Molecular Biology and Genomics Practice Questions

Q: Which enzyme is known to prevent cellular aging?

Telomerase. **Explanation:** **Telomerase** is the correct answer because it is a specialized ribonucleoprotein reverse transcriptase that maintains chromosomal stability. In somatic cells, DNA polymerase cannot replicate the extreme 3' ends of linear chromosomes (the **"End Replication Problem"**), leading to progressive shortening of telomeres with each cell division. When telomeres reach a critical minimum length, the cell enters **senescence** (cellular aging) or apoptosis. Telomerase prevents this by adding repetitive TTAGGG sequences to the ends of chromosomes, effectively acting as a "cellular fountain of youth." It is highly active in germ cells, stem cells, and cancer cells, but inactive in most mature somatic cells. **Incorrect Options:** * **DNA ligase:** Responsible for joining Okazaki fragments by creating phosphodiester bonds; it is essential for DNA repair and replication but does not prevent telomere shortening. * **DNA polymerase alpha:** Involved in the initiation of DNA replication by synthesizing an RNA primer; it cannot solve the end-replication problem. * **RNA polymerase II:** Responsible for the transcription of DNA into mRNA; it has no role in maintaining chromosomal length or preventing aging. **High-Yield Clinical Pearls for NEET-PG:** * **Composition:** Telomerase contains an RNA template (**TERC**) and a catalytic protein subunit (**TERT**). * **Cancer Link:** Approximately 85–90% of cancer cells upregulate telomerase to achieve **replicative immortality**. * **Shelterin Complex:** A protein complex that protects telomeres from being recognized as DNA double-strand breaks. * **Dyskeratosis Congenita:** A genetic disorder caused by telomerase deficiency, leading to premature aging signs, bone marrow failure, and mucosal leukoplakia.

Q: A mutation in which of the following sequences in eukaryotic mRNA will affect the process by which the poly-A tail is added to the 3' end of mRNA?

AAUAAA. **Explanation:** The correct answer is **AAUAAA**. This sequence is known as the **Polyadenylation Signal**, located approximately 10–30 nucleotides upstream of the cleavage site at the 3' end of eukaryotic pre-mRNA. **1. Why AAUAAA is correct:** In eukaryotes, the maturation of mRNA involves the addition of a poly-A tail (200–250 adenine residues). This process is triggered when the **Cleavage and Polyadenylation Specificity Factor (CPSF)** recognizes the highly conserved **AAUAAA** sequence. Once recognized, the pre-mRNA is cleaved, and the enzyme **Poly-A Polymerase (PAP)** adds the adenine residues. A mutation in this sequence prevents proper cleavage and polyadenylation, leading to unstable mRNA that is rapidly degraded. **2. Why other options are incorrect:** * **CCA:** This sequence is added to the **3' end of tRNA** (not mRNA) post-transcriptionally. it is the site where amino acids attach. * **CAAT:** This is the **CAAT box**, a conserved promoter sequence located upstream of the transcription start site that regulates the frequency of transcription initiation. * **GU...A...AG:** These are conserved sequences at the **splice sites**. GU is the 5' donor site, AG is the 3' acceptor site, and 'A' represents the branch point. They are essential for splicing, not polyadenylation. **Clinical Pearls for NEET-PG:** * **Poly-A Tail Function:** Increases mRNA stability, facilitates nucleocytoplasmic transport, and enhances translation efficiency. * **Poly-A Polymerase:** Unlike RNA polymerase, it does **not** require a DNA template. * **Clinical Correlation:** Mutations in the AAUAAA signal are associated with certain types of **α-thalassemia**, where failure to polyadenylate α-globin mRNA leads to its deficiency.

Q: What is the number of Barr bodies in XXY males?

1. ### Explanation **1. The Correct Answer (A):** The number of Barr bodies is determined by the formula: **n – 1**, where ‘n’ is the total number of X chromosomes present in the cell. This phenomenon is known as the **Lyon Hypothesis**. In a male with Klinefelter syndrome (47, XXY), there are two X chromosomes. Applying the formula (2 – 1 = 1), there is **one Barr body**. The Barr body represents a highly condensed, transcriptionally inactive form of the X chromosome called **facultative heterochromatin**. This inactivation occurs early in embryonic development to ensure **dosage compensation**, preventing females (or individuals with extra X chromosomes) from having double the amount of X-linked gene products compared to XY males. **2. Why Other Options are Incorrect:** * **Option B (2):** This would be seen in individuals with three X chromosomes, such as **47, XXX (Triple X syndrome)** or **48, XXXY**. * **Option C (3):** This would be seen in individuals with four X chromosomes, such as **48, XXXX** or **49, XXXXY**. * **Option D (0):** This is the normal finding in a **46, XY male** or a female with **45, XO (Turner syndrome)**, as they possess only one X chromosome. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Lyonization:** The process of X-inactivation is random, fixed, and incomplete (some genes on the short arm of the X chromosome escape inactivation). * **Xist Gene:** The *X-inactive specific transcript* (Xist) gene, located on the X inactivation center (Xic), produces a non-coding RNA that coats and silences the chromosome. * **Staining:** Barr bodies are typically visualized as dark-staining masses at the periphery of the nucleus in buccal smear preparations or as "drumsticks" in polymorphonuclear leukocytes (neutrophils). * **Rule of Thumb:** Regardless of the number of Y chromosomes, the cell always maintains only **one active X chromosome**; all others become Barr bodies.

Q: What is the approximate number of genes contained in the human genome?

30,000. ### Explanation The human genome consists of approximately **3.2 billion base pairs**, but only a small fraction (about 1.5%) codes for proteins. According to the findings of the **Human Genome Project (HGP)**, the estimated number of protein-coding genes is significantly lower than historical predictions, currently placed between **20,000 and 30,000**. **1. Why Option B is Correct:** While the exact number is refined as technology improves (recent estimates suggest ~21,000), **30,000** remains the standard benchmark for medical examinations like NEET-PG. This figure highlights the "G-value paradox," where the complexity of an organism does not necessarily correlate with the total number of genes, but rather with how those genes are regulated and spliced. **2. Why Other Options are Incorrect:** * **Option A (40,000):** This was an early post-genomic estimate that has since been revised downward as researchers identified many sequences as non-coding pseudogenes. * **Options C & D (80,000 and 1,00,000):** Prior to the completion of the HGP in 2003, scientists predicted much higher numbers based on the vast diversity of human proteins. We now know that **alternative splicing** allows a single gene to produce multiple protein isoforms, explaining how 30,000 genes can generate over 100,000 distinct proteins. **High-Yield Clinical Pearls for NEET-PG:** * **Coding vs. Non-coding:** Only **1.5%** of the genome is exonic (protein-coding). * **Largest Gene:** **Dystrophin** (2.4 million bases). * **Smallest Gene:** **TATA-binding protein** (or SRY on the Y chromosome). * **Chromosome with most genes:** Chromosome 1 (approx. 2,968 genes). * **Chromosome with fewest genes:** Y chromosome (approx. 231 genes). * **Single Nucleotide Polymorphisms (SNPs):** Occur at about 10 million locations; these account for most human genetic variation.

Q: Which of the following trinucleotide repeat mutations is present in Huntington's disease?

CAG. **Explanation:** **Huntington’s Disease (HD)** is an autosomal dominant neurodegenerative disorder characterized by chorea, psychiatric symptoms, and dementia. The underlying molecular defect is the expansion of the **CAG** trinucleotide repeat within the *Huntingtin (HTT)* gene on chromosome 4. 1. **Why CAG is correct:** The CAG sequence codes for the amino acid **Glutamine**. In HD, the expansion leads to a "Polyglutamine (polyQ) tract" in the huntingtin protein. This results in a **toxic gain-of-function**, leading to neuronal death, particularly in the caudate nucleus and putamen (striatum). 2. **Analysis of Incorrect Options:** * **CGG (Option A):** Associated with **Fragile X Syndrome**. It occurs in the 5' untranslated region (UTR) of the *FMR1* gene, leading to gene silencing via hypermethylation. * **GAA (Option B):** Associated with **Friedreich Ataxia**. This is an intronic expansion in the *FXN* gene (encoding Frataxin), leading to impaired mitochondrial function. * **CTG (Option C):** Associated with **Myotonic Dystrophy (Type 1)**. It occurs in the 3' UTR of the *DMPK* gene. **High-Yield Clinical Pearls for NEET-PG:** * **Anticipation:** HD exhibits anticipation (earlier onset/increased severity in successive generations), typically when inherited from the **father** (paternal transmission), due to instability during spermatogenesis. * **Threshold:** Normal repeats are 40 repeats are fully penetrant for the disease. * **Neuroimaging:** Classic finding is **atrophy of the caudate nucleus**, leading to "boxcar ventricles" (enlargement of the frontal horns of lateral ventricles). * **Mnemonic:** "Huntington's **CAG**es **4** **C**audate" (CAG repeat, Chromosome 4, Caudate atrophy).

Q: What is the effect of UV radiation on cells?

Stimulates formation of pyrimidine dimers. ### Explanation **Correct Option: B. Stimulates formation of pyrimidine dimers** Ultraviolet (UV) radiation, specifically UV-B (280–320 nm), is a potent mutagen. When DNA is exposed to UV light, it causes the formation of **covalent bonds** between adjacent pyrimidine bases (Cytosine or Thymine) on the same strand. The most common lesion is the **Thymine-Thymine (T-T) dimer**, also known as a cyclobutane pyrimidine dimer. These dimers create a "kink" in the DNA backbone, which obstructs DNA polymerase during replication and RNA polymerase during transcription, leading to mutations or cell death if left unrepaired. **Analysis of Incorrect Options:** * **Option A & C:** UV radiation specifically targets pyrimidines because their chemical structure (single-ring) is more susceptible to photochemical excitation than the double-ring structure of purines (Adenine and Guanine). Purine dimers are not a standard pathological feature of UV damage. * **Option D:** Since only pyrimidine dimer formation is the characteristic mechanism, "All of the above" is incorrect. **Clinical Pearls for NEET-PG:** 1. **Repair Mechanism:** Pyrimidine dimers are normally repaired by the **Nucleotide Excision Repair (NER)** pathway. 2. **Clinical Correlation:** A defect in the NER pathway (specifically the UV-specific endonuclease) leads to **Xeroderma Pigmentosum**. Patients present with extreme photosensitivity and a high risk of skin cancers (Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma). 3. **Enzyme involved:** In bacteria, the enzyme **DNA Photolyase** can directly reverse this damage (Photoreactivation), but this enzyme is absent in humans.

Q: What is the melting temperature of DNA proportional to?

Proportional to GC pairs. **Explanation:** The melting temperature ($T_m$) of DNA is defined as the temperature at which 50% of the DNA double helix is denatured into single strands. This process involves breaking the hydrogen bonds between complementary base pairs. **1. Why Option B is Correct:** The stability of the DNA duplex is primarily determined by the base composition. **Guanine-Cytosine (GC) pairs** are held together by **three hydrogen bonds**, whereas Adenine-Thymine (AT) pairs are held by only two. Because more energy (heat) is required to break three bonds than two, DNA with a higher GC content has a higher $T_m$. Therefore, $T_m$ is directly proportional to the GC content. **2. Why Other Options are Incorrect:** * **Option A:** $T_m$ is inversely proportional to AT content. Higher AT pairs mean fewer hydrogen bonds, leading to a lower melting temperature. * **Option C:** Base pair composition is the primary intrinsic factor determining $T_m$; claiming no relation is biochemically incorrect. * **Option D:** While length does influence stability in very short oligonucleotides, for genomic DNA, the **base ratio (GC%)** and **ionic strength** of the solution are the dominant factors. $T_m$ is not a simple linear function of length in long DNA molecules. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **Hyperchromicity:** Denaturation of DNA leads to an increase in UV light absorption at **260 nm**. This is known as the hyperchromic effect. * **Formamide & Urea:** These are chemical denaturants that lower the $T_m$ by disrupting hydrogen bonds. * **Ionic Strength:** High salt concentration ($Na^+$ ions) increases $T_m$ by neutralizing the negatively charged phosphate backbone, reducing repulsion between strands. * **TATA Box:** Promoters are often rich in AT pairs (like the TATA box) because they need to be easily "melted" or opened by RNA polymerase to initiate transcription.

Q: The two strands of DNA are held together by:

Hydrogen bond. **Explanation:** The stability and structure of the DNA double helix are primarily maintained by **Hydrogen bonds** between complementary nitrogenous bases. According to Watson-Crick base pairing, Adenine (A) pairs with Thymine (T) via **two** hydrogen bonds, while Guanine (G) pairs with Cytosine (C) via **three** hydrogen bonds. These bonds are weak enough to allow "unzipping" during replication and transcription but strong enough to hold the strands together under physiological conditions. **Analysis of Incorrect Options:** * **Van-der-Waal bonds:** While these forces contribute to the "base-stacking" stability between adjacent bases on the *same* strand, they are not the primary force holding the two strands together. * **Covalent bonds:** These are strong bonds found in the **phosphodiester backbone** (linking the 3' carbon of one sugar to the 5' carbon of the next). If strands were held by covalent bonds, they could not be easily separated for biological processes. * **Ionic interaction:** DNA is negatively charged due to phosphate groups, but these charges actually cause repulsion between strands, which is neutralized by magnesium ions ($Mg^{2+}$) rather than holding the strands together. **High-Yield Clinical Pearls for NEET-PG:** * **GC Content & $T_m$:** DNA with higher G-C content has a higher **Melting Temperature ($T_m$)** because G-C pairs have three hydrogen bonds compared to the two in A-T pairs. * **Chargaff’s Rule:** In double-stranded DNA, the amount of A = T and G = C; therefore, Purines = Pyrimidines. * **Denaturation:** Agents like heat, formamide, and urea disrupt hydrogen bonds, leading to DNA denaturation.

Question 1

Which enzyme is known to prevent cellular aging?

Accepted Answer

Telomerase

Answer

DNA ligase

Answer

DNA polymerase alpha

Answer

RNA polymerase II

Question 2

A mutation in which of the following sequences in eukaryotic mRNA will affect the process by which the poly-A tail is added to the 3' end of mRNA?

Accepted Answer

AAUAAA

Answer

CCA

Answer

CAAT

Answer

GU...A...AG

Question 3

What is the number of Barr bodies in XXY males?

Accepted Answer

1

Answer

2

Answer

3

Answer

0

Question 4

What is the approximate number of genes contained in the human genome?

Accepted Answer

30,000

Answer

40,000

Answer

80,000

Answer

1,00,000

Question 5

Histone undergoes post-translational modification by which of the following mechanisms, except?

Accepted Answer

Glycosylation

Answer

Acetylation

Answer

Methylation

Answer

Phosphorylation

Question 6

Which of the following trinucleotide repeat mutations is present in Huntington's disease?

Accepted Answer

CAG

Answer

CGG

Answer

GAA

Answer

CTG

Question 7

What is the effect of UV radiation on cells?

Accepted Answer

Stimulates formation of pyrimidine dimers

Answer

Stimulates formation of purine dimers

Answer

Prevents formation of purine dimers

Answer

All of the above

Question 8

Which of the following is NOT a method used in gene therapy?

Accepted Answer

Fluorescence in situ hybridization (FISH)

Answer

Transfection

Answer

Electroporation

Answer

Bacteriophage transduction

Question 9

What is the melting temperature of DNA proportional to?

Accepted Answer

Proportional to GC pairs

Answer

Proportional to AT pairs

Answer

No relation with base pair composition

Answer

Directly proportional to length of DNA

Question 10

The two strands of DNA are held together by:

Accepted Answer

Hydrogen bond

Answer

Van-der-Waal bond

Answer

Covalent bond

Answer

Ionic interaction

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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