DNA Replication and Repair Mechanisms — MCQs
Which of the following is true about ataxia telangiectasia?
What is a key similarity between the processes of replication and transcription?
The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:
Mutations are due to changes in:
Which of the following is the etiology of Werner syndrome?
Which one of the following statements about chromatin is not true?
Which of the following doesn't occur in 5' to 3' direction?
Which of the following doesn't occur in 5' to 3' direction?
During DNA replication, if the template strand has the sequence 5'-GATTACA-3', what is the sequence of the complementary strand?
What is the most important tool used in genetic engineering?
DNA Replication and Repair Mechanisms Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is true about ataxia telangiectasia?
- A. There is absence of amphicytes in different organs
- B. It is an autosomal recessive disease (Correct Answer)
- C. It is associated with normal immune function
- D. Serum levels of IgA are increased
- E. Serum alpha-fetoprotein levels are decreased
Explanation: ***It is an autosomal recessive disease*** - Ataxia telangiectasia is caused by mutations in the **ATM gene**, which is inherited in an **autosomal recessive** pattern. - This genetic defect leads to a deficiency in a protein crucial for DNA repair, causing systemic effects. *There is absence of amphicytes in different organs* - This statement is incorrect; **ataxia telangiectasia** is not characterized by an absence of amphicytes. - The term "amphicytes" is not typically associated with the defining pathological features of ataxia telangiectasia. *It is associated with normal immune function* - Ataxia telangiectasia is associated with **immunodeficiency**, particularly affecting T- and B-cell function. - Patients often experience recurrent infections due to impaired adaptive immunity, which is not a characteristic of normal immune function. *Serum levels of IgA are increased* - Patients with ataxia telangiectasia typically have **decreased serum levels of IgA**, and often IgG and IgE, leading to immunodeficiency. - Increased IgA levels are characteristic of other conditions and not ataxia telangiectasia. *Serum alpha-fetoprotein levels are decreased* - In ataxia telangiectasia, serum **alpha-fetoprotein (AFP) levels are characteristically elevated**, not decreased. - Elevated AFP is a useful diagnostic marker for this condition.
Question 2: What is a key similarity between the processes of replication and transcription?
- A. Use RNA primers for initiation.
- B. Use ribonucleotides as precursors.
- C. Are semi-conservative events.
- D. Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction. (Correct Answer)
Explanation: ***Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction.*** - Both DNA replication and RNA transcription synthesize nucleic acid polymers by forming **phosphodiester bonds** between incoming nucleotides. - The new strand in both processes is always elongated in the **5' to 3' direction**, as new nucleotides are added to the 3' hydroxyl group of the growing strand. *Use RNA primers for initiation.* - **DNA replication** requires **RNA primers** to initiate synthesis of new DNA strands, as DNA polymerase cannot start a new strand *de novo*. - **Transcription (RNA synthesis)** does not require a primer; **RNA polymerase** can initiate transcription *de novo* at a promoter sequence. *Use ribonucleotides as precursors.* - **Transcription** uses **ribonucleotides** (ATP, UTP, CTP, GTP) as precursors to synthesize RNA. - **Replication** primarily uses **deoxyribonucleotides** (dATP, dTTP, dCTP, dGTP) to synthesize DNA, although it temporarily uses ribonucleotides for RNA primers. *Are semi-conservative events.* - **DNA replication** is a **semi-conservative process**, meaning each new DNA molecule consists of one original strand and one newly synthesized strand. - **Transcription** is **not semi-conservative**; it involves synthesizing an RNA molecule from a DNA template, leaving the original DNA template unchanged.
Question 3: The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:
- A. DNA Ligase (Correct Answer)
- B. DNA Helicase
- C. DNA Phosphorylase
- D. DNA Topoisomerase
Explanation: ***DNA Ligase*** - **DNA ligase** forms a **phosphodiester bond** between the **3'-OH group** of one Okazaki fragment and the **5'-phosphate group** of the adjacent fragment, effectively sealing the nicks. - After **DNA polymerase I** removes the **RNA primers** and fills in the gaps, DNA ligase completes the synthesis of the **lagging strand** during DNA replication. - This enzyme is essential for maintaining the **integrity of the DNA backbone**. *DNA Helicase* - **DNA helicase** functions to **unwind the DNA double helix**, separating the two strands to create a replication fork. - It does not participate in joining DNA fragments. *DNA Phosphorylase* - **DNA phosphorylase** is not a standard enzyme involved in the direct sealing of DNA fragments during replication. - This is not the enzyme responsible for ligating Okazaki fragments. *DNA Topoisomerase* - **DNA topoisomerase** relieves the **supercoiling tension** that builds up in the DNA double helix ahead of the replication fork due to unwinding. - It does not have a role in forming phosphodiester bonds between newly synthesized DNA fragments.
Question 4: Mutations are due to changes in:
- A. DNA nucleotide sequence (Correct Answer)
- B. RNA nucleotide sequence
- C. Amino acid sequence of ribonuclease
- D. Cell membrane
Explanation: ***DNA nucleotide sequence*** - **Mutations** are defined as changes in the **genetic material**, which is primarily composed of **DNA**. - These changes in the **nucleotide sequence** of DNA can alter the genetic code, leading to changes in **protein structure and function**. *RNA nucleotide sequence* - While RNA can have its nucleotide sequence altered, these changes are generally not considered true **mutations** in the heritable sense for most organisms. - RNA is typically a temporary molecule, and changes to its sequence are usually not passed down to subsequent generations. *Amino acid sequence of ribonuclease* - An altered **amino acid sequence** in a protein like ribonuclease is a consequence of a **mutation in the DNA**, not the mutation itself. - **Ribonucleases** are enzymes that catalyze the degradation of RNA, and their structure is determined by the **DNA sequence**. *Cell membrane* - The cell membrane is a **lipid bilayer** with embedded proteins that regulates cellular transport and communication. - While its components can be affected by genetic mutations, alterations in the cell membrane itself do not constitute the primary definition of a **mutation**.
Question 5: Which of the following is the etiology of Werner syndrome?
- A. Increased length of telomere
- B. Increased advanced glycation end products
- C. Decreased lipid peroxidation
- D. Short telomere with damaged DNA and loss of helicase (Correct Answer)
Explanation: ***Short telomere with damaged DNA and loss of helicase*** - **Werner syndrome** is an **autosomal recessive disorder** characterized by **premature aging** due to mutations in the *WRN* gene, which codes for a **RecQ-type DNA helicase**. - The dysfunctional helicase leads to defects in **DNA replication, repair, and transcription**, resulting in **genomic instability**, **damaged DNA**, and **accelerated telomere attrition** (short telomeres). *Increased length of telomere* - **Increased telomere length** is generally associated with a **reduced rate of cellular aging** and is not characteristic of Werner syndrome or other premature aging disorders. - In most aging processes and syndromes like Werner, **telomeres tend to shorten** over time due to incomplete replication and oxidative stress. *Increased advanced glycation end products* - **Advanced glycation end products (AGEs)** accumulate in various tissues during normal aging and in conditions like diabetes, contributing to vascular and organ damage. - While AGEs play a role in the broader aging process, they are not the primary underlying genetic defect or direct etiology of Werner syndrome, which is a **DNA repair disorder**. *Decreased lipid peroxidation* - **Lipid peroxidation** is a process by which **free radicals** attack lipids, leading to cellular damage and is often associated with oxidative stress and aging. - A **decrease in lipid peroxidation** would generally be considered protective against aging-related damage, which is the opposite of what is seen in Werner syndrome, where there's an accelerated aging phenotype and increased cellular stress.
Question 6: Which one of the following statements about chromatin is not true?
- A. DNA winds approximately 1.75 times around the nucleosomes
- B. Covalent modification of histones influence chromatin compaction
- C. Non-histone proteins are part of mitotic chromosomes
- D. H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes (Correct Answer)
Explanation: ***H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes*** - This statement is **not entirely true** as presented because while **H2A-H2B dimers** do make contacts with DNA near entry/exit regions, they do not bind **exclusively** at these ends. - In the nucleosome structure, two H2A-H2B dimers flank the central **(H3-H4)₂ tetramer** and interact with DNA throughout approximately **30 base pairs on each side**. - The **entry and exit points** of nucleosomal DNA are primarily stabilized by **linker histones (H1)**, which bind to the dyad axis and linker DNA regions. - The statement oversimplifies the complex three-dimensional interactions within the nucleosome core particle. *DNA winds approximately 1.75 times around the nucleosomes* - This statement is **true**; approximately **1.65 to 1.75 turns** of DNA (about 146-147 base pairs) wrap around the **histone octamer** to form the core nucleosome particle. - This precise winding is crucial for the compaction of DNA into eukaryotic chromatin and represents the fundamental repeating unit of chromatin structure. *Covalent modification of histones influence chromatin compaction* - This statement is **true**; **post-translational modifications** (PTMs) such as acetylation, methylation, phosphorylation, and ubiquitination on histone tails significantly impact **chromatin structure and accessibility**. - For example, **histone acetylation** generally leads to a more open chromatin conformation (euchromatin) by neutralizing positive charges, facilitating gene expression. - **Histone methylation** can lead to either open or compact chromatin depending on the specific residue modified (e.g., H3K4me3 for activation, H3K9me3 for repression). *Non-histone proteins are part of mitotic chromosomes* - This statement is **true**; mitotic chromosomes contain numerous **non-histone proteins** essential for chromosome structure and function. - Examples include **structural maintenance of chromosomes (SMC) proteins** like condensin and cohesin, topoisomerases (DNA topoisomerase II), and kinetochore proteins. - These non-histone proteins are crucial for chromosome condensation, sister chromatid cohesion, segregation, and proper mitotic progression.
Question 7: Which of the following doesn't occur in 5' to 3' direction?
- A. DNA repair
- B. Transcription
- C. DNA replication
- D. RNA editing (Correct Answer)
Explanation: ***RNA editing*** - **RNA editing** involves modifications to **RNA molecules** after transcription, such as base insertions, deletions, or substitutions. - This process does not follow a 5' to 3' synthesis direction, unlike DNA or RNA synthesis. *DNA repair* - **DNA repair mechanisms**, such as **excision repair**, involve synthesizing new DNA to replace damaged sections. - This synthesis occurs in the **5' to 3' direction** by **DNA polymerases**. *Transcription* - **Transcription** is the process where **RNA polymerase** synthesizes an **RNA molecule** from a **DNA template**. - This synthesis always occurs in the **5' to 3' direction**, adding nucleotides to the 3' end of the growing RNA strand. *DNA replication* - **DNA replication** involves the synthesis of new **DNA strands** from a **template strand**. - **DNA polymerase** adds nucleotides exclusively in the **5' to 3' direction**, requiring a primer for initiation.
Question 8: Which of the following doesn't occur in 5' to 3' direction?
- A. DNA replication
- B. RNA editing (Correct Answer)
- C. DNA repair
- D. Transcription
Explanation: ***RNA editing*** - **RNA editing** involves modifications to RNA molecules after transcription, such as base insertions, deletions, or substitutions. These processes **do not occur in a specific 5' to 3' direction** characteristic of polymerization. - Unlike synthesis processes, RNA editing is a post-transcriptional modification that alters pre-existing RNA molecules at specific sites. *DNA replication* - **DNA replication** always proceeds in the **5' to 3' direction** for the synthesis of new DNA strands. - DNA polymerase can only add nucleotides to the **3'-hydroxyl end** of a growing strand. *DNA repair* - Many forms of **DNA repair**, such as nucleotide excision repair and base excision repair, involve the synthesis of new DNA segments. - This resynthesis step, carried out by DNA polymerase, occurs in the **5' to 3' direction**, similar to replication. *Transcription* - **Transcription** involves the synthesis of an RNA strand from a DNA template. - RNA polymerase adds ribonucleotides to the **3'-hydroxyl end** of the nascent RNA molecule, thus proceeding in the **5' to 3' direction**.
Question 9: During DNA replication, if the template strand has the sequence 5'-GATTACA-3', what is the sequence of the complementary strand?
- A. 5'-GATTACA-3'
- B. 3'-GATTACA-5'
- C. 5'-ACATTAG-3'
- D. 5'-TGTAATC-3' (Correct Answer)
Explanation: **5'-TGTAATC-3'** - DNA replication involves **base pairing rules**: **adenine (A)** pairs with **thymine (T)**, and **guanine (G)** pairs with **cytosine (C)**. - The complementary strand is synthesized in an **antiparallel direction**: if the template is 5'-GATTACA-3', the new strand will be 3'-CTAATGT-5'. When written in the conventional 5' to 3' direction, this becomes 5'-TGTAATC-3'. *5'-GATTACA-3'* - This sequence is identical to the template strand, which would only occur if the DNA were to replicate in a **non-complementary manner**, violating base pairing rules. - Direct duplication of the template sequence does not produce a complementary strand. *3'-GATTACA-5'* - This sequence is the **template sequence written in the antiparallel direction** but is not the complementary strand. - It fails to apply the correct base pairing rules (A with T, G with C). *5'-ACATTAG-3'* - This sequence incorrectly pairs the bases and does not maintain the **antiparallel orientation** correctly. - For example, the first base G in the template would pair with C, not A.
Question 10: What is the most important tool used in genetic engineering?
- A. Topoisomerase
- B. DNA Ligase
- C. Restriction endonuclease (Correct Answer)
- D. Helicase
Explanation: ***Restriction endonuclease*** - **Restriction endonucleases** are crucial for genetic engineering as they specifically cut DNA at particular recognition sites, allowing the insertion or deletion of genes. - This precise cutting ability is fundamental for creating **recombinant DNA** molecules. *Helicase* - **Helicase** is primarily involved in unwinding the DNA double helix during processes like DNA replication and transcription. - While essential for cellular functions, it does not directly manipulate DNA for gene insertion or modification in the way restriction enzymes do. *Topoisomerase* - **Topoisomerase** enzymes are responsible for managing DNA supercoiling, preventing tangling during DNA replication and transcription by cutting and rejoining DNA strands. - It plays a role in DNA structure but is not directly used for targeted gene editing or insertion. *DNA Ligase* - **DNA ligase** is essential for joining DNA fragments, which is a critical step in genetic engineering after restriction endonucleases have cut the DNA. - However, while it acts as a "molecular glue" to seal nicks and re-form phosphodiester bonds, it cannot initiate the precise cutting required to isolate genes.
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