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Nucleotide

Nucleotide Definition

Nucleotides can be defined as the organic molecules that act as the building blocks of the genetic materials – DNA and RNA. But, the function of the nucleotides is not limited only to this. They also have other roles to play. For example, they form a part of cell signaling, enzyme reactions, as well as body’s metabolism.

Structure and Components of Nucleotides:

As such, each and every nucleotide will be composed of three parts, which are namely,

  • A 5-sided sugar molecule (or five-carbon sugar or pentose)
  • A phosphate group
  • A nitrogen-containing base (or nucleobase or nitrogenous bases)

In this, the sugar and the phosphate components would make up the backbone of the double helix in the DNA and the bases will be present in the center. As such, the backbone will be held together by the chemical bonds that are formed between the phosphate component of one nucleotide and the sugar component of the other nucleotide. And, the two strands of the double helix will be held together by the chemical bonds that are formed between the nitrogenous bases.

It’s to be noted here that the difference between the RNA and DNA lies in the fact that the 5-sided sugar in RNA is ribose, while that of in the DNA is deoxyribose.

Types of Nucleobases:

The nucleobases that are found in the structure of nucleotides can be divided into two major groups: Purines and Pyrimidines. In DNA, there are two kinds of purine molecules, which are namely, adenine (A) and guanine (G) and there are two kinds of pyrimidine molecules, which are namely, thymine (T) and cytosine (C). On the other hand, in RNA, the same purine molecules will be found. But, when it comes to pyrimidines, in addition to cytosine (C), uracil (U) will be present. Yes, the difference lies in the presence of uracil instead of thymine here.

Base Pairing:

As mentioned earlier, the two strands of DNA, as well as the helical regions of RNA are joined together by chemical bonds formed between the nitrogenous bases. This phenomenon is called base pairing and the rules of base pairing are as follows:

  • Adenine always pairs with Thymine or Uracil
  • Cytosine always pairs with Guanine

As such, these nitrogenous bases are joined together via hydrogen bonds. In this, there will be two hydrogen bonds between A & T/U, whereas, there will be three hydrogen bonds between C and G. This way, the rules of base pairing tells us that if the sequence of nucleotides on one strand of the DNA is read, the complementary sequence on the other strand can be easily deduced.

Chargaff’s Rule:

According to the Chargaff’s Rule, whatever the amount of A is present in the DNA of an organism, the amount of T will be the same. Likewise, whatever the amount of G is present in the DNA, the amount of C is also the same.

Thus, A = T and C = G, but C + G : A + T will differ from one organism to another.

Nucleotide Functions:

As mentioned earlier, in addition to forming the basic units of genetic material, the nucleotides have other roles to play. They are:

  • Found in other molecules like ATP
  • Found in coenzymes like NAD and NADP
  • Found in messenger molecules like cAMP
  • Take part in chemical reactions
  • Found to be activated intermediates in several biosyntheses
  • Act as metabolic regulators
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