Molecular Biology and Genomics Practice Questions

Q: Which of the following statements regarding satellite DNA is incorrect?

It is transcriptionally active.. ### Explanation **1. Why Option D is the Correct Answer:** Satellite DNA consists of highly repetitive, non-coding DNA sequences. In the hierarchy of chromatin structure, satellite DNA is the primary component of **constitutive heterochromatin**. Heterochromatin is characterized by being highly condensed and tightly packed, which makes it inaccessible to RNA polymerase. Therefore, satellite DNA is **transcriptionally inactive** (silent). It does not code for proteins or functional RNA, making the statement in Option D incorrect. **2. Analysis of Incorrect Options:** * **Option A:** Satellite DNA is defined by **tandem repeats** (head-to-tail orientation) of short nucleotide sequences. These repeats can range from a few base pairs (microsatellites) to hundreds of base pairs (macrosatellites). * **Option B & C:** These sequences are structural rather than informational. They are predominantly localized at the **centromeres** (providing a scaffold for the kinetochore) and **telomeres** (protecting chromosome ends). In humans, Alpha-satellite DNA is the most common type found at the centromere of all chromosomes. **3. High-Yield Clinical Pearls for NEET-PG:** * **Density Gradient:** The term "Satellite" comes from their behavior during **Cesium Chloride (CsCl) density gradient centrifugation**, where they form small "satellite" bands separate from the main bulk of genomic DNA due to differences in G-C content. * **DNA Profiling:** Variations in the number of tandem repeats (VNTRs and STRs) are the basis for **DNA fingerprinting** and forensic analysis. * **Microsatellite Instability (MSI):** Defective DNA mismatch repair (MMR) leads to MSI, a hallmark of **Lynch Syndrome** (Hereditary Non-Polyposis Colorectal Cancer). * **Telomeres:** Human telomeric repeat sequence is **TTAGGG**.

Q: What is true about the coding strand of DNA?

It matches the RNA transcript that encodes the protein.. ### Explanation In DNA transcription, the two strands of the double helix serve different roles. The **Coding Strand** (also known as the **Sense** or **Plus [+]** strand) has the same base sequence and polarity (5'→3') as the synthesized mRNA, with the only difference being that DNA contains Thymine (T) while mRNA contains Uracil (U). **Why Option D is Correct:** The mRNA transcript is synthesized using the template strand as a guide via complementary base pairing. Because the coding strand is also complementary to the template strand, it ends up being a "mirror image" of the template, thus matching the mRNA sequence. This is why the coding strand is used by researchers to represent a gene's sequence. **Analysis of Incorrect Options:** * **Option A:** This describes the **Template Strand** (Antisense/Minus strand). DNA polymerase and RNA polymerase read the template strand to synthesize a new complementary strand. * **Option B:** The **Minus (–) strand** is a synonym for the Template strand, not the coding strand. * **Option C:** While the coding strand *is* known as the plus (+) strand, Option D is the more definitive functional description of its role in protein encoding as per standard molecular biology definitions. (Note: In many competitive exams, if two options are technically true, the one describing the functional relationship to the genetic code is preferred). **High-Yield Clinical Pearls for NEET-PG:** * **Directionality:** RNA Polymerase reads the template strand in the **3' → 5'** direction but synthesizes mRNA in the **5' → 3'** direction. * **TATA Box:** Found on the coding strand, typically 25-35 base pairs upstream of the transcription start site. * **Codons:** The sequence of codons listed in genetic code tables corresponds to the sequence of the **Coding Strand**.

Q: Which of the following amino acids does not exhibit degeneracy in its genetic code?

Methionine. **Explanation:** The genetic code is described as **degenerate** (or redundant) because most amino acids are specified by more than one codon. This occurs because there are 61 sense codons but only 20 standard amino acids. **Why Methionine is the Correct Answer:** Methionine and Tryptophan are the only two amino acids that are **non-degenerate**. They are encoded by a single specific codon each: * **Methionine:** AUG (also serves as the "Start Codon") * **Tryptophan:** UGG Because Methionine has only one codon, it does not exhibit degeneracy. **Analysis of Incorrect Options:** * **A. Serine:** Exhibits high degeneracy; it is encoded by **six** different codons (UCU, UCC, UCA, UCG, AGU, AGC). * **B. Valine:** Encoded by **four** codons (GUU, GUC, GUA, GUG). * **C. Alanine:** Encoded by **four** codons (GCU, GCC, GCA, GCG). **High-Yield Clinical Pearls for NEET-PG:** 1. **Wobble Hypothesis:** Degeneracy usually involves the **third base** of the codon (the "wobble" position). This minimizes the effects of mutations, as a change in the third base often results in the same amino acid (silent mutation). 2. **Initiation:** In eukaryotes, the initiation codon (AUG) codes for Methionine. In prokaryotes, it codes for **N-formylmethionine (fMet)**. 3. **Universality Exceptions:** While the genetic code is nearly universal, exceptions exist in **Mitochondria**, where UGA (normally a stop codon) codes for Tryptophan. 4. **Non-overlapping & Commaless:** The code is read sequentially without skipping any bases or sharing bases between adjacent codons.

Q: How many base pairs are encoded by a prokaryotic polypeptide of 250 amino acids?

750 bp. ### Explanation **1. Why Option C (750 bp) is Correct:** The fundamental principle governing this calculation is the **Genetic Code**, which is a **triplet code**. This means that three consecutive nucleotides (a codon) in mRNA—and by extension, three base pairs (bp) in the double-stranded DNA—encode for a single amino acid. * **Calculation:** 250 amino acids × 3 base pairs/amino acid = **750 base pairs**. In prokaryotes, genes are generally **colinear** with their protein products because they lack introns (non-coding intervening sequences). Therefore, the number of base pairs in the coding region directly corresponds to the number of amino acids multiplied by three. **2. Why Other Options are Incorrect:** * **Option A (250 bp):** This assumes a 1:1 ratio, which would only be possible if a single base encoded an amino acid (singlet code). This would only allow for 4 possible amino acids ($4^1$), insufficient for the 20 standard amino acids. * **Option B (500 bp):** This assumes a doublet code ($4^2 = 16$), which still falls short of the 20 amino acids required for life. * **Option D (1000 bp):** This would imply 4 base pairs per amino acid, which is not the biological standard. **3. High-Yield NEET-PG Clinical Pearls:** * **Introns vs. Exons:** While this calculation is straightforward for prokaryotes, in **eukaryotes**, the genomic DNA would be much longer than 750 bp due to the presence of **introns** (spliced out during mRNA processing). * **Stop Codons:** In a more complex version of this question, you might add 3 bp for the "Stop Codon" (UAA, UAG, UGA), which does not code for an amino acid but is part of the gene. * **Wobble Hypothesis:** Proposed by Francis Crick, it explains why 61 codons can be recognized by fewer than 61 tRNAs, primarily due to flexible base pairing at the 3rd position of the codon.

Q: Restriction fragment length polymorphism is used for what purpose?

Analysis of chromosome structures. **Explanation:** **Restriction Fragment Length Polymorphism (RFLP)** is a molecular biology technique used to detect variations in homologous DNA sequences. It relies on the principle that specific enzymes (Restriction Endonucleases) cut DNA at specific recognition sites. If a mutation or variation exists at these sites, the resulting DNA fragments will differ in length. **Why Option A is Correct:** RFLP is used for the **analysis of chromosome structures** because it identifies variations in the physical structure and sequence of DNA. By comparing fragment patterns, clinicians can map genes, identify genetic polymorphisms, and detect structural changes or mutations associated with hereditary diseases. It serves as a "genetic fingerprint" for specific chromosomal regions. **Why Other Options are Incorrect:** * **B. DNA estimation:** This is typically done using **Spectrophotometry** (measuring absorbance at 260 nm) or Fluorometry, not RFLP. * **C. Synthesis of nucleic acids:** This refers to processes like **PCR (Polymerase Chain Reaction)** or automated DNA synthesis, whereas RFLP is an analytical/diagnostic tool. * **D. Detecting proteins in a cell:** Protein detection is achieved via **Western Blotting** or ELISA. RFLP is strictly a DNA-based technique. **NEET-PG High-Yield Pearls:** * **Mechanism:** Uses Restriction Endonucleases (e.g., EcoRI) followed by Gel Electrophoresis and Southern Blotting. * **Clinical Applications:** Paternity testing, forensic "DNA fingerprinting," and prenatal diagnosis of disorders like **Sickle Cell Anemia** (where a mutation abolishes a restriction site). * **Key Limitation:** It requires a large amount of high-quality DNA compared to PCR-based methods. * **Related Technique:** **VNTRs** (Variable Number Tandem Repeats) are the molecular basis for the polymorphisms observed in RFLP.

Q: Co-translational insertion is seen with which of the following?

Translocon. **Explanation:** **Co-translational insertion** is the process where the synthesis of a protein and its translocation into the Endoplasmic Reticulum (ER) occur simultaneously. This is the primary pathway for secreted proteins, integral membrane proteins, and lysosomal enzymes. * **Why Translocon is correct:** As a ribosome begins translating an mRNA with a Signal Recognition Particle (SRP), the entire complex docks onto the ER membrane. The nascent polypeptide chain is then threaded through a specialized protein-lined channel called the **Translocon** (specifically the **Sec61 complex**). The Translocon acts as the "gatekeeper," allowing the growing protein to enter the ER lumen or integrate into the membrane while translation is still ongoing. **Analysis of Incorrect Options:** * **Chaperones:** These are proteins (e.g., HSP70) that assist in the proper folding of proteins or prevent aggregation. They act *after* or during synthesis but do not facilitate the insertion process itself. * **Ubiquitin:** This is a small regulatory protein that marks misfolded or unneeded proteins for degradation via the proteasome (the "kiss of death"). * **Mannose 6-Phosphate (M6P):** This is a post-translational modification added in the Golgi apparatus. It acts as a chemical "tag" to target acid hydrolases specifically to the **lysosomes**. **High-Yield Clinical Pearls for NEET-PG:** * **Signal Hypothesis:** Proposed by Günter Blobel, it states that proteins have intrinsic signals (Signal Peptide) that govern their transport and localization. * **I-Cell Disease:** Caused by a deficiency in the enzyme required to add the M6P tag, leading to lysosomal enzymes being secreted extracellularly rather than being targeted to lysosomes. * **Sec61:** The specific name of the translocon protein often tested in advanced molecular biology questions.

Q: This is an example of which type of mutation?

Transversion. ***Transversion*** - A **transversion** mutation involves substitution between **purines** (A, G) and **pyrimidines** (C, T), such as A↔C, A↔T, G↔C, or G↔T. - This type of mutation changes the **chemical structure** from a two-ring base to a single-ring base or vice versa. *Transition* - A **transition** mutation involves substitution between bases of the **same chemical class** (purine to purine or pyrimidine to pyrimidine). - Examples include **A↔G** (both purines) or **C↔T** (both pyrimidines), which are more common than transversions. *Insertion* - An **insertion** mutation involves adding one or more **nucleotides** into the DNA sequence. - This is a completely different type of mutation that **increases** the total number of bases in the sequence. *Frameshift* - A **frameshift** mutation occurs when insertions or deletions are **not multiples of three** nucleotides. - This shifts the **reading frame** during translation, altering all downstream amino acids in the protein.

Q: Which of the following statements about telomerase is true?

Causes carcinogenesis. **Explanation:** Telomerase is a specialized **ribonucleoprotein reverse transcriptase** enzyme responsible for maintaining the length of telomeres (repetitive TTAGGG sequences at the ends of chromosomes). **Why Option B is Correct:** In normal somatic cells, telomeres shorten with each cell division [1], eventually leading to senescence (the "Hayflick limit"). However, in approximately **90% of human cancers**, telomerase is reactivated or upregulated. This prevents telomere shortening, granting cancer cells **replicative immortality**, a hallmark of carcinogenesis. By maintaining chromosomal stability during rapid division, telomerase allows malignant cells to evade apoptosis and divide indefinitely. **Analysis of Incorrect Options:** * **Option A:** Telomerase has **Reverse Transcriptase** activity (RNA-dependent DNA polymerase), not RNA polymerase activity. It uses its own internal RNA template to synthesize DNA. * **Option C:** Telomerase is generally **absent or expressed at very low levels** in most differentiated somatic cells, which is why they have a limited lifespan [1]. * **Option D:** Telomerase is **highly active in germ cells** (sperm and ova), as well as embryonic stem cells and hematopoietic stem cells, to ensure that telomere length is preserved across generations. **NEET-PG High-Yield Pearls:** * **Components:** It consists of **TERT** (Telomerase Reverse Transcriptase - the catalytic protein) and **TERC** (Telomerase RNA - the template). * **Clinical Link:** Telomerase inhibitors are being researched as potential anti-cancer therapies. * **Progeria:** Mutations leading to premature telomere shortening are linked to aging syndromes like Werner syndrome.

Q: What is the aim of the ENCODE project?

To identify the functional elements of the human genome. **Explanation:** The **ENCODE (Encyclopedia of DNA Elements)** project was launched by the National Human Genome Research Institute (NHGRI) as a follow-up to the Human Genome Project. While the Human Genome Project focused on "reading" the sequence, ENCODE aims to **identify and characterize all functional elements** within the human genome. This includes protein-coding regions, non-coding RNA genes, regulatory elements (promoters, enhancers, silencers), and regions of chromatin modification. A key finding of ENCODE is that approximately 80% of the genome has some biochemical function, debunking the older concept of "junk DNA." **Analysis of Incorrect Options:** * **Option A:** Sequencing of the human genome was the primary goal of the **Human Genome Project (HGP)**, completed in 2003. * **Option B:** Metagenome analysis of intestinal flora is the focus of the **Human Microbiome Project (HMP)**, which studies the microbial communities inhabiting the human body. * **Option D:** Analysis of the mitochondrial genome (the 16.5 kb circular DNA) is a specific area of genetics but is not the objective of the ENCODE project, which focuses on the 3.2 billion base pairs of the nuclear genome. **High-Yield Facts for NEET-PG:** * **Functional Elements:** ENCODE identifies elements like **H3K4me3** (associated with promoters) and **CTCF-binding sites** (insulators). * **Non-Coding DNA:** Most disease-associated SNPs (Single Nucleotide Polymorphisms) identified in GWAS studies lie in the non-coding functional regions identified by ENCODE, not in exons. * **HapMap Project:** Another related project that maps common patterns of human genetic variation (haplotypes).

Question 1

Which of the following statements regarding satellite DNA is incorrect?

Accepted Answer

It is transcriptionally active.

Answer

It consists of repeated DNA sequences arranged in tandem.

Answer

It is often found clustered around centromeres.

Answer

It is also found clustered around telomeres.

Question 2

What is true about the coding strand of DNA?

Accepted Answer

It matches the RNA transcript that encodes the protein.

Answer

DNA acts as a template for mRNA synthesis.

Answer

It is also known as the minus strand.

Answer

It is also known as the plus strand.

Question 3

Which of the following statements is NOT TRUE regarding the initiation of protein synthesis in eukaryotes?

Accepted Answer

Binding of mRNA to the 60S preinitiation complex to form the 43S initiation complex

Answer

Dissociation of the ribosome into its 40S and 60S subunits

Answer

Binding of a ternary complex to the 40S ribosome

Answer

Combination of the 48S initiation complex with the 60S ribosomal subunit

Question 4

Which of the following amino acids does not exhibit degeneracy in its genetic code?

Accepted Answer

Methionine

Answer

Serine

Answer

Valine

Answer

Alanine

Question 5

How many base pairs are encoded by a prokaryotic polypeptide of 250 amino acids?

Accepted Answer

750 bp

Answer

250 bp

Answer

500 bp

Answer

1000 bp

Question 6

Restriction fragment length polymorphism is used for what purpose?

Accepted Answer

Analysis of chromosome structures

Answer

DNA estimation

Answer

Synthesis of nucleic acids

Answer

Detecting proteins in a cell

Question 7

Co-translational insertion is seen with which of the following?

Accepted Answer

Translocon

Answer

Chaperons

Answer

Ubiquitin

Answer

Mannose 6-Phosphate

Question 8

This is an example of which type of mutation?

Accepted Answer

Transversion

Answer

Transition

Answer

Insertion

Answer

Frameshift

Question 9

Which of the following statements about telomerase is true?

Accepted Answer

Causes carcinogenesis

Answer

Has RNA polymerase activity

Answer

Is present in somatic cells

Answer

Is absent in germ cells

Question 10

What is the aim of the ENCODE project?

Accepted Answer

To identify the functional elements of the human genome

Answer

Sequencing of the human genome

Answer

Metagenome (genome of intestinal flora) analysis

Answer

Analysis of the mitochondrial genome

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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