Gene Expression I: Transcription – Study Notes (Becker's World of the Cell, Ch. 18)

Gene Expression I: Transcription

Introduction to Transcription

Transcription is the fundamental process by which the genetic information stored in DNA is used to synthesize RNA molecules. This process is essential for gene expression, as it enables the production of proteins and functional RNAs that carry out cellular activities.

• Transcription: The synthesis of RNA from a DNA template.

• Gene expression: The process by which information from a gene is used to synthesize a functional gene product (RNA or protein).

• Key question: How is DNA expressed to guide the production of RNA and proteins?

Key Terms

• Transcription Replication Fork: The region where DNA is unwound for transcription.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• RNA Polymerase: Enzyme that synthesizes RNA from DNA.

• Sigma Factor: Bacterial protein that aids RNA polymerase binding to promoters.

• Transcription Factor: Eukaryotic proteins required for RNA polymerase binding and initiation.

• Pribnow Box: Bacterial promoter sequence (~10 bp upstream, TATAAT).

• Rho-factor: Protein involved in terminating bacterial transcription.

• RNA Polymerase I, II, III: Eukaryotic enzymes for different RNA classes.

• RNA Splicing: Removal of introns from pre-mRNA.

• 5' Cap/Poly A tail: Modifications to eukaryotic mRNA for stability and export.

• Exon: Coding sequence retained in mature RNA.

• Intron: Non-coding sequence removed during RNA processing.

18.1 The Directional Flow of Genetic Information

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information in cells: DNA is transcribed into RNA, which is then translated into protein. This principle underlies all molecular biology.

• DNA → RNA → Protein: The standard flow of genetic information.

• RNA as a final product: Some RNAs (e.g., rRNA, tRNA) are not translated but function directly.

• Central dogma exceptions: Reverse transcription (RNA → DNA) occurs in retroviruses and via retrotransposons; some viruses replicate RNA from RNA templates.

Diagram: The central dogma can be summarized as:

• DNA replication: $DNA \rightarrow DNA$

• Transcription: $DNA \rightarrow RNA$

• Translation: $RNA \rightarrow Protein$

• Reverse transcription: $RNA \rightarrow DNA$

• RNA replication: $RNA \rightarrow RNA$ (in some viruses)

Transcription and Translation

Definitions and Differences

Transcription and translation are two distinct steps in gene expression:

• Transcription: RNA synthesis using a DNA template; the "language" remains nucleic acid.

• Translation: Protein synthesis using RNA as a template; the "language" changes from nucleotides to amino acids.

Example: The sequence of bases in mRNA determines the sequence of amino acids in a protein.

Types of RNA Involved

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes for protein synthesis.

• Ribosomal RNA (rRNA): Structural and catalytic component of ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome during translation.

Transcription and Translation in Prokaryotes vs. Eukaryotes

Compartmentalization and Coupling

While the molecular details of transcription and translation are similar in prokaryotes and eukaryotes, their cellular organization differs:

• Prokaryotes: No nuclear envelope; transcription and translation are coupled (occur simultaneously).

• Eukaryotes: Transcription occurs in the nucleus; translation occurs in the cytoplasm, separated by the nuclear envelope.

Polyribosome formation: In prokaryotes, multiple ribosomes can translate a single mRNA molecule at once, forming a polyribosome.

Reverse Transcription

RNA as a Template for DNA Synthesis

In some cases, RNA serves as a template for DNA synthesis, a process called reverse transcription.

• Reverse transcriptase: Enzyme that catalyzes RNA → DNA synthesis.

• Retroviruses: Viruses that use reverse transcription to replicate.

• Retrotransposons: Eukaryotic genetic elements that use reverse transcription without viral infection.

18.2 Mechanisms of Transcription

RNA Structure and Properties

RNA is chemically similar to DNA but has key differences:

• Ribose instead of deoxyribose.

• Uracil replaces thymine.

• Usually single-stranded.

Stages of Transcription

Transcription occurs in four main stages:

1. Binding: RNA polymerase binds to the promoter region of DNA.

2. Initiation: RNA synthesis begins; DNA is unwound and the first nucleotides are added.

3. Elongation: RNA polymerase moves along the DNA, synthesizing RNA in the 5' → 3' direction.

4. Termination: RNA polymerase releases the completed RNA and dissociates from DNA.

Bacterial Transcription: Promoters and Sigma Factor

In bacteria, transcription initiation requires specific promoter sequences and the sigma factor:

• Promoter: DNA region upstream of the transcription start site; includes the Pribnow box (~10 bp, TATAAT) and the -35 sequence (TTGACA).

• Sigma factor (σ): Subunit of RNA polymerase that recognizes promoter elements and facilitates binding.

• Upstream/Downstream: Terms describing direction relative to the transcription start site (5' is upstream, 3' is downstream).

RNA Polymerase Structure (Bacteria)

• Single type of RNA polymerase synthesizes all major RNA classes.

• Composed of two α subunits, two β subunits (β and β'), and a dissociable sigma (σ) factor.

• Holoenzyme: Complete enzyme with all subunits, required for proper initiation.

Initiation and Abortive Synthesis

• RNA polymerase initiates synthesis, often producing short abortive transcripts before successful elongation.

• Scrunching: Polymerase pulls downstream DNA into its interior during initiation.

• Once a transcript reaches ~10 nucleotides, the sigma factor is released and elongation proceeds.

Elongation and Proofreading

• RNA is synthesized in the 5' → 3' direction; each new nucleotide is added to the 3' end.

• Topoisomerases prevent supercoiling during transcription.

• RNA polymerase has weak proofreading abilities, including forward/reverse reactions and backtracking to remove incorrect nucleotides.

Equation: RNA chain elongation:

$(NTP)_n + NTP \xrightarrow{RNA\,polymerase} (NTP)_{n+1} + PP_i$

where NTP = nucleoside triphosphate, $PP_i$ = pyrophosphate.

Termination in Bacteria

• Termination occurs when RNA polymerase encounters a termination signal.

• Rho-independent termination: GC-rich hairpin loop followed by U's causes release of RNA.

• Rho-dependent termination: Rho factor binds to RNA and unwinds it from DNA, causing termination.

Summary Table: Comparison of Prokaryotic and Eukaryotic Transcription

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
RNA Polymerase Types	One	Three (I, II, III)
Promoter Elements	Pribnow box, -35 sequence	TATA box, BRE, DPE, etc.
Initiation Factors	Sigma factor	Transcription factors (TFs)
RNA Processing	Minimal	Extensive (capping, polyadenylation, splicing)

Additional info:

• Further details on eukaryotic transcription, RNA processing, and regulation are covered in subsequent slides and chapters.

• Understanding transcription is foundational for topics such as gene regulation, RNA processing, and molecular biology techniques.