Lecture 10: DNA Replication, Telomeres, and Transcription

Conventions for Reading, Writing, and Analyzing DNA Sequences

Sequence Convention & Directionality:
- DNA sequences are standardly written in the $5' \rightarrow 3'$ direction from left to right.
- When showing double-stranded DNA, the top strand is depicted in the $5' \rightarrow 3'$ orientation, and the antiparallel complementary strand is depicted underneath in the $3' \rightarrow 5'$ orientation (e.g., top strand $5'\text{-GATCCATGGC-}3'$ , complementary strand $3'\text{-CTAGGTACCG-}5'$ ).
Consensus Sequences:
- A consensus sequence consists of the most commonly encountered nucleotides at each position based on evolutionary comparisons of conserved nucleic acid regions.
- Sequence conservation across organisms or genomic loci strongly implies functional importance.
- Consensus sequences reflect average patterns; individual functional sites are generally similar but not necessarily identical to the consensus.
Standard IUPAC Nucleic Acid Abbreviations:
- Purines ( $Pu$ or $R$ ): Adenine ( $A$ ) and Guanine ( $G$ ).
- Pyrimidines ( $Py$ or $Y$ ): Cytosine ( $C$ ) and Thymine ( $T$ ).
- Any nucleotide ( $N$ ): Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), or Thymine ( $T$ ) present with equal or unspecified frequency.
- Equally common bases: Indicated as fractional notations, such as $\text{C/G}$ , meaning Cytosine and Guanine occur with equal frequency at that specific locus.

Molecular Basis of Bacterial DNA Replication Initiation

Replicons and Origins of Replication:
- A replicon is a specific unit of DNA that undergoes replication as a discrete entity, regulated by a specific replication origin.
- Escherichia coli possesses a single circular chromosome with one origin of replication designated as $oriC$ .
- Replication from $oriC$ proceeds in both directions simultaneously, establishing bidirectional replication with two advancing replication forks.

Structural Architecture of oriC:
- The $oriC$ site spans $245\,\text{bp}$ and contains highly conserved consensus sequence arrays:
  - 13-mer Tandem Repeat Array: Contains three repeating $13\text{-mer}$ consensus sequences ( $5'\text{-GATCTATTTATTT-}3'$ ) that are exceptionally rich in Adenine-Thymine ( $\text{A-T}$ ) base pairs.
  - 9-mer Tandem Repeat Array: Contains four repeating $9\text{-mer}$ consensus sequences ( $5'\text{-TTATCCACA-}3'$ ).

Initiation Protein Cascade:
1. DnaA Binding: Multiple copies of the initiator protein DnaA bind specifically to the four $9\text{-mer}$ repeats within $oriC$ .
2. DNA Bending & Open Complex Formation: DnaA oligomerizes and bends the DNA around the protein core. This structural distortion breaks the hydrogen bonds in the adjacent $\text{A-T}$ -rich $13\text{-mer}$ sequences, unwinding the duplex to form an open complex.
3. DnaC Loading: The helicase loader protein DnaC binds to the hexameric helicase DnaB and delivers it directly to the unwound single-stranded DNA of the open complex.
4. DnaB Helicase Activity: DnaB is a $5' \rightarrow 3'$ helicase that uses ATP hydrolysis energy to break hydrogen bonds between complementary base pairs, unwinding the double helix and expanding the replication bubble in both directions.
5. Single-Stranded Binding Proteins (SSBs): SSB proteins bind selectively to exposed single-stranded template DNA to prevent re-annealing and secondary structure formation.

DNA Unwinding and Topoisomerase Function

Torsional Stress & Supercoiling:
- As helicase (DnaB) unwinds the double helix at the replication fork, positive supercoiling and torsional tension build up in the overwound DNA ahead of the fork.
Mechanism of DNA Topoisomerase (DNA Gyrase):
1. Topoisomerase binds to supercoiled DNA upstream of the replication fork.
2. The enzyme transiently cleaves phosphodiester bonds in one or both DNA strands.
3. The severed DNA strands rotate around each other to relieve torsional strain and eliminate supercoils.
4. Topoisomerase reseals (religates) the broken phosphodiester backbone.
Antibiotic Targeting:
- Bacterial topoisomerases are critical for survival and serve as the direct cellular target for fluoroquinolone antibiotics (e.g., ciprofloxacin), which block religation and trap DNA-enzyme complexes.

Leading and Lagging Strand DNA Synthesis

RNA Priming Requirement:
- All known DNA polymerases require a pre-existing $3'\text{-OH}$ group to elongate a polynucleotide chain; they cannot initiate DNA strand synthesis de novo.
- Primase (DnaG) is a specialized RNA polymerase that joins DnaA and other proteins at $oriC$ to form the primosome.
- Primase synthesizes short RNA primers ( $5' \rightarrow 3'$ ) providing the free $3'\text{-OH}$ group necessary for DNA polymerase extension.
Replisome Architecture & DNA Polymerase III:
- Daughter DNA synthesis in E. coli is carried out by the DNA Polymerase III (pol III) holoenzyme.
- A holoenzyme is a multiprotein complex containing a core enzyme associated with all necessary accessory subunits required for complete catalytic activity.
- The replisome is a macromolecular machine situated at each replication fork containing two catalytic copies of DNA polymerase III.
Semi-Discontinuous Synthesis Mechanism:
- Leading Strand: Synthesized continuously by one copy of pol III in the same direction as replication fork progression ( $5' \rightarrow 3'$ direction toward the fork).
- Lagging Strand: Synthesized discontinuously by the second copy of pol III in short segments called Okazaki fragments in the direction opposite to fork progression ( $5' \rightarrow 3'$ direction away from the fork).

Bidirectional Replication Bubble Showing Leading and Lagging Strands

Role of the Sliding Clamp:
- The sliding clamp ( $\beta\text{-clamp}$ ) is a ring-shaped homodimer with an inner diameter of approximately $35\,\text{Å}$ and an outer diameter of $75\,\text{Å}$ , encircling double-stranded DNA.
- It tethers DNA polymerase III to the template strand, preventing enzyme dissociation.
- This dramatically increases the processivity of DNA polymerase, defined as the speed/rate of synthesis (number of nucleotides added per second) without detaching from the template.

Structure and Operation of the Sliding Clamp

RNA Primer Removal and Okazaki Fragment Ligation

DNA Polymerase I (pol I) Dual Activities:
- Following Okazaki fragment synthesis by pol III, DNA Polymerase I processes the RNA primers using two distinct enzymatic activities:
  1. $5' \rightarrow 3'$ Exonuclease Activity: Cleaves ribonucleotides one by one from the $5'$ end of the RNA primer.
  2. $5' \rightarrow 3'$ Polymerase Activity: Adds deoxyribonucleotides (DNA) to the exposed $3'\text{-OH}$ end of the preceding upstream Okazaki fragment to fill the gap.

DNA Polymerase I Primer Removal and Replacement Steps

DNA Ligase Action:
- Once pol I replaces the final RNA nucleotide with a DNA nucleotide, a single-stranded nick (break in the phosphodiester backbone) remains between adjacent DNA fragments.
- DNA ligase replaces pol I at the nick site and catalyzes the formation of a phosphodiester bond between the free $3'\text{-OH}$ group and the adjacent $5'\text{-monophosphate}$ ( $\text{p}$ ) group, sealing the DNA backbone.

Summary of Bacterial Replication Proteins:

Protein / Enzyme	Gene	Essential Molecular Role
DNA Topoisomerase / Gyrase	gyrA, gyrB	Relaxes positive supercoiling ahead of the replication fork by cutting and religating DNA strands.
Helicase	dnaB	Unwinds the double helix using ATP energy to break base pair hydrogen bonds.
Helicase Loader	dnaC	Delivers and loads DnaB helicase onto unwound DNA at $oriC$ .
Single-Stranded Binding Protein (SSB)	ssb	Binds single-stranded DNA to prevent re-annealing and hairpin formation.
Primase	dnaG	Specialized RNA polymerase that synthesizes short RNA primers ( $5' \rightarrow 3'$ ).
DNA Polymerase III	dnaE, dnaN, etc.	Primary replicative enzyme; elongates leading and lagging strands continuously and discontinuously.
DNA Polymerase I	polA	Removes RNA primers ( $5' \rightarrow 3'$ exonuclease) and replaces them with DNA ( $5' \rightarrow 3'$ polymerase).
DNA Ligase	lig	Catalyzes phosphodiester bond formation to seal single-stranded backbone nicks.

Comprehensive Summary of Bacterial DNA Replication Components

DNA Proofreading Mechanisms

$3' \rightarrow 5'$ Exonuclease Proofreading:
- Replicative DNA polymerases incorporate incorrect bases at extremely low frequencies, but fidelity is further increased by intrinsic $3' \rightarrow 5'$ exonuclease proofreading activity.
- Mechanism:
  1. When a mismatched base pair is incorporated into the growing daughter strand at the catalytic polymerase site, the abnormal geometry disrupts standard base pairing.
  2. Polymerization stalls, and the $3'$ end of the newly synthesized daughter strand rotates out of the polymerase active domain and into the active $3' \rightarrow 5'$ exonuclease catalytic domain.
  3. The exonuclease cleaves the mismatched nucleotide from the $3'$ end.
  4. The corrected daughter strand rotates back into the polymerase active site to resume $5' \rightarrow 3'$ DNA synthesis.

Comparison of Bacterial and Eukaryotic DNA Replication

Shared Features:
- Both systems require topoisomerases to relieve supercoiling and helicases to unwind DNA strands.
- Both utilize DNA polymerases that synthesize strictly in the $5' \rightarrow 3'$ direction and require primers.
- Both rely on DNA ligases to join DNA fragments.
Key Differences:
- Origins of Replication: Bacterial chromosomes contain a single origin ( $oriC$ ), whereas eukaryotic chromosomes contain thousands of origins of replication across linear chromosomes.
- Polymerase Diversity: Eukaryotes utilize more than a dozen distinct DNA polymerases (e.g., pol $\alpha$ , pol $\delta$ , pol $\epsilon$ ) compared to five major types in bacteria.
- Primer Synthesis: In eukaryotes, DNA Polymerase $\alpha$ possesses primase activity and synthesizes the hybrid RNA-DNA primer.
- RNA Primer Removal: Eukaryotes utilize RNase H (along with flap endonucleases) to degrade RNA primers, rather than a single $5' \rightarrow 3'$ exonuclease activity like bacterial pol I.
- Telomere Maintenance: Linear eukaryotic chromosome ends require specialized telomerase activity to prevent chromosome shortening, a problem absent in circular bacterial DNA.

Multiple Replication Origins on Eukaryotic Chromosomes

Telomeres and Telomerase Biology

The End-Replication Problem:
- Because DNA polymerases synthesize exclusively $5' \rightarrow 3'$ and require a primer, removal of the terminal RNA primer on the lagging strand leaves a single-stranded overhang at the $5'$ end of the daughter strand.
- Without a mechanism to replicate linear chromosome ends, chromosomes progressive erode with each round of cell division.
Telomere Structure:
- Telomeres act like terminal protective end-caps ("aglets") on linear eukaryotic chromosomes.
- They consist of short, repetitive tandem sequences (e.g., human repeat sequence $5'\text{-TTAGGG-}3'$ on the G-rich strand paired with $3'\text{-AATCCC-}5'$ ).
- The single-stranded $3'$ overhang folds back to form higher-order structures called t-loops (telomeric loops) and D-loops (displacement loops) that sequester the chromosome end from DNA damage repair systems.

Human Telomeric Repeats and t-loop / D-loop Structural Model

Structure and Catalytic Function of Telomerase:
- Telomerase is a ribonucleoprotein complex comprising:
  1. TERT (Telomerase Reverse Transcriptase): The catalytic protein subunit exhibiting RNA-dependent DNA polymerase activity.
  2. TERC / TER (Telomerase RNA Component): An integral RNA molecule that provides a template sequence complementary to the telomeric repeat (e.g., $3'\text{-CAAUCCCAAUC-}5'$ ).
- Telomerase operates as an RNA-dependent DNA polymerase (reverse transcriptase), violating the classic central dogma by translating an internal RNA sequence into DNA extension.
Mechanism of Telomere Lengthening:
1. Telomerase RNA hybridizes with the exposed $3'$ overhang of the telomeric DNA.
2. TERT synthesizes new telomeric DNA sequence ( $5' \rightarrow 3'$ ) onto the $3'$ end of the chromosome using the internal RNA as template.
3. Telomerase translocates to the newly synthesized $3'$ end and repeats the elongation process (multiple rounds of telomerase activity).
4. Primase and DNA polymerase subsequently synthesize a complementary strand using the extended $3'$ overhang as a template, preserving chromosome length.

Telomerase Binding and Elongation Mechanism

Biological Significance: Cellular Lifespan, Stem Cells, and Cancer:
- Hayflick Limit: Primary human somatic cells lack active telomerase. With each cell cycle, telomeres shorten until reaching a critical threshold after $50\text{--}75$ population doublings (the Hayflick limit), triggering cell senescence or apoptosis.
- TERT Knockout Effects: Homozygous loss-of-function mutations in TERT produce developmental defects due to progressive telomere loss. Phenotypic pathology typically becomes critical by the 4th and 5th generations, inducing widespread apoptosis.
- Stem Cells & Germline: Active telomerase expression is maintained in germ-line cells and specific adult stem cell populations to preserve chromosomal integrity across generations.
- Cancer Reactivation: Abnormal reactivation of telomerase (specifically TERT reactivation) occurs in up to $85\text{--}90\%$ of all human cancers, allowing malignant cells to bypass the Hayflick limit and achieve replicative immortality.

Transcription and RNA Processing

Structural Comparison of DNA and RNA:

Structural Characteristic	DNA	RNA
Nucleotide Composition	Deoxyribonucleotides	Ribonucleotides
Sugar Type	Deoxyribose	Ribose
Presence of $2'\text{-OH}$ Group	No ( $2'\text{-H}$ )	Yes ( $2'\text{-OH}$ )
Nitrogenous Bases	Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), Thymine ( $T$ )	Adenine ( $A$ ), Guanine ( $G$ ), Cytosine ( $C$ ), Uracil ( $U$ )
Backbone Linkage	Phosphodiester bonds	Phosphodiester bonds
Strandedness	Usually double-stranded	Usually single-stranded
Secondary Structure	Double helix	Highly diverse (hairpins, loops, tertiary folds)
Chemical Stability	Chemically stable	Chemically labile / easily degraded

Structural Comparison Table Between DNA and RNA

Chemical Mechanism of Transcription:
- RNA Polymerase catalyzes transcription by adding ribonucleoside triphosphates (rNTPs: ATP, UTP, GTP, CTP) to the $3'\text{-OH}$ end of a growing RNA chain based on template strand complementarity ( $A$ pairs with $U$ , $G$ pairs with $C$ ).
- Reaction joins the $5'\text{-phosphate}$ of the incoming rNTP to the $3'\text{-OH}$ of the preceding ribonucleotide, creating a phosphodiester bond and releasing a pyrophosphate group ( $\text{PP}_i$ ).
- Unlike DNA polymerases, RNA polymerases synthesize RNA de novo without requiring a primer.

Chemical Mechanism of Transcription and Phosphodiester Bond Formation

Experimental Identification of Messenger RNA:
- Pulse-Chase Analysis: Pulse-labeling eukaryotic cells with radioactive uracil ( $^{3}\text{H-uracil}$ ) demonstrated that newly synthesized RNA forms initially in the nucleus and subsequently translocates to the cytoplasm.
- Brenner, Jacob, and Meselson Experiment: Demonstrated that E. coli infected with bacteriophage T2 produced an unstable, rapidly degraded RNA intermediate that programmed pre-existing bacterial ribosomes to produce viral proteins. This unstable intermediate was named messenger RNA (mRNA).

Functional Classes of RNA

Protein-Coding RNA:
- Messenger RNA (mRNA): Transcribed from protein-coding genes; serves as a short-lived genetic intermediate delivering encoded sequence information from nuclear DNA to cytoplasmic ribosomes for translation into polypeptide chains.
Functional (Non-Coding) RNAs:
- Transfer RNA (tRNA): Encoded by dozens of distinct genes; folds into cloverleaf/L-shaped structures responsible for binding specific amino acids and matching their anticodons to mRNA codons during translation.
- Ribosomal RNA (rRNA): Combines with ribosomal proteins to form the primary structural and catalytic components (peptidyl transferase ribozyme activity) of the ribosome.
- Small Nuclear RNA (snRNA): Complexed with proteins in the eukaryotic nucleus to form snRNPs, which process pre-mRNA by removing introns during RNA splicing.
- Micro RNA (miRNA): Endogenous small regulatory RNAs in plants and animals that regulate post-transcriptional gene expression through mRNA degradation or translational repression.
- Small Interfering RNA (siRNA): Exogenous or endogenous double-stranded small RNAs that protect plant and animal genomes by silencing viral nucleic acids and preventing transposon mobility.
- Ribozymes: Specialized RNA molecules possessing biological catalytic activity independent of protein enzymes.