Molecular Biology and Genomics Practice Questions

Q: The primary defect in Xeroderma pigmentosa is related to which cellular process?

Formation of thymidine dimers. **Explanation:** **Xeroderma Pigmentosum (XP)** is an autosomal recessive genetic disorder characterized by extreme sensitivity to ultraviolet (UV) radiation. **Why Option A is Correct:** The primary defect in XP is a deficiency in the **Nucleotide Excision Repair (NER)** pathway. When skin is exposed to UV light (specifically UV-B), it causes the covalent cross-linking of adjacent pyrimidine bases, most commonly leading to the **formation of thymidine dimers** (cyclobutane pyrimidine dimers). In healthy individuals, the NER pathway identifies these bulky lesions, and specific endonucleases (XP proteins A through G) excise the damaged DNA segment. In XP patients, this repair mechanism fails, leading to the accumulation of mutations and a high risk of skin malignancies. **Why Other Options are Incorrect:** * **Option B:** Defective Poly ADP Ribose Polymerase (PARP) activity is associated with impaired Base Excision Repair (BER) and is a target in cancer therapies (PARP inhibitors), but it is not the primary defect in XP. * **Option C:** Defective 3'→5' exonuclease activity (proofreading) is typically associated with **Hereditary Non-Polyposis Colorectal Cancer (HNPCC/Lynch Syndrome)**, which involves Mismatch Repair (MMR) defects. * **Option D:** While other dimers can form, **thymidine dimers** are the hallmark lesion caused by UV radiation and the specific substrate for the NER pathway relevant to XP. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Severe sunburn with minimal sun exposure, "parchment-like" skin (xeroderma), hyperpigmented macules, and telangiectasias. * **Malignancy Risk:** 1000-fold increased risk of Basal Cell Carcinoma, Squamous Cell Carcinoma, and Melanoma. * **Neurological Involvement:** Seen in the **De Sanctis-Cacchione syndrome** variant of XP. * **Key Enzyme:** The specific enzyme often cited as deficient is **UV-specific endonuclease**.

Q: A promoter sequence is an example of which type of factor?

Cis acting factor. **Explanation:** In molecular biology, gene expression is regulated by the interaction between DNA sequences and regulatory proteins. **1. Why Cis-acting factor is correct:** A **cis-acting factor** (or element) is a specific sequence of DNA that regulates the expression of genes located on the **same chromosome** (the word "cis" means "on the same side"). A **promoter** is a classic example; it is a DNA sequence located upstream of the structural gene where RNA polymerase binds to initiate transcription. Because the promoter is a physical part of the DNA molecule it regulates, it is classified as a cis-acting element. Other examples include enhancers and silencers. **2. Why the other options are incorrect:** * **Trans-acting factor:** These are typically **proteins** (like transcription factors) encoded by genes located elsewhere in the genome. They are produced in the cytosol, enter the nucleus, and can diffuse to bind to any relevant DNA sequence regardless of its chromosomal location. * **Coactivator:** These are proteins that increase the rate of transcription by binding to transcription factors rather than binding directly to the DNA promoter sequence itself. * **Mediator:** This is a large multi-protein complex that acts as a bridge between gene-specific transcription factors (bound to enhancers) and the RNA polymerase II machinery. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic:** **C**is is **C**ontiguous (DNA sequence); **T**rans is **T**ransferred (Protein/Factor). * **Mutations:** A mutation in a *cis*-acting element (like a TATA box mutation) usually affects only the gene on that specific allele, whereas a mutation in a *trans*-acting factor can affect the expression of multiple genes throughout the genome. * **TATA Box:** The most common promoter element in eukaryotes, located approximately 25-35 base pairs upstream of the transcription start site.

Q: How many nonsense or genetic codons are there?

3. ### Explanation **1. Why Option B is Correct:** In the standard genetic code, there are **64 possible codons** (4³ combinations of the four nitrogenous bases). Out of these, **61 are sense codons** that code for specific amino acids. The remaining **3 codons** do not code for any amino acid and are known as **Nonsense codons** or **Stop codons**. Their primary function is to signal the termination of polypeptide chain synthesis during translation. The three nonsense codons are: * **UAA** (Ochre) * **UAG** (Amber) * **UGA** (Opal) **2. Why Other Options are Incorrect:** * **Option A (2):** While there are two types of codons (Sense and Nonsense), the numerical count of nonsense codons is three. * **Option C (4):** There are 4 distinct nitrogenous bases (A, U, G, C), but this does not correlate to the number of stop codons. * **Option D (5):** There is no biological basis for 5 nonsense codons in the standard genetic code. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Initiation Codon:** **AUG** (Methionine) is the universal start codon. In prokaryotes, it codes for N-formylmethionine. * **Degeneracy/Redundancy:** Multiple codons can code for a single amino acid (e.g., Leucine has 6 codons), but one codon never codes for more than one amino acid (**Unambiguous**). * **Nonsense Mutation:** A point mutation that changes a sense codon into a nonsense codon, leading to premature termination of the protein and usually resulting in a non-functional product (e.g., certain types of β-thalassemia). * **Exceptions:** In human mitochondria, **UGA** codes for Tryptophan rather than acting as a stop codon.

Q: The disorder shown in the illustration is related to which DNA repair mechanism?

Nucleotide excision repair. ***Nucleotide excision repair*** - The illustration shows **Xeroderma Pigmentosum (XP)**, a disorder caused by defective **nucleotide excision repair (NER)** mechanism that removes **UV-induced DNA lesions** like pyrimidine dimers. - NER defects result in **extreme photosensitivity**, **premature aging**, and **high risk of skin cancers** due to inability to repair **bulky DNA adducts** caused by UV radiation. *Mismatch repair* - Defects in **mismatch repair** cause **Lynch syndrome** (hereditary nonpolyposis colorectal cancer), not the photosensitive skin disorder shown. - This mechanism corrects **base-base mismatches** and **insertion-deletion loops** during DNA replication, not UV-induced damage. *Base excision repair* - **Base excision repair (BER)** removes **small base modifications** like **8-oxoguanine** and **uracil** through glycosylases. - BER defects are not associated with **photosensitivity** or the characteristic skin manifestations seen in this illustration. *SOS repair* - **SOS repair** is an **error-prone DNA repair** system in bacteria activated under extreme DNA damage conditions. - This mechanism does not exist in **human cells** and is not related to any human genetic disorders or photosensitivity syndromes.

Q: Mutations are due to changes in:

DNA nucleotide sequence. **Explanation:** **1. Why Option A is Correct:** A mutation is defined as a permanent, heritable change in the **DNA nucleotide sequence**. DNA serves as the primary genetic blueprint of the cell. Any alteration in its sequence—whether through substitutions (point mutations), insertions, or deletions—can lead to changes in the genetic code. These changes are then propagated through replication and, if they occur in germ cells, are passed to offspring. **2. Why Other Options are Incorrect:** * **Option B (RNA nucleotide sequence):** While errors can occur during transcription (DNA to RNA), these are transient and not considered mutations because they are not inherited or permanent. If the underlying DNA is normal, subsequent RNA transcripts will be correct. * **Option C (Amino acid sequence):** Changes in amino acids are the *result* of a mutation (at the protein level), not the mutation itself. Ribonuclease is simply a specific enzyme; mutations affect the genome, not just one specific protein's structure directly. * **Option D (Cell walls):** Cell walls are structural components (primarily in bacteria and plants). Changes here are phenotypic consequences of genetic or environmental factors, not the source of genetic mutation. **Clinical Pearls for NEET-PG:** * **Transition vs. Transversion:** Transition is a point mutation where a purine is replaced by a purine (A↔G) or pyrimidine by pyrimidine (C↔T). Transversion is purine ↔ pyrimidine. * **Silent Mutation:** A change in DNA that does not change the amino acid (due to degeneracy of the genetic code). * **Missense Mutation:** Results in a different amino acid (e.g., Sickle Cell Anemia: Glutamic acid replaced by Valine at the 6th position of the β-globin chain). * **Nonsense Mutation:** Creates a premature stop codon (UAG, UAA, UGA), leading to a truncated protein.

Q: A mutation in a codon that causes a change in the coded amino acid is known as what?

Missense mutation. **Explanation:** **1. Why Missense Mutation is Correct:** A **missense mutation** is a type of point mutation where a single nucleotide change results in a codon that codes for a **different amino acid**. This can alter the tertiary structure and function of the resulting protein. A classic clinical example is **Sickle Cell Anemia**, where a point mutation (GAG → GTG) causes Glutamate to be replaced by Valine at the 6th position of the β-globin chain. **2. Why the Other Options are Incorrect:** * **Mitogenesis (A):** This refers to the induction of mitosis (cell division), typically by a mitogen. It is a physiological process, not a type of genetic mutation. * **Somatic Mutation (B):** This describes *where* a mutation occurs (in non-germline body cells) rather than the *effect* on the genetic code. Somatic mutations are not passed to offspring but can lead to cancer. * **Recombination (D):** This is the process by which DNA strands are broken and repaired to produce new combinations of alleles (e.g., crossing over during meiosis). It is a mechanism for genetic diversity, not a specific point mutation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Silent Mutation:** A nucleotide change that codes for the *same* amino acid (due to the degeneracy of the genetic code). * **Nonsense Mutation:** A change that results in a **premature stop codon** (UAA, UAG, UGA), leading to a truncated, usually non-functional protein (e.g., β-thalassemia). * **Frameshift Mutation:** Insertion or deletion of nucleotides (not in multiples of three), altering the entire reading frame downstream (e.g., Duchenne Muscular Dystrophy). * **Transition vs. Transversion:** Transition is Purine to Purine (A↔G) or Pyrimidine to Pyrimidine (C↔T); Transversion is Purine to Pyrimidine or vice versa.

Question 1

The primary defect in Xeroderma pigmentosa is related to which cellular process?

Accepted Answer

Formation of thymidine dimers

Answer

Defective poly ADP ribose polymerase activity

Answer

Defective exonuclease activity

Answer

Formation of adenine dimers

Question 2

A promoter sequence is an example of which type of factor?

Accepted Answer

Cis acting factor

Answer

Trans acting factor

Answer

Coactivator

Answer

Mediator

Question 3

Which of the following is true about DNA polymerase in eukaryotes?

Accepted Answer

Components include alpha, beta, TS, delta, and epsilon subunits.

Answer

Beta subunit is associated with DNA repair.

Answer

TS subunit is associated with DNA repair.

Answer

Delta subunit is associated with the synthesis of mitochondrial DNA.

Question 4

Senescent cells are deficient in what?

Accepted Answer

Telomerase

Answer

RNA polymerase

Answer

DNA polymerase

Answer

Helicase

Question 5

Aminoacyl t-RNA is required for all except?

Accepted Answer

Hydroxyproline

Answer

Methionine

Answer

Cystine

Answer

Lysine

Question 6

The DNA segment from which the primary mRNA transcript is copied or transcribed is called:

Accepted Answer

Template strand

Answer

Coding strand

Answer

Initiator methionine domain

Answer

Translation unit

Question 7

How many nonsense or genetic codons are there?

Accepted Answer

3

Answer

2

Answer

4

Answer

5

Question 8

The disorder shown in the illustration is related to which DNA repair mechanism?

Accepted Answer

Nucleotide excision repair

Answer

Mismatch repair

Answer

Base excision repair

Answer

SOS repair

Question 9

Mutations are due to changes in:

Accepted Answer

DNA nucleotide sequence

Answer

RNA nucleotide sequence

Answer

Amino acid sequence of ribonuclease

Answer

Cell walls

Question 10

A mutation in a codon that causes a change in the coded amino acid is known as what?

Accepted Answer

Missense mutation

Answer

Mitogenesis

Answer

Somatic mutation

Answer

Recombination

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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