Deoxyribonucleic Acid

Nucleic Acid Structure and Eukaryotic DNA Packaging

The Basics

  • Composition of DNA: DNA consists of four bases:
    • Adenosine (A)
    • Thymine (T)
    • Guanine (G)
    • Cytosine (C)
  • RNA Differences: In RNA, uracil (U) replaces thymine (T).
  • Base Pairing Ratio:
    • Adenosine pairs with Thymine (A-T) or Uracil (A-U in RNA).
    • Guanine pairs with Cytosine (G-C).
  • Structural Forms:
    • DNA is structured as a double helix.
    • RNA typically appears as a single strand.
  • Replication Capability: Each strand can be replicated, and a complementary strand can be assembled.

Nucleic Acid Structure: The Backbone

Nucleotide Sugar-Phosphate Backbone

  • Polymers: DNA and RNA are linear polymers connected end-to-end.
  • Each nucleotide monomer contains:
    • A sugar
    • A phosphate group
    • A base
  • Basis of Genetic Information: The sequence of bases characterizes the information held within the nucleic acids.

DNA vs RNA Structure

  • Sugar Components:
    • DNA contains deoxyribose.
    • RNA contains ribose.
    • The prefix “deoxy” indicates the loss of an oxygen atom from ribose.
  • Linkage Type:
    • Sugars in nucleic acids are linked by phosphodiester bridges.
    • This sugar linkage forms the backbone of nucleic acids.
  • Backbone and Variation:
    • While the backbone is consistent in both DNA and RNA, the bases differ among nucleotides.

Base Types

  • Purine Derivatives:
    • Adenine (A)
    • Guanine (G)
  • Pyrimidine Derivatives:
    • Cytosine (C)
    • Thymine (T) (only in DNA)
    • Uracil (U) (only in RNA)

Structural Differences

  • RNA Differences from DNA:
    • Sugar units in RNA are riboses.
    • One base in RNA is uracil instead of thymine.
    • Both DNA and RNA have phosphodiester linkages that carry a negative charge.
  • Reaction Resistance:
    • The negative charge helps repel nucleophilic species, maintaining the integrity of stored information.
    • The absence of a hydroxyl group in DNA increases its resistance to hydrolysis, enhancing stability.
  • Role of Stability:
    • This greater stability of DNA is crucial because it serves as the hereditary material in cells and viruses.

Nucleotide Monomeric Units

  • Nomenclature for DNA Nucleosides:
    • Deoxyadenosine
    • Deoxyguanosine
    • Deoxycytidine
    • Thymidine
  • Nomenclature for RNA Nucleosides:
    • Adenosine
    • Guanosine
    • Cytidine
    • Uridine
  • Bond Type:
    • The linkage of the base to the sugar occurs through a β-glycosidic bond.
  • Formation of Nucleotides:
    • Nucleotides are formed by adding one or multiple phosphoryl groups.

DNA Structure

  • Directionality:
    • DNA strands are long and possess directionality.
  • Notation Method:
    • Notation can change from complex to simple forms as follows:
    • pApCpG
    • pACG
    • ACG (most commonly used)
  • Trinucleotide Representation:
    • This represents deoxyadenylate monophosphate, deoxycytidylate monophosphate, and deoxyguanylate monophosphate linked by phosphodiester linkages.
    • The “p” signifies the phosphoryl group.

Strand Ends and Directionality

  • End Orientation:
    • The 5′ end has a phosphoryl group attached to the hydroxyl group at carbon 5 of the sugar.
  • Directionality of Sequence:
    • The base sequence is read from 5′ to 3′.
  • Significance:
    • This directionality indicates that sequences such as ACG and GCA correspond to different amino acids, highlighting the basis of genetic coding, which will be discussed further.

The DNA Double Helix

Introduction to the Double Helix

  • Discovery: The double helix model was proposed by Watson and Crick, heavily utilizing X-ray imaging work conducted by Franklin and Wilkins.

Watson-Crick Model Features

  • Structural Features:
    • Two helical DNA strands coil around a common axis, forming a right-handed helix.
    • The strands run in opposite (complementary) directions.
    • The sugar-phosphate backbone is on the exterior, while bases reside inside the helix.
    • Bases are generally perpendicular to the helix axis.
    • The diameter of the helix measures roughly 20 Å.

Base Pairing and Stability

  • Base Pair Combinations:
    • A-T and C-G pairs are referred to as Watson-Crick base pairs.
    • Hydrogen bonds hold the pairs together, enhancing stability.
  • Implication of Base Sequence:
    • Knowing the sequence of one strand defines the sequence of the complementary strand, which also helps stabilize the double helix.

Stability Factors

  • Hydrophobic Effect:
    • Hydrophobic interactions between bases drive them towards the more protected interior of the helix.
  • Base Stacking:
    • Bases attract one another through van der Waals forces, contributing further to stability.
  • Genetic Information Protection:
    • The base sequence holding genetic information is shielded by being located centrally within the helix, safeguarding it.

Genetic Information Transmission

  • Transmission Process:
    • The sequence from one strand determines the sequence of the complementary strand during replication, where two identical single strands act as templates.
    • The newly synthesized DNA strands originate from each original strand, resulting in two new double helices.
    • This method is known as semiconservative replication.

Dynamics of DNA Replication and Transcription

  • Local Separation:
    • During DNA replication and transcription, a reversible, localized separation of the double helix occurs.
  • Laboratory Techniques:
    • In lab settings, DNA denaturation occurs at the melting temperature (Tm).
    • This critical process involves dissociation of strands, termed denaturation.
    • Strands can spontaneously re-associate when temperature is lowered, which is known as annealing.

Major and Minor Grooves in DNA

  • Groove Formation:
    • The distinct grooves in DNA arise because glycosidic bonds are not diametrically opposed.
  • Lining of Grooves:
    • Each groove features potential hydrogen bond donor and acceptor atoms, crucial for interactions involving proteins during replication and transcription.
  • Accessibility of Major Groove:
    • The larger major groove is more accessible for DNA-binding protein interactions compared to the minor groove.

DNA Supercoiling

Structural Changes for Compaction

  • Fitting DNA:
    • To fit compactly within a cell or nucleus, structural modifications such as twisting into a superhelix (supercoiling) take place.
  • Unwinding Requirement:
    • Circular DNA strands must be unwound to enable supercoiling.

Stability of Topological Isomers

  • Isomer Stability:
    • D is unstable due to unpaired base pairs, while E is stable with paired bases.
    • D and E are categorized as topological isomers or topoisomers, sharing the same sequence but differing in structure.

Compact Nature of Supercoiled DNA

  • Comparative Compactness:
    • Supercoiled DNA is more compact than its relaxed states.
    • Negative supercoiling naturally occurs more often than positive supercoiling.

Effects of Supercoiling

  • Functionality of Negative Supercoiling:
    • Negative supercoiling results from unwinding, enhancing ease of strand separation for DNA replication and translation.
  • Challenges of Positive Supercoiling:
    • Positive supercoiling condenses the DNA, creating weaknesses during strand separation.
  • Occurrence:
    • Circular DNA is predominantly found in prokaryotic cells, while supercoiling can be observed in linear DNA within eukaryotes.

Eukaryotic DNA

General Characteristics

  • DNA Length:
    • Each cell contains approximately 3.6 meters of DNA, which is organized into 46 chromosomes.
  • Nucleus Size:
    • The entire DNA content is contained within a nucleus measuring roughly 5 µm in diameter.
  • Supercoiling Necessity:
    • While supercoiling assists in compaction, it is insufficient for fitting the lengthy DNA within the nucleus.

Role of Specialized Proteins

  • Assistance with Compaction:
    • Specialized proteins are critical in compacting DNA while ensuring accessibility for translation processes.
  • Formation of Chromosomes:
    • This DNA-protein complex is collectively referred to as a chromosome.

DNA Packaging Structures

  • Association with Histones:
    • Eukaryotic DNA is closely associated with proteins known as histones, generating a structure referred to as chromatin.
    • Chromatin is composed of isolated chromosomes.
    • Histone Types:
    • There are five major histones involved:
      • H2A
      • H2B
      • H3
      • H4
      • H1 acts as a linker histone.

Nucleosome Formation

  • Composition of Nucleosome:
    • The four core histones form a histone octamer, defining the nucleosome's repeating unit.
  • Appearance of Nucleosome:
    • When extended, the DNA and nucleosomes appear in a “beads-on-a-string” configuration.

Evidence from DNA Digestion Studies

  • Isolation of Beads:
    • Studies involving DNA digestion with DNase isolate individual nucleosome “beads.”
    • Each bead comprises eight histones bound to approximately 200 base pairs of DNA, consisting of core and linker DNA.
    • Further digestion reveals the nucleosome core particle (H3)2(H4)2 tetramer and two H2A-H2B dimers bound to about 146 base pairs of DNA.

DNA-Wrapping Mechanism

  • Superhelix Formation:
    • DNA wraps into a left-handed superhelix around the histone octamer, forming contacts primarily along the phosphodiester backbone and mainly in the minor groove.
  • Function of H1 Histone:
    • Histone H1 seals the nucleosome where the linker DNA enters and exits.
    • H1 has a distinct structure compared to the other histones.

Overall Chromatin Structure

  • Chromatin Fiber Width:
    • Compacting linear DNA around histones results in a structure width of about 100 Å.
    • Further condensation is necessary to fit within the nucleus.
  • Higher-Order Chromatin Structures:
    • Nucleosomes arrange into 30 nm fibers, collectively referred to as higher-order chromatin.
    • The mechanisms of this higher-order structure are under investigation, but predictive models suggest they also must be left-handed.

Nucleosome Packing Model

  • Model Prediction:
    • One model suggests that nucleosomes are packed into intertwining left-handed helical stacks, with linker DNA bridging between nucleosomes.

RNA Structures

  • RNA Complexity: Despite being single-stranded, RNA can fold back on itself, forming complex, well-defined structures, which directly relate to its function, often rivaling protein complexity.

Stem-Loop Motif

  • Fundamental Structure:
    • The simplest RNA structure is the stem-loop, formed when complementary sequences within a single strand create a double-helical area.
    • These areas can include both Watson-Crick and mismatched base pairs, with mismatches being critical for functional folding.

Interactions and Stabilization

  • Interaction Types: More intricate RNA structures result from interactions between widely separated bases, wherein three or more bases can stabilize the structure.
  • Nonstandard Pairing: Nonstandard base pairing occurs through hydrogen bonding, often facilitated by metal ions that stabilize the structure.

Summary

  • Overview of Nucleic Acids: Nucleic acids contain bases linked to a sugar-phosphate backbone.
  • Double-Helix Formation: Nucleic acid strands can form double-helices, which can adopt various structural forms.
  • Eukaryotic DNA and Histones: Eukaryotic DNA complexes with histones.
  • RNA Complexity: RNA is capable of adopting complex structures relative to its function.