Nucleic Acid Biochemistry Practice Questions

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 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: 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 of the following is not a component of a nucleotide?

Fatty acid. ***Fatty acid*** - A **fatty acid** is a component of **lipids**, such as triglycerides and phospholipids, which are structurally and functionally distinct from **nucleotides**. - **Nucleotides** are the building blocks of nucleic acids (DNA and RNA), whereas fatty acids are essential for cell membranes and energy storage. *Sugar* - A **pentose sugar** (either **deoxyribose** in DNA or **ribose** in RNA) is a fundamental component of every nucleotide. - This sugar forms the backbone of the nucleic acid strand, covalently linked to the phosphate group and the nitrogenous base. *Phosphate* - A **phosphate group** is a crucial component of a nucleotide, providing the negative charge and forming the phosphodiester bonds that link nucleotides together into a nucleic acid chain. - The number of phosphate groups (mono-, di-, or triphosphate) determines the nucleotide's energy state and function. *Base* - A **nitrogenous base** (adenine, guanine, cytosine, thymine, or uracil) is an essential component of a nucleotide, responsible for genetic information storage and pairing. - This base is attached to the pentose sugar and determines the specific identity of the nucleotide within the DNA or RNA sequence.

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: The Watson-Crick model is for which type of DNA?

B DNA. ***B DNA*** - The **Watson-Crick model** describes the most common and stable form of DNA found in living organisms under physiological conditions. - **B-DNA** is a right-handed double helix, characterized by a **major and minor groove**, with approximately 10-10.5 base pairs per turn. *A DNA* - **A-DNA** is a **right-handed double helix** that forms under dehydrating conditions and is shorter and wider than B-DNA. - It has a more tilted base pair arrangement and is not the primary form described by the Watson-Crick model. *C DNA* - **C-DNA** is a less common **right-handed double helix** that forms under even lower hydration conditions and in the presence of certain ions. - It has fewer base pairs per turn than B-DNA, typically around 9.3 base pairs. *Z DNA* - **Z-DNA** is unique because it is a **left-handed double helix**, unlike the right-handed forms of A, B, and C DNA. - It is transiently formed in regions with a high concentration of **GC base pairs** and has a zigzag backbone, hence its name.

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: Which of the following is not a precursor in the synthesis of pyrimidines?

Thymidine. ***Thymidine*** - **Thymidine** is a *nucleoside* consisting of deoxyribose and thymine. It is a *product* and a component of DNA, not a precursor in the *de novo synthesis* of pyrimidine bases. - While it can be incorporated into DNA via the *salvage pathway*, it does not serve as an initial building block for the pyrimidine ring itself. *Glutamine* - **Glutamine** provides the **nitrogen atoms** crucial for the formation of the pyrimidine ring, specifically N3 in the pyrimidine base. - It is a key donor of *amino groups* in various anabolic pathways, including nucleotide synthesis. *Carbon dioxide (CO2)* - **Carbon dioxide (CO2)** contributes one of the carbon atoms (C2) to the pyrimidine ring. - It combines with **ammonia** (derived from glutamine) to form **carbamoyl phosphate**, an essential intermediate. *Aspartic acid* - **Aspartic acid** provides four atoms (N1, C4, C5, C6) of the pyrimidine ring. - Its carbon skeleton and amino group are directly incorporated into the pyrimidine structure during the *de novo synthesis* pathway.

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: 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.

Question 1

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 2

What is the primary function of exonuclease in DNA replication?

Accepted Answer

Proofreading

Answer

Polymerization

Answer

Chain elongation

Answer

Termination

Question 3

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 4

Which of the following is not a component of a nucleotide?

Accepted Answer

Fatty acid

Answer

Sugar

Answer

Base

Answer

Phosphate

Question 5

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

Accepted Answer

Intron

Answer

m-RNA

Answer

ATP

Answer

Exon

Question 6

The Watson-Crick model is for which type of DNA?

Accepted Answer

B DNA

Answer

A DNA

Answer

C DNA

Answer

Z DNA

Question 7

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

Accepted Answer

Translation

Answer

Transcription

Answer

DNA replication

Answer

RNA splicing

Question 8

Which of the following is not a precursor in the synthesis of pyrimidines?

Accepted Answer

Thymidine

Answer

Glutamine

Answer

Carbon dioxide (CO2)

Answer

Aspartic acid

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

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

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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