Nucleotides

A nucleotide is a basic unit of DNA, which looks like the image given
aside. From the picture, it is clear that a nucleotide is made up of :
    1. A nitrogenous base.
    2. A pentose sugar.
    3. And, a phosphate group.

OK, time to derive them,

NITROGENOUS BASE :

A nitrogen containing compound. They are important as they make up the building blocks of nucleic acids. The term is especially used due to presence of organic ring in them. Well, there are two types of N-bases:

(i) Purines : 

• Heterocyclic aromatic organic compound, consists of a pyrimidine ring fused to an imidazole ring. 
• IUPAC Name = 9H-purine. 
• Chemical formula = C₅H₄N₄. 



Purine

• Well there are 9 notable purines occuring naturally, but there are only two purines associated with the nucleic acids. They are Adenine and Guanine. 

*Adenine and Guanine characteristics :

1.  Both are purine derivative. They have variety of roles in biochemistry like cellular  respiration, in the form of both the energy-rich adenosine triphosphate (ATP) and the  cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide  (FAD). 
2.  Have functions in protein synthesis and as a chemical component of DNA and RNA. 
3.  The shape of adenine is complementary to either thymine in DNA or uracil in RNA. 
4.  The shape of guanine is complementary to cytosine.

I will represents them throughout the posts as (A) and (G).

(ii) Pyrimidines :

• Aromatic heterocyclic organic compound same as pyridine. 
• Six-membered single ring with two nitrogen atoms at 1 and 3 (positions). IUPAC Name = Pyrimidine. 
• Chemical formula = C₄H₄N₂. 
• There are only found as derivatives (also k/a Diazines), like nucleotides, thiamine (vitaminB₁) and alloxan. And in nucleotides as Cytosine, Thymine and Uracil. 

Pyrimidine

*Cytosine, Thymine and Uracil Characteristics :

1. In nucleotide, these bases form hydrogen bonds with their complementary purines. Thus, in DNA, the purines adenine (A) and guanine (G) pair up with the pyrimidines thymine (T) and cytosine (C), respectively.
2. In RNA, the complement of adenine (A) is uracil (U) instead of thymine (T), so the pairs that form are adenine:uracil and guanine:cytosine.
3. Very rarely, thymine can appear in RNA, or uracil in DNA, but when the other three major pyrimidine bases are represented, some minor pyrimidine bases can also occur in nucleic acids. 
4. These minor pyrimidines are usually methylated versions of major ones and are postulated to have regulatory functions.

Still have doubts, then contact me below.