Nucleic Acid Biochemistry Practice Questions

Q: What is the function of the UGA codon?

Terminates protein synthesis. ***Terminates protein synthesis*** - The **UGA codon** is one of the three **stop codons** (UAA, UAG, UGA) that signal the termination of translation. - When a ribosome encounters a UGA codon, there is no corresponding **tRNA** with an anticodon, leading to the binding of release factors and dissociation of the ribosomal complex. *Initiates transcription* - **Transcription initiation** involves RNA polymerase binding to a promoter region, which is a DNA sequence, not a specific mRNA codon. - The UGA codon is part of an mRNA sequence and functions during translation. *Translates* - While translation is the process of synthesizing protein from an mRNA template, the **UGA codon** specifically acts as a signal to **stop** this process. - It does not directly code for an amino acid, unlike other codons that are "translated" into specific amino acids. *None of the options* - This option is incorrect because **UGA** has a very specific and critical function in **terminating protein synthesis**.

Q: Which of the following statements is true about silent mutations in a gene?

No change in Amino acid sequence in protein. ***No change in Amino acid sequence in protein*** - A **silent mutation** is a type of point mutation that results in a **substituted nucleotide** but does not alter the **amino acid sequence** of the protein. - This occurs due to the **redundancy (degeneracy) of the genetic code**, where multiple codons can specify the same amino acid. - Example: UUA → UUG both code for Leucine, making this a silent mutation. *No change in mRNA sequence* - A silent mutation *does* involve a **change in the DNA sequence**, which consequently leads to a **change in the mRNA sequence** (a different nucleotide at the mutation site). - The change in mRNA sequence is at the nucleotide level, but the amino acid encoded remains the same. *No change in protein expression* - While a silent mutation does not change the amino acid sequence, it *can* affect **protein expression** through several mechanisms: - **Codon usage bias**: Different codons are translated at different rates - **mRNA stability**: Sequence changes may affect secondary structure - **Translation efficiency**: Some synonymous codons are translated faster than others - Therefore, this statement is incorrect as silent mutations can affect protein expression levels. *None of the above* - This option is incorrect because there is one correct statement among the choices provided.

Q: DNA and RNA both contain which pyrimidine base?

Cytosine. ***Cytosine*** - **Cytosine** is a **pyrimidine** base found in both **DNA** and **RNA**. - In DNA, cytosine pairs with **guanine**; in RNA, it also pairs with guanine. *Uracil* - **Uracil** is a **pyrimidine** base found only in **RNA**, replacing thymine. - It pairs with **adenine** in RNA. *Thymine* - **Thymine** is a **pyrimidine** base found only in **DNA**. - It pairs with **adenine** in DNA and is replaced by uracil in RNA. *Guanine* - **Guanine** is a **purine** base, not a pyrimidine. - It is present in both **DNA** and **RNA**, and pairs with cytosine.

Q: RNA with enzymatic activity is

Peptidyl transferase. ***Peptidyl transferase*** - The **peptidyl transferase** activity, resident in the **large ribosomal subunit (50S in prokaryotes, 60S in eukaryotes)**, is actually carried out by **ribosomal RNA (rRNA)**, making it a ribozyme. - This enzymatic activity catalyzes the formation of a **peptide bond** between the amino acid at the P-site and the incoming amino acyl-tRNA at the A-site during protein synthesis. *Peptidase* - **Peptidases** are enzymes that catalyze the **hydrolysis of peptide bonds**, breaking down proteins into smaller peptides or amino acids. - While essential for protein turnover and digestion, peptidases are typically **protein enzymes**, not RNA molecules. *Cytidine deaminase* - **Cytidine deaminase** is an enzyme that catalyzes the **deamination of cytidine** to uridine, a reaction crucial for pyrimidine metabolism and often targeted in cancer therapy. - This enzyme is a **protein**, not an RNA molecule with enzymatic activity. *Aminoacyl tRNA synthetase* - **Aminoacyl-tRNA synthetases** are a family of enzymes responsible for **attaching the correct amino acid to its corresponding tRNA molecule**, a critical step in ensuring the accuracy of protein synthesis. - These are **protein enzymes** and do not possess RNA-based catalytic activity.

Q: Splicing is a process of what?

Removal of introns from pre-mRNA. ***Removal of introns from pre-mRNA*** - **Splicing** is a crucial step in **RNA processing** where non-coding sequences (**introns**) are excised from a newly synthesized **pre-mRNA** molecule. - This process ensures that only the protein-coding regions (**exons**) are joined together to form a mature mRNA. *Protein synthesis from mRNA* - This describes **translation**, the process where **ribosomes** read the genetic code in mRNA to synthesize a protein polypeptide chain. - Translation occurs after mRNA has been processed and exported from the nucleus. *Activation of proteins during gene expression* - This refers to **post-translational modifications** or **protein folding**, which are steps that occur after protein synthesis to make a protein functional. - Splicing is an upstream process, preceding protein synthesis. *DNA replication process* - **DNA replication** is the biological process of producing two identical replicas of DNA from one original DNA molecule. - This process is distinct from gene expression and RNA processing, focusing instead on the duplication of the entire genome.

Q: According to Chargaff's rules, which of the following base pairs shows equal numbers in double-stranded DNA?

G=C. ***G=C*** - Chargaff's rules state that in double-stranded DNA, the amount of **guanine (G)** equals the amount of **cytosine (C)**. - This is due to **complementary base pairing** where G always pairs with C via three hydrogen bonds. - Both G=C and A=T are valid answers according to Chargaff's rules. *A=T* - According to Chargaff's rules, **adenine (A)** equals **thymine (T)** in double-stranded DNA. - A pairs with T via two hydrogen bonds in complementary base pairing. - This is also a correct statement of Chargaff's rules (equally valid as G=C). *A=G* - **Adenine (A)** and **guanine (G)** are both purines but do not pair with each other. - Their amounts are **not necessarily equal** in double-stranded DNA. *A=C* - **Adenine (A)** is a purine and **cytosine (C)** is a pyrimidine, but they do not form complementary base pairs. - Their amounts are **not equal** in double-stranded DNA.

Q: Action of Telomerase is:

Longevity of cell - Aging. ***Longevity of cell - Aging*** - **Telomerase** is an enzyme that adds repetitive DNA sequences (telomeres) to the ends of eukaryotic chromosomes, protecting genetic information during cell division. - By restoring telomeres, telomerase allows cells to divide more times, thereby **increasing cellular longevity** and impacting the aging process. - This is the **primary function** of telomerase in cellular biology. *DNA repair* - **DNA repair mechanisms** are involved in correcting mutations and damage to DNA throughout the genome. - While telomerase maintains the ends of chromosomes, its primary function is not to repair damaged DNA sequences within the chromosome itself. - DNA repair enzymes include DNA polymerases, ligases, and nucleases that fix base mismatches and strand breaks. *Breakdown of telomere* - The **breakdown or shortening of telomeres** naturally occurs with each cell division in most somatic cells due to the \"end-replication problem.\" - **Telomerase** actively counteracts this shortening by adding to telomeres, rather than causing their breakdown. - This represents the opposite of telomerase action. *Replication of DNA* - While telomerase does synthesize DNA sequences, its action is **highly specific to telomeric repeats** at chromosome ends. - General DNA replication is carried out by **DNA polymerase**, not telomerase. - Telomerase is a specialized reverse transcriptase with a distinct function from standard replicative machinery.

Q: The Feulgen reaction is a cytochemical test used to detect which of the following?

Deoxyribonucleic acid (DNA). ***Deoxyribonucleic acid (DNA)*** - The **Feulgen reaction** is a specific histochemical technique used to detect and quantify **DNA** in cells. - It relies on the hydrolysis of purine bases from the DNA molecule, which exposes aldehyde groups that then react with **Schiff's reagent** to produce a magenta color. *Transfer RNA (t-RNA)* - tRNA is a type of **RNA** molecule that is involved in protein synthesis. - The Feulgen reaction is specific for **DNA** and does not detect **RNA** because RNA lacks the deoxyribose sugars that are critical for the reaction mechanism. *None of the options* - This option is incorrect because **DNA** is a correct answer, as the Feulgen reaction specifically targets and stains DNA. - The specificity of the Feulgen reaction for DNA makes it a widely used tool in cytogenetics and cell biology. *Messenger RNA (m-RNA)* - mRNA is a type of **RNA** that carries genetic information from DNA to ribosomes for protein synthesis. - Like other RNA molecules, **mRNA** does not react with the Feulgen stain due to the presence of **ribose sugar** instead of deoxyribose, which prevents the formation of aldehyde groups needed for the reaction.

Question 1

What is the function of the UGA codon?

Accepted Answer

Terminates protein synthesis

Answer

Initiates transcription

Answer

Translates

Answer

None of the options

Question 2

Which of the following statements is true about silent mutations in a gene?

Accepted Answer

No change in Amino acid sequence in protein

Answer

No change in mRNA sequence

Answer

No change in protein expression

Answer

None of the above

Question 3

DNA and RNA both contain which pyrimidine base?

Accepted Answer

Cytosine

Answer

Uracil

Answer

Thymine

Answer

Guanine

Question 4

Which of the following statements about base excision repair is correct?

Accepted Answer

Uses the enzyme DNA glycosylase to remove damaged bases and create an apurinic site

Answer

Used for the repair of deaminated bases only

Answer

Removes approximately 1 nucleotide at a time

Answer

Repairs bulky helix-distorting lesions caused by UV radiation

Question 5

Which one of the following statements about chromatin is not true?

Accepted Answer

H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes

Answer

DNA winds approximately 1.75 times around the nucleosomes

Answer

Covalent modification of histones influence chromatin compaction

Answer

Non-histone proteins are part of mitotic chromosomes

Question 6

RNA with enzymatic activity is

Accepted Answer

Peptidyl transferase

Answer

Cytidine deaminase

Answer

Peptidase

Answer

Aminoacyl tRNA synthetase

Question 7

Splicing is a process of what?

Accepted Answer

Removal of introns from pre-mRNA

Answer

Protein synthesis from mRNA

Answer

Activation of proteins during gene expression

Answer

DNA replication process

Question 8

According to Chargaff's rules, which of the following base pairs shows equal numbers in double-stranded DNA?

Accepted Answer

G=C

Answer

A=C

Answer

A=G

Answer

A=T

Question 9

Action of Telomerase is:

Accepted Answer

Longevity of cell - Aging

Answer

DNA repair

Answer

Replication of DNA

Answer

Breakdown of telomere

Question 10

The Feulgen reaction is a cytochemical test used to detect which of the following?

Accepted Answer

Deoxyribonucleic acid (DNA)

Answer

Transfer RNA (t-RNA)

Answer

None of the options

Answer

Messenger RNA (m-RNA)

Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry — MCQs

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