Unit 3 lecture #4

Cellular Signaling and Gene Expression Fundamentals

• Nuclear versus Cytoplasmic Cellular Responses:

  • Cellular signaling pathways trigger either cytoplasmic or nuclear responses depending on the pathway targets.

  • Alterations in gene expression represent nuclear responses.

  • In a linear signal transduction pathway lacking divergence, signal transmission requires sequential modification (such as phosphorylation) of each intermediate target protein.

  • If a target protein cannot be phosphorylated or cross-correlated, signal transduction halts immediately at that step, preventing downstream nuclear responses and blocking changes in gene expression.

  • Pathways that maintain active signaling through branching or divergence can still evoke responses even if a single branch is disrupted.

• Definition and Stages of Gene Expression:

  • Gene expression is the multi-step biochemical process through which cells extract genetic instructions stored in DNA to produce functional gene products.

  • The process consists of two primary stages:

  • Transcription: The enzymatic synthesis of an RNA molecule whose sequence is complementary to a specific segment of the DNA.

  • Translation: The synthesis of a polypeptide chain (protein) based on the nucleotide sequence encoded within a messenger RNA (mRNA) molecule.

  • Deoxyribonucleic acid (DNA) functions as an exceptionally stable repository of genetic instructions necessary for building and maintaining cellular structures.

  • Ribonucleic acid (RNA) acts as an active, working copy of the genetic material that carries out cellular functions or directs protein synthesis.

Gene Classification and Genomic Composition

• Protein-Coding versus Non-Coding Genes:

  • All genes undergo transcription from DNA into RNA, but only a subset undergoes translation into protein.

  • Protein-Coding Genes:

  • Transcribed into messenger RNA (mRNA).

  • Function as the direct template for translation into polypeptides.

  • Non-Coding Genes:

  • Transcribed into non-coding RNAs (ncRNAs) that are never translated into polypeptides.

  • Functional non-coding RNA classes include:

    • Ribosomal RNA (rRNA): Serves catalytic and structural roles within the ribosome.

    • Transfer RNA (tRNA): Transports specific amino acids to the ribosome during protein synthesis.

    • Regulatory RNA: Participates in modulating gene expression and transcript stability.

• Proportional Distribution in the Human Genome:

  • Protein-coding genes comprise less than $2\%$ of the entire human genome.

  • Non-coding sequences and non-coding RNA genes constitute at least $80\%$ of the human genome.

  • These vast non-coding regions control and coordinate essential aspects of cellular function and gene regulation.

The Central Dogma and Enzymatic Machinery of Transcription

• The Central Dogma of Molecular Biology:

  • Formulates the universal directional flow of genetic information within all living systems across all domains of life (Bacteria, Archaea, Eukarya):     $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

  • Transcription represents an information transfer within the same chemical language: from nucleic acid ($5'$ to $3'$ DNA) to nucleic acid ($5'$ to $3'$ RNA).

  • Translation represents an information transfer between distinct biochemical languages: from the language of nucleic acids to the language of amino acids (proteins).

• Catalytic Function and Properties of RNA Polymerase:

  • RNA polymerase is the primary enzyme responsible for synthesizing polymers of ribonucleotides.

  • Catalyzes the formation of phosphodiester bonds linking ribonucleoside triphosphates together into an RNA chain.

  • Generates an immediate product termed the primary transcript, which is always complementary and antiparallel to the DNA template strand.

  • Differential binding affinity requirements:

  • RNA polymerase must maintain relatively low affinity for DNA to glide freely and processively along the template strand during elongation.

  • If RNA polymerase possessed high binding affinity for DNA, it would remain immobilized and fail to transcribe the gene.

  • Specialized regulatory proteins known as transcription factors possess high affinity for specific promoter sequences and recruit RNA polymerase to the DNA.

• Domain-Specific Varieties of RNA Polymerase:

  • Prokaryotes: Utilize a single multi-subunit RNA polymerase to synthesize all cellular RNA classes.

  • Eukaryotes: Utilize three distinct RNA polymerases, each dedicated to transcribing specific types of genes:

  • RNA Polymerase I (RNA Pol I): Transcribes genes encoding ribosomal RNAs (rRNAs).

  • RNA Polymerase II (RNA Pol II): Transcribes protein-coding genes into messenger RNAs (mRNAs).

  • RNA Polymerase III (RNA Pol III): Transcribes genes encoding transfer RNAs (tRNAs).

Mechanics of Template Selection and Directionality

• Chemical Directionality of Synthesis:

  • Nucleic acid synthesis occurs strictly in the $5'$ to $3'$ direction across all organisms.

  • DNA and RNA polymerases strictly require a free $3'$ hydroxyl group ($-\text{OH}$) to form a phosphodiester bond with the incoming $5'$ phosphate of the next nucleotide.

  • Because synthesized nucleic acids pair in an antiparallel orientation, RNA polymerase must read the DNA template strand in the $3'$ to $5'$ direction.

• Strand Terminology and Complementarity:

  • Coding Strand (Sense Strand / Non-Template Strand):

  • Contains the exact nucleotide sequence corresponding to the generated RNA transcript (with thymine [$\text{T}$] present in place of uracil [$\text{U}$]).

  • Oriented in the $5'$ to $3'$ direction.

  • Template Strand (Antisense Strand / Non-Coding Strand):

  • Serves as the physical template read directly by RNA polymerase.

  • Runs antiparallel ($3'$ to $5'$) to the coding strand and the nascent RNA.

  • Base Pairing Rules during RNA Synthesis:

  • Adenine ($\text{A}$) on the template pairs with Uracil ($\text{U}$) in the RNA transcript.

  • Thymine ($\text{T}$) on the template pairs with Adenine ($\text{A}$) in the RNA transcript.

  • Guanine ($\text{G}$) on the template pairs with Cytosine ($\text{C}$) in the RNA transcript.

  • Cytosine ($\text{C}$) on the template pairs with Guanine ($\text{G}$) in the RNA transcript.

  • Example sequence comparison:

  • Coding Strand: $5'\text{-ATG-ATC-}\dots-3'$

  • Template Strand: $3'\text{-TAC-TAG-}\dots-5'$

  • Resulting RNA Transcript: $5'\text{-AUG-AUC-}\dots-3'$

• Chromosomal Strand Selection:

  • Either strand of a double-stranded DNA molecule can function as the template strand for a given gene.

  • On the same chromosome, adjacent genes can utilize opposite strands as their template.

  • Strand determination is dictated entirely by the orientation and positioning of the promoter:

  • If a promoter positions RNA polymerase to transcribe toward the right, the bottom strand running $3'$ to $5'$ in that direction serves as the template.

  • If a promoter positions RNA polymerase to transcribe toward the left, the top strand running $3'$ to $5'$ in that direction serves as the template.

Architecture of the Transcription Unit and Promoter Elements

• Structure of a Transcription Unit:

  • A transcription unit is a defined stretch of DNA that contains all sequences necessary to initiate, execute, and terminate transcription, yielding an RNA transcript.

  • Core components include:

  • Promoter: DNA sequence upstream of the coding region that directs transcription factor binding and recruits RNA polymerase; the promoter sequence itself is not transcribed into the final RNA molecule.

  • RNA Coding Region: The continuous DNA segment transcribed by RNA polymerase into the RNA molecule.

  • Terminator: Specific downstream DNA sequence that directs the cessation of transcription and release of RNA polymerase.

• Positional Nomenclature (Upstream versus Downstream):

  • Transcription Start Site ($+1$): The precise nucleotide position where RNA polymerase adds the first ribonucleotide of the RNA transcript.

  • Upstream: Nucleotide positions located before the $+1$ transcription start site, designated by negative integer values ($-1$, $-10$, $-35$, etc.).

  • Downstream: Nucleotide positions located after the $+1$ transcription start site, designated by positive integer values ($+1$, $+2$, etc.).

  • All positional coordinates are referenced relative to the $5'$ to $3'$ orientation of the coding strand.

• Prokaryotic Promoter Organization:

  • Sigma Factor ($\sigma$ Factor):

  • Specialized bacterial transcription factor that binds promoter sequences with high affinity and directly recruits RNA polymerase.

  • $-35$ Box:

  • A conserved regulatory sequence situated $35$ base pairs upstream of the $+1$ site that is recognized and bound by the sigma factor.

  • $-10$ Box (Pribnow Box):

  • A conserved regulatory sequence located $10$ base pairs upstream of the $+1$ site, characterized by the consensus sequence $\text{TATAAT}$.

  • Energetics of Strand Separation:

  • Adenine-Thymine ($\text{A-T}$) base pairs are joined by $2$ hydrogen bonds.

  • Guanine-Cytosine ($\text{G-C}$) base pairs are joined by $3$ hydrogen bonds.

  • The $\text{A-T}$-rich composition of the Pribnow box requires less energy to denature, facilitating local melting and separation of the DNA duplex.

  • Unwinding at the Pribnow box positions the $+1$ nucleotide precisely within the catalytic active site of RNA polymerase.

• Eukaryotic Promoter Elements:

  • TATA Box:

  • An $\text{A-T}$-rich consensus sequence located upstream of the $+1$ start site in eukaryotic core promoters.

  • Functions analogously to the prokaryotic Pribnow box by facilitating localized duplex melting through weaker double hydrogen bonds.

  • General Transcription Factors (GTFs):

  • A collection of accessory proteins that recognize and bind eukaryotic promoter elements (including the TATA box) to assemble the transcription pre-initiation complex and recruit RNA Polymerase II.

  • Promoter-Proximal Elements:

  • Regulatory sequence elements situated near the core promoter that interact with specific regulatory proteins to stimulate or repress transcription initiation.

  • Polyadenylation Signal:

  • Downstream signal sequence in eukaryotic transcripts that directs endonuclease cleavage and transcription termination.

Comparative Gene Organization: Operons versus Monocistronic Genes

• Prokaryotic Spatial and Genomic Organization:

  • Prokaryotes lack a nuclear membrane; transcription and translation both take place within the cytosol.

  • Concurrent Processes: Translation initiates on the nascent RNA transcript while transcription is still actively occurring on the DNA template, producing rapid central dogma kinetics.

  • No post-transcriptional RNA processing is performed on the primary transcript.

  • Operon Organization:

  • Bacterial genes that participate in the same metabolic or physiological pathway are clustered into operons (e.g., the lac operon containing lacZ, lacY, and lacA).

  • A single promoter located upstream controls the transcription of the entire cluster of genes.

  • Transcription yields a single continuous polycistronic mRNA that encodes multiple distinct polypeptides.

  • Transcriptional Regulatory Sites:

  • Activator Binding Sites: Bound by transcriptional activator proteins to enhance RNA polymerase recruitment.

  • Operator Sites: Bound by repressor proteins to physically impede RNA polymerase movement and suppress transcription.

• Eukaryotic Spatial and Genomic Organization:

  • Eukaryotes maintain complete spatial and temporal separation between transcription and translation via the nuclear envelope:

  • Transcription occurs inside the nucleus.

  • Pre-mRNA processing occurs inside the nucleus.

  • Mature mRNA exits the nucleus into the cytosol via nuclear pore complexes via passive diffusion mechanisms.

  • Translation occurs exclusively within the cytosol.

  • Monocistronic Organization:

  • Eukaryotic genomes do not organize genes into operons.

  • Each individual gene is regulated by its own distinct promoter, producing monocistronic mRNA that encodes a single polypeptide.

  • Exon and Intron Composition:

  • Eukaryotic coding sequences are discontinuous, containing:

    • Exons: Nucleotide segments that contain the coding sequence for protein synthesis.

    • Introns: Non-coding intervening sequences interspersed between exons.

  • RNA Polymerase II transcribes the entire gene continuously, including all exons and introns, yielding a pre-mRNA primary transcript.

  • Pre-mRNA must undergo RNA processing (including splicing to excise introns and ligate exons) to form a mature, export-competent mRNA.

Questions & Discussion

• Classroom Examination Timing and Preparation:

  • Prompt / Concern: Comparison between examination time constraints and classroom response question limits.

  • Detail: University examinations provide an average of $1\text{ minute }25\text{ seconds}$ ($1:25$) per question. Dr. Lupe strictly enforces a $1\text{ minute}$ limit on iClicker response questions, whereas other classroom contexts permit $1\text{ minute }10\text{ seconds}$ to $1\text{ minute }15\text{ seconds}$. Training to answer questions within a strict $1\text{ minute}$ limit reserves an extra $25\text{ seconds}$ buffer on formal examinations for critical reading and evaluation of complex choices.

• Signal Transduction Pathway Inhibition:

  • Prompt / Question: Identification of why specific pathway alterations prevent changes in gene expression.

  • Detail: In an evaluated question, choice B represented the only scenario where gene expression was completely blocked. In that scenario, the target protein could not be phosphorylated or cross-correlated, terminating signal transduction. Because no pathway divergence was present to bypass the blocked target protein, downstream nuclear responses were fully prevented.

• Human Genome Non-Coding Proportions:

  • Question: What sequences comprise the remaining $20\%$ of the human genome beyond the $<2\%$ protein-coding genes and the at least $80\%$ functional elements?

  • Detail: The human genome is overwhelmingly non-coding, contradicting the historical misconception that protein-coding genes constitute the majority or roughly $50/50$ of the genome. While protein-coding sequences make up less than $2\%$ of total DNA, at least $80\%$ of the genome possesses biochemical activity and regulatory functions controlling cellular processes.

• Directional Requirements of RNA Synthesis:

  • Question: Explanation of the absolute requirement for $5'$ to $3'$ RNA synthesis.

  • Detail: Chemical elongation requires an attack by the $3'$ hydroxyl group ($-\text{OH}$) of the nascent strand on the $5'$ alpha-phosphate of an incoming ribonucleoside triphosphate. Therefore, synthesis cannot occur in the reverse direction. To satisfy antiparallel complementary base pairing, the underlying template strand must be read in the $3'$ to $5'$ orientation.

• Inclusion of Regulatory Sequences in Transcripts:

  • Question: Are promoter and terminator sequences incorporated into the transcribed RNA molecule?

  • Detail: The promoter is an upstream regulatory region located before the $+1$ transcription start site; it serves as a recognition site for transcription factors and polymerase recruitment but is not transcribed into the RNA transcript. The terminator sequence directs RNA polymerase dissociation at the end of the transcription unit.

• Polycistronic Output and Operon Expression:

  • Question: Can multiple distinct proteins be produced from a single mRNA molecule without having multiple genes encoded on the transcript?

  • Detail: Standard eukaryotic genes produce monocistronic mRNAs that yield one protein product per gene. In contrast, prokaryotic operons transcribe multiple clustered structural genes (such as lacZ, lacY, and lacA) into a single polycistronic mRNA, where each gene region retains its own translational signals to generate separate polypeptides.