Central Dogma: Transcription and Translation in Bacteria & Eukaryotes

Central Dogma: Bacteria vs Eukaryotes

Prokaryotes

The central dogma in prokaryotes involves the processes of transcription and translation, which are tightly coupled and occur in the cytoplasm. Key regulatory elements and mechanisms differ from those in eukaryotes.

Operon and Regulon

• Operon: A cluster of genes under the control of a single promoter, transcribed together as a polycistronic mRNA. Monocistronic mRNA encodes a single protein, while polycistronic mRNA encodes multiple proteins.

• Regulon: A collection of genes or operons controlled by a common regulatory protein, allowing coordinated expression.

Transcription

• Template vs Coding Strand: The template strand is used by RNA polymerase to synthesize RNA, which is complementary to the template and identical (except for uracil) to the coding strand.

• Initiation:

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

  • Sigma Factor: Protein that helps RNA polymerase recognize promoter regions.

  • Sigma Factor Binding Sites: Specific DNA sequences recognized by sigma factors; upstream refers to sequences before the gene, downstream after.

• Elongation:

  • RNA polymerase adds ribonucleotides; uracil replaces thymine in RNA.

  • Primers are not required for RNA polymerase initiation.

• Termination:

  • Transcription ends when RNA polymerase encounters a terminator sequence; the fate of RNA polymerase varies (may dissociate or recycle).

Translation

• Codon: Three-nucleotide sequence in mRNA specifying an amino acid.

• Stop Codon: Codons (UAA, UAG, UGA) that signal termination of translation.

• Start Codon: Usually AUG, signals initiation of translation.

• Reading Frame: The way nucleotides are grouped into codons; correct frame is essential for proper protein synthesis.

Ribosome

• Prokaryotic ribosomes are 70S, composed of 50S and 30S subunits.

• Polyribosomes (polysomes) are multiple ribosomes translating a single mRNA simultaneously.

tRNA

• Anticodon: Three-nucleotide sequence on tRNA complementary to mRNA codon.

• tRNA brings specific amino acids to the ribosome during translation.

Release Factor

• Proteins that recognize stop codons and promote release of the newly synthesized polypeptide.

Initiation

• Shine-Dalgarno Sequence: Ribosome binding site in prokaryotic mRNA, located upstream of the start codon.

• Ensures correct positioning of ribosome for translation initiation.

Elongation

• Ribosome moves along mRNA, adding amino acids to the growing polypeptide chain.

• EPA sites: Exit, Peptidyl, Aminoacyl sites on the ribosome where tRNA binds during translation.

Termination

• Release factors recognize stop codons, leading to dissociation of the ribosome and release of the polypeptide.

Eukaryotes

In eukaryotes, transcription and translation are separated by the nuclear envelope. Transcription occurs in the nucleus, and translation in the cytoplasm. Additional RNA processing steps are required before translation.

DNA Transcription

• RNA Polymerases: Eukaryotes have three main RNA polymerases:

  • RNA Polymerase I: synthesizes rRNA

  • RNA Polymerase II: synthesizes mRNA

  • RNA Polymerase III: synthesizes tRNA and other small RNAs

• Transcription Factors: Proteins required for RNA polymerase binding and initiation of transcription.

• Type of mRNA Product: Eukaryotic mRNA is typically monocistronic (encodes a single protein).

RNA Processing

• 5' Cap and 3' Poly A Tail: Added to pre-mRNA to protect from degradation and assist in translation initiation.

• RNA Splicing: Removal of introns (non-coding regions) and joining of exons (coding regions). Alternative Splicing: Allows a single gene to produce multiple protein variants.

• Exons vs Introns: Exons are expressed sequences; introns are intervening sequences removed during splicing.

mRNA Translation

• 5' Cap: Facilitates ribosome binding and translation initiation.

• Kozak Sequence: Consensus sequence surrounding the start codon in eukaryotic mRNA, important for efficient translation initiation.

Comparison Table: Prokaryotic vs Eukaryotic Transcription & Translation

Feature	Prokaryotes	Eukaryotes
Location of Transcription	Cytoplasm	Nucleus
mRNA Structure	Polycistronic	Monocistronic
RNA Processing	None	5' cap, poly A tail, splicing
Translation Initiation	Shine-Dalgarno sequence	Kozak sequence
Coupling of Transcription & Translation	Yes	No
Number of RNA Polymerases	One	Three

Key Equations

• Central Dogma: $\text{DNA} \xrightarrow{\text{Transcription}} \text{RNA} \xrightarrow{\text{Translation}} \text{Protein}$

Example

• In Escherichia coli, the lac operon is a classic example of polycistronic mRNA, where multiple genes involved in lactose metabolism are transcribed together.

• In humans, alternative splicing of the beta-globin gene allows for different hemoglobin variants.