A polynucleotide chain is composed of monomers of nucleotide molecules. The structure of DNA is two nucleotide molecules wound together. Even RNA is made of a single chain of polynucleotides. The design of the DNA and RNA polynucleotide chain are given below –
- Every monomer has three sections, namely a pentose sugar, a nitrogenous base and a phosphate group.
- The pentose sugar and the phosphate groups are linked by a phosphodiester linkage.
- The four nitrogenous bases are adenine, cytosine, guanine and thymine or uracil.
Also Read: Nucleotide
DNA structure
DNA has two polynucleotide chains wound to form a double helix structure. The pentose sugar molecule is deoxyribose. The hydroxyl group is not present in the deoxyribose. Also, the base pairs are adenine, cytosine, guanine and thymine.
In DNA, the two strands are wound in the opposite direction. Thus, the strands are firmly held by the hydrogen bonds between the complementary base pairs.
Also see: Double helix structure of DNA
RNA structure
The RNA is a single-chain structure of polynucleotides. Here, ribose is the pentose sugar. The ribose has the hydroxyl group on the 2’ end of the carbon. Also, the base pairs here are adenine, cytosine, guanine and uracil.
Formation of Polynucleotide Chain
- The nitrogenous bases can be grouped as either purines or pyrimidines. The purines include adenine and guanine. The pyrimidines include cytosine, thymine and uracil.
- The first carbon of the pentose sugar is attached to the nitrogenous base with the help of an N-glycosidic linkage.
- The third carbon atom of the pentose sugar combines with the phosphate group by a covalent bond. This linkage is the phosphodiester bond.
- The sugar and phosphate form a solid backbone to the polynucleotide chain.
- This polynucleotide monomer (sugar, phosphate and nitrogenous base) combines to form a polynucleotide chain.
Chargaff’s Rule
Chargaff discovered the nitrogenous bases in DNA. He stated that the ratio of purines to pyrimidines is always 1:1. This means the number of adenine (A) should be equal to thymine (T), and the number of guanine (G) equal to cytosine (C).
Frequently Asked Questions
What is a nucleoside?
The nucleoside is a sub-unit of the nucleotide. The pentose sugar and the nitrogenous base are together termed as a nucleoside. Whereas a pentose sugar, nitrogenous base and phosphate together form the nucleotide.
Extended Reading: Difference between nucleotide and nucleoside
What is base pairing?
The complementary nitrogenous base combines with the help of a hydrogen bond. This complementary pairing is called base pairing. Usually, the purines pair with the pyrimidines. The A (purine) pairs with T (pyrimidine). Likewise, C (pyrimidine) pairs with G (purine).
What is the use of polynucleotide?
Polynucleotide chains are used in DNA sequencing or PCR (polymerase chain reaction). Thus, they aid in numerous biochemical analyses.
What is the role of the nucleases enzyme?
This enzyme is specially designed to cleave the nucleotides of DNA and RNA molecules. The enzyme nucleases can easily break the phosphodiester bond. They also play a vital role in the DNA repair mechanism.
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