6.1 DNA Structure, Chromatin & Replication

Deoxyribonucleic acid (DNA) is the primary repository of genetic information in biological systems. Understanding its chemical architecture, packaging, and high-fidelity replication is essential for mastering MCAT molecular biology.

DNA Primary & Secondary Structure

Deoxyribonucleotides & Chemical Composition

DNA is a linear polymer composed of monomeric deoxyribonucleotides. Each nucleotide consists of three structural components:

1. A 2'-deoxyribose sugar (a pentose ring lacking a hydroxyl group at the 2' position, which confers chemical resistance against alkaline hydrolysis relative to RNA).
2. An inorganic phosphate group esterified to the 5'-hydroxyl ($5'-OH$) group of the sugar.
3. A nitrogen-containing nitrogenous base attached via a $\beta$-$N$-glycosidic bond to the 1' carbon of the sugar ring.

Nitrogenous bases are classified into two structural families:

• Purines: Bicyclic 9-membered double-ring molecules containing Adenine (A) and Guanine (G). Mnemonic: PURe As Gold.
• Pyrimidines: Monocyclic 6-membered single-ring molecules containing Cytosine (C), Thymine (T), and Uracil (U in RNA). Mnemonic: CUT the Py.

Phosphodiester Linkages & Strand Polarity

Individual nucleotides are covalently linked together by $5'$ to $3'$ phosphodiester bonds. The $3'-OH$ group of a preceding sugar undergoes condensation with the $5'-PO_4^{3-}$ group of the adjacent sugar, generating a repeating sugar-phosphate backbone. Polarity is strictly defined: the $5'$ end terminates with a free phosphate group (or triphosphate), while the $3'$ end terminates with a free hydroxyl group ($3'-OH$). By universal convention, nucleic acid sequences are written in the $5' \rightarrow 3'$ direction.

The Watson-Crick Antiparallel Double Helix (B-DNA)

In 1953, James Watson and Francis Crick elucidated the secondary structure of B-DNA, the predominant physiological conformation under aqueous conditions:

• Antiparallel Alignment: Two complementary polynucleotide strands run in opposite directions ($5' \rightarrow 3'$ opposite $3' \rightarrow 5'$).
• Right-Handed Helix: The duplex coils clockwise along its longitudinal axis, exhibiting a diameter of 2.0 nm, a pitch of 3.4 nm per complete $360^\circ$ turn, and $10.5$ base pairs per turn ($0.34\text{ nm}$ axial distance between adjacent base pairs).
• Major & Minor Grooves: Helical coiling creates alternating major grooves ($2.2\text{ nm}$ wide) and minor grooves ($1.2\text{ nm}$ wide). Sequence-specific DNA-binding proteins (e.g., transcription factors, zinc-finger motifs) interact primarily with exposed hydrogen-bonding groups in the wider major groove.

Alternative secondary structures include A-DNA (a wider, more compact right-handed helix with 11 bp/turn formed under dehydrated conditions or in RNA-DNA hybrids) and Z-DNA (a elongated left-handed helix with 12 bp/turn and a zig-zag phosphate backbone formed in GC-rich regions under high salt concentration; transcriptionally inactive).

Complementary Base Pairing & Thermodynamic Stability

Double-stranded DNA stability is dictated by Chargaff's Rules ($%A = %T$ and $%G = %C$, meaning $%\text{Purines} = %\text{Pyrimidines} = 50%$):

• Adenine pairs with Thymine via 2 hydrogen bonds ($A=T$).
• Guanine pairs with Cytosine via 3 hydrogen bonds ($G \equiv C$).

The melting temperature ($T_m$) is the temperature at which $50%$ of double-stranded DNA denatures into single strands. High GC-content significantly elevates $T_m$ due to two factors:

1. The extra hydrogen bond per $G \equiv C$ base pair.
2. Superior base-stacking interactions (van der Waals and $\pi-\pi$ orbital overlap between adjacent aromatic rings stacked along the core of the helix).

Additionally, high ionic strength (increased monovalent cations such as $\text{Na}^+$ or $\text{K}^+$) increases $T_m$ by shielding the negatively charged polyanionic phosphate backbones, reducing intrastrand electrostatic repulsion.

---

Chromatin Organization & Epigenetic Regulation

To accommodate approximately $2\text{ meters}$ of linear DNA within a $10\text{ }\mu\text{m}$ eukaryotic nucleus, DNA is complexed with specialized nuclear proteins to form chromatin.

Nucleosome Architecture

The nucleosome is the fundamental structural subunit of chromatin. It consists of $147\text{ base pairs}$ of double-stranded DNA wrapped $1.65\text{ turns}$ in a left-handed superhelix around a central histone octamer core consisting of two copies each of four core histones: H2A, H2B, H3, and H4 ($2 \times [\text{H2A, H2B, H3, H4}]$).

Histones are small, highly basic proteins rich in positively charged amino acid residues (Lysine and Arginine). At physiological $\text{pH } 7.4$, these positive charges form tight electrostatic ionic bonds with the negatively charged polyanionic phosphate backbone of DNA. Histone H1 (the linker histone) binds to the entry/exit DNA sites and linker DNA regions ($20\text{--}80\text{ bp}$), compacting 10-nm "beads-on-a-string" nucleosomes into a dense 30-nm chromatin fiber.

Euchromatin vs. Heterochromatin

Chromatin exists in two functional and structural states:

• Euchromatin: Lightly packed, transcriptionally active chromatin located toward the nuclear interior. Histone tails are highly acetylated by Histone Acetyltransferases (HATs). Acetylation transfers an acetyl group ($-\text{COCH}_3$) from Acetyl-CoA to the $\epsilon$-amino group of lysine residues, neutralizing their positive charge. This weakens histone-DNA electrostatic interactions, relaxing chromatin structure to allow access for RNA polymerase and transcription factors.
• Heterochromatin: Highly condensed, transcriptionally silent chromatin localized predominantly at nuclear margins, centromeres, and telomeres. Histone tails are deacetylated by Histone Deacetylases (HDACs) and methylated by Histone Methyltransferases (HMTs) (e.g., $\text{H3K9me3}$, $\text{H3K27me3}$), which recruit repressive chromodomain proteins (such as HP1) to enforce tight condensation.

---

DNA Replication Mechanism

The Semi-Conservative Model (Meselson-Stahl Experiment)

DNA replication occurs during the S phase of the eukaryotic cell cycle via a semi-conservative mechanism: each daughter DNA double helix consists of one original intact parental strand and one newly synthesized daughter strand. In 1958, Matthew Meselson and Franklin Stahl confirmed this model in E. coli using heavy isotope ($^{15}\text{N}$) and light isotope ($^{14}\text{N}$) pulse-chase labeling combined with $\text{CsCl}$ density gradient ultracentrifugation:

• Generation 0 ($^{15}\text{N}$ parent): Single heavy band ($^{15}\text{N}\text{--}^{15}\text{N}$).
• Generation 1 (after 1 round in $^{14}\text{N}$): Single intermediate-density hybrid band ($^{15}\text{N}\text{--}^{14}\text{N}$), ruling out conservative replication.
• Generation 2 (after 2 rounds in $^{14}\text{N}$): Equal amounts of light band ($^{14}\text{N}\text{--}^{14}\text{N}$) and hybrid band ($^{15}\text{N}\text{--}^{14}\text{N}$), definitively confirming semi-conservative replication.

---

Enzymatic Machinery of Replication

Replication initiates at specific genomic locus sequences termed Origins of Replication (ori). While circular prokaryotic genomes possess a single origin (oriC), linear eukaryotic chromosomes contain thousands of origins operating simultaneously to replicate large genomes within the S phase.

   5' --- [Topoisomerase] --- [Helicase] ---> [Leading Strand Synthesis 5'->3'] ---> 3'
                                  |
                                  v (Replication Fork)
   3' <--- [Okazaki Fragments] <--- [Primase/Pol δ] <--- [Lagging Strand 3'<-5'] <--- 5'

Key Replication Enzymes & Their Functions

1. DNA Helicase (DnaB in prokaryotes; MCM complex in eukaryotes): Unwinds double-stranded parental DNA at the replication fork by hydrolyzing ATP to break interstrand hydrogen bonds.
2. Single-Strand Binding Proteins (SSBs in prokaryotes; RPA in eukaryotes): Bind cooperatively to exposed single-stranded DNA to prevent re-annealing and protect against nucleolytic cleavage.
3. DNA Topoisomerase / DNA Gyrase: Relieves positive supercoiling strain and torsional tension generated ahead of the advancing replication fork. Type I topoisomerases cleave a single strand; Type II topoisomerases (e.g., bacterial DNA Gyrase) cleave both strands, pass an intact duplex through the break, and religate the ends using ATP.
   • Pharmacological Note: Fluoroquinolone antibiotics (e.g., ciprofloxacin) target bacterial DNA gyrase. Chemotherapeutic agents (e.g., etoposide, doxorubicin, irinotecan) inhibit human topoisomerases I and II, inducing lethal double-strand breaks in cancer cells.
4. Primase (DnaG in prokaryotes; Pol $\alpha$-primase in eukaryotes): An RNA polymerase that synthesizes short (~10 nucleotide) RNA primers, supplying the free $3'-OH$ group required for DNA polymerases to initiate synthesis.
5. DNA Polymerases: Catalyze $5' \rightarrow 3'$ DNA synthesis by directing nucleophilic attack of the $3'-OH$ terminus of the growing strand onto the $\alpha$-phosphate of an incoming dNTP, releasing inorganic pyrophosphate ($PP_i$). Pyrophosphate is rapidly hydrolyzed into $2 P_i$ by pyrophosphatase, driving synthesis forward ($\Delta G \ll 0$).
   • Leading Strand: Synthesized continuously in the $5' \rightarrow 3'$ direction toward the advancing replication fork.
   • Lagging Strand: Synthesized discontinuously in the $5' \rightarrow 3'$ direction away from the replication fork, producing short segments called Okazaki fragments ($1000\text{--}2000\text{ bp}$ in bacteria; $100\text{--}200\text{ bp}$ in eukaryotes).

1. DNA Ligase: Catalyzes covalent phosphodiester bond formation between the $3'-OH$ of one Okazaki fragment and the $5'-PO_4^{3-}$ of an adjacent fragment, sealing nicks in the backbone.

Telomeres & Telomerase

Because linear eukaryotic chromosomes require RNA primers for lagging strand synthesis, removal of the terminal primer leaves a single-stranded gap at the extreme 5' end that cannot be filled by DNA polymerases—a phenomenon termed the end-replication problem. Without compensating mechanisms, linear chromosomes shorten with each round of division.

Telomeres consist of non-coding tandem hexameric nucleotide repeats ($5'-TTAGGG-3'$ in humans) bound by shelterin protein complexes at chromosome ends. Telomerase is a specialized reverse transcriptase ribonucleoprotein enzyme carrying an internal RNA template (TERC). Telomerase uses its TERC template to synthesize new $5'-TTAGGG-3'$ repeats onto the single-stranded 3' overhang of the template strand, allowing primase and DNA polymerase $\delta$ to complete lagging strand synthesis.

Telomerase is highly active in germline cells, embryonic stem cells, and ~90% of human cancer cells (conferring cellular immortality). It is silent in most differentiated somatic cells, resulting in progressive telomere attrition, activation of p53 damage responses, and ultimate cellular senescence (Hayflick limit after ~50–70 divisions).