Gene Expression: Transcription, Translation, and Regulation in Prokaryotes and Eukaryotes

Gene Expression Overview

Introduction

Gene expression is the process by which genetic information encoded in DNA is converted into functional products, such as proteins or RNA molecules. This process involves transcription, translation, and various regulatory mechanisms, and differs between prokaryotic and eukaryotic cells.

• Gene: A segment of DNA that encodes a functional product, typically a protein or RNA.

• Central Dogma: DNA → RNA → Protein

• Key Steps: Transcription, RNA processing (in eukaryotes), translation, and post-translational modification.

Transcription

Initiation

Transcription begins when RNA polymerase binds to the promoter region of a gene, a specific DNA sequence that signals where transcription should start.

• Promoter: Directs RNA polymerase to the correct DNA strand.

Elongation

During elongation, RNA polymerase synthesizes a complementary RNA strand using DNA as a template.

• RNA polymerase adds RNA nucleotides complementary to the DNA template.

• RNA is synthesized in the 5' to 3' direction.

• No primer is required for RNA synthesis (unlike DNA replication).

Termination

Transcription ends when RNA polymerase encounters a termination site downstream of the gene.

• In prokaryotes, termination often involves a specific DNA sequence.

• In eukaryotes, termination is linked to polyadenylation signals.

RNA Processing (Eukaryotes)

Overview

Eukaryotic pre-mRNA undergoes several modifications before becoming mature mRNA.

• 5' Cap: Addition of a modified guanine nucleotide to the 5' end.

• Poly-A Tail: Addition of a stretch of adenine nucleotides to the 3' end.

• Splicing: Removal of non-coding introns and joining of coding exons.

Translation

Ribosome Structure

Translation occurs at the ribosome, a complex of rRNA and proteins.

• Consists of a small subunit (binds mRNA) and a large subunit (catalyzes peptide bond formation).

mRNA Features

• Start codon: AUG (codes for methionine).

• Stop codons: UAA, UAG, UGA (signal termination; no amino acid added).

Codons & Genetic Code

The genetic code consists of triplet codons, each specifying one amino acid.

• Redundancy: Multiple codons can encode the same amino acid.

• No ambiguity: Each codon corresponds to only one amino acid.

Amino Acid	Example Codons
Methionine (Met)	AUG
Tryptophan (Trp)	UGG
Stop	UAA, UAG, UGA

Role of tRNA

Transfer RNA (tRNA) brings amino acids to the ribosome and matches them to the codons in mRNA via its anticodon region.

• Each tRNA is specific for one amino acid.

• tRNA anticodon pairs with mRNA codon, adding the amino acid to the growing polypeptide chain.

Polypeptide Maturation

After translation, polypeptides may undergo folding and chemical modifications to become functional proteins.

• Examples: Cleavage of signal peptides, disulfide bond formation.

Prokaryotic vs. Eukaryotic Gene Expression

Comparison Table

Feature	Prokaryotes (e.g., E. coli)	Eukaryotes (e.g., human cells)
Location of transcription	Cytoplasm (no nucleus)	Nucleus
Transcription-translation coupling	Immediate; ribosomes bind mRNA as it is transcribed	mRNA must be processed and exported from nucleus
mRNA processing	None required	5' capping, poly-A tail, splicing
Promoter complexity	Simple	More complex/regulatory
Example of regulation	Lactose operon induction by lactose	Hormone-mediated activation of insulin gene

Example: Lactose Metabolism in E. coli

Lactose metabolism in E. coli is regulated by the lac operon, which is activated in the presence of lactose.

• The bacterium imports lactose, which binds to the lac repressor and allows transcription of genes needed for lactose breakdown.

• Enzyme produced: β-galactosidase

Example: Insulin Synthesis in Pancreatic β-Cells

In humans, insulin synthesis is regulated by gene expression in pancreatic β-cells.

• Gene for insulin is transcribed in the nucleus.

• RNA polymerase initiates transcription.

• Pre-mRNA undergoes capping, poly-A tail addition, and splicing.

• Mature mRNA is exported to the cytoplasm, where ribosomes translate it into insulin protein.

• Insulin protein is processed (signal peptide removal, disulfide bond formation) to become functional.

Key Terms

Term	Definition
Gene	Segment of DNA that encodes a functional product.
Transcription	Synthesis of RNA from a DNA template by RNA polymerase.
Translation	Assembly of a polypeptide chain using mRNA codons and ribosome-mediated synthesis.
Codon	Three-nucleotide sequence in mRNA that specifies an amino acid.
Start Codon	AUG; signals the beginning of translation.
Stop Codon	UAA, UAG, UGA; signal termination of translation.
Redundancy	Multiple codons encoding the same amino acid.
tRNA	Transfer RNA that matches an amino acid to a codon via its anticodon.
RNA Processing	Modification (capping, poly-A tail, splicing) of pre-mRNA in eukaryotes.
Ribosome	Molecular machine composed of small and large subunits that synthesizes proteins.

Transcription Elongation & Polymerase Dynamics

Speed & Proofreading

RNA polymerase incorporates nucleotides rapidly and has proofreading ability to ensure accuracy.

• In eukaryotes, RNA polymerase incorporates ~60 RNA nucleotides per second.

• Proofreading removes misincorporated nucleotides before elongation proceeds.

Active-site Capacity

The polymerase catalytic site can hold multiple nucleotides, allowing rapid synthesis.

• Multiple polymerases can initiate transcription simultaneously on the same gene.

Transcription Termination

Prokaryotic Termination

• RNA polymerase encounters a termination site and releases the newly synthesized RNA.

Eukaryotic Termination & Polyadenylation Signal

• Polyadenylation signal (e.g., AAUAAA) directs cleavage of the transcript and addition of a poly-A tail.

mRNA Processing in Eukaryotes

5' Cap (GTP cap)

• Protects mRNA from degradation and assists export from nucleus.

3' Poly-A Tail

• ~200 adenine nucleotides added to the 3' end, enhancing stability and export.

Splicing: Introns vs. Exons

• Introns: Non-coding regions removed during processing.

• Exons: Coding regions retained in mature mRNA.

Prokaryotic vs. Eukaryotic Transcription & Processing

Feature	Prokaryotes	Eukaryotes
Transcription-translation coupling	Occurs simultaneously; ribosomes bind mRNA as it is transcribed	Separate processes; mRNA exported from nucleus
Termination signal	Sequence in DNA	Polyadenylation signal (AAUAAA)
mRNA processing	None required	5' cap, poly-A tail, splicing
Multiple (tandem) polymerases	Form along DNA template	Form along DNA template
RNA polymerase variety	Single type (RNA Pol)	Multiple RNA polymerases with specialized functions

Key Terminology Recap

Term	Definition
RNA polymerase	Enzyme that synthesizes RNA using DNA as a template.
Proofreading	Error-checking activity that removes misincorporated ribonucleotides.
Terminator	DNA sequence that signals the end of transcription.
Poly-adenylation signal	Motif that directs 3' end formation and poly-A tail addition.
5' cap	Modified guanine added to the 5' end of eukaryotic mRNA.
Poly-A tail	Adenine-rich stretch added to the 3' end of eukaryotic mRNA.
Spliceosome	Complex that removes introns and joins exons.
Intron	Non-coding RNA segment removed during processing.
Exon	Coding RNA segment retained in mature mRNA.

$\text{Central Dogma: DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$ $\text{Transcription:} \quad \text{DNA} \rightarrow \text{mRNA}$ $\text{Translation:} \quad \text{mRNA} \rightarrow \text{Polypeptide}$ $\text{Start Codon:} \quad \text{AUG}$ $\text{Stop Codons:} \quad \text{UAA, UAG, UGA}$