Molecular Biology and Genomics Practice Questions

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

45 X0 genome. ***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.

Q: Effect of environment on genes is called?

Epigenetics. ***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.

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

Hybridization. ***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.

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

Has a lower molecular weight than hn-RNA.. ***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.

Q: What is the function of primase?

Synthesizing small RNA fragments during DNA synthesis. ***Synthesizing small RNA fragments during DNA synthesis*** - **Primase** is an enzyme that synthesises short **RNA primers** which are crucial for initiating DNA replication. - These **RNA primers** provide a free 3'-hydroxyl group, which **DNA polymerase** requires to start adding deoxyribonucleotides. *Joining DNA fragments* - This function is primarily carried out by **DNA ligase**, which forms phosphodiester bonds between adjacent nucleotides to join DNA fragments. - **DNA ligase** is essential for repairing DNA breaks and joining **Okazaki fragments** on the lagging strand during replication. *Synthesising small RNA fragments during translation* - Small RNA fragments are generally involved in **gene regulation** (e.g., microRNAs) or structural components of ribosomes (e.g., ribosomal RNA) during translation, but primase is not involved in their synthesis for this purpose. - The synthesis of **mRNA**, **tRNA**, and **rRNA** during translation is carried out by **RNA polymerases**, not **primase**. *Unwinding of DNA* - The **unwinding of the DNA double helix** is primarily performed by an enzyme called **DNA helicase**. - **DNA helicase** breaks the hydrogen bonds between complementary base pairs, separating the two strands to allow replication or transcription to proceed.

Q: What is the primary function of reverse transcription?

Synthesis of DNA from an RNA template. ***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**.

Q: Which of the following usually require a RNA intermediate for cloning/replication?

Retroviruses. ***Retroviruses*** - **All retroviruses require an RNA intermediate** for their replication cycle, making this the correct answer. - Retroviruses have an **RNA genome** that must be **reverse transcribed into DNA** by reverse transcriptase enzyme before integration into the host genome. - The integrated DNA (provirus) is then transcribed back to RNA, which serves both as mRNA for viral proteins and as genomic RNA for new virions. - Examples include **HIV, HTLV**, and other retroviruses that definitively use this RNA → DNA → RNA replication strategy. *Transposons* - This option is **too broad** to be correct. Only **retrotransposons** (Class I transposons) use RNA intermediates via a "copy-and-paste" mechanism involving reverse transcription. - However, **DNA transposons** (Class II) move by a "cut-and-paste" DNA mechanism **without any RNA intermediate**. - Since the question asks what "usually requires" RNA intermediate, and many common transposons (like bacterial Tn5, Tn10) are DNA transposons, this answer is imprecise. *Cosmids* - Cosmids are **hybrid cloning vectors** containing cos sites from bacteriophage lambda combined with plasmid sequences. - They replicate as **DNA plasmids** in bacteria using DNA-dependent DNA polymerase. - No RNA intermediate is involved in their replication mechanism. *Plasmids* - Plasmids are **extrachromosomal circular DNA molecules** that replicate independently within bacterial or yeast cells. - Replication occurs via **DNA-to-DNA synthesis** using DNA polymerase. - No RNA intermediate is required for plasmid propagation.

Q: What is the role of nonsense codons in protein synthesis?

Termination of protein synthesis. ***Termination of protein synthesis*** - **Nonsense codons**, also known as **stop codons** (UAA, UAG, UGA), signal the end of translation. - When a ribosome encounters a nonsense codon, it binds **release factors** instead of an aminoacyl-tRNA, leading to the dissociation of the polypeptide chain. *Elongation of the polypeptide chain* - **Elongation** involves the sequential addition of amino acids to the growing polypeptide chain, guided by sense codons. - Nonsense codons do not code for any amino acid and thus do not contribute to chain elongation. *Pre-translational modification of proteins* - **Pre-translational modifications** refer to events like protein folding and disulfide bond formation that occur as the polypeptide is being synthesized. - Nonsense codons are involved in halting the synthesis, not in modifying the protein. *Initiation of protein synthesis* - **Initiation** of protein synthesis begins at the **start codon** (AUG), which codes for methionine. - Nonsense codons are distinct from the start codon and fulfill a different role in the translation process.

Q: What is the primary role of telomerase in cellular biology?

It contributes to cellular immortality.. ***It contributes to cellular immortality*** - Telomerase maintains **telomere length** in stem cells and cancer cells, allowing them to divide indefinitely without undergoing senescence. - This activity is crucial for the **immortality** observed in certain cell types and the unchecked proliferation characteristic of cancer. - This is the **primary functional role** of telomerase in cellular biology. *It is a reverse transcriptase that adds DNA sequences* - While telomerase is indeed a **reverse transcriptase**, this describes its **mechanism of action** rather than its primary cellular role. - Its specific role is to add repetitive **telomeric DNA sequences** (TTAGGG repeats) to chromosome ends, maintaining telomere length. *It is present in most somatic cells* - **Telomerase activity** is generally very low or absent in most differentiated somatic cells. - This limited activity contributes to the **Hayflick limit** and cellular aging, as telomeres shorten with each cell division. *It is absent in most somatic cells* - While telomerase activity is **low or undetectable** in the vast majority of differentiated somatic cells, it is not entirely absent in all of them. - Some somatic cells, like certain progenitor cells, may retain very **minimal telomerase activity**, although not enough to prevent telomere shortening over time. - The more accurate statement is that it has "low or absent activity" rather than being completely absent.

Q: The Shine-Dalgarno sequence is primarily associated with which biological process?

Translation. ***Translation*** - The **Shine-Dalgarno sequence** is a **ribosome-binding site** in prokaryotic messenger RNA (mRNA) that helps recruit the ribosome to the mRNA to initiate protein synthesis. - Its interaction with the **16S rRNA** of the small ribosomal subunit positions the start codon (AUG) correctly for **translation initiation**. - This sequence is located approximately **8 base pairs upstream** of the start codon in bacterial mRNA. *Transcription* - **Transcription** is the process of synthesizing RNA from a DNA template. - It involves elements like **promoters** and **enhancers**, not the Shine-Dalgarno sequence. *DNA replication* - **DNA replication** is the process by which DNA makes a copy of itself. - This process involves origins of replication, helicases, and DNA polymerases, with no role for the Shine-Dalgarno sequence. *RNA splicing* - **RNA splicing** is a eukaryotic process that removes introns from pre-mRNA. - The Shine-Dalgarno sequence is found in **prokaryotes**, which lack splicing machinery and introns.

Question 1

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

Accepted Answer

45 X0 genome

Answer

46 XX genome

Answer

47 XXX

Answer

None of the options

Question 2

Effect of environment on genes is called?

Accepted Answer

Epigenetics

Answer

Euthenics

Answer

Positive Eugenics

Answer

Negative Eugenics

Question 3

DNA microarrays allow detection of gene mutations through which process?

Accepted Answer

Hybridization

Answer

Polymerase Chain Reaction

Answer

Cloning

Answer

Southern Blotting

Question 4

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

Accepted Answer

Has a lower molecular weight than hn-RNA.

Answer

Transcribed from nuclear DNA.

Answer

Contains uracil instead of thymine.

Answer

Sugar is ribose.

Question 5

What is the function of primase?

Accepted Answer

Synthesizing small RNA fragments during DNA synthesis

Answer

Joining DNA fragments

Answer

Synthesizing small RNA fragments during translation

Answer

Unwinding of DNA

Question 6

What is the primary function of reverse transcription?

Accepted Answer

Synthesis of DNA from an RNA template

Answer

Synthesis of RNA from a DNA template

Answer

Synthesis of DNA from a DNA template

Answer

Synthesis of RNA from an RNA template

Question 7

Which of the following usually require a RNA intermediate for cloning/replication?

Accepted Answer

Retroviruses

Answer

Cosmids

Answer

Plasmids

Answer

Transposons

Question 8

What is the role of nonsense codons in protein synthesis?

Accepted Answer

Termination of protein synthesis

Answer

Elongation of the polypeptide chain

Answer

Pre-translational modification of proteins

Answer

Initiation of protein synthesis

Question 9

What is the primary role of telomerase in cellular biology?

Accepted Answer

It contributes to cellular immortality.

Answer

It is a reverse transcriptase that adds DNA sequences.

Answer

It is present in most somatic cells.

Answer

It is absent in most somatic cells.

Question 10

The Shine-Dalgarno sequence is primarily associated with which biological process?

Accepted Answer

Translation

Answer

Transcription

Answer

DNA replication

Answer

RNA splicing

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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