7.1 Deoxyribonucleic Acid and Ribonucleic Acid Structure

4 min read•august 22, 2024

DNA and RNA are the building blocks of life, storing and transmitting genetic information. Their unique structures, composed of nucleotides with specific bases, sugars, and phosphate groups, enable them to carry out their essential functions in cells.

Understanding the differences between DNA and RNA is crucial. DNA's double-stranded helix and RNA's single-stranded nature, along with their distinct bases and sugar components, influence their roles in genetic processes and cellular activities.

DNA and RNA Nucleotide Structure

Composition of DNA and RNA Nucleotides

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  • DNA and RNA nucleotides are composed of three main components: a nitrogenous base, a pentose sugar (deoxyribose in DNA and ribose in RNA), and a phosphate group
  • The nitrogenous bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T), while in RNA, thymine is replaced by uracil (U)
  • The pentose sugar in DNA is deoxyribose, which lacks a hydroxyl group at the 2' carbon, while RNA contains ribose, which has a hydroxyl group at the 2' carbon
  • The phosphate group is attached to the 5' carbon of the pentose sugar, forming a phosphodiester bond with the 3' carbon of the adjacent nucleotide

Nitrogenous Base Pairing

  • In DNA, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C) through hydrogen bonding
    • A and T form two hydrogen bonds, while G and C form three hydrogen bonds
  • In RNA, adenine (A) pairs with uracil (U) instead of thymine, forming two hydrogen bonds
  • The specific base pairing (A with T/U and G with C) is essential for maintaining the stability and complementarity of the nucleic acid strands

DNA vs RNA Structure

Similarities between DNA and RNA

  • Both DNA and RNA are composed of nucleotides, each containing a nitrogenous base, a pentose sugar, and a phosphate group
  • DNA and RNA share three common nitrogenous bases: adenine (A), guanine (G), and cytosine (C)
  • The phosphate group is attached to the 5' carbon of the pentose sugar in both DNA and RNA, forming the backbone of the nucleic acid strand

Differences between DNA and RNA

  • DNA is typically double-stranded, forming a right-handed helix, while RNA is usually single-stranded and can form various secondary structures (hairpins and loops)
  • DNA contains the nitrogenous bases adenine, guanine, cytosine, and thymine (T), while RNA contains adenine, guanine, cytosine, and uracil (U) instead of thymine
  • The pentose sugar in DNA is deoxyribose, while RNA contains ribose, which has an additional hydroxyl group at the 2' carbon
  • DNA is more stable than RNA due to the absence of the 2' hydroxyl group in deoxyribose, which makes RNA more susceptible to hydrolysis

DNA Strand Directionality

5' and 3' Ends

  • DNA strands have a directionality, with one end designated as the 5' end and the other as the 3' end, based on the orientation of the pentose sugar
  • The 5' end has a free phosphate group attached to the 5' carbon of the pentose sugar, while the 3' end has a free hydroxyl group attached to the 3' carbon

Antiparallel Nature of DNA Strands

  • The two strands of DNA run in opposite directions, with the 5' end of one strand aligned with the 3' end of the complementary strand, forming an antiparallel structure
  • The antiparallel nature of DNA strands is essential for the formation of the double helix structure and the complementary base pairing between the strands

Directionality in DNA Replication and Transcription

  • During DNA replication and transcription, the directionality of the strands determines the direction of synthesis, with new strands being synthesized in the 5' to 3' direction
  • DNA polymerase and RNA polymerase enzymes synthesize new strands by adding nucleotides to the growing 3' end of the strand

DNA Conformations: A-DNA, B-DNA, Z-DNA

Factors Influencing DNA Conformation

  • DNA can adopt different conformations depending on the environmental conditions (hydration, salt concentration) and the sequence of the nitrogenous bases
  • The conformation of DNA can affect its interactions with proteins and other molecules, influencing processes such as DNA packaging, gene regulation, and protein-DNA interactions

B-DNA

  • The most common conformation of DNA is B-DNA, which is a right-handed double helix with about 10 base pairs per turn and a diameter of approximately 2 nm
  • B-DNA is the predominant form of DNA under physiological conditions and is the conformation typically depicted in textbooks and scientific illustrations

A-DNA

  • A-DNA is a shorter and wider right-handed double helix compared to B-DNA, with about 11 base pairs per turn and a diameter of approximately 2.6 nm
  • A-DNA is observed under dehydrating conditions or in DNA-RNA hybrids, such as those formed during transcription or in certain experimental settings

Z-DNA

  • Z-DNA is a left-handed double helix with a zigzag phosphate backbone and about 12 base pairs per turn
  • Z-DNA is favored by sequences with alternating purine-pyrimidine bases, especially alternating guanine and cytosine (GC repeats)
  • The biological significance of Z-DNA is not fully understood, but it may play a role in regulating gene expression or in facilitating certain protein-DNA interactions
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