Nucleic acids Questions in English

(A) Nitrogenous bases are classified into two categories based on their chemical structure:
$1$. Purines: These are double-ring structures. Adenine $(A)$ and Guanine $(G)$ are purines.
$2$. Pyrimidines: These are single-ring structures. Cytosine $(C)$,Uracil $(U)$,and Thymine $(T)$ are pyrimidines.
Therefore,the correct classification is: Purines $(A, G)$ and Pyrimidines $(C, U, T)$.

(A) In a nucleotide structure:
$1$. The bond labeled $P$ connects the nitrogenous base (adenine in this case) to the $1'$ carbon of the pentose sugar. This is known as an $N-$glycosidic bond.
$2$. The bond labeled $Q$ connects the phosphate group to the $5'$ carbon of the pentose sugar. This is known as a phosphoester bond.
Therefore,$P$ is an $N-$glycosidic bond and $Q$ is a phosphoester bond.

(B) In the structure of a nucleic acid ($DNA$ or $RNA$),nucleotides are linked together to form a polynucleotide chain.
Specifically,the $3'$-$OH$ group of the sugar of one nucleotide reacts with the $5'$-phosphate group of the adjacent nucleotide.
This reaction results in the formation of a $3'-5'$ phosphodiester bond,which acts as the backbone of the nucleic acid strand.
Therefore,the correct answer is the phosphodiester bond.

(C) The correct matches are as follows:
$1$. Adenosine is a nucleoside,which consists of a nitrogenous base attached to a sugar molecule $(A-III)$.
$2$. Adenylic acid is a nucleotide,which consists of a nitrogenous base,a sugar molecule,and a phosphate group $(B-II)$.
$3$. Adenine is a nitrogenous base (specifically a purine) $(C-I)$.
$4$. Alanine is an amino acid $(D-IV)$.
Therefore,the correct sequence is $A-III, B-II, C-I, D-IV$.

(A) The correct matching is as follows:
$1$. Thymine is a nitrogenous base (pyrimidine),so $(A)$ matches with $(iv)$.
$2$. Deoxythymidine is a nucleoside (base + sugar),so $(B)$ matches with $(ii)$.
$3$. Guanylic acid is a nucleotide (base + sugar + phosphate),so $(C)$ matches with $(iii)$.
$4$. Poly guanylic acid is a polymer of nucleotides,which is a nucleic acid,so $(D)$ matches with $(i)$.
Therefore,the correct sequence is $A-iv, B-ii, C-iii, D-i$.

(B) Nucleases are enzymes that catalyze the hydrolysis of the phosphodiester bonds in nucleic acids ($DNA$ or $RNA$).
The reaction proceeds as follows:
$\text{Nucleic acids} \xrightarrow{\text{Nucleases}} \text{Nucleotides}$
Therefore,$A$ represents Nucleic acids and $B$ represents Nucleotides.

(A) nitrogenous base is a nitrogen-containing molecule that has the chemical properties of a base. Adenine,Cytosine,and Thymine are nitrogenous bases.
Guanosine is a nucleoside,which is formed by the attachment of a nitrogenous base (Guanine) to a sugar molecule (Ribose).
Therefore,Guanosine is not a nitrogenous base.

(B) In a nucleic acid,the sugar of one nucleotide is linked to the phosphate group of the adjacent nucleotide.
This linkage is formed between the $3'$-carbon of the sugar of one nucleotide and the $5'$-carbon of the sugar of the next nucleotide via a phosphate group.
This specific covalent linkage is known as a $3'-5'$ phosphodiester bond.
Therefore,the correct option is $B$.

(D) nucleoside is formed by the attachment of a nitrogenous base to a pentose sugar. The bond formed between the nitrogen of the nitrogenous base and the $1'$ carbon of the sugar is known as an $N$-glycosidic bond.
- Disaccharides contain glycosidic bonds (between two sugars).
- Dipeptides contain peptide bonds.
- Triglycerides contain ester bonds.
Therefore,the correct option is $D$.

(B) The structure shown in the image is $Uracil$.
$Uracil$ is a nitrogenous base,specifically a pyrimidine derivative,which is found in $RNA$ but not in $DNA$ (where it is replaced by $Thymine$).
The chemical structure consists of a six-membered ring containing two nitrogen atoms and two carbonyl groups,which matches the provided image.
Therefore,the correct option is $B$.