Molecular Biology and Genomics Practice Questions

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: The Central Dogma of molecular biology includes all except?

Reverse transcription. ***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.

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: 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 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 does not play a role in protein synthesis?

Intron. ***Intron*** - Introns are **non-coding regions** within a gene that are transcribed into RNA but are subsequently **spliced out** before translation. - They do not carry genetic information for protein synthesis; their removal ensures the correct sequence of amino acids is produced. *Exon* - Exons are the **coding regions** of a gene that contain the genetic information for protein synthesis. - After introns are removed, exons are ligated together to form the **mature mRNA** that is translated into protein. *m-RNA* - **Messenger RNA (mRNA)** carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm. - It serves as the **template** for protein synthesis through the process of translation. *ATP* - **Adenosine triphosphate (ATP)** provides the **energy** required for various steps in protein synthesis, including mRNA transcription, amino acid activation, and ribosome movement. - It is a crucial energy currency that fuels the process of forming peptide bonds and assembling the polypeptide chain.

Q: Function of reverse transcriptase is

RNA dependent DNA synthesis. ***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.

Question 1

What is the primary function of exonuclease in DNA replication?

Accepted Answer

Proofreading

Answer

Polymerization

Answer

Chain elongation

Answer

Termination

Question 2

The Central Dogma of molecular biology includes all except?

Accepted Answer

Reverse transcription

Answer

Transcription

Answer

RNA replication

Answer

Translation

Question 3

Which of the following is a termination codon?

Accepted Answer

UAA

Answer

AUG

Answer

AUA

Answer

AGG

Question 4

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

Accepted Answer

Poly-A tail

Answer

CCA

Answer

Intron

Answer

7-methylguanosine

Question 5

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 6

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 7

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

Accepted Answer

Topoisomerase

Answer

DNA polymerase

Answer

DNA ligase

Answer

Helicase

Question 8

Which RNA is used in RNA splicing?

Accepted Answer

Small nuclear RNA (snRNA)

Answer

mRNA

Answer

tRNA

Answer

rRNA

Question 9

Which of the following does not play a role in protein synthesis?

Accepted Answer

Intron

Answer

m-RNA

Answer

ATP

Answer

Exon

Question 10

Function of reverse transcriptase is

Accepted Answer

RNA dependent DNA synthesis

Answer

DNA dependent RNA synthesis

Answer

DNA dependent DNA synthesis

Answer

RNA dependent RNA synthesis

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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