Genetic Code, Transcription, and Translation: From DNA to Protein

Genetic Code and the Central Dogma

Overview of the Genetic Code

The genetic code is the set of rules by which information encoded in DNA and transcribed into mRNA is translated into proteins. It is written in linear sequences of ribonucleotide bases, which are read in groups of three called codons. Each codon specifies a particular amino acid or a stop signal during protein synthesis.

• Triplet Code: Each codon consists of three ribonucleotide bases (triplet).

• Start Codon: AUG (methionine) signals the start of translation.

• Stop Codons: UAA, UAG, UGA signal termination of translation.

• Degeneracy: Most amino acids are specified by more than one codon.

• Unambiguous: Each codon specifies only one amino acid.

Example: The codon AUG codes for methionine and also serves as the start codon.

The Central Dogma of Molecular Biology

The central dogma describes the flow of genetic information within a biological system:

• DNA stores genetic information.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into protein (amino acid sequence).

Summary Flow: DNA → RNA → Protein

Transcription: DNA to RNA

Definition and Process

Transcription is the process by which a cell makes an RNA copy of a gene. It involves several key steps:

1. DNA is used as a template.

2. RNA polymerase "reads" the DNA strand and synthesizes a complementary mRNA molecule (A pairs with U, C with G).

The result is an mRNA molecule containing the coded instructions for building a protein.

Key Enzymes and Strands

• RNA Polymerase: Synthesizes RNA from the DNA template. Does not require a primer.

• Template Strand: The DNA strand that is transcribed.

• Coding Strand: The non-transcribed DNA strand, similar in sequence to the RNA transcript (except T is replaced by U).

Promoters and Initiation

Promoters are DNA sequences where RNA polymerase binds to initiate transcription. In bacteria, common promoter elements include the -10 (TATAAT, Pribnow box) and -35 (TTGACA) regions.

• Cis-elements: DNA sequences near a gene that control its expression (do not move).

• Trans-acting factors: Proteins (such as sigma factors) that bind to cis-elements to regulate transcription.

Transcription in Prokaryotes vs. Eukaryotes

Feature	Prokaryotes	Eukaryotes
Location	Cytoplasm	Nucleus
RNA Polymerase Binding	Directly to promoter (with sigma factor)	Requires transcription factors
Promoter Elements	-10, -35 regions	TATA box
Chromatin	Not present	Must be open (euchromatin)
mRNA Processing	None	5' cap, poly-A tail, splicing

Elongation and Termination

• Elongation: RNA polymerase adds nucleotides 5' → 3' direction, using complementary base pairing (A-U, C-G).

• Termination in Prokaryotes:

  • Intrinsic termination: Formation of a hairpin structure in RNA followed by a stretch of U's causes dissociation.

  • Rho-dependent termination: Rho protein disrupts the RNA-DNA hybrid, ending transcription.

• Termination in Eukaryotes: Polyadenylation signal (AAUAAA) triggers cleavage and poly-A tail addition.

mRNA Processing (Eukaryotes Only)

After transcription, eukaryotic pre-mRNA undergoes several modifications before translation:

• 5' Cap Addition: Protects mRNA and aids ribosome binding.

• Poly-A Tail Addition: Increases stability and facilitates export.

• Splicing: Removal of non-coding introns and joining of exons, performed by the spliceosome.

In prokaryotes, mRNA is ready for translation immediately after transcription.

Translation: RNA to Protein

Overview

Translation is the process by which the sequence of codons in mRNA is converted into a sequence of amino acids, forming a polypeptide (protein). This process occurs at the ribosome and involves tRNA and rRNA.

Translation Initiation

Step	Eukaryotes	Prokaryotes
Ribosome Binding	Small subunit binds 5' cap of mRNA	Small subunit binds Shine-Dalgarno sequence
Start Codon	Scans for AUG	No scanning; binds directly
First tRNA	Methionine	Formyl-methionine (fMet)

Translation Elongation

1. New tRNA enters the A site, matching the next mRNA codon.

2. Peptide bond forms between amino acids (catalyzed by rRNA).

3. Ribosome shifts (translocation): A → P, P → E sites.

4. Chain grows one amino acid at a time.

Translation Termination

• Ribosome reaches a stop codon (UAA, UAG, UGA).

• Release factor binds, not a tRNA.

• Polypeptide is released and translation ends.

Roles of Different RNA Types

• mRNA (messenger RNA): Carries codons, template for protein synthesis, made by transcription.

• rRNA (ribosomal RNA): Makes up ribosomes, catalyzes peptide bond formation, acts as a ribozyme.

• tRNA (transfer RNA): Brings amino acids to the ribosome, matches codons via anticodon, delivers amino acids to the growing chain.

Summary Flow: Eukaryotes vs. Prokaryotes

Step	Eukaryotes	Prokaryotes
Transcription Location	Nucleus	Cytoplasm
mRNA Processing	Yes (capping, poly-A, splicing)	No
Translation Timing	After export to cytoplasm	Can begin before transcription ends
Protein Synthesis Speed	Slower, more regulated	Rapid

Key Terms and Definitions

• Codon: A sequence of three nucleotides in mRNA that specifies an amino acid or stop signal.

• Anticodon: A sequence of three nucleotides in tRNA complementary to a codon in mRNA.

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

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

• Spliceosome: Complex of RNA and protein that removes introns from pre-mRNA.

• Polyribosome: Multiple ribosomes translating a single mRNA simultaneously.

Additional info:

• In prokaryotes, transcription and translation are coupled, allowing rapid protein synthesis.

• Different sigma factors in bacteria allow for regulation of gene expression in response to environmental changes.

• Hairpin secondary structures in RNA can signal termination of transcription in prokaryotes.