Molecular Biology and Genomics Practice Questions

Q: Which type of mutation usually involves mutation of a gene coding for the following molecule?

Nonsense suppressor mutation. ***Nonsense suppressor mutation*** - Involves mutations in **tRNA genes** that alter the **anticodon sequence** to read through **stop codons** (UAG, UAA, UGA). - The **tRNA structure** shown is directly involved in this mechanism, where mutant tRNA can insert an amino acid at a **nonsense codon** instead of terminating translation. *Silent mutation* - Changes a **codon** without altering the **amino acid sequence** due to the **degeneracy of the genetic code**. - Does not typically involve **tRNA gene mutations** but rather changes in protein-coding genes that don't affect protein function. *Nonsense mutation* - Creates a **premature stop codon** (UAG, UAA, UGA) in a **protein-coding gene**, leading to **truncated proteins**. - This type of mutation occurs in **mRNA-coding sequences**, not in **tRNA genes** like the molecule shown. *Missense mutation* - Results in a **single amino acid substitution** in the protein sequence due to a **point mutation** in the coding sequence. - Involves changes in **protein-coding genes** rather than **tRNA genes**, and doesn't affect the tRNA structure or anticodon recognition.

Q: Which of the following is the cofactor for prokaryotic DNA ligase?

NAD. **Explanation:** DNA ligase is the essential enzyme responsible for sealing "nicks" in the phosphodiester backbone by catalyzing the formation of a bond between a 3'-hydroxyl group and a 5'-phosphate group. This process requires an energy source to activate the 5' phosphate. **1. Why NAD+ is correct:** In **prokaryotes** (such as *E. coli*), DNA ligase specifically utilizes **Nicotinamide Adenine Dinucleotide (NAD+)** as the cofactor. The enzyme cleaves NAD+ into Nicotinamide Mononucleotide (NMN) and AMP; the AMP is then transferred to the enzyme to initiate the ligation reaction. **2. Why the other options are incorrect:** * **ATP:** This is the cofactor for **Eukaryotic DNA ligase** and T4 bacteriophage ligase. While prokaryotes use NAD+, humans and other eukaryotes use ATP. * **Tetrahydrobiopterin (BH4):** This is a cofactor for hydroxylation reactions involving amino acids (e.g., Phenylalanine hydroxylase, Tyrosine hydroxylase). It is not involved in DNA replication. * **FAD:** This is a redox cofactor involved in the Electron Transport Chain and various metabolic dehydrogenation reactions (e.g., Succinate dehydrogenase), but not in DNA ligation. **High-Yield Clinical Pearls for NEET-PG:** * **DNA Ligase Function:** Essential for joining **Okazaki fragments** on the lagging strand during replication and for DNA repair mechanisms (like Nucleotide Excision Repair). * **The "Glue":** DNA ligase is often referred to as "molecular glue" in recombinant DNA technology. * **Deficiency:** A deficiency in DNA Ligase I in humans leads to **46BR syndrome**, characterized by growth retardation, immunodeficiency, and sensitivity to DNA-damaging agents. * **Distinction:** Remember the mnemonic: **P**rokaryotes = **P**rimitive = **N**AD+; **E**ukaryotes = **E**volved = **A**TP.

Q: Which of the following is NOT a characteristic of the genetic code?

Overlapping. The genetic code is a set of rules by which information encoded in genetic material is translated into proteins. Understanding its properties is fundamental for molecular biology and genetics. ### **Why "Overlapping" is the Correct Answer** The genetic code is **non-overlapping**. This means that in a sequence like ABCDEF, the first codon is ABC, the second is DEF, and so on. Each nucleotide is part of only one codon. If the code were overlapping, a single mutation could affect multiple amino acids in a protein chain, which is not what occurs in nature. ### **Explanation of Incorrect Options** * **Non-ambiguous:** This means that one specific codon always codes for the same amino acid (e.g., UGG always codes for Tryptophan). It is "specific." * **Universal:** The genetic code is nearly identical in all known living organisms, from bacteria to humans. (Note: Minor exceptions exist in mitochondrial DNA). * **Degeneracy (Redundancy):** Most amino acids are coded by more than one codon (e.g., Leucine is coded by six different codons). This acts as a protective mechanism against minor mutations. ### **High-Yield Clinical Pearls for NEET-PG** * **Commaless:** There are no "punctuations" or gaps between codons; the mRNA is read continuously. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why the third base of a codon can sometimes vary without changing the amino acid, accounting for degeneracy. * **Initiation Codon:** **AUG** (Methionine in eukaryotes, Formyl-methionine in prokaryotes). * **Stop Codons (Nonsense Codons):** **UAA** (Ochre), **UAG** (Amber), and **UGA** (Opal). These do not code for any amino acid. * **Exceptions to Universality:** In human mitochondria, **UGA** codes for Tryptophan (instead of Stop) and **AUA** codes for Methionine (instead of Isoleucine).

Q: Gene therapy is most effectively given in which of the following conditions?

Severe combined immunodeficiency. **Explanation:** **Severe Combined Immunodeficiency (SCID)** is considered the "poster child" for successful gene therapy because it fulfills the ideal criteria for genetic intervention: it is caused by a single gene defect, affects hematopoietic stem cells (which are easily accessible), and provides a **selective survival advantage** to corrected cells. 1. **Why SCID is the Correct Answer:** The most common form treated via gene therapy is **ADA-SCID** (Adenosine Deaminase deficiency). When the functional ADA gene is inserted into the patient's autologous hematopoietic stem cells and re-infused, the corrected T-cells have a natural proliferative advantage over the defective ones. This allows for the restoration of the immune system without the high risks associated with allogeneic bone marrow transplants. 2. **Why Other Options are Less Effective:** * **Cystic Fibrosis:** Delivery is the main hurdle. The thick mucus in the lungs prevents viral vectors from reaching the target epithelial cells, and the turnover of these cells means the effect is often transient. * **Thalassemia & Sickle Cell Anemia:** These involve the hemoglobin molecule. Achieving the precise, high-level expression of the globin gene required to balance the alpha/beta chain ratio is biochemically complex compared to the simple enzyme replacement needed in SCID. **High-Yield Clinical Pearls for NEET-PG:** * **First Gene Therapy:** Performed in 1990 by William French Anderson on a 4-year-old girl (Ashanthi DeSilva) with **ADA-SCID**. * **Vectors:** Retroviruses and Lentiviruses are commonly used for SCID to ensure integration into the host genome. * **Recent Advance:** CRISPR-Cas9 is currently being explored for Sickle Cell Anemia, but SCID remains the classic historical and clinical success story in medical textbooks.

Q: What percentage of mitochondrial DNA constitutes total cellular DNA?

1%. **Explanation:** **1. Why Option A is Correct:** In human cells, the vast majority of genetic material is housed within the nucleus (Nuclear DNA). Mitochondrial DNA (mtDNA) is a small, circular, double-stranded molecule located within the mitochondria. Despite there being hundreds to thousands of mitochondria per cell (and multiple copies of mtDNA per mitochondrion), the actual size of the mitochondrial genome is very small—approximately 16.5 kb compared to the 3.2 billion base pairs of the nuclear genome. Consequently, mtDNA accounts for approximately **1% of the total cellular DNA**. **2. Why Other Options are Incorrect:** * **Option B (1.30%):** This is a distractor often confused with the percentage of the human genome that codes for proteins (exons), which is approximately 1.5%. * **Options C & D (3% and 5%):** These values significantly overestimate the mass of mitochondrial DNA. While mitochondria are numerous, their individual genomes are too compact to reach these percentages of total cellular DNA mass. **3. High-Yield Clinical Pearls for NEET-PG:** * **Maternal Inheritance:** mtDNA is inherited exclusively from the mother because the sperm's mitochondria are degraded during fertilization. * **Lack of Introns:** Unlike nuclear DNA, mtDNA is highly economical; it contains no introns and very little non-coding sequences. * **Mutation Rate:** The mutation rate in mtDNA is **10–20 times higher** than in nuclear DNA due to the lack of protective histones and proximity to reactive oxygen species (ROS) generated during oxidative phosphorylation. * **Heteroplasmy:** This refers to the presence of a mixture of more than one type of organellar genome (mutant vs. wild-type) within a cell, which explains the variable clinical severity of mitochondrial diseases (e.g., MELAS, LHON).

Q: In humans, what is the sequence of telomeres?

5'-TTAGGG-3'. **Explanation:** **1. Why the Correct Answer is Right:** Telomeres are specialized nucleoprotein structures located at the ends of linear eukaryotic chromosomes. They consist of tandem repeats of a specific hexanucleotide sequence. In humans (and all vertebrates), this conserved sequence is **5'-TTAGGG-3'**. The primary function of telomeres is to protect the chromosome ends from degradation, fusion, and being recognized as double-stranded DNA breaks. Because DNA polymerase cannot fully replicate the 3' end of a linear DNA molecule (the "end-replication problem"), telomeres shorten with each cell division. The enzyme **Telomerase**, a ribonucleoprotein, uses its internal RNA template to add these TTAGGG repeats back to the ends, thereby maintaining chromosomal stability. **2. Analysis of Incorrect Options:** * **Option A, C, and D:** These are arbitrary sequences that do not correspond to the highly conserved human telomeric repeat. While different species have different telomeric sequences (e.g., *Tetrahymena* uses TTGGGG), the sequence **TTAGGG** is specific and universal for human genetics. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hayflick Limit:** The finite number of times a normal somatic cell population will divide before cell division stops (senescence), dictated by telomere shortening. * **Cancer Connection:** Approximately 85–90% of cancer cells upregulate **Telomerase**, allowing them to achieve "replicative immortality." * **Shelterin Complex:** A group of six proteins (including TRF1 and TRF2) that binds to TTAGGG repeats to form a protective "T-loop," preventing DNA repair machinery from attacking the chromosome ends. * **Disease Link:** Mutations in telomerase components (TERT or TERC) lead to **Dyskeratosis Congenita**, characterized by premature aging, bone marrow failure, and mucosal leukoplakia.

Question 1

Which type of mutation usually involves mutation of a gene coding for the following molecule?

Accepted Answer

Nonsense suppressor mutation

Answer

Silent mutation

Answer

Nonsense mutation

Answer

Missense mutation

Question 2

What is the most likely effect of a 2bp insertion in the middle of an intron in a typical eukaryotic gene?

Accepted Answer

Normal transcription, normal translation

Answer

Normal transcription, altered translation

Answer

Defective termination of transcription, normal translation

Answer

Normal transcription, defective mRNA splicing

Question 3

Which of the following is the cofactor for prokaryotic DNA ligase?

Accepted Answer

NAD

Answer

Tetrahydrobiopterin

Answer

ATP

Answer

FAD

Question 4

Which of the following is NOT a characteristic of the genetic code?

Accepted Answer

Overlapping

Answer

Nonambiguous

Answer

Universal

Answer

Degeneracy

Question 5

Gene therapy is most effectively given in which of the following conditions?

Accepted Answer

Severe combined immunodeficiency

Answer

Cystic fibrosis

Answer

Thalassemia

Answer

Sickle cell anemia

Question 6

Single gene defect causing multiple unrelated problems is known as:

Accepted Answer

Pleiotropism

Answer

Pseudodominance

Answer

Penetrance

Answer

Anticipation

Question 7

What percentage of mitochondrial DNA constitutes total cellular DNA?

Accepted Answer

1%

Answer

1.30%

Answer

3%

Answer

5%

Question 8

Okazaki fragments formed during DNA replication are structurally:

Accepted Answer

DNA

Answer

mRNA

Answer

rRNA

Answer

tRNA

Question 9

The DNA nucleotide sequence to which RNA polymerase binds to initiate transcription is called the:

Accepted Answer

Promoter region

Answer

Exon

Answer

Enhancer region

Answer

Repressor region

Question 10

In humans, what is the sequence of telomeres?

Accepted Answer

5'-TTAGGG-3'

Answer

5'-GGCTTG-3'

Answer

5'-TAACGT-3'

Answer

5'-GTAGGC-3'

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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