Transcription and RNA Processing: The Central Dogma of Molecular Biology

Central Dogma of Molecular Biology

Overview

The Central Dogma of biology describes the flow of genetic information within a cell, from DNA to RNA to protein. This process is fundamental to gene expression and the functioning of all living organisms.

• Transcription: The process by which RNA is synthesized from a DNA template. DNA serves as the coding template for building RNA.

• Translation: The process by which proteins are synthesized using the encoded message of mRNA.

• Gene Expression: Sometimes transcription and translation together are referred to as gene expression, the process by which genetic information is used to produce functional products (proteins).

Example: DNA → RNA → Protein

• DNA Replication: DNA can be replicated; RNA can be transcribed into DNA, but the transfer of nucleic acid to protein is irreversible.

Practice Example

• According to the central dogma, the intermediate molecule involved in the flow of information in a cell is mRNA (messenger RNA).

• The full process by which genotype becomes expressed as phenotype is called gene expression.

Transcription: From DNA to RNA

Definition and Key Terms

Transcription is the process that builds an RNA molecule using DNA within a gene as the coding template. Genes are small units of DNA that encode a product, such as a protein.

• Promoter: DNA sequence where transcription begins; site of RNA polymerase attachment.

• RNA Polymerase: Enzyme that synthesizes RNA from scratch (no primer needed).

• Terminator: DNA sequence where transcription ends.

"Upstream" refers to DNA sequences in the 5' direction of transcription; "downstream" refers to DNA sequences in the 3' direction.

Practice Example

• The best definition of a gene: A segment of DNA that encodes a product (such as a protein).

• Transcription is the process that creates an RNA product from a sequence of DNA.

• A promoter is a sequence of DNA within a gene where RNA polymerase can begin transcription.

Overview of Transcription

DNA Strands and Base Pairing

DNA in a gene consists of two strands:

• Coding Strand: Has the same sequence as the RNA molecule being created (except replacing T with U).

• Template Strand: Serves as the template for RNA synthesis.

During transcription, RNA is built from 5' to 3' end by pairing free RNA nucleotides to the DNA template. Nucleotide pairing follows Watson & Crick base-pairing rules:

• A pairs with U (in RNA)

• T pairs with A

• C pairs with G

• G pairs with C

Practice Example

• The strand of DNA that has the same sequence as the RNA molecule being created is the coding strand.

• Transcription is sometimes described as a process in which RNA is "copied" from the template strand of DNA. The RNA transcript and the DNA template strand are antiparallel.

Steps of Transcription

Three Main Steps

The process of transcription consists of three steps:

1. Initiation

2. Elongation

3. Termination

1) Initiation of Transcription

Initiation begins when RNA polymerase binds to the promoter on DNA and unwinds the two DNA strands.

• In prokaryotes, RNA polymerase binds on its own.

• In eukaryotes, transcription factor proteins are required for RNA polymerase to bind to the promoter.

Unwinding of the DNA exposes the DNA template strand, which RNA polymerase uses as a guide to build RNA.

2) Elongation of Transcription

During elongation, RNA polymerase synthesizes an RNA molecule by pairing RNA nucleotides with the DNA template. RNA polymerase moves along the DNA, unwinding and building the RNA in the 5' to 3' direction.

• A single gene can be transcribed simultaneously by several RNA polymerases to make more RNA.

3) Termination of Transcription

Termination is the process that results in the end of transcription to produce an RNA molecule.

• Prokaryotes and eukaryotes differ in how they terminate transcription.

• Eukaryotic termination forms a pre-mRNA molecule requiring further modification (RNA processing).

Practice Example

• Transcription in eukaryotes requires transcription factors in addition to RNA polymerase.

• During transcription, RNA polymerase moves along the DNA in the 5' to 3' direction.

• Transcription is always initiated at a "start codon" (Additional info: In reality, transcription starts at the promoter, not a start codon; start codons are relevant for translation).

RNA Processing in Eukaryotes

Pre-mRNA and mRNA Modification

Unlike prokaryotic mRNA, eukaryotic mRNA requires further modification upon transcription termination. The initial RNA transcript is called pre-mRNA, which undergoes RNA processing and splicing to become mature mRNA ready for translation.

1) RNA Processing

Eukaryotic RNA processing involves several events that alter both ends of the pre-mRNA:

• 5' Cap Addition: A modified guanine nucleotide is added to the 5' end of the pre-mRNA. This cap stabilizes the mRNA and helps with export from the nucleus.

• Poly-A Tail Addition: A sequence of adenine nucleotides is added to the 3' end of the pre-mRNA. The poly-A tail stabilizes the mRNA and aids in translation.

Splicing: Introns (non-coding regions) are removed, and exons (coding regions) are joined together.

Practice Example

• In eukaryotic gene expression, mRNA, rRNA, and tRNA are translated.

• A cap is added to the 5' end of the mRNA; a poly-A tail is added to the 3' end.

Key Properties of RNA Polymerase

Comparison with DNA Polymerase

• RNA polymerase can initiate strand synthesis without a primer.

• RNA polymerase is dependent on a DNA sequence template.

• RNA polymerase catalyzes phosphodiester bond formation.

• RNA polymerase cannot proofread using 3' to 5' exonuclease activity (unlike DNA polymerase).

• RNA polymerase polymerizes nucleotides in a 5' to 3' direction.

Summary Table: Comparison of Transcription in Prokaryotes and Eukaryotes

Feature	Prokaryotes	Eukaryotes
RNA Polymerase Binding	Binds directly to promoter	Requires transcription factors
RNA Processing	None (mRNA is ready for translation)	Pre-mRNA requires capping, poly-A tail, and splicing
Location	Cytoplasm	Nucleus
Termination	Direct termination	Pre-mRNA formation, then processing

Key Equations and Concepts

• Base Pairing Rule:

$A \leftrightarrow T$ (in DNA) $A \leftrightarrow U$ (in RNA) $C \leftrightarrow G$

• Direction of Synthesis:

$\text{RNA synthesis: 5' \rightarrow 3'}$

• Central Dogma Equation:

$\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

Additional info:

• Start codons (AUG) are relevant for translation, not for the initiation of transcription.

• Transcription factors are proteins that help RNA polymerase bind to the promoter in eukaryotes.

• Splicing removes introns and joins exons in eukaryotic pre-mRNA.