Tools of recombinant DNA technology Questions in English

(C) $DNA$ ligase is the enzyme responsible for joining the broken strands of $DNA$ by catalyzing the formation of a phosphodiester bond between the $3'-hydroxyl$ end of one nucleotide and the $5'-phosphate$ end of another. This process is essential for $DNA$ replication and repair.

(C) Plasmids are small,circular,double-stranded $DNA$ molecules that are distinct from a cell's chromosomal $DNA$.
They are naturally found in bacterial cells and some eukaryotes.
Plasmids are considered extrachromosomal genetic material because they exist and replicate independently of the bacterial chromosome.
They often carry genes that provide advantages to the bacteria,such as antibiotic resistance.

(B) In recombinant $DNA$ technology,vectors are essential for transferring foreign $DNA$ into a host cell. Plasmids are small,circular,extrachromosomal $DNA$ molecules found in bacteria that can replicate independently. They are widely used as vectors to carry and clone genes of interest. Therefore,plasmids are the correct choice.

(C) $DNA$ ligase is an enzyme that facilitates the joining of $DNA$ strands together by catalyzing the formation of a phosphodiester bond.
It is commonly referred to as the 'molecular glue' in biotechnology.
$DNA$ polymerase is used for synthesis,while endonucleases are used to cut $DNA$ at specific sites.

(B) The process of synthesizing $DNA$ from an $mRNA$ template is known as reverse transcription.
This process is catalyzed by the enzyme $Reverse$ $transcriptase$.
$DNA$ probes are often synthesized using this method to create complementary $DNA$ $(cDNA)$ from $mRNA$ sequences.
Therefore,the correct option is $B$.

(B) The synthesis of $cDNA$ (complementary $DNA$) from an $mRNA$ template is a process known as reverse transcription.
This process is catalyzed by the enzyme $Reverse \text{ } transcriptase$.
$Reverse \text{ } transcriptase$ uses the $mRNA$ strand as a template to synthesize a complementary $DNA$ strand, which is then used to create a double-stranded $cDNA$ molecule.

(C) Restriction enzymes are a class of enzymes that recognize specific nucleotide sequences in $DNA$ and cut the $DNA$ at those specific sites,known as recognition sites. These enzymes are essential tools in recombinant $DNA$ technology for creating $DNA$ fragments. $DNA$ Polymerase is used for synthesis,$DNA$ Ligase is used for joining $DNA$ fragments,and $DNA$ Gyrase is involved in supercoiling.

(B) Restriction endonucleases are known as the molecular scissors of $DNA$.
These enzymes recognize specific nucleotide sequences in the $DNA$ molecule and cut the $DNA$ at specific locations.
$DNA$ ligase is known as the molecular glue,as it joins $DNA$ fragments together.
Exonucleases remove nucleotides from the ends of $DNA$ molecules,and peptidases are enzymes that break down proteins.

(C) The enzyme $DNA$ ligase is a specific type of enzyme that facilitates the joining of $DNA$ strands together by catalyzing the formation of a phosphodiester bond. It is commonly referred to as the 'molecular glue' in biotechnology and genetic engineering,as it is essential for sealing nicks in the $DNA$ backbone,particularly during $DNA$ replication,repair,and recombinant $DNA$ technology.

(B) Nucleases are enzymes that catalyze the hydrolysis of phosphodiester bonds in the sugar-phosphate backbone of nucleic acids ($DNA$ or $RNA$).
Specifically,they act by breaking the phosphodiester bonds that link the nucleotides together in a polynucleotide chain.
Therefore,the correct function is to break the phosphodiester bonds.

(B) Restriction endonucleases are enzymes that cut $DNA$ molecules at specific nucleotide sequences,known as recognition sites. These enzymes are essential tools in recombinant $DNA$ technology for creating $DNA$ fragments. In contrast,$DNA$ polymerase is involved in $DNA$ replication,and $DNA$ ligase is responsible for joining $DNA$ fragments together.

(B) Myelomas are cancerous plasma cells derived from a tumor of the bone marrow. In biotechnology,specifically in the production of monoclonal antibodies,myeloma cells are fused with antibody-producing $B$-lymphocytes to create hybridoma cells. Since the question asks for the cells obtained from a cancerous tumor (specifically in the context of hybridoma technology),the correct term for the cancerous cell line used is myeloma cells.

(C) Restriction Fragment Length Polymorphism $(RFLP)$ is a technique that exploits variations in homologous $DNA$ sequences.
Step $1$: The $DNA$ is first treated with restriction enzymes,a process known as restriction digestion $(4)$,to cut the $DNA$ into fragments of varying lengths.
Step $2$: These $DNA$ fragments are then separated based on their size using gel electrophoresis $(1)$.
Therefore,both restriction digestion and electrophoresis are essential steps in $RFLP$ analysis.
Thus,the correct combination is $1$ and $4$.

(A) Agrobacterium tumefaciens is a soil bacterium that naturally infects plants. It contains a large plasmid called the Tumor-inducing $(Ti)$ plasmid. This plasmid carries the $T-DNA$ (Transfer $DNA$),which is integrated into the plant genome,causing the formation of crown gall tumors. Therefore,the correct option is $A$.

(C) Restriction enzymes are classified based on the type of ends they produce after cutting $DNA$.
$1$. Sticky ends (cohesive ends) are produced by enzymes like $EcoRI$,$BamHI$,and $HindIII$,which create staggered cuts.
$2$. Blunt ends (flush ends) are produced by enzymes that cut both strands of $DNA$ at the same position.
$3$. $SmaI$ is a classic example of a restriction enzyme that recognizes the sequence $5'-CCCGGG-3'$ and cuts between the $C$ and $G$ to produce blunt ends.

(C) Plasmids are small,circular,double-stranded $DNA$ molecules that are distinct from a cell's chromosomal $DNA$.
They are widely used as vectors in biotechnology because they possess an origin of replication $(ori)$,which allows them to replicate independently within the host cell.
This property ensures that the recombinant $DNA$ inserted into the plasmid is copied many times as the host cell divides.

(A) cloning vector is a $DNA$ molecule used as a vehicle to artificially carry foreign genetic material into another cell.
Key features of a cloning vector include:
$1$. Origin of replication $(ori)$: This is the sequence where replication starts. $A$ vector should have only one $ori$ to control the copy number of the linked $DNA$.
$2$. Selectable marker: Helps in identifying and eliminating non-transformants.
$3$. Cloning sites: The vector should have very few,preferably single,recognition sites for commonly used restriction enzymes.
Therefore,having 'two origins of replication' is not a characteristic of a standard cloning vector,as it would lead to instability and difficulty in controlling copy number.
Thus,option $A$ is the correct answer.

(B) The $PCR$ (Polymerase Chain Reaction) technique is used to amplify a specific segment of $DNA$.
This process requires a thermostable $DNA$ polymerase enzyme that can withstand high temperatures during the denaturation step.
$Taq$ polymerase,isolated from the bacterium $Thermus$ $aquaticus$,is the enzyme used in $PCR$ because it remains active at high temperatures required for the separation of $DNA$ strands.

(A) Restriction endonucleases are enzymes that recognize specific nucleotide sequences in $DNA$ and cut the $DNA$ at those specific sites,known as recognition sequences.
Ligases are used to join $DNA$ fragments.
General endonucleases cut $DNA$ at random positions,not at specific sites.
Alkaline phosphatase is used to remove phosphate groups from the $5'$ end of $DNA$ molecules to prevent self-ligation.
Therefore,the correct answer is $A$.

(B) The correct answer is $B$.
$DNA$ ligase is an enzyme that joins $DNA$ fragments by forming phosphodiester bonds,often referred to as 'molecular glue'.
Restriction enzymes (endonucleases) are responsible for cutting $DNA$ at specific recognition sequences to create sticky or blunt ends.
$RFLPs$ (Restriction Fragment Length Polymorphisms) are produced by the action of restriction enzymes.
$DNA$ polymerase is used in $PCR$ to synthesize new $DNA$ strands.
Reverse transcriptase is used to synthesize $cDNA$ (complementary $DNA$) from an $mRNA$ template.

(D) Restriction endonucleases are enzymes that recognize specific palindromic nucleotide sequences in a $DNA$ molecule and cut the $DNA$ at these specific sites.
This property is essential in genetic engineering for creating recombinant $DNA$ molecules by inserting a gene of interest into a vector.
Therefore,they are known as 'molecular scissors' because they cut $DNA$ at precise locations rather than randomly.

(D) The $Ti$ plasmid (Tumor-inducing plasmid) is a circular $DNA$ molecule found in the soil bacterium $Agrobacterium$ $tumefaciens$.
This bacterium has the natural ability to transfer a portion of its $DNA$,known as $T$-$DNA$,into the genome of infected plant cells,causing crown gall disease.
Due to this unique property,scientists use the $Ti$ plasmid as a vector to deliver foreign genes into plant cells for genetic engineering and crop improvement.
Therefore,the correct answer is $Agrobacterium$.

(A) In genetic engineering,the ability to cut $DNA$ at specific sites and join them to vectors is fundamental.
Restriction endonucleases are enzymes that act as 'molecular scissors' to cut $DNA$ at specific recognition sequences.
$DNA$ ligase is the enzyme responsible for joining $DNA$ fragments together.
However,the foundational breakthrough that allowed for the precise manipulation and insertion of foreign $DNA$ into a host genome was the discovery of restriction endonucleases,which enable the creation of recombinant $DNA$ molecules.
Therefore,the correct answer is $A$.

(B) In genetic engineering,$Retroviruses$ are used as vectors to deliver genes into higher organisms,including animals. These viruses have the ability to infect host cells and integrate their genetic material into the host genome,making them effective tools for gene therapy and genetic modification.

(D) Plasmids are small,circular,double-stranded $DNA$ molecules that are distinct from a cell's chromosomal $DNA$. They are naturally found in bacteria and some eukaryotes like yeast. In biotechnology,plasmids are extensively used as vectors to carry foreign genetic material into another cell,making them essential tools in genetic engineering.

(C) palindromic $DNA$ sequence is a sequence of base pairs in $DNA$ that reads the same on the two strands when the orientation of reading is kept the same (e.g.,$5' \rightarrow 3'$ direction).
For option $C$:
Strand $1$: $5'-GAATTC-3'$
Strand $2$: $3'-CTTAAG-5'$
If we read Strand $2$ in the $5' \rightarrow 3'$ direction,it becomes $5'-GAATTC-3'$.
Since both strands read the same in the $5' \rightarrow 3'$ direction,it is a palindromic sequence.

(B) $DNA$ probe is a single-stranded sequence of $DNA$ or $RNA$ that is labeled with a radioactive isotope or a fluorescent tag.
It is used to detect the presence of complementary sequences in a sample of $DNA$ or $RNA$ through the process of hybridization.
By binding specifically to the target gene sequence,the probe allows researchers to identify and isolate the gene of interest.

(B) $DNA$ ligase is an enzyme commonly referred to as the 'molecular glue'.
It functions by catalyzing the formation of phosphodiester bonds between the $3'-OH$ end of one $DNA$ fragment and the $5'-phosphate$ end of another $DNA$ fragment.
This process effectively joins or ligates $DNA$ fragments together,which is a crucial step in recombinant $DNA$ technology for creating recombinant molecules.

(D) $BACs$ (Bacterial Artificial Chromosomes) and $YACs$ (Yeast Artificial Chromosomes) are engineered $DNA$ molecules used as cloning vectors.
They are constructed using sequences derived from bacteria and yeast,respectively,to carry large fragments of foreign $DNA$.
These vectors are widely used in genomic studies and for gene transformation in eukaryotic organisms.
Therefore,both statements $B$ and $C$ are correct.

(B) Genetic engineering is a branch of biotechnology that involves the direct manipulation of an organism's genes.
This process is made possible primarily through Recombinant $DNA$ $(rDNA)$ technology.
$rDNA$ technology allows scientists to isolate,modify,and insert specific genes into a host organism,thereby altering its genetic makeup to express desired traits.
Hybridization is a traditional breeding method,while $X$-ray diffraction and heavy isotope labeling are analytical techniques used in structural biology and molecular research,respectively.

(C) In genetic engineering,antibiotic resistance genes are used as selectable markers. These genes allow for the identification and elimination of non-transformants and selectively permit the growth of the transformants. When a host cell is transformed with a recombinant $DNA$ molecule containing an antibiotic resistance gene,it becomes resistant to that specific antibiotic. Thus,when grown on a medium containing the antibiotic,only the transformed cells will survive,while non-transformed cells will die.

(A) The naming convention for restriction endonucleases follows specific rules. The first letter of the name comes from the genus,and the second two letters come from the species of the prokaryotic cell from which they were isolated. For example,in $EcoRI$,'$E$' stands for $Escherichia$ (genus),and '$co$' stands for $coli$ (species). Therefore,the second part of the name is derived from the species of the organism.

(C) Molecular scissors are enzymes that cut $DNA$ at specific sites.
Restriction endonucleases are the enzymes responsible for this action,as they recognize specific palindromic nucleotide sequences and cleave the $DNA$ strands.
Therefore,they are referred to as molecular scissors in recombinant $DNA$ technology.

(C) Restriction enzymes,also known as molecular scissors,are enzymes that cut $DNA$ at specific recognition sequences.
These enzymes specifically recognize and cleave double-stranded $DNA$ molecules at specific sites,known as restriction sites.
Therefore,the substrate for restriction enzymes is double-stranded $DNA$.

(A) palindromic $DNA$ sequence is a sequence of nucleotides in $DNA$ that is the same when read from $5'$ to $3'$ on one strand and from $5'$ to $3'$ on the complementary strand.
In the given options,the sequence $5'-GAATTC-3'$ / $3'-CTTAAG-5'$ is a classic palindromic sequence recognized by the restriction enzyme $EcoRI$.
When read in the $5' \rightarrow 3'$ direction,both strands show the sequence $GAATTC$.
Therefore,option $A$ is the correct palindromic sequence.

(B) The $DNA$ polymerase used in $PCR$ is known as $Taq$ polymerase.
It is isolated from a thermophilic bacterium called $Thermus$ $aquaticus$.
Because $PCR$ involves repeated cycles of heating (denaturation) to separate $DNA$ strands,the enzyme must be thermostable.
Therefore,$Taq$ polymerase remains active at high temperatures,allowing the amplification process to continue without needing to add fresh enzyme after every cycle.

(C) $Agrobacterium$ $tumefaciens$ is a soil bacterium that naturally infects plants.
It contains a $Ti$ (tumor-inducing) plasmid,which it uses to transfer a specific segment of its $DNA$ (known as $T-DNA$) into the host plant's genome.
In genetic engineering,scientists have modified this $Ti$ plasmid to remove its pathogenic properties while retaining its ability to transfer foreign $DNA$ into plants.
Therefore,it acts as a natural vector for gene transfer in plants.

(B) Restriction endonucleases are enzymes that cut $DNA$ at specific recognition sequences. In bacteria,these enzymes serve as a defense mechanism to degrade foreign $DNA$ (such as viral $DNA$) that enters the cell,thereby protecting the bacterial genome. While they are widely used in genetic engineering for recombinant $DNA$ technology,their primary biological function is the protection of the bacterial cell from foreign $DNA$.

(B) $Agrobacterium \text{ } tumefaciens$ is a soil bacterium that naturally infects plants and transfers a piece of $DNA$ known as $T-DNA$ into the plant genome.
Because of this natural ability to transfer genes, it is widely used in genetic engineering as a vector to introduce foreign genes into plants.
$Bacillus \text{ } thuringiensis$ is used for producing $Bt$ toxins, and $Pseudomonas \text{ } putida$ is often used for bioremediation, but neither acts as a natural genetic vector for plants.

(A) Genetic engineering,also known as recombinant $DNA$ technology,relies heavily on the ability to cut $DNA$ at specific sites.
Restriction endonucleases are enzymes that act as 'molecular scissors' by recognizing specific nucleotide sequences and cutting the $DNA$ molecule at those precise locations.
This discovery allowed scientists to isolate specific genes and insert them into vectors,forming the foundation of modern biotechnology.

(D) The cell wall of fungi is primarily composed of chitin. To isolate $DNA$ from fungal cells, the cell wall must be degraded to release the cellular contents. The enzyme $Chitinase$ is specifically responsible for breaking down the chitinous cell wall of fungi. Therefore, $Chitinase$ is used for this purpose.

(D) Bacteria possess a defense mechanism known as the restriction-modification system to protect themselves from invading foreign $DNA$ (such as bacteriophage $DNA$).
They produce restriction endonucleases that cut specific sequences of foreign $DNA$.
To prevent these enzymes from cutting their own $DNA$,bacteria add methyl groups $(-CH_3)$ to specific bases within those same recognition sequences on their own genome.
This process is called $DNA$ methylation.
Therefore,the methylation of bacterial $DNA$ serves to protect the host's genome from being degraded by its own restriction enzymes.

(C) Insertional inactivation is a technique used to identify recombinant clones.
In this process,a foreign $DNA$ fragment is inserted into a gene,which results in the inactivation of that gene.
For example,the gene encoding the enzyme $\beta$-galactosidase can be used as a selectable marker.
If a foreign $DNA$ is inserted into the coding sequence of the $\beta$-galactosidase gene,the enzyme becomes inactivated.
Consequently,the colonies do not produce any color in the presence of a chromogenic substrate,allowing for the identification of recombinant colonies.

(A) Restriction enzymes are a class of enzymes that cut $DNA$ at specific recognition sequences.
$EcoRI$ is a specific type of restriction enzyme known as a restriction endonuclease.
It recognizes the palindromic sequence $5'-GAATTC-3'$ and cuts the $DNA$ between the $G$ and $A$ bases.
Therefore,$EcoRI$ functions as an endonuclease,which cleaves the phosphodiester bonds within the $DNA$ strand.

(C) Restriction endonucleases are a class of enzymes that recognize specific base sequences in $DNA$ and cut the $DNA$ duplex at specific locations. These enzymes are commonly referred to as 'molecular scissors' in biotechnology. They do not remove nucleotides from ends (that is the function of exonucleases) nor do they inhibit $DNA$ polymerase. Therefore,the correct function is to cut $DNA$ at specific sites.

(D) Restriction enzymes are specialized nucleases that cleave $DNA$ molecules at specific recognition sequences.
They are considered essential tools in genetic engineering and recombinant $DNA$ technology because they allow for the precise cutting of $DNA$ strands.
Therefore,both statements $(B)$ and $(C)$ are correct.

(B) $Taq$ polymerase is a thermostable $DNA$ polymerase enzyme isolated from the thermophilic bacterium $Thermus$ $aquaticus$.
It is a crucial component of the Polymerase Chain Reaction $(PCR)$ technique.
In $PCR$,this enzyme is used to synthesize new $DNA$ strands by extending primers,as it can withstand the high temperatures required for the denaturation of $DNA$ strands during the amplification process.

(C) Restriction enzymes are known as molecular scissors because they cut $DNA$ molecules at specific nucleotide sequences. These enzymes are essential tools in recombinant $DNA$ technology as they allow for the precise cleavage of $DNA$ strands to insert foreign genes.

(C) The naming convention for restriction enzymes follows a specific pattern:
$1$. The first letter is derived from the genus name of the organism ($E$ for $Escherichia$).
$2$. The next two letters are derived from the species name ($co$ for $coli$).
$3$. The fourth letter represents the strain of the organism ($R$ for $RY13$).
$4$. The Roman numeral indicates the order in which the enzyme was isolated from that strain ($I$ for the first enzyme isolated).
Therefore,in $EcoRI$,$R$ stands for the strain.