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Nucleotides & Nucleosides: Structures, Properties, and Nomenclature

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Nucleotides and Nucleosides: Structures, Properties, and Nomenclature

Introduction

Nucleotides and nucleosides are fundamental building blocks of nucleic acids, which are essential for the storage, transmission, and expression of genetic information. Understanding their structure and properties is crucial for comprehending the chemistry of DNA and RNA, as well as the molecular basis of heredity.

Basic Structure of Nucleotides and Nucleosides

Definitions

  • Nucleic acids: Polymers of nucleotides specialized for the storage, transmission, and use of genetic information.

  • Nucleotide: Consists of three components:

    • Sugar (pentose)

    • Nitrogenous base

    • Phosphate group

  • Nucleoside: Consists of two components:

    • Sugar (pentose)

    • Nitrogenous base

The Monosaccharide Component

  • RNA contains D-ribose.

  • DNA contains 2'-deoxy-D-ribose (lacking an -OH group at the 2' position).

  • Carbon atoms in the sugar are numbered with primes (1', 2', 3', etc.).

  • The 1' carbon forms a β-N-glycosidic linkage to the nitrogenous base.

The Nitrogenous Base

  • Nitrogenous bases are classified as pyrimidines or purines.

  • Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)

  • Purines: Adenine (A), Guanine (G)

  • In DNA: A, C, G, T; in RNA: A, C, G, U

  • The base is joined to the sugar by a β-N-glycosidic bond.

Summary Table: Nitrogenous Bases, Nucleosides, and Nucleotides

Nitrogenous Base

Nucleoside

Nucleotide

Adenine

Adenosine

Adenosine 5'-monophosphate

Guanine

Guanosine

Guanosine 5'-monophosphate

Thymine

Thymidine

Thymidine 5'-monophosphate

Cytosine

Cytidine

Cytidine 5'-monophosphate

Uracil

Uridine

Uridine 5'-monophosphate

Phosphate Esters and Nucleotide Structure

Phosphate Ester Functional Group

  • A phosphate ester is a functional group formed by the reaction of a phosphoric acid with an alcohol group of the sugar.

  • In nucleotides, the phosphate group is typically attached to the 5'-hydroxyl group of the sugar.

ATP and Nucleotide Triphosphates

  • ATP (adenosine triphosphate) is a nucleotide with three phosphate groups.

  • NTPs (nucleotide triphosphates) are high-energy molecules due to weak phosphoanhydride bonds between phosphate groups.

  • These molecules are essential for energy transfer in biological systems.

Example: The structure of ATP can be represented as:

Constructing Polynucleotides from Nucleotides

Formation of Polynucleotides

  • DNA and RNA are linear polymers of nucleotides linked by phosphodiester bonds.

  • Each nucleotide is joined to the next via a monophosphate diester linkage between the 3'-hydroxyl of one sugar and the 5'-phosphate of the next.

  • Polynucleotide chains have directionality:

    • 5' end: Free 5'-phosphate group

    • 3' end: Free 3'-hydroxyl group

  • Sequences are written from 5' to 3'.

  • New nucleotides are added to the 3' end during synthesis.

Phosphodiester Bond Structure

Structure of DNA: Double Helix and Base Pairing

Base Pairing and Stability

  • The DNA double helix is stabilized by two main interactions:

  • Hydrogen bonding (base pairing):

    • Adenine (A) pairs with Thymine (T) via two hydrogen bonds.

    • Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.

    • In RNA, Uracil (U) replaces Thymine (T).

  • Base stacking: Attractive forces between adjacent base pairs add stability to the double helix.

Example: Base Pairing

  • A-T base pair: 2 hydrogen bonds

  • G-C base pair: 3 hydrogen bonds

Conformations of the DNA Double Helix: A-, B-, and Z-Forms

Overview of DNA Forms

  • The DNA double helix can exist in several conformations, primarily A-DNA, B-DNA, and Z-DNA.

  • These forms differ in handedness, sugar pucker, number of base pairs per turn, and groove dimensions.

Key Features of DNA Forms

Feature

A Form

B Form

Z Form

Type of Helix

Right-handed

Right-handed

Left-handed

Sugar Pucker

C3'-endo

C2'-endo

C2'-endo (C), C3'-endo (G)

Base Pairs per Turn

11

10.4

12

Diameter (nm)

2.3

2.0

1.8

Major Groove

Narrow, deep

Wide, deep

Flat

Minor Groove

Wide, shallow

Narrow, shallow

Narrow, deep

Structural Differences

  • B-DNA: Most common form in cells; right-handed helix; C2'-endo sugar pucker.

  • A-DNA: Right-handed helix; C3'-endo sugar pucker; found in dehydrated samples and some RNA-DNA hybrids.

  • Z-DNA: Left-handed helix; occurs in regions with alternating purine-pyrimidine sequences (e.g., GC repeats); C2'-endo for C, C3'-endo for G.

Sugar Puckering

  • The sugar ring in nucleotides is not planar; it can adopt different puckered conformations (C2'-endo or C3'-endo).

  • Sugar pucker affects the overall geometry and stability of the DNA helix.

Summary

  • Nucleotides and nucleosides are essential components of nucleic acids.

  • DNA and RNA differ in their sugar and nitrogenous base composition.

  • The double helix can exist in multiple forms (A, B, Z), each with distinct structural features.

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