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Polynucleotide Chain

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 A polynucleotide chain is a long chain of nucleotides, which are the building blocks of DNA and RNA molecules. Nucleotides are composed of a sugar molecule, a phosphate group, and a nitrogenous base. The nitrogenous base can be one of four types: adenine (A), guanine (G), cytosine (C), or thymine (T) in DNA, or uracil (U) in RNA. The sequence of these nitrogenous bases determines the genetic information stored in DNA and RNA and ultimately governs the structure and function of all living organisms. The polynucleotide chain is held together by phosphodiester bonds between the sugar and phosphate groups, and the nitrogenous bases extend from the sugar-phosphate backbone, forming the rungs of the DNA or RNA ladder. Understanding the properties and functions of polynucleotide chains is fundamental to the study of genetics and molecular biology.

Nucleotide

 

DNA and RNA Structure

DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are two types of nucleic acids that play a critical role in the storage, transmission, and expression of genetic information in all living organisms.

DNA is a double-stranded helix composed of four types of nucleotides: adenine (A), cytosine (C), guanine (G), and thymine (T). The nucleotides are connected by covalent bonds between the sugar (deoxyribose) and phosphate groups, forming a sugar-phosphate backbone. The nucleotide bases are held together by hydrogen bonds: A always pairs with T, and C always pairs with G. These base pairs form the rungs of the DNA ladder, which twists to form the double helix.

RNA is typically single-stranded and is composed of nucleotides that are similar to those in DNA, but with the base uracil (U) instead of thymine. RNA can also form base pairs with itself or with other RNA molecules to form complex structures. There are three types of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), each with distinct roles in gene expression.

The structure of DNA and RNA is critical to their function in genetic information storage and expression. The double-stranded nature of DNA allows for the precise replication of genetic information during cell division, while the single-stranded nature of RNA allows for flexibility and versatility in its function as a messenger between DNA and protein synthesis. Additionally, the base-pairing rules ensure that genetic information is accurately and faithfully transmitted from one generation to the next.

Formation of Nucleotide Chain

A polynucleotide chain, such as DNA or RNA, is composed of three components: a nitrogenous base, a pentose sugar (ribose in RNA and deoxyribose in DNA), and a phosphate group. There are two types of nitrogenous bases: purines (adenine and guanine) and pyrimidines (cytosine, uracil, and thymine). Cytosine is found in both DNA and RNA, while thymine is present only in DNA, and uracil is present in RNA in place of thymine. A nitrogenous base is connected to the 1′ carbon of the pentose sugar through an N-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine, and uridine or deoxythymidine. When a phosphate group links to the 5′ carbon of a nucleoside through a phosphodiester linkage, a nucleotide (or deoxynucleotide, depending on the sugar present) is formed. Two nucleotides are linked through a 3′-5′ phosphodiester linkage to form a dinucleotide. More nucleotides can be linked to form a polynucleotide chain. The resulting polymer has a free phosphate moiety at the 5′-end of the sugar and a free 3′-OH group at the other end. The backbone of a polynucleotide chain is formed by sugars and phosphates, with nitrogenous bases projecting from the backbone.

Polynucleotide Chain

 

Characteristics of DNA’s Double-Helix Structure

  • DNA is made up of two chains of polynucleotides, with sugar-phosphate forming the backbone and the bases projecting inwards. 
  • The two chains have opposite polarity, meaning if one chain has 5’à3′ polarity, the other has 3’à5′ polarity. 
  • The bases in the two strands are paired through hydrogen bonds, forming base pairs. Adenine is bonded to Thymine with two H-bonds, while Guanine is bonded to Cytosine with three H-bonds. This arrangement results in a uniform distance between the two strands of the helix. 
  • The two chains are coiled in a right-handed fashion, with a pitch of 3.4 nm and approximately 10 base pairs in each turn. The stability of the helical structure is due to the stacking of one base pair over the other. 
  • The proposal of the double helix structure for DNA was revolutionary, and Francis Crick proposed the Central dogma in molecular biology, which states that genetic information flows from DNA to RNA to Protein. However, in some viruses, the flow of information is in a reverse direction, from RNA to DNA.

Chargaff’s Rule

Chargaff’s rule is a fundamental principle in the study of DNA. It states that the amount of adenine (A) always equals the amount of thymine (T) in the DNA of any given species, and likewise, the amount of guanine (G) always equals the amount of cytosine (C). This means that the base composition of DNA is consistent within a single organism, but can vary between different species.

This rule was first observed by Erwin Chargaff, an Austrian biochemist, in the 1940s. It was a crucial discovery that contributed to the eventual unraveling of the structure of DNA by Watson and Crick in 1953. Understanding Chargaff’s rule is important because it helps explain the complementary nature of the two strands of DNA and the mechanism of DNA replication. It also serves as the basis for understanding the genetic code and how DNA carries and transmits genetic information.

FAQs on Polynucleotide Chain

Question 1: What are the three Components of a Polynucleotide Chain?

Answer: 

A nitrogenous base, a pentose sugar, and a phosphate group.

Question 2: How are Nucleotides and Nucleosides formed?

Answer: 

A nucleoside is formed when a nitrogenous base is connected to the 1′ carbon of a pentose sugar. A nucleotide is formed when a phosphate group links to the 5′ carbon of a nucleoside through a phosphodiester linkage.

Question 3: What is the backbone of a Polynucleotide Chain?

Answer: 

The backbone of a polynucleotide chain is formed by sugars and phosphates, with nitrogenous bases projecting from the backbone.


Last Updated : 15 Mar, 2023
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