Molecular Biology and Genomics Practice Questions

Q: What type of molecule does mRNA represent a complementary copy of?

A single strand of DNA. ### Explanation **Why the Correct Answer is Right:** The process of **transcription** involves the synthesis of RNA from a DNA template. During this process, the enzyme RNA polymerase reads the **template strand** (antisense strand) of DNA in a 3’ to 5’ direction to synthesize a complementary mRNA molecule in a 5’ to 3’ direction. Because mRNA is formed by base-pairing rules (where Uracil replaces Thymine), it represents a **complementary copy of a single strand of DNA**. Specifically, the mRNA sequence is complementary to the template strand and identical (except for U/T) to the coding (sense) strand. **Analysis of Incorrect Options:** * **A & B (tRNA & rRNA):** These are distinct types of non-coding RNA. While mRNA, tRNA, and rRNA all originate from DNA templates, they do not represent copies of each other. They work together during translation but are transcribed from different genomic loci. * **C (Ribosomal DNA):** rDNA refers to the DNA sequences that code for ribosomal RNA (rRNA). mRNA is transcribed from structural genes (protein-coding genes), not from the specific regions designated as rDNA. **NEET-PG High-Yield Pearls:** * **Directionality:** RNA synthesis always occurs in the **5’ → 3’** direction. * **Template vs. Coding:** The mRNA is complementary to the **Template (Antisense)** strand and identical to the **Coding (Sense)** strand. * **Post-transcriptional Modification:** In eukaryotes, the initial product is **hnRNA** (heterogeneous nuclear RNA), which undergoes 5’ capping, 3’ polyadenylation, and splicing to become mature mRNA. * **Enzyme:** In eukaryotes, **RNA Polymerase II** is responsible for synthesizing mRNA.

Q: To which group does the X chromosome belong?

Group C. ### Explanation Human chromosomes are classified into seven groups (**A to G**) based on the **Denver Classification System**, which categorizes them according to their size and the position of the centromere. **Why Group C is Correct:** The **X chromosome** is classified under **Group C**. This group consists of medium-sized submetacentric chromosomes (Pairs 6 to 12 and the X chromosome). Because the X chromosome is similar in size and centromere position to chromosomes 6–12, it is grouped with them. **Analysis of Incorrect Options:** * **Group A (Chromosomes 1–3):** These are the largest chromosomes and are primarily metacentric (except for chromosome 2, which is submetacentric). * **Group B (Chromosomes 4–5):** These are large chromosomes with submetacentric centromeres. * **Group D (Chromosomes 13–15):** These are medium-sized **acrocentric** chromosomes that possess satellites (important for forming nucleolus organizer regions). **High-Yield Clinical Pearls for NEET-PG:** * **The Y Chromosome:** Belongs to **Group G** (along with chromosomes 21 and 22). These are the smallest acrocentric chromosomes. * **Acrocentric Chromosomes:** Groups D (13, 14, 15) and G (21, 22) are acrocentric. These are the sites for **Robertsonian translocations**, commonly associated with Down Syndrome. * **Denver Classification Summary:** * **Group A:** 1–3 (Largest) * **Group B:** 4–5 * **Group C:** 6–12 + **X** (Medium, Submetacentric) * **Group D:** 13–15 (Medium, Acrocentric) * **Group E:** 16–18 * **Group F:** 19–20 * **Group G:** 21–22 + **Y** (Smallest, Acrocentric)

Q: In DNA transfer, what is the order of vectors from smallest to largest?

Plasmids, Bacteriophage, Cosmids. In molecular biology, vectors are DNA molecules used as vehicles to carry foreign genetic material into another cell. The capacity of a vector to carry an insert (foreign DNA) determines its classification and utility in genomic studies. ### **Explanation of the Correct Answer** The correct order from smallest to largest carrying capacity is **Plasmids < Bacteriophage < Cosmids**. 1. **Plasmids:** These are extra-chromosomal, circular DNA found in bacteria. They have the smallest capacity, typically carrying inserts of **0.5 to 8 kb**. 2. **Bacteriophages (e.g., Lambda phage):** These are viruses that infect bacteria. They can accommodate larger fragments of DNA, usually between **8 to 25 kb**. 3. **Cosmids:** These are hybrid vectors combining properties of plasmids (ori site) and phage λ (cos sites). They are designed to carry significantly larger genomic fragments, ranging from **30 to 45 kb**. ### **Analysis of Incorrect Options** * **Options A, C, and D:** These are incorrect because they misplace the hierarchy of insert sizes. Plasmids always represent the smallest capacity, while Cosmids represent a "middle-ground" between simple phages and massive artificial chromosomes (like BACs or YACs). ### **High-Yield Clinical Pearls for NEET-PG** To master questions on genomic libraries, remember the expanded hierarchy of vector capacities: * **Plasmids:** <10 kb * **Bacteriophage (λ):** ~10–25 kb * **Cosmids:** ~30–45 kb * **Bacterial Artificial Chromosomes (BAC):** 100–300 kb (Derived from F-plasmid) * **Yeast Artificial Chromosomes (YAC):** 200–2000 kb (Largest capacity; used for Human Genome Project) **Mnemonic:** **P**lease **B**ring **C**offee **B**efore **Y**oga (**P**lasmid < **B**acteriophage < **C**osmid < **B**AC < **Y**AC).

Q: All of the following are true about DNA methylation except:

It has no role in carcinogenesis.. **Explanation** DNA methylation is a key epigenetic mechanism that involves the addition of a methyl group (–CH₃) to DNA, typically at the 5th carbon of a cytosine ring. **Why Option D is the correct answer (The Exception):** DNA methylation plays a **critical role in carcinogenesis**. Aberrant methylation patterns are hallmarks of cancer: * **Hypermethylation** of promoter regions in tumor suppressor genes (e.g., *p16*, *BRCA1*) leads to gene silencing, allowing uncontrolled cell growth. * **Global Hypomethylation** can lead to genomic instability and the activation of oncogenes. Therefore, stating it has "no role" is factually incorrect. **Analysis of Incorrect Options:** * **Option A:** Methylation primarily occurs at **cytosine residues** within **CpG islands** (regions with a high frequency of Cytosine-guanine phosphodiester bonds). This is catalyzed by DNA methyltransferases (DNMTs). * **Option B:** It is a major regulator of **gene expression**. Generally, high levels of methylation in promoter regions correlate with **transcriptional silencing** (gene "switching off"). * **Option C:** It is the fundamental mechanism behind **genomic imprinting**, where certain genes are expressed in a parent-of-origin-specific manner (e.g., Prader-Willi and Angelman syndromes). **High-Yield Clinical Pearls for NEET-PG:** * **S-Adenosylmethionine (SAM):** The universal methyl donor for DNA methylation. * **5-Azacytidine:** A hypomethylating agent used in the treatment of Myelodysplastic Syndrome (MDS). * **Maintenance vs. De Novo:** DNMT1 maintains methylation patterns during replication, while DNMT3a/3b are responsible for new (de novo) methylation. * **Gene Silencing:** Remember: **Methylation = Mute**; **Acetylation = Active** (referring to histones).

Q: The process underlying differences in the expression of a gene, based on which parent transmitted it, is called what?

Genomic imprinting. **Explanation:** The correct answer is **Genomic Imprinting (Option D)**. **1. Why Genomic Imprinting is Correct:** Genomic imprinting is an epigenetic phenomenon where certain genes are expressed in a parent-of-origin-specific manner. Although an individual inherits two copies of a gene (one from each parent), imprinting causes one copy to be "silenced" (usually via **DNA methylation**), while the other remains active. Therefore, the phenotype of the offspring depends entirely on which parent transmitted the functional allele. **2. Why Other Options are Incorrect:** * **Anticipation (A):** Refers to the phenomenon where a genetic disorder (e.g., Huntington’s disease, Fragile X) becomes more severe or appears at an earlier age in successive generations, typically due to **trinucleotide repeat expansions**. * **Mosaicism (B):** Occurs when an individual has two or more genetically different cell lines derived from a single zygote, usually due to post-zygotic mutations or non-disjunction. * **Non-penetrance (C):** A situation where an individual carries a dominant disease-causing mutation but does not manifest any clinical symptoms of the disease. **3. Clinical Pearls for NEET-PG:** * **Classic Examples:** The best-known examples of imprinting involve **Chromosome 15q11-q13**. * **Prader-Willi Syndrome:** Deletion of the *paternal* allele (Maternal imprinting). * **Angelman Syndrome:** Deletion of the *maternal* allele (Paternal imprinting). * **Mechanism:** Primarily involves **DNA Methylation** (adding methyl groups to cytosine residues in CpG islands) and histone modification. * **Uniparental Disomy (UPD):** Imprinting disorders can also occur if a child inherits two copies of a chromosome from one parent and none from the other.

Question 1

What type of molecule does mRNA represent a complementary copy of?

Accepted Answer

A single strand of DNA

Answer

tRNA

Answer

rRNA

Answer

Ribosomal DNA

Question 2

Which molecular technique utilizes an oligonucleotide probe with a single base pair substitution to detect DNA variations?

Accepted Answer

Restriction Fragment Length Polymorphism (RFLP)

Answer

Polymerase Chain Reaction (PCR)

Answer

Error-coded mutation analysis

Answer

None of the above

Question 3

To which group does the X chromosome belong?

Accepted Answer

Group C

Answer

Group A

Answer

Group B

Answer

Group D

Question 4

In DNA transfer, what is the order of vectors from smallest to largest?

Accepted Answer

Plasmids, Bacteriophage, Cosmids

Answer

Cosmids, Plasmids, Bacteriophage

Answer

Bacteriophage, Cosmides, Plasmids

Answer

Cosmids, Bacteriophage, Plasmids

Question 5

Which of the following statements is true regarding the lac operon?

Accepted Answer

Catabolite repression is mediated by CRP.

Answer

The repressor is a dimer and a positive regulator.

Answer

CRP is a gratuitous inducer.

Answer

Lactose is a positive regulator.

Question 6

What is a palindrome in the context of DNA?

Accepted Answer

A sequence of DNA that reads the same forwards and backward

Answer

Synthesis of DNA

Answer

Extrachromosomal molecule of DNA

Answer

Small nuclear RNA

Question 7

All of the following are true about DNA methylation except:

Accepted Answer

It has no role in carcinogenesis.

Answer

It usually occurs at cytosine residues.

Answer

It can alter gene expression patterns in cells.

Answer

It plays a role in genomic imprinting.

Question 8

DNA repair defect is seen in which of the following conditions?

Accepted Answer

All of these

Answer

Xeroderma pigmentosa

Answer

Bloom's syndrome

Answer

Ataxia telangiectasia

Question 9

What technique is used to identify the exact location of a genetic loci?

Accepted Answer

Fluorescent in-situ hybridization

Answer

Polymerase chain reaction

Answer

Chromosome painting

Answer

Comparative genomic hybridization

Question 10

The process underlying differences in the expression of a gene, based on which parent transmitted it, is called what?

Accepted Answer

Genomic imprinting

Answer

Anticipation

Answer

Mosaicism

Answer

Non-penetrance

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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