Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics Indian Medical PG Practice Questions and MCQs

Question 811: In which of the following conditions is a Barr body absent in females?

A. 46 XX genome
B. 45 X0 genome (Correct Answer)
C. 47 XXX
D. None of the options

Explanation: ***45 X0 genome*** - A Barr body is a **condensed, inactivated X chromosome** found in somatic cells of females with at least two X chromosomes. - Individuals with a **45 X0 genome** (Turner Syndrome) have only one X chromosome, therefore no Barr body is formed. *46 XX genome* - Individuals with a **46 XX genome** are typical females and will have one Barr body per somatic cell, as one of the two X chromosomes is inactivated. - This is the normal female karyotype. *47 XXX* - Individuals with a **47 XXX genome** (triple X syndrome) have two Barr bodies per somatic cell, as two of their three X chromosomes are inactivated. - The number of Barr bodies is typically one less than the number of X chromosomes. *None of the options* - This option is incorrect because the 45 X0 genome indeed leads to the absence of a Barr body in females. - There is a specific condition listed among the options where a Barr body is absent.

Question 812: Effect of environment on genes is called?

A. Euthenics
B. Positive Eugenics
C. Negative Eugenics
D. Epigenetics (Correct Answer)

Explanation: ***Epigenetics*** - **Epigenetics** refers to heritable changes in **gene expression** that do not involve alterations to the underlying **DNA sequence**. - These changes are often influenced by **environmental factors**, such as diet, stress, and exposure to toxins, which can affect how genes are turned on or off. *Positive Eugenics* - **Positive eugenics** aims to improve the human population by encouraging the reproduction of individuals deemed to have "desirable" traits. - This concept is ethically controversial and focuses on directed breeding rather than environmental gene influence. *Negative Eugenics* - **Negative eugenics** aims to reduce the prevalence of "undesirable" traits in the human population by discouraging or preventing the reproduction of individuals deemed to possess them. - Like positive eugenics, this is a highly controversial concept focused on limiting reproduction based on perceived genetic quality. *Euthenics* - **Euthenics** is a movement focused on improving human well-being and development by improving living conditions and the environment. - While it acknowledges the impact of the environment, it focuses on societal and lifestyle improvements rather than the direct molecular changes in gene expression.

Question 813: In which stage of cell division is chromosomal study best carried out?

A. Metaphase (Correct Answer)
B. Telophase
C. Anaphase
D. Prophase

Explanation: ***Metaphase*** - During **metaphase**, chromosomes are maximally condensed and align at the cell's equatorial plate, making them easily visible and distinguishable under a microscope. - This arrangement allows for clear visualization of **chromosome number**, **size**, and **morphology**, which is crucial for genetic analysis. *Prophase* - In **prophase**, chromosomes begin to condense, but they are still diffuse and not fully compact, making detailed study difficult. - The nuclear envelope is also still present for most of prophase, obstructing a clear view of the chromosomes. *Telophase* - During **telophase**, chromosomes decondense and arrive at opposite poles, becoming less distinct and harder to analyze individually. - New nuclear envelopes form around the separated chromosomes, further obscuring their view for detailed study. *Anaphase* - In **anaphase**, sister chromatids separate and move towards opposite poles, but they are in motion and not aligned, making them difficult to capture and analyze individually. - The separated chromatids are also stretched and elongated, which makes their morphological assessment challenging.

Question 814: The Central Dogma of molecular biology includes all except?

A. Reverse transcription (Correct Answer)
B. Transcription
C. RNA replication
D. Translation

Explanation: ***Reverse transcription*** - The **original Central Dogma** as proposed by Francis Crick described the unidirectional flow of genetic information: **DNA → RNA → Protein**. - **Reverse transcription** (RNA → DNA) was **not part of the original Central Dogma** and was only discovered later with the identification of **reverse transcriptase enzyme** in retroviruses by Baltimore and Temin (1970). - While reverse transcription is now recognized as an important biological process, it represents an **exception to the classical Central Dogma** rather than a core component. *Transcription* - **Transcription** (DNA → RNA) is a **fundamental process** within the Central Dogma. - It involves synthesis of RNA from a DNA template and is the first step in gene expression in all living organisms. *Translation* - **Translation** (RNA → Protein) is the **second major step** of the Central Dogma. - This process occurs on ribosomes where mRNA is decoded into a specific sequence of amino acids to form proteins. *RNA replication* - **RNA replication** (RNA → RNA), while primarily seen in RNA viruses, is considered part of the **expanded Central Dogma**. - It represents one of the possible flows of genetic information in biological systems, though not universal to all organisms.

Question 815: What is the classification of the Y chromosome?

A. Metacentric
B. Submetacentric (Correct Answer)
C. Acrocentric
D. None of the options

Explanation: ***Submetacentric*** - The **Y chromosome** is classified as submetacentric because its **centromere** is located off-center, resulting in two arms of unequal length. - The short arm (Yp) is smaller than the long arm (Yq), but not as disproportionate as in acrocentric chromosomes. - The **X chromosome** is also submetacentric, making both sex chromosomes belong to this category. *Metacentric* - A **metacentric chromosome** has its **centromere** located in the middle, resulting in two arms of approximately equal length. - Examples include chromosomes 1, 3, 16, 19, and 20, which have nearly equal arm ratios unlike the Y chromosome. *Acrocentric* - An **acrocentric chromosome** has its **centromere** located very close to one end, creating one very short arm and one very long arm. - The five acrocentric human chromosomes are **13, 14, 15, 21, and 22**, which possess satellite DNA and nucleolar organizing regions (NORs) on their short arms. - The **Y chromosome is NOT acrocentric** despite historical confusion; it has a more centrally positioned centromere than true acrocentric chromosomes. *None of the options* - This option is incorrect because the Y chromosome has a specific and well-established classification as **submetacentric** based on its centromere position and arm ratio.

Question 816: Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?

A. Transcribed from nuclear DNA.
B. Has a lower molecular weight than hn-RNA. (Correct Answer)
C. Contains uracil instead of thymine.
D. Sugar is ribose.

Explanation: ***Has a lower molecular weight than hn-RNA.*** - **Mature mRNA** undergoes **splicing**, which removes **introns** (non-coding regions) from the heterogeneous nuclear RNA (hnRNA) precursor. - The removal of these introns results in a **shorter, more compact molecule** with a lower molecular weight compared to the original hnRNA. *Transcribed from nuclear DNA.* - While mRNA is indeed **transcribed from DNA**, this statement is true for **all types of RNA (rRNA, tRNA, and mRNA)**, not just mature cytoplasmic mRNA specifically, and does not differentiate it. - The initial transcript is **hnRNA**, which is then processed into mature mRNA. *Contains uracil instead of thymine.* - This is a characteristic of **all RNA molecules**, not just mature cytoplasmic mRNA, and is a fundamental difference between RNA and DNA. - DNA contains **thymine**, while RNA contains **uracil**. *Sugar is ribose.* - This is a distinguishing feature of **all RNA molecules**, indicating that the sugar component of its nucleotides is **ribose**, whereas DNA contains **deoxyribose**. - This statement is not unique to mature cytoplasmic mRNA.

Question 817: Who invented the technique for identifying individuals by their DNA?

A. Shapiro
B. Lewis
C. Jeffreys (Correct Answer)
D. Pasteur

Explanation: ***Jeffreys*** - **Alec Jeffreys** developed the technique of **DNA fingerprinting** in 1984, which revolutionized forensic science and paternity testing. - His method involved analyzing **variable number tandem repeats (VNTRs)** in DNA to create a unique profile for each individual. *Shapiro* - **Robert Shapiro** is a legal figure, famously associated with the O. J. Simpson murder trial, not directly with the invention of DNA identification techniques. - While he was involved in cases where DNA evidence was used, he did not contribute to its scientific development. *Lewis* - **Edward B. Lewis** was a Nobel Prize-winning geneticist known for his work on **developmental genetics** in *Drosophila melanogaster*, not for DNA identification techniques. - His research focused on gene clusters and their role in embryonic development. *Pasteur* - **Louis Pasteur** was a pioneering microbiologist and chemist, famous for his discoveries related to **vaccination**, microbial fermentation, and pasteurization. - His work predates the discovery and application of DNA for individual identification by over a century.

Question 818: DNA microarrays allow detection of gene mutations through which process?

A. Polymerase Chain Reaction
B. Cloning
C. Southern Blotting
D. Hybridization (Correct Answer)

Explanation: ***Hybridization*** - DNA microarrays detect mutations by **hybridizing labeled patient DNA** to **thousands of oligonucleotide probes** containing known DNA sequences immobilized on a solid surface. - A mismatch between the patient's DNA and the probe results in **reduced or absent hybridization**, indicating a **mutation or genetic variation**. - This principle allows **high-throughput screening** of multiple genes simultaneously. *Polymerase Chain Reaction* - PCR is used to **amplify specific DNA sequences exponentially**, creating millions of copies from minimal starting material. - While PCR may be used to **prepare DNA samples** before microarray analysis, it is **not the detection mechanism** on the chip itself. *Cloning* - Cloning involves creating **identical copies of DNA fragments, cells, or organisms** using vectors and host cells. - It's a method for **producing large quantities** of specific DNA sequences but **not a detection technique** for mutations on microarrays. *Southern Blotting* - Southern blotting detects specific DNA sequences through **gel electrophoresis, membrane transfer, and probe hybridization**. - While it also uses hybridization, it is a **low-throughput technique** analyzing one sample at a time, unlike the **high-throughput parallel analysis** of DNA microarrays.

Question 819: What is the primary function of reverse transcription?

A. Synthesis of DNA from an RNA template (Correct Answer)
B. Synthesis of RNA from a DNA template
C. Synthesis of DNA from a DNA template
D. Synthesis of RNA from an RNA template

Explanation: ***Synthesis of DNA from an RNA template*** - **Reverse transcription** is catalyzed by the enzyme **reverse transcriptase**, which uses an **RNA template** to synthesize a complementary DNA (cDNA) strand. - This process is fundamental in the life cycle of **retroviruses** like HIV, allowing them to integrate their genetic material into the host genome. *Synthesis of RNA from a DNA template* - This process is known as **transcription**, where genetic information is copied from **DNA to RNA**, not the reverse. - It is a key step in gene expression, leading to the production of various types of RNA molecules. *Synthesis of DNA from a DNA template* - This describes **DNA replication**, the process by which DNA makes copies of itself, ensuring genetic continuity during cell division. - It involves enzymes like **DNA polymerase** and creates two identical DNA molecules from one original DNA molecule. *Synthesis of RNA from an RNA template* - This process is known as **RNA replication** and is characteristic of certain **RNA viruses** (e.g., influenza virus), where RNA serves as both the template and the genetic material. - It involves an enzyme called **RNA-dependent RNA polymerase**.

Question 820: Function of reverse transcriptase is

A. RNA dependent DNA synthesis (Correct Answer)
B. DNA dependent RNA synthesis
C. DNA dependent DNA synthesis
D. RNA dependent RNA synthesis

Explanation: ***RNA dependent DNA synthesis*** - **Reverse transcriptase** is an enzyme that catalyzes the formation of **DNA from an RNA template**, a process known as reverse transcription. - This function is crucial for **retroviruses** like HIV, which use it to integrate their RNA genome into the host cell's DNA. *DNA dependent RNA synthesis* - This process is known as **transcription**, carried out by **RNA polymerase**. - It involves the synthesis of **RNA from a DNA template**, the opposite of reverse transcription. *DNA dependent DNA synthesis* - This process is known as **DNA replication**, carried out by **DNA polymerase**. - It involves the synthesis of **new DNA strands from existing DNA templates**. *RNA dependent RNA synthesis* - This process is characteristic of **RNA viruses**, specifically those with **RNA replicase** enzymes. - These enzymes synthesize **new RNA strands from an RNA template** for viral replication.

Practice by Chapter

DNA Replication and Repair Mechanisms

Practice Questions

Transcription Factors and Gene Regulation

Practice Questions

Epigenetics and DNA Methylation

Practice Questions

RNA Processing and Splicing

Practice Questions

miRNA and RNA Interference

Practice Questions

Protein Synthesis and Post-Translational Modifications

Practice Questions

Genomics and Human Genome Project

Practice Questions

Single Nucleotide Polymorphisms

Practice Questions

Gene Therapy Approaches

Practice Questions

CRISPR-Cas9 and Genome Editing

Practice Questions

DNA Fingerprinting and Forensics

Practice Questions

Molecular Basis of Genetic Diseases

Practice Questions

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