP) predicted over 100,000 genes based on protein diversity, current genomic sequencing and proteomic data have refined this number significantly. **1. Why Option B is Correct:** The current consensus, supported by databases like GENCODE and RefSeq, identifies approximately **19,000 to 20,000 protein-coding genes**. This accounts for only about **1.5% to 2%** of the total genome. The vast complexity of humans compared to simpler organisms (like *C. elegans*, which has a similar gene count) is attributed to **alternative splicing** and post-translational modifications, which allow one gene to produce multiple distinct proteins. **2. Analysis of Incorrect Options:** * **Option A (12,000):** This is an underestimate; even simpler eukaryotes like *Drosophila* have approximately 14,000 genes. * **Option C (24,000-25,000):** This was the widely accepted estimate in the mid-2000s (post-HGP completion) before more stringent criteria for "protein-coding" status were applied. * **Option D (30,000):** This was an early 2001 estimate. As sequencing technology improved, many sequences previously thought to be genes were reclassified as non-coding RNAs or pseudogenes. **High-Yield NEET-PG Pearls:** * **Exome:** The total collection of exons (coding regions); it is the target for **Whole Exome Sequencing (WES)**. * **Non-coding DNA:** Makes up ~98% of the genome, including introns, regulatory sequences, and "junk DNA" (transposons). * **Gene Density:** Chromosome 19 has the highest gene density, while Chromosome 13 and the Y chromosome have the lowest. * **Largest Gene:** Dystrophin (*DMD* gene), spanning 2.4 million base pairs.

Q: What type of mutation involves the substitution of a purine for a pyrimidine or vice versa?

Transversion. ### Explanation **1. Why Transversion is Correct:** Point mutations (single base substitutions) are classified into two categories: transitions and transversions. A **transversion** occurs when a **purine (A, G)** is replaced by a **pyrimidine (C, T)** or vice versa. Structurally, this involves swapping a two-ringed structure for a single-ringed structure (or vice versa), which causes a more significant distortion of the DNA helix than a transition. **2. Why the Other Options are Incorrect:** * **B. Transition:** This is a substitution where a purine is replaced by another purine (A ↔ G) or a pyrimidine is replaced by another pyrimidine (C ↔ T). Transitions are more common in the genome than transversions. * **C. Insertion:** This involves the addition of one or more extra nucleotides into the DNA sequence. If not in multiples of three, it leads to a **frameshift mutation**. * **D. Deletion:** This involves the removal of one or more nucleotides. Like insertions, these are categorized as "indels" and often result in truncated or non-functional proteins. **3. High-Yield Clinical Pearls for NEET-PG:** * **Frequency:** Although there are twice as many possible transversion pathways as transitions, **transitions** occur more frequently in nature due to the spontaneous deamination of 5-methylcytosine to thymine. * **Sickle Cell Anemia:** A classic example of a transversion mutation (GAG → GTG), where Adenine (purine) is replaced by Thymine (pyrimidine) in the β-globin gene, leading to Glutamate being replaced by Valine. * **Silent vs. Missense:** Substitutions can be **silent** (no amino acid change), **missense** (different amino acid), or **nonsense** (creates a premature stop codon: UAA, UAG, UGA).

Question 1

What determines the function of a gene?

Accepted Answer

Inserting or knocking out a gene

Answer

Southern blot

Answer

Western blot

Answer

Inoculating in transgenic mice

Question 2

Transfer RNA (tRNA) primarily interacts with which of the following molecules during protein synthesis?

Accepted Answer

Messenger ribonucleic acid (mRNA)

Answer

Deoxyribonucleic acid (DNA)

Answer

Micro ribonucleic acid (miRNA)

Answer

Small interfering ribonucleic acid (siRNA)

Question 3

Which of the following is NOT required for Polymerase Chain Reaction (PCR)?

Accepted Answer

Dideoxyribonucleotides

Answer

Deoxyribonucleotides

Answer

Taq polymerase

Answer

Template DNA

Question 4

What is the result of a transition mutation in the DNA sequence GATCCT?

Accepted Answer

GGTCCT

Answer

GTTCCT

Answer

GABCCT

Answer

GrUGGT

Question 5

What does a Poly(A) tail translate into?

Accepted Answer

Polysine

Answer

Polyproline

Answer

Polyalanine

Answer

Polyglycine

Question 6

Okazaki segments are required for:

Accepted Answer

DNA synthesis

Answer

RNA synthesis

Answer

Protein synthesis

Answer

None of the above

Question 7

What is the estimated total number of genes in the human genome?

Accepted Answer

19,000-20,000

Answer

12,000

Answer

24,000-25,000

Answer

30,000

Question 8

Which of the following statements is true regarding the genetics of cancer?

Accepted Answer

Topoisomerase II is useful for breaking double-stranded DNA.

Answer

Increased telomerase activity enhances the anti-tumour effect.

Answer

Maximum synthesis of DNA occurs during the G2 phase.

Answer

Transition from G2 to M phase is important in the blockage of cancer spread.

Question 9

What is a chromosome inversion?

Accepted Answer

A single chromosome undergoes breakage and rearranges within itself.

Answer

A portion of the chromosome is missing or deleted.

Answer

A portion of the chromosome is duplicated, resulting in extragenetic material.

Answer

A portion of one chromosome has been deleted from its normal place and inserted into another chromosome.

Question 10

What type of mutation involves the substitution of a purine for a pyrimidine or vice versa?

Accepted Answer

Transversion

Answer

Transition

Answer

Insertion

Answer

Deletion

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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