Gene Expression: From DNA to Protein – Transcription in Prokaryotes and Eukaryotes

Gene Expression: From DNA to Protein

Overview of Gene Expression

Gene expression is the process by which information encoded in DNA is used to direct the synthesis of proteins, which perform most life functions. The first major step in gene expression is transcription, where a segment of DNA is copied into messenger RNA (mRNA). This process is fundamental to all living cells and is highly regulated.

Stages of Transcription

1. Initiation

Transcription begins at a specific region of DNA called the promoter. The promoter is a DNA sequence located upstream (toward the 5' end) of the gene to be transcribed. It serves as the binding site for RNA polymerase, the enzyme responsible for synthesizing RNA.

• Promoter: A segment of DNA where RNA polymerase binds to initiate transcription.

• Upstream: Refers to the direction toward the 5' end of the DNA, before the gene sequence.

Example: In both prokaryotes and eukaryotes, the promoter is essential for the correct initiation of transcription.

2. Elongation

During elongation, RNA polymerase moves along the DNA template strand, adding complementary RNA nucleotides in the 5' to 3' direction. The growing mRNA strand is synthesized using the DNA template as a guide.

• Directionality: mRNA is always synthesized in the 5' to 3' direction.

• Template strand: The DNA strand that is read by RNA polymerase to build the mRNA.

Equation:

$\text{DNA (template strand)} \xrightarrow{\text{RNA polymerase}} \text{mRNA (5' \rightarrow 3')}$

3. Termination

Transcription ends when RNA polymerase reaches a specific DNA sequence called the terminator. At this point, the newly synthesized mRNA molecule dissociates from the DNA template and is released.

• Terminator: A DNA sequence signaling the end of transcription.

• Release: The mRNA transcript detaches from the DNA, completing the process.

Example: In prokaryotes, the mRNA is often immediately available for translation after termination.

Transcription in Prokaryotic vs. Eukaryotic Cells

Prokaryotic Cells

• Transcription and translation occur simultaneously in the cytoplasm.

• mRNA does not require processing and is immediately translated by ribosomes.

Example: In Escherichia coli, as soon as mRNA is synthesized, ribosomes begin translating it into protein.

Eukaryotic Cells

• Transcription occurs in the nucleus; translation occurs in the cytoplasm.

• mRNA undergoes processing (capping, polyadenylation, splicing) before translation.

• Transcription involves additional regulatory steps and factors.

Initiation of Transcription in Eukaryotes

Promoter Region and the TATA Box

The promoter region in eukaryotic genes is characterized by the presence of a TATA box, a conserved DNA sequence (TATAAA) found in the non-template strand. The TATA box helps position RNA polymerase II for accurate transcription initiation.

• TATA box: A DNA sequence found about 25-35 base pairs upstream of the transcription start site in many eukaryotic promoters.

Role of Transcription Factors

Transcription factors are proteins that bind to specific DNA sequences in the promoter region, including the TATA box, to help recruit RNA polymerase II. These factors are essential for the assembly of the transcription initiation complex.

• Transcription factors bind first to the TATA box.

• Additional transcription factors and RNA polymerase II then assemble to form the transcription initiation complex.

Example: The assembly of transcription factors and RNA polymerase II at the promoter is required for the accurate initiation of mRNA synthesis in human cells.

Summary Table: Key Differences in Transcription

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
mRNA Processing	None	Capping, polyadenylation, splicing
Promoter Elements	-35 and -10 regions	TATA box
Transcription Factors	Not required (RNA polymerase binds directly)	Required for initiation
Coupling with Translation	Yes (simultaneous)	No (separated by nuclear envelope)

Additional info: In eukaryotes, after transcription, the pre-mRNA undergoes several modifications before it is exported to the cytoplasm for translation. These modifications include the addition of a 5' cap, a poly-A tail, and the removal of introns via splicing.