The DNA Questions in English

(B) According to the Watson-Crick model of $DNA$ structure, the $DNA$ molecule consists of two polynucleotide chains that form a double helix.
The diameter or width of this double helix is constant throughout its length.
The distance between the two strands of the $DNA$ helix is $20 \, \text{Å}$ (or $2 \, nm$).

(B) nucleoside is formed by the attachment of a nitrogenous base to a pentose sugar.
When the pentose sugar is deoxyribose,the resulting molecule is called a deoxyribonucleoside.
$A$ nucleotide is formed only when a phosphate group is also attached to the nucleoside.

(B) $DNA$ (Deoxyribonucleic acid) consists of two polynucleotide chains that are coiled around a common axis to form a double helix structure.
These two strands are antiparallel to each other,meaning they run in opposite directions ($5' \rightarrow 3'$ and $3' \rightarrow 5'$).
The strands are held together by hydrogen bonds between the nitrogenous bases.

(C) The nitrogenous bases found in $RNA$ are Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Uracil $(U)$.
Thymine $(T)$ is a nitrogenous base found in $DNA$ but is replaced by Uracil in $RNA$.
Therefore,Thymine is not present in $RNA$.

(A) The $DNA$ double helix structure,as proposed by Watson and Crick,consists of a repeating unit called a pitch or a full turn.
Each full turn of the $DNA$ helix consists of $10$ base pairs.
The distance between two adjacent base pairs is $3.4\ \mathring{A}$ $(0.34\ nm)$.
Therefore,the length of one full turn is $10 \times 3.4\ \mathring{A} = 34\ \mathring{A}$ (or $3.4\ nm$).

(A) In a polynucleotide chain,nucleotides are joined by $3'-5'$ phosphodiester bonds.
Specifically,the phosphate group connects the $3'$-carbon atom of the sugar of one nucleotide to the $5'$-carbon atom of the sugar of the adjacent nucleotide.
Therefore,the linkage occurs between the $3^{rd}$ carbon of the first nucleotide and the $5^{th}$ carbon of the second nucleotide.

(D) $DNA$ (Deoxyribonucleic acid) is the primary genetic material in almost all living organisms. It carries the genetic instructions necessary for the development,functioning,growth,and reproduction of all known living organisms. While some viruses use $RNA$ as their genetic material,$DNA$ is the universal hereditary molecule for cellular life.

(D) nucleotide is the basic structural unit of nucleic acids like $DNA$ and $RNA$.
It is composed of three distinct chemical components:
$1$. $A$ nitrogenous base (purine or pyrimidine).
$2$. $A$ pentose sugar (ribose in $RNA$ or deoxyribose in $DNA$).
$3$. $A$ phosphate group.
When a nitrogenous base is attached to a sugar,it is called a nucleoside. When a phosphate group is added to a nucleoside,it becomes a nucleotide.

(C) $DNA$ $(Deoxyribonucleic acid)$ and $RNA$ $(Ribonucleic acid)$ are both nucleic acids.
Both are long-chain polymers composed of repeating units called nucleotides.
$A$ nucleotide consists of a nitrogenous base,a pentose sugar,and a phosphate group.
Therefore,the fundamental structural similarity is that both are polymers of nucleotides.

(D) In the $B-DNA$ model proposed by Watson and Crick,the structure is a double helix.
Each full turn of the helix consists of $10$ base pairs.
The distance between two adjacent base pairs is $3.4\ \mathring{A}$.
Therefore,the length of one full turn is calculated as $10 \times 3.4\ \mathring{A} = 34\ \mathring{A}$.

(A) The hereditary material in almost all living organisms is $Deoxyribonucleic Acid$ $(DNA)$.
$DNA$ stores the genetic information required for the development,functioning,growth,and reproduction of organisms.
While some viruses use $RNA$ as their genetic material,$DNA$ is the universal hereditary molecule for all cellular life forms.

(A) $DNA$ stands for Deoxyribonucleic acid.
It contains a pentose sugar known as $2$-deoxyribose.
In this sugar,the hydroxyl group $(-OH)$ is absent at the $2'$ carbon position,which distinguishes it from the ribose sugar found in $RNA$.

(A) Nitrogenous bases are classified into two categories based on their structure: purines and pyrimidines.
Purines have a double-ring structure (a six-membered ring fused to a five-membered ring),while pyrimidines have a single-ring structure.
The purines are Adenine $(A)$ and Guanine $(G)$.
The pyrimidines are Cytosine $(C)$,Thymine $(T)$,and Uracil $(U)$.
Among the given options,Guanine is a purine and therefore possesses a double-ring structure.

(C) Nitrogenous bases are classified into two types based on their ring structure:
$1$. Purines: These are double-ring structures (a six-membered ring fused to a five-membered ring). Examples include Adenine $(A)$ and Guanine $(G)$.
$2$. Pyrimidines: These are single-ring structures (a six-membered ring). Examples include Cytosine $(C)$,Thymine $(T)$,and Uracil $(U)$.
Since Uracil is a pyrimidine,it possesses a single-ring structure.

(B) Nitrogenous bases in $DNA$ and $RNA$ are classified into two categories: Purines and Pyrimidines.
$1$. Purines are double-ringed structures,which include Adenine $(A)$ and Guanine $(G)$.
$2$. Pyrimidines are single-ringed structures,which include Cytosine $(C)$,Thymine $(T)$,and Uracil $(U)$.
Since Thymine $(T)$ is a single-ringed nitrogenous base found in $DNA$,it is classified as a Pyrimidine.

(A) $DNA$ (Deoxyribonucleic acid) consists of four nitrogenous bases: Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Thymine $(T)$.
Uracil $(U)$ is a nitrogenous base found exclusively in $RNA$ (Ribonucleic acid) instead of Thymine.
Therefore,Uracil is not present in the structure of $DNA$.

(A) nucleoside consists of a nitrogenous base attached to a pentose sugar. When a phosphate group is added to a nucleoside,it forms a nucleotide. Specifically,when a deoxyribonucleoside (which contains deoxyribose sugar) combines with a phosphate group,it forms a deoxyribonucleotide.

(B) In a nucleic acid,two consecutive nucleotides are linked by a $3'-5'$ phosphodiester bond.
This bond is formed between the $3'$-hydroxyl group of the sugar of one nucleotide and the $5'$-phosphate group of the next nucleotide.
This linkage creates the backbone of the polynucleotide chain.

(D) dinucleotide is formed when two nucleotides are linked together by a phosphodiester bond.
In a nucleotide,the phosphate group is attached to the $C_5$ carbon of the sugar.
When joining two nucleotides,the phosphate group attached to the $C_5$ carbon of the second nucleotide forms a bond with the $C_3$ hydroxyl $(-OH)$ group of the sugar of the first nucleotide.
This linkage is known as the $3'-5'$ phosphodiester bond.
Therefore,the bond is formed between the $C_3$ carbon of one sugar and the $C_5$ carbon of the next sugar.

(B) The structure of $DNA$ (Deoxyribonucleic acid) is a double helix.
It consists of two polynucleotide chains that run in an anti-parallel direction.
These two chains are coiled around a common axis and are held together by hydrogen bonds between the nitrogenous bases.

(C) The $DNA$ (Deoxyribonucleic acid) molecule consists of two polynucleotide chains that are coiled around a common axis to form a double helix structure.
These two chains run in an anti-parallel direction.
The nitrogenous bases of the two strands are linked by hydrogen bonds,which stabilize the helical structure.

(D) $DNA$ stands for Deoxyribonucleic Acid.
It is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development,functioning,growth,and reproduction of all known organisms and many viruses.

(A) $DNA$ was first identified and isolated by the Swiss chemist Friedrich Miescher in $1869$. He isolated a substance from the nuclei of pus cells,which he called 'nuclein'. Since $1869$ falls in the latter half of the $19^{th}$ century (which spans from $1851$ to $1900$),option $A$ is the correct answer.

(A) According to the Watson and Crick model of $DNA$ structure,the $DNA$ molecule exists as a double helix.
In this structure,the two polynucleotide strands are coiled around a common axis.
The diameter of the $DNA$ double helix is constant throughout its length.
The distance between the two strands of the $DNA$ double helix is $20 \ \mathring A$ $(2 \ nm)$.

(B) In $1953$,James Watson and Francis Crick proposed the double-helical model of $DNA$ based on $X$-ray diffraction data produced by Maurice Wilkins and Rosalind Franklin. This model provided a systematic explanation for the structure of $DNA$,including base pairing and the antiparallel nature of the strands.

(D) In the $DNA$ double helix model proposed by Watson and Crick,the two polynucleotide chains are coiled around a common axis in a right-handed fashion. This structure is described as a double helix,which resembles a twisted ladder where the sugar-phosphate backbones form the sides and the base pairs form the rungs. Therefore,the structure is best described as a double helical ladder-like structure.

(B) In the $DNA$ double helix structure,the two strands are held together by hydrogen bonds between the nitrogenous bases.
According to Chargaff's rules and the Watson-Crick model,a purine base (Adenine or Guanine) always pairs with a pyrimidine base (Thymine or Cytosine) on the opposite strand.
Specifically,Adenine $(A)$ pairs with Thymine $(T)$ via two hydrogen bonds,and Guanine $(G)$ pairs with Cytosine $(C)$ via three hydrogen bonds.
These hydrogen bonds are relatively weak compared to covalent bonds,which allows the strands to separate during processes like replication and transcription.

(A) According to the base-pairing rules established by $Chargaff$,purines always pair with pyrimidines in a $DNA$ double helix.
Specifically,$Adenine$ $(A)$ pairs with $Thymine$ $(T)$,and $Guanine$ $(G)$ pairs with $Cytosine$ $(C)$.
Since $Adenine$ and $Guanine$ are purines,they must pair with a pyrimidine ($Thymine$ or $Cytosine$) to maintain the constant width of the $DNA$ molecule.
Therefore,a purine base always pairs with a pyrimidine base.

(C) According to the base-pairing rules established by Erwin Chargaff and the structure of $DNA$ proposed by Watson and Crick,adenine $(A)$ always pairs with thymine $(T)$ via two hydrogen bonds.
Cytosine $(C)$ pairs with guanine $(G)$ via three hydrogen bonds.
Since the question asks for the base that pairs with adenine in $DNA$,the correct answer is thymine.

(B) According to the base-pairing rules proposed by Watson and Crick for $DNA$ structure,nitrogenous bases pair specifically through hydrogen bonds.
Specifically,Guanine $(G)$ always pairs with Cytosine $(C)$ via three hydrogen bonds.
Adenine $(A)$ pairs with Thymine $(T)$ via two hydrogen bonds.
Therefore,the correct nitrogenous base that pairs with Guanine is Cytosine.

(A) In the structure of $DNA$,nitrogenous bases follow Chargaff's rule of base pairing.
According to this rule,Adenine $(A)$ always pairs with Thymine $(T)$ via two hydrogen bonds,and Guanine $(G)$ always pairs with Cytosine $(C)$ via three hydrogen bonds.
Therefore,the correct complementary base pair among the given options is $A-T$.

(D) In the structure of $DNA$,nitrogenous bases follow Chargaff's rule of base pairing.
According to this rule,Adenine $(A)$ always pairs with Thymine $(T)$ via two hydrogen bonds,and Guanine $(G)$ always pairs with Cytosine $(C)$ via three hydrogen bonds.
Therefore,both $A-T$ and $G-C$ are correct complementary base pairings found in $DNA$.

(C) In the structure of $DNA$,nitrogenous bases pair with each other via hydrogen bonds.
Specifically,$A$ (Adenine) pairs with $T$ (Thymine) through two hydrogen bonds.
$G$ (Guanine) pairs with $C$ (Cytosine) through three hydrogen bonds.
Therefore,the correct answer is hydrogen bond.

(A) In the structure of $DNA$,nitrogenous bases pair with each other through hydrogen bonds. $Adenine$ $(A)$ always pairs with $Thymine$ $(T)$ via two hydrogen bonds,while $Guanine$ $(G)$ pairs with $Cytosine$ $(C)$ via three hydrogen bonds. Therefore,there are two hydrogen bonds between $A$ and $T$.

(B) In the structure of $DNA$,nitrogenous bases pair with each other through hydrogen bonds.
According to Watson and Crick's model,Cytosine $(C)$ always pairs with Guanine $(G)$ through $3$ hydrogen bonds.
Conversely,Adenine $(A)$ pairs with Thymine $(T)$ through $2$ hydrogen bonds.
Therefore,the number of hydrogen bonds between $C$ and $G$ is $3$.

(C) According to Chargaff's rule, in a double-stranded $DNA$ molecule, the amount of purine bases $(Adenine + Guanine)$ is always equal to the amount of pyrimidine bases $(Thymine + Cytosine)$.
Therefore, the ratio of purines to pyrimidines is $1:1$, which means they are present in an equal ratio.

(B) Assertion $(S)$ is true because in $DNA$,Adenine $(A)$ always pairs with Thymine $(T)$ via two hydrogen bonds.
Reason $(R)$ is also true because $DNA$ contains Thymine instead of Uracil,which is found in $RNA$.
However,the reason why $A$ pairs with $T$ is due to the specific chemical structure and hydrogen bonding capacity of these bases,not simply because $DNA$ lacks uracil. Therefore,$R$ is not the direct explanation for $S$.

(B) $1$. $RNA$ (Ribonucleic acid) contains the nitrogenous bases Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Uracil $(U)$. It does not contain Thymine $(T)$. Thus,Assertion $(S)$ is true.
$2$. $DNA$ (Deoxyribonucleic acid) contains the nitrogenous bases Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Thymine $(T)$. It does not contain Uracil $(U)$. Thus,Reason $(R)$ is true.
$3$. While both statements are scientifically accurate,the reason $(R)$ explains the composition of $DNA$,whereas the assertion $(S)$ describes the composition of $RNA$. Therefore,$R$ is not the direct explanation for $S$.

(D) The nitrogenous bases found in $RNA$ are adenine,guanine,cytosine,and uracil. Thymine is absent in $RNA$ and is replaced by uracil.
In contrast,$DNA$ contains adenine,guanine,cytosine,and thymine. Uracil is absent in $DNA$.
Therefore,the statement $S$ is false because $RNA$ does not contain thymine.
The reason $R$ is also false because $DNA$ does contain thymine.
Thus,both $S$ and $R$ are false.

(B) According to Chargaff's rules and the structure of $DNA$ proposed by Watson and Crick,nitrogenous bases pair specifically through hydrogen bonds.
In $DNA$,Adenine $(A)$ always pairs with Thymine $(T)$ via two hydrogen bonds $(A=T)$.
Guanine $(G)$ always pairs with Cytosine $(C)$ via three hydrogen bonds $(G≡C)$.
Therefore,the correct complementary base pairing is Adenine and Thymine.

(B) $DNA$ molecule consists of a deoxyribose sugar,a phosphate group,and a nitrogenous base.
In all $DNA$ molecules,the sugar (deoxyribose) and the phosphate backbone remain constant.
The variation between different $DNA$ molecules arises from the specific sequence and arrangement of the nitrogenous bases ($Adenine$,$Thymine$,$Cytosine$,and $Guanine$).
Therefore,the nitrogenous bases are the part that varies.

(B) The $DNA$ double helix model,proposed by $Watson$ and $Crick$,consists of two polynucleotide strands.
These strands run in opposite directions,meaning one strand runs in the $5' \rightarrow 3'$ direction while the other runs in the $3' \rightarrow 5'$ direction; this orientation is known as antiparallel.
Furthermore,the nitrogenous bases on the two strands are complementary,where $Adenine$ $(A)$ always pairs with $Thymine$ $(T)$ via two hydrogen bonds,and $Guanine$ $(G)$ always pairs with $Cytosine$ $(C)$ via three hydrogen bonds.
Therefore,the strands are antiparallel and complementary.

(C) $DNA$ (Deoxyribonucleic acid) and $RNA$ (Ribonucleic acid) differ in two main structural components:
$1$. Sugar: $DNA$ contains $2'$-deoxyribose sugar,while $RNA$ contains ribose sugar.
$2$. Pyrimidines: $DNA$ contains cytosine and thymine,whereas $RNA$ contains cytosine and uracil. Thymine is replaced by uracil in $RNA$.

(D) The nitrogenous bases found in $RNA$ are Adenine $(A)$,Guanine $(G)$,Cytosine $(C)$,and Uracil $(U)$.
Thymine $(T)$ is a nitrogenous base found in $DNA$ but is replaced by Uracil in $RNA$.
Therefore,Thymine is absent in the structure of $RNA$.

(C) $RNA$ (Ribonucleic acid) is typically a single-stranded molecule.
$DNA$ (Deoxyribonucleic acid) is typically a double-stranded molecule consisting of two polynucleotide chains coiled around each other to form a double helix.
Therefore,the number of polynucleotide strands in $RNA$ is $1$ and in $DNA$ is $2$.

(B) The $DNA$ molecule consists of two polynucleotide chains that form a double helix structure.
These two chains are antiparallel,meaning they run in opposite directions ($5' \rightarrow 3'$ and $3' \rightarrow 5'$).
They are held together by hydrogen bonds between the nitrogenous bases,keeping the chains at a constant distance from each other.
Therefore,the two polynucleotide chains are the structures that are positioned opposite to each other,at an equal distance,and in opposite directions.

(B) In the $DNA$ double helix structure,nitrogenous bases are paired according to $Chargaff's$ rule.
$Adenine$ $(A)$ always pairs with $Thymine$ $(T)$ via two hydrogen bonds.
$Guanine$ $(G)$ pairs with $Cytosine$ $(C)$ via three hydrogen bonds.
Therefore,the correct answer is that $Adenine$ and $Thymine$ are connected by two weak hydrogen bonds.

(C) In the $B-DNA$ model proposed by Watson and Crick,the structure is a double helix.
$1$. The length of one full turn of the $DNA$ helix is $34\ \mathring{A}$ (or $3.4\ nm$).
$2$. The distance between the two polynucleotide strands (the diameter of the helix) is $20\ \mathring{A}$ (or $2.0\ nm$).
Therefore,the correct values are $34\ \mathring{A}$ and $20\ \mathring{A}$.

(B) In the $DNA$ double helix structure, nitrogenous bases are linked by hydrogen bonds.
According to Chargaff's rules and the Watson-Crick model, Guanine $(G)$ always pairs with Cytosine $(C)$.
This pairing is stabilized by three hydrogen bonds $(G \equiv C)$.
Conversely, Adenine $(A)$ pairs with Thymine $(T)$ via two hydrogen bonds $(A = T)$.

(D) In a typical cell,there are three main types of $RNA$: $m-RNA$,$t-RNA$,and $r-RNA$.
$r-RNA$ (ribosomal $RNA$) constitutes about $80\%$ of the total cellular $RNA$.
$t-RNA$ (transfer $RNA$) accounts for approximately $15\%$ of the total $RNA$.
$m-RNA$ (messenger $RNA$) makes up only about $5\%$ of the total $RNA$.
Therefore,$r-RNA$ is the most abundant type of $RNA$ in the cell.