Molecular Biology and Genomics Practice Questions

Q: By which enzyme is cDNA synthesized from RNA?

Reverse transcriptase. ***Reverse transcriptase*** - **Reverse transcriptase** is a unique enzyme that synthesizes a **complementary DNA (cDNA)** strand from an **RNA template**. - This process, known as **reverse transcription**, is crucial in retroviruses and molecular biology techniques like RT-PCR. *Helicase* - **Helicase** enzymes are responsible for **unwinding nucleic acid double helices**, separating DNA strands during replication and transcription. - It does not synthesize DNA from an RNA template. *DNA-dependent DNA polymerase* - **DNA-dependent DNA polymerase** synthesizes new **DNA strands using an existing DNA template** during DNA replication. - It cannot use RNA as a template to synthesize DNA. *Topoisomerase* - **Topoisomerase** enzymes are involved in **managing DNA supercoiling** by creating transient breaks in the DNA backbone. - They do not synthesize DNA from any template.

Q: The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:

DNA Ligase. ***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.

Q: What is the most important tool used in genetic engineering?

Restriction endonuclease. ***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.

Q: Location of gene on chromosome is identified by

Genetic mapping. ***Genetic mapping*** - **Genetic mapping** (also called chromosome mapping) uses various techniques to determine the **physical location (locus)** of genes on a chromosome. - Techniques include **linkage analysis**, **FISH (Fluorescence In Situ Hybridization)**, chromosomal banding, and analysis of **inheritance patterns** of traits and genetic markers. - This identifies both the **relative positions** between genes and their **absolute chromosomal addresses**. *Karyotyping* - **Karyotyping** is a technique that visualizes the entire set of chromosomes in an organism. - While it can identify **large chromosomal abnormalities** like aneuploidy or major deletions/insertions, it does not pinpoint the exact location of a specific gene. *Microarray* - **Microarray** technology is used to study the expression levels of thousands of genes simultaneously or to detect specific genetic variations. - It does not directly map the physical location of a gene on a chromosome. *Genomic imprinting* - **Genomic imprinting** is an epigenetic phenomenon where certain genes are expressed in a **parent-of-origin-specific manner**. - It describes a mechanism of gene regulation rather than a method for identifying the location of a gene on a chromosome.

Q: A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?

3. ***3*** - A **frameshift mutation** occurs when nucleotides are inserted or deleted in a number not divisible by three, altering the **reading frame** of the codons. - If insertions or deletions occur in multiples of **three**, the reading frame is restored after the mutation, largely preserving the downstream amino acid sequence. *1* - An insertion or deletion of a single nucleotide (1) definitively causes a **frameshift mutation**. - This alters all subsequent **codons**, leading to a completely different amino acid sequence downstream from the mutation. *2* - An insertion or deletion of two nucleotides (2) also results in a **frameshift mutation**. - This change shifts the **reading frame**, leading to the production of an altered protein or a premature stop codon. *None of the options* - This option is incorrect because a specific number, **three**, can allow for a frameshift mutation to not affect the complete amino acid sequence. - Multiples of three maintain the original **reading frame** (although potentially adding or removing a specific amino acid), whereas other numbers guarantee a frameshift.

Q: In eukaryotic cells, where does the majority of functional RNA activity occur?

Cytoplasm. ***Cytoplasm*** - The **cytoplasm** is the cellular compartment where the **majority of functional RNA activity** occurs, including **translation** (protein synthesis) involving mRNA, tRNA, and rRNA. - **Ribosomes** (the sites of translation) are located in the cytoplasm, either free-floating or bound to the endoplasmic reticulum. - Many types of **regulatory RNAs** such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) exert their functions in the cytoplasm by targeting mRNAs for degradation or translational repression. - **mRNA degradation** and **RNA interference pathways** primarily operate in the cytoplasm. - The question asks for the broader location rather than the specific molecular machinery, making cytoplasm the most comprehensive answer. *Nucleus* - While RNA is **transcribed** from DNA and **processed** (capping, polyadenylation, splicing) in the nucleus, these are preparatory steps. - The nucleus is primarily the site of **RNA synthesis**, not where most RNA performs its functional roles. - Only a small fraction of functional RNA activity (like rRNA processing in the nucleolus) occurs here compared to the cytoplasm. *Ribosome* - While **ribosomes are the specific sites of translation** and are composed of rRNA and proteins, they represent molecular machinery rather than a cellular location. - Ribosomes themselves are located **within the cytoplasm**, making cytoplasm the more inclusive answer for where RNA activity occurs. - The question asks "where" in terms of cellular compartment, not which molecular complex. *None of the options* - This is incorrect as the cytoplasm is indeed the primary site where the majority of functional RNA activities occur in eukaryotic cells.

Q: Which type of RNA is primarily involved in gene silencing?

miRNA. ***miRNA*** - **miRNA** (microRNA) is a small non-coding RNA molecule that plays a crucial role in **post-transcriptional regulation of gene expression**. - It functions by binding to complementary messenger RNA (mRNA) molecules, leading to **mRNA degradation** or **inhibition of translation**, thereby silencing genes. - miRNA is the primary RNA type involved in **gene silencing** through the RNA interference (RNAi) pathway. *rRNA* - **rRNA** (ribosomal RNA) is a primary component of **ribosomes**, the cellular machinery responsible for protein synthesis. - Its main function is to **catalyze peptide bond formation** and provide structural integrity to the ribosome, not gene silencing. *tRNA* - **tRNA** (transfer RNA) is responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - It acts as an adapter molecule, translating the **genetic code** in mRNA into an amino acid sequence. *mRNA* - **mRNA** (messenger RNA) carries genetic information from **DNA to ribosomes** for protein synthesis. - While mRNA can be targeted by gene silencing mechanisms (like miRNA), it is not the RNA type that performs the silencing function itself.

Q: Which of the following is NOT a characteristic of the genetic code?

Overlapping. ***Overlapping*** - The genetic code is generally **non-overlapping**, meaning each nucleotide is part of only one codon, and codons are read sequentially. - An overlapping code would mean that a single nucleotide could be part of multiple codons, which is not how protein synthesis typically occurs. *Nonambiguous* - This statement IS a characteristic; each codon specifies **only one amino acid**, meaning there is no ambiguity about which amino acid will be added. - While multiple codons can specify the same amino acid, a single codon never specifies more than one different amino acid. *Universal* - This statement IS a characteristic; the genetic code is largely **universal** across almost all organisms, from bacteria to humans. - The same codons typically specify the same amino acids in different species, which supports the idea of common ancestry. *Degeneracy* - This statement IS a characteristic; the genetic code is **degenerate**, meaning that most amino acids are specified by more than one codon. - This redundancy helps protect against the effects of single-nucleotide mutations.

Q: Which enzyme polymerises Okazaki fragments?

DNA polymerase III. ***DNA polymerase III*** - **DNA polymerase III** is the primary replicative enzyme in **prokaryotes (bacteria)** responsible for synthesizing new DNA strands, including the **polymerization of Okazaki fragments** on the lagging strand. - It possesses high processivity (can add ~500 nucleotides without dissociating), essential for rapid and efficient DNA synthesis during replication, adding nucleotides in a **5' to 3' direction**. - In **eukaryotes**, DNA polymerase δ (delta) performs the analogous function of polymerizing Okazaki fragments. *DNA polymerase I* - **DNA polymerase I** in prokaryotes primarily functions in **removing RNA primers** left by primase and **filling the resulting gaps** with DNA nucleotides. - It has 5' to 3' exonuclease activity for primer removal and polymerase activity for gap filling, but is **not the main enzyme for elongating Okazaki fragments**. - Its role is in **DNA repair and finishing replication**, not the extensive synthesis of Okazaki fragments. *DNA polymerase II* - **DNA polymerase II** in prokaryotes is primarily involved in **DNA repair mechanisms**, particularly in **restarting stalled replication forks** and responding to DNA damage. - It is not the main enzyme responsible for the polymerization of **Okazaki fragments** during normal DNA replication. *RNA polymerase* - **RNA polymerase** (specifically **primase**, a specialized RNA polymerase) synthesizes short **RNA primers** (8-12 nucleotides) during DNA replication, which provide the 3'-OH group necessary to initiate DNA synthesis. - It does not synthesize DNA or polymerize **Okazaki fragments**; its function is to create RNA primers, not extend DNA strands.

Question 1

What is a key similarity between the processes of replication and transcription?

Accepted Answer

Involve phosphodiester bond formation with elongation occurring in the 5' - 3' direction.

Answer

Use RNA primers for initiation.

Answer

Use ribonucleotides as precursors.

Answer

Are semi-conservative events.

Question 2

By which enzyme is cDNA synthesized from RNA?

Accepted Answer

Reverse transcriptase

Answer

Helicase

Answer

DNA-dependent DNA polymerase

Answer

Topoisomerase

Question 3

The gaps between Okazaki fragments on the lagging strand during DNA replication are rejoined and sealed by:

Accepted Answer

DNA Ligase

Answer

DNA Helicase

Answer

DNA Phosphorylase

Answer

DNA Topoisomerase

Question 4

What is the most important tool used in genetic engineering?

Accepted Answer

Restriction endonuclease

Answer

Topoisomerase

Answer

DNA Ligase

Answer

Helicase

Question 5

Location of gene on chromosome is identified by

Accepted Answer

Genetic mapping

Answer

Karyotyping

Answer

Microarray

Answer

Genomic imprinting

Question 6

A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?

Accepted Answer

3

Answer

1

Answer

2

Answer

None of the options

Question 7

In eukaryotic cells, where does the majority of functional RNA activity occur?

Accepted Answer

Cytoplasm

Answer

Nucleus

Answer

Ribosome

Answer

None of the options

Question 8

Which type of RNA is primarily involved in gene silencing?

Accepted Answer

miRNA

Answer

rRNA

Answer

tRNA

Answer

mRNA

Question 9

Which of the following is NOT a characteristic of the genetic code?

Accepted Answer

Overlapping

Answer

Universal

Answer

Degeneracy

Answer

Nonambiguous

Question 10

Which enzyme polymerises Okazaki fragments?

Accepted Answer

DNA polymerase III

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

RNA polymerase

Molecular Biology and Genomics — MCQs

Molecular Biology and Genomics — MCQs

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