Molecular Biology and Genomics Practice Questions

Q: Multiple proteins from a single gene may be generated by:

Alternative splicing. ### Explanation **Correct Option: A. Alternative Splicing** The central dogma states that one gene typically codes for one protein; however, **alternative splicing** is the primary mechanism that allows for proteomic diversity. During post-transcriptional modification, different combinations of **exons** (coding regions) from a single pre-mRNA transcript are joined together, while introns are removed. This results in multiple unique mRNA isoforms, which are then translated into different proteins with distinct functions or tissue specificities. * *Example:* The **Calcitonin gene** produces Calcitonin in the thyroid but undergoes alternative splicing to produce Calcitonin Gene-Related Peptide (CGRP) in neural tissue. **Why Incorrect Options are Wrong:** * **B. Mutations in the promoter:** The promoter region controls the *initiation and rate* of transcription. Mutations here typically lead to increased, decreased, or abolished protein synthesis, but they do not change the protein's primary structure or create multiple variants. * **C. RNA interference (RNAi):** This is a regulatory mechanism (involving siRNA or miRNA) that leads to **gene silencing** by degrading mRNA or inhibiting translation. It reduces protein expression rather than generating new protein variants. * **D. mRNA capping:** This involves adding a 7-methylguanosine cap to the 5' end. It is essential for mRNA stability, nuclear export, and translation initiation, but it does not alter the coding sequence. **High-Yield Clinical Pearls for NEET-PG:** * **Spliceosomes:** These are the molecular machines (composed of snRNPs) that perform splicing. * **Clinical Correlation:** Mutations in splice sites are responsible for approximately 15% of genetic diseases, including **Beta-thalassemia** and **Spinal Muscular Atrophy (SMA)**. * **Isoforms:** Different proteins produced from the same gene via alternative splicing are called isoforms.

Q: Friedreich's ataxia is caused due to triplet repeats of which nucleotide sequence?

GAA. **Explanation:** **Friedreich's Ataxia (FRDA)** is an autosomal recessive neurodegenerative disorder characterized by progressive ataxia, loss of deep tendon reflexes, and hypertrophic cardiomyopathy. **Why GAA is correct:** The molecular basis of Friedreich's Ataxia is the **expansion of a GAA trinucleotide repeat** located in the first intron of the **FXN gene** on chromosome 9. This gene encodes **Frataxin**, a mitochondrial protein involved in iron-sulfur cluster biogenesis. The expansion leads to transcriptional silencing (via heterochromatin formation), resulting in a deficiency of Frataxin. This causes mitochondrial iron overload, increased oxidative stress, and subsequent damage to the spinal cord (spinocerebellar tracts) and heart. **Analysis of Incorrect Options:** * **AGG:** This sequence is not typically associated with a major trinucleotide repeat disorder. However, **CGG** repeats are seen in Fragile X Syndrome. * **UAA:** This is a **stop codon** (Ochre). While mutations can create premature stop codons (nonsense mutations), it is not the basis for triplet repeat expansion diseases. * **AUG:** This is the **start codon** (codes for Methionine). It initiates translation and is not associated with expansion disorders. **High-Yield Clinical Pearls for NEET-PG:** * **Inheritance:** Unlike most triplet repeat disorders (which are Autosomal Dominant), Friedreich's Ataxia is **Autosomal Recessive**. * **Anticipation:** It does *not* typically show significant clinical anticipation, unlike Huntington’s or Fragile X. * **Clinical Triad:** Progressive limb ataxia, absent knee/ankle jerks, and **Hypertrophic Cardiomyopathy** (the most common cause of death). * **Skeletal Deformities:** Kyphoscoliosis and **Pes Cavus** (high arched feet) are classic findings. * **Diabetes:** About 10-20% of patients develop Diabetes Mellitus due to pancreatic beta-cell dysfunction.

Q: Which of the following is a classic example of a missense mutation?

Sickle cell disease. **Explanation:** **Sickle Cell Disease (SCD)** is the classic prototype of a **missense mutation**. In SCD, a single base substitution occurs in the 6th codon of the $\beta$-globin gene (GAG $\rightarrow$ GTG). This point mutation results in the replacement of **Glutamic acid** (a polar, negatively charged amino acid) with **Valine** (a non-polar, hydrophobic amino acid). This single amino acid change causes the hemoglobin (HbS) to polymerize under deoxygenated conditions, leading to the characteristic "sickling" of red blood cells. **Analysis of Incorrect Options:** * **Thalassemia:** Most commonly results from **splice-site mutations** or **nonsense mutations** (leading to premature stop codons) in $\beta$-thalassemia, or large **gene deletions** in $\alpha$-thalassemia. It is a quantitative defect in globin chain synthesis, rather than a qualitative structural change. * **Sideroblastic Anemia:** This is primarily a defect in heme synthesis (often due to ALAS2 enzyme deficiency) rather than a specific point mutation in the globin gene. * **Hemochromatosis:** While the most common cause (HFE gene mutation, C282Y) is technically a missense mutation, it is not considered the "classic" teaching example for this genetic concept in medical biochemistry compared to Sickle Cell Disease. **High-Yield Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** The SCD mutation (A $\rightarrow$ T) is a **transversion** (purine to pyrimidine). * **Electrophoresis:** On alkaline electrophoresis, HbS moves **slower** than HbA toward the anode because it loses the negative charge of glutamic acid. * **Sticky Patches:** The substitution of Valine creates "sticky" hydrophobic patches on the surface of the hemoglobin molecule.

Q: Which of the following is NOT a function of restriction endonucleases?

They are non-specific to base pairs.. **Explanation:** Restriction Endonucleases (REs), often called "molecular scissors," are enzymes primarily derived from bacteria where they serve as a defense mechanism against viral DNA. **Why Option D is the Correct Answer:** The defining characteristic of restriction endonucleases is their **high specificity**. They recognize and bind to specific DNA sequences, typically 4 to 8 base pairs long, known as **palindromic sequences** (reading the same 5'→3' on both strands). Because they only cut at these precise recognition sites, saying they are "non-specific" is factually incorrect, making it the right choice for this "NOT" question. **Analysis of Incorrect Options:** * **Option A:** REs function by hydrolyzing the phosphodiester bonds on **both strands** of the DNA double helix at specific points within or near the recognition site. * **Option B:** Many REs (like *EcoRI*) make staggered cuts, leaving short, single-stranded overhangs known as **sticky (cohesive) ends**. These are highly useful in recombinant DNA technology as they can easily hybridize with complementary sequences. * **Option C:** Some REs (like *SmaI*) cut straight across the DNA at the same position on both strands, resulting in **blunt ends**. These lack overhangs and are generally more difficult to ligate. **High-Yield Clinical Pearls for NEET-PG:** * **Type II REs** are the most commonly used in genetic engineering because they cut exactly at the recognition site and do not require ATP. * **Naming Convention:** The first letter is the Genus, the next two are the species, and the Roman numeral indicates the order of discovery (e.g., *EcoRI*: *Escherichia coli*, strain RY13, 1st enzyme). * **Restriction Fragment Length Polymorphism (RFLP):** A technique using REs to detect genetic variations/mutations, crucial in forensic medicine and prenatal diagnosis of diseases like Sickle Cell Anemia.

Q: What is the primary function of Type II restriction enzymes?

Cleavage of specific palindromic DNA sequences.. **Explanation:** **1. Why Option B is Correct:** Type II restriction enzymes (Restriction Endonucleases) are fundamental tools in molecular biology and recombinant DNA technology. Their primary function is to recognize specific, short nucleotide sequences—typically **4 to 8 base pairs long and palindromic** (reading the same 5' to 3' on both strands)—and cleave the phosphodiester backbone at or near that specific site. Unlike Type I or III, Type II enzymes do not require ATP and cut precisely, making them predictable for gene cloning and DNA mapping. **2. Why the Other Options are Incorrect:** * **Option A:** Methylation is performed by **Methyltransferases**. In bacteria, this serves as a defense mechanism (Restriction-Modification System) to protect host DNA from being degraded by its own restriction enzymes. * **Option C:** Protein digestion is the role of **proteases** (e.g., pepsin, trypsin), not restriction enzymes, which act exclusively on nucleic acids. * **Option D:** Maintaining protein unfolding is the function of **molecular chaperones** (e.g., Heat Shock Proteins like HSP70), which prevent premature folding or aggregation. **3. NEET-PG High-Yield Pearls:** * **Nomenclature:** The first letter is the genus, the next two are the species, and the Roman numeral denotes the order of discovery (e.g., *EcoRI* from *E. coli*). * **Blunt vs. Sticky Ends:** Some enzymes (like *HpaI*) produce blunt ends, while others (like *EcoRI*) produce cohesive "sticky" ends, which are more efficient for ligation in genetic engineering. * **Clinical Application:** Restriction Fragment Length Polymorphism (**RFLP**) uses these enzymes to detect mutations (e.g., Sickle Cell Anemia, where a mutation abolishes a *MstII* recognition site).

Q: What are the small DNA segments termed that are produced during discontinuous DNA replication?

Okazaki fragments. **Explanation:** DNA replication is **semi-discontinuous** because DNA polymerase can only synthesize DNA in the **5' to 3' direction**. While the leading strand is synthesized continuously toward the replication fork, the lagging strand must be synthesized in the opposite direction (away from the fork). **1. Why Okazaki fragments is correct:** To accommodate the 5' to 3' synthesis requirement on the lagging strand, DNA is synthesized in short, discrete segments. These segments are called **Okazaki fragments** (named after Reiji and Tsuneko Okazaki). Each fragment begins with an RNA primer synthesized by **Primase**, which is later removed and replaced with DNA by DNA Polymerase I, and the nicks are sealed by **DNA Ligase**. **2. Why other options are incorrect:** * **Crick strands / Watson fragments:** These are distractors named after Francis Crick and James Watson, who discovered the double-helix structure of DNA. There are no specific "fragments" or "strands" named after them in the context of replication. * **Tsuneko strands:** While Tsuneko Okazaki was the co-discoverer of these fragments, the term "Tsuneko strands" is not standard biological nomenclature. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **DNA Ligase:** The enzyme responsible for joining Okazaki fragments. A deficiency in DNA ligase can lead to impaired DNA repair and replication (e.g., Bloom Syndrome). * **Directionality:** Remember that both strands are *read* 3'→5' but *synthesized* 5'→3'. * **Length:** In eukaryotes, Okazaki fragments are typically 100–200 nucleotides long, whereas in prokaryotes, they are much longer (1000–2000 nucleotides). * **Topoisomerase:** Relieves torsional strain (supercoiling) ahead of the replication fork; targeted by drugs like **Fluoroquinolones** (DNA Gyrase) and **Etoposide/Irinotecan**.

Question 1

Which enzyme is NOT involved in DNA replication?

Accepted Answer

Reverse transcriptase

Answer

Telomerase

Answer

Restriction endonuclease

Answer

DNA ligase

Question 2

Multiple proteins from a single gene may be generated by:

Accepted Answer

Alternative splicing

Answer

Mutations in the promoter of the gene

Answer

RNA interference

Answer

mRNA capping

Question 3

Defects in snRNPs cause which of the following genetic disorders?

Accepted Answer

Thalassemia

Answer

Sickle cell anemia

Answer

Marfan syndrome

Answer

Ehlers-Danlos syndrome

Question 4

Friedreich's ataxia is caused due to triplet repeats of which nucleotide sequence?

Accepted Answer

GAA

Answer

AGG

Answer

UAA

Answer

AUG

Question 5

Which of the following is a classic example of a missense mutation?

Accepted Answer

Sickle cell disease

Answer

Thalassemia

Answer

Sideroblastic anemia

Answer

Hemochromatosis

Question 6

Which of the following is NOT a function of restriction endonucleases?

Accepted Answer

They are non-specific to base pairs.

Answer

They cut both strands of double-stranded DNA.

Answer

The cut ends produced by restriction endonucleases can be sticky.

Answer

The cut ends produced by restriction endonucleases can be blunt.

Question 7

What is the primary function of Type II restriction enzymes?

Accepted Answer

Cleavage of specific palindromic DNA sequences.

Answer

Methylation of specific DNA sequences.

Answer

Facilitation of protein digestion.

Answer

Maintenance of nascent protein unfolding.

Question 8

Which of the following is required for certain types of eukaryotic protein synthesis but not for prokaryotic protein synthesis?

Accepted Answer

Signal recognition particle

Answer

Ribosomal RNA

Answer

Messenger RNA

Answer

Peptidyl transferase

Question 9

Termination of translation is caused by all of the following except:

Accepted Answer

Peptidyl transferase

Answer

48S complex

Answer

RF-1

Answer

UAA

Question 10

What are the small DNA segments termed that are produced during discontinuous DNA replication?

Accepted Answer

Okazaki fragments

Answer

Crick strands

Answer

Watson fragments

Answer

Tsuneko strands

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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