Transcription and RNA Processing: From DNA to Functional RNA

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Transcription and RNA Processing

Introduction: The Flow of Genetic Information

Genetic information is stored in DNA and must be accurately expressed to produce functional molecules. This process involves the regulated transcription of DNA into RNA, followed by RNA processing to yield mature, functional RNA species. The central dogma of molecular biology describes the directional flow of genetic information from DNA to RNA to protein, with some exceptions such as reverse transcription in retroviruses.

Storage of Information: DNA contains the genetic blueprint for all cellular functions.
Copying Information: DNA replication ensures accurate transmission of genetic material.
Generating Variation: Mutations introduce genetic diversity.
Using Information: Gene expression is tightly regulated to ensure proper cellular function.

Central dogma and RNA types

The Central Dogma and Its Refinements

The central dogma, first articulated by Francis Crick, states that genetic information flows from DNA to RNA (transcription) and from RNA to protein (translation). However, not all genes encode proteins; many produce functional RNAs, and some information can flow from RNA back to DNA (reverse transcription).

Transcription: DNA is transcribed into various RNA types, including mRNA, rRNA, tRNA, and regulatory RNAs.
Translation: mRNA is translated into protein by ribosomes.
Reverse Transcription: Retroviruses can convert RNA back into DNA.

Central dogma and RNA types

RNA: Structure, Chemistry, and Function

RNA Chemistry and Structure

RNA is chemically distinct from DNA, featuring a ribose sugar with a 2'-OH group and the base uracil (U) instead of thymine (T). RNA is usually single-stranded but can form complex secondary and tertiary structures through intramolecular base pairing.

Ribose Sugar: The presence of a 2'-OH group makes RNA more susceptible to hydrolysis.
Bases: RNA contains adenine (A), guanine (G), cytosine (C), and uracil (U).
Single-Stranded Nature: Allows RNA to fold into diverse structures, enabling catalytic and regulatory functions.

Purine and pyrimidine ribonucleotides

Major Types of Cellular RNA

Cells contain several classes of RNA, each with distinct roles:

mRNA (messenger RNA): Encodes protein sequences.
rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.
tRNA (transfer RNA): Adaptor molecule for translation.
snRNA (small nuclear RNA): Involved in pre-mRNA splicing.
miRNA/siRNA (micro/small interfering RNA): Regulate gene expression.
Other RNAs: Include snoRNA (rRNA processing), telomerase RNA, and more.

Gene Expression in Prokaryotes and Eukaryotes

Prokaryotic vs. Eukaryotic Gene Expression

Gene expression differs fundamentally between prokaryotes and eukaryotes, primarily due to cellular compartmentalization in eukaryotes.

Prokaryotes: Transcription and translation are coupled; both occur in the cytoplasm.
Eukaryotes: Transcription occurs in the nucleus, and RNA processing precedes export and translation in the cytoplasm.

Prokaryotic gene expression: coupled transcription and translation Eukaryotic gene expression: compartmentalized transcription and translation

Transcription: Mechanism and Regulation

Transcription Initiation in Prokaryotes

Transcription in prokaryotes begins when RNA polymerase holoenzyme binds to promoter sequences, typically at the -35 and -10 regions relative to the transcription start site (+1). The sigma (σ) factor confers promoter specificity.

Promoter Elements: Consensus sequences at -35 (TTGACA) and -10 (TATAAT) positions.
Initiation: RNA polymerase unwinds DNA and begins RNA synthesis at +1.
Elongation: After promoter escape, the core enzyme synthesizes RNA 5' to 3'.

E. coli promoter consensus sequences RNA polymerase synthesizing RNA from DNA template

Transcription Initiation in Eukaryotes

Eukaryotic transcription is more complex, involving three major RNA polymerases and multiple general transcription factors (GTFs). RNA polymerase II transcribes protein-coding genes and requires assembly of a preinitiation complex at the promoter, often involving a TATA box.

RNA Polymerase II: Main enzyme for mRNA synthesis.
General Transcription Factors: TFIID (with TBP), TFIIB, TFIIE, TFIIF, TFIIH, etc.
Promoter Escape: Phosphorylation of the C-terminal domain (CTD) of Pol II triggers transition to elongation.

Transcription Elongation and Termination

During elongation, RNA polymerase synthesizes RNA in the 5' to 3' direction, reading the DNA template 3' to 5'. Termination mechanisms differ between prokaryotes and eukaryotes.

Prokaryotes: Intrinsic (Rho-independent) and Rho-dependent termination mechanisms.
Eukaryotes: Cleavage and polyadenylation signals direct 3' end formation; RNA Pol II is released after downstream degradation of the transcript.

RNA polymerase elongation complex Transcription termination mechanisms

RNA Processing in Eukaryotes

Overview of RNA Processing

Eukaryotic pre-mRNA undergoes several processing steps before becoming mature mRNA:

5' Capping: Addition of a 7-methylguanosine cap early during transcription.
Splicing: Removal of introns and joining of exons by the spliceosome.
3' Polyadenylation: Addition of a poly(A) tail after cleavage at the polyadenylation signal.

Splicing and the Spliceosome

Splicing is catalyzed by the spliceosome, a dynamic complex of snRNAs and proteins (snRNPs). Splicing occurs via two transesterification reactions, removing introns as lariats and ligating exons.

Key Sequence Elements: 5' splice site (GU), branch-point A, 3' splice site (AG).
Major snRNPs: U1, U2, U4/U6, U5.
Alternative Splicing: Allows a single gene to produce multiple mRNA and protein isoforms.

Spliceosome assembly and splicing reactions Alternative splicing of rat α-tropomyosin

Mitochondrial Transcription

Processing of Mitochondrial Transcripts

Mitochondrial DNA is transcribed into long polycistronic RNAs, which are then processed to release individual mRNAs, rRNAs, and tRNAs. This organization reflects the compact and efficient nature of the mitochondrial genome.

Human mitochondrial DNA map and transcription

Summary Table: Major Types of Cellular RNA

RNA Type	Size (nt)	Types/Cell	Abundance	Major Function
mRNA	100s–1000s	1000s	Variable	Codes for protein
rRNA	100–5300	3	Very high	Component of ribosome
snoRNA	60–400	10–100	Low to moderate	rRNA processing
snRNA	100–300	5	Moderate	Component of spliceosome
tRNA	75–90	40–60	High	Protein synthesis
miRNA/siRNA	22	?	?	Gene regulation

Key Equations and Concepts

Direction of Synthesis: RNA is synthesized 5' to 3'; the DNA template is read 3' to 5'.
Transcription Reaction:

Splicing Reactions: Two sequential transesterification reactions join exons and remove introns as lariats.

Practice Questions

Which features distinguish transcription from replication in eukaryotes?
Label processes as exclusive to prokaryotes, eukaryotes, both, or neither (e.g., sigma factor binding, alternative splicing, capping, etc.).