Molecular Biology and Genomics Practice Questions

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.

Q: What is the primary function of exonuclease in DNA replication?

Proofreading. ***Proofreading*** - Exonucleases, particularly those associated with **DNA polymerases**, are crucial for **proofreading** during DNA replication. - They remove incorrectly paired nucleotides from the 3' end of the growing DNA strand, ensuring high fidelity of replication. *Polymerization* - **DNA polymerase** is primarily responsible for the **polymerization** of new DNA strands by adding nucleotides. - While exonucleases can be part of the polymerase complex, their main function is not polymerization itself. *Chain elongation* - **Chain elongation** refers to the process of adding nucleotides to the growing DNA strand, which is performed by **DNA polymerase**. - Exonucleases act as a quality control mechanism during this elongation process, rather than carrying out the elongation. *Termination* - **Termination** of DNA replication involves specific sequences and proteins that signal the end of replication, not the primary function of exonucleases. - Exonucleases are active throughout the replication process to maintain accuracy.

Q: Which RNA is used in RNA splicing?

Small nuclear RNA (snRNA). ***Small nuclear RNA (snRNA)*** - **snRNAs** are key components of **spliceosomes**, the molecular machines that catalyze the removal of introns from pre-mRNA. - They bind to specific sequences within the pre-mRNA and facilitate the splicing reactions. *mRNA* - **mRNA (messenger RNA)** carries the genetic code from DNA to the ribosomes for **protein synthesis**. - While it is the molecule that gets spliced, it does not directly participate in the splicing machinery itself. *rRNA* - **rRNA (ribosomal RNA)** is a structural and catalytic component of **ribosomes**, where protein synthesis occurs. - It plays no direct role in the process of RNA splicing. *tRNA* - **tRNA (transfer RNA)** molecules are responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - They are involved in translation, not in the processing of RNA by splicing.

Q: Which of the following is a termination codon?

UAA. ***UAA*** - **UAA** is one of the three **stop codons** (UAA, UAG, UGA) that signals the termination of protein synthesis during translation. - When the ribosome encounters a UAA codon, no corresponding tRNA with an anticodon binds, and release factors bind instead, leading to the dissociation of the polypeptide chain. *AUG* - **AUG** is the universal **start codon** in most organisms, encoding for methionine in eukaryotes and N-formylmethionine in prokaryotes. - Its presence signals the initiation of protein synthesis, not its termination. *AUA* - **AUA** is a codon that codes for the amino acid **Isoleucine**. - It is a **sense codon** and does not act as a signal for termination. *AGG* - **AGG** is a codon that codes for the amino acid **Arginine**. - Similar to AUA, it is a **sense codon** and participates in elongating the polypeptide chain, rather than terminating it.

Q: What is attached to the 3' end of mRNA after transcription?

Poly-A tail. ***Poly-A tail*** - A **poly-A tail**, consisting of multiple adenosine monophosphates, is added to the **3' end of mRNA** after transcription to protect it from degradation. - This modification aids in the **transport of mRNA from the nucleus to the cytoplasm** and in its translation. *CCA* - The **CCA sequence** is found at the **3' end of tRNA**, not mRNA, and is critical for amino acid attachment. - It is added post-transcriptionally to tRNA molecules by the enzyme **tRNA nucleotidyltransferase**. *Intron* - **Introns** are non-coding regions within a gene that are transcribed into mRNA but are subsequently removed during **RNA splicing**, not added to the 3' end. - Their removal ensures that only the **coding regions (exons)** are translated into protein. *7-methylguanosine* - **7-methylguanosine** forms the **5' cap** of mRNA, which is added to the 5' end, not the 3' end. - This cap is important for **mRNA stability**, ribosome binding, and protection against degradation.

Q: What is the primary function of a primer in DNA replication?

Initiation of DNA replication. ***Initiation of DNA replication*** - DNA polymerase cannot synthesize new DNA strands de novo; it requires a pre-existing 3'-hydroxyl group to begin adding nucleotides. - The **primer**, a short RNA sequence, provides this necessary **3'-hydroxyl group**, allowing DNA polymerase to start synthesizing the new DNA strand. *Transcription* - This process involves synthesizing **RNA from a DNA template**, primarily carried out by **RNA polymerase**. - While primers are involved in DNA synthesis, they do not directly initiate the process of transcription. *Translation* - **Translation** is the process of synthesizing **proteins from mRNA templates** using ribosomes, tRNA, and amino acids. - This process is distinct from DNA synthesis and does not involve primers; its initiation involves start codons and ribosomal subunits. *Termination of DNA replication* - **Termination of DNA replication** occurs when replication forks meet or at specific termination sequences, often with the involvement of specialized proteins. - Primers are involved in the *start* of replication, not its conclusion.

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: Which enzyme is required for cutting the DNA strand during synthesis?

Topoisomerase. ***Topoisomerase*** - **Topoisomerases** are enzymes essential for DNA replication; they induce temporary **single- or double-strand breaks** in DNA to relieve **supercoiling** ahead of the replication fork. - This cutting and rejoining activity prevents the DNA from becoming excessively tangled and facilitates the unwinding process required for synthesis. *DNA polymerase* - **DNA polymerase** is responsible for **synthesizing new DNA strands** by adding nucleotides, not for cutting the DNA backbone. - It works by moving along the template strand, reading the bases, and then adding complementary nucleotides to the growing DNA strand. *DNA ligase* - **DNA ligase** functions to **join DNA fragments** together by forming phosphodiester bonds, especially in sealing Okazaki fragments during lagging strand synthesis. - Its role is to ligate (join) cut strands, not to initiate cuts in the DNA. *Helicase* - **Helicase** unwinds the DNA double helix into single strands using ATP hydrolysis; it **separates the two strands** but does not cut the phosphodiester backbone. - This enzyme creates the replication fork by disrupting hydrogen bonds between base pairs, making the DNA accessible for replication machinery.

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.

Question 1

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

Accepted Answer

Translation

Answer

Transcription

Answer

DNA replication

Answer

RNA splicing

Question 2

What is the primary function of exonuclease in DNA replication?

Accepted Answer

Proofreading

Answer

Polymerization

Answer

Chain elongation

Answer

Termination

Question 3

Which RNA is used in RNA splicing?

Accepted Answer

Small nuclear RNA (snRNA)

Answer

mRNA

Answer

tRNA

Answer

rRNA

Question 4

Which of the following is a termination codon?

Accepted Answer

UAA

Answer

AUG

Answer

AUA

Answer

AGG

Question 5

What is attached to the 3' end of mRNA after transcription?

Accepted Answer

Poly-A tail

Answer

CCA

Answer

Intron

Answer

7-methylguanosine

Question 6

What is the primary function of a primer in DNA replication?

Accepted Answer

Initiation of DNA replication

Answer

Transcription

Answer

Translation

Answer

Termination of DNA replication

Question 7

What is the function of DNA ligase?

Accepted Answer

Joins the Okazaki fragments on the lagging strand by sealing the nicks between them.

Answer

Unwinding (denaturation) of dsDNA to provide an ssDNA template

Answer

Initiation of DNA synthesis and elongation

Answer

Initiates synthesis of RNA primers

Question 8

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 9

Which enzyme is required for cutting the DNA strand during synthesis?

Accepted Answer

Topoisomerase

Answer

DNA polymerase

Answer

DNA ligase

Answer

Helicase

Question 10

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

Accepted Answer

Retroviruses

Answer

Cosmids

Answer

Plasmids

Answer

Transposons

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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