Gene Expression: Protein Synthesis (Translation) – Chapter 19 Study Notes

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Gene Expression: II. Protein Synthesis

Overview of Protein Synthesis

Protein synthesis is a fundamental process in cell biology, converting genetic information encoded in mRNA into functional polypeptides. This process, known as translation, involves several key molecular players and occurs differently in prokaryotic and eukaryotic cells.

For some genes, the RNA transcript is the final product (e.g., rRNA, tRNA).
For most genes, the ultimate product is a protein.
mRNAs encode instructions for translation, assembling amino acids into a polypeptide.

Transcription and Translation: Cellular Locations

Transcription and translation are spatially separated in eukaryotes but coupled in prokaryotes.

Prokaryotes: Both processes occur in the cytoplasm; translation can begin before transcription is complete.
Eukaryotes: Transcription occurs in the nucleus; mRNA is processed and exported to the cytoplasm for translation.

Translation: The Cast of Characters

Ribosomes: Complexes of rRNA and protein that synthesize polypeptides.
tRNA molecules: Adaptors that align amino acids in the correct order by matching anticodons to mRNA codons.
Aminoacyl-tRNA synthetases: Enzymes that attach amino acids to their appropriate tRNA molecules.
mRNA: Encodes the amino acid sequence information.

Ribosomes

Structure and Function

Ribosomes are the molecular machines of translation, composed of dissociable subunits.

Made of: rRNA and protein.
Location in eukaryotes: Free in cytoplasm, bound to endoplasmic reticulum (ER), or outer nuclear envelope.
Subunits: Large and small subunits (e.g., 60S and 40S in eukaryotes; 50S and 30S in prokaryotes).

Table: Eukaryotic Ribosome Subunits

Subunit	rRNA Components	Proteins	Molecular Weight
60S (Large)	28S, 5.8S, 5S	~49	~2,800,000
40S (Small)	18S	~33	~1,400,000
80S (Complete)	All above	~82	~4,200,000

Functional Sites of Ribosomes

mRNA-binding site: Binds mRNA for translation.
A (aminoacyl) site: Entry point for aminoacyl-tRNA.
P (peptidyl) site: Holds tRNA with growing polypeptide chain.
E (exit) site: Releases empty tRNA after amino acid transfer.

Transfer RNA (tRNA)

Structure and Function

tRNA molecules are adaptors that bring specific amino acids to the ribosome, matching them to codons in mRNA.

Each tRNA binds a specific amino acid and recognizes specific mRNA codons via its anticodon.
Aminoacyl tRNA: tRNA attached to an amino acid (charged tRNA).
Bond type: Ester bond links amino acid to tRNA.
Naming: tRNAs are named for their attached amino acid (e.g., tRNAAla).

tRNA Structure

Secondary structure: Cloverleaf with acceptor stem, anticodon loop, D loop, TψC loop.
Tertiary structure: L-shaped 3D conformation.

Codon Recognition and Wobble Hypothesis

Anticodon: Region of tRNA that base-pairs with mRNA codon.
Wobble hypothesis: Flexibility in base pairing at the third position of the codon allows some tRNAs to recognize multiple codons.
Inosine (I): Modified nucleotide in tRNA anticodon that can pair with U, C, or A in mRNA.

Table: Wobble Base Pairing

Base in Anticodon	Bases Recognized in Codon (3rd position)
A	U
C	G
G	C or U
U	A or G
I (Inosine)	U, C, or A

Aminoacyl-tRNA Synthetases

Function and Mechanism

Aminoacyl-tRNA synthetases are enzymes that attach the correct amino acid to its corresponding tRNA, ensuring fidelity in translation.

20 different synthetases: One for each amino acid.
Special synthetases: For nontraditional amino acids.
Reaction: Catalyze amino acid attachment via ester bond, using ATP hydrolysis.

Equation:

Proofreading: Synthetases check the final product for accuracy.
Recognition: Both the anticodon and 3' end of tRNA are needed for correct amino acid attachment.

Messenger RNA (mRNA)

Structure and Coding Information

Codon sequence: Directs the order of amino acids in the polypeptide.
Export: mRNA must be exported from nucleus to cytoplasm in eukaryotes.
Start codon: Usually AUG (methionine).
Stop codons: UAG, UAA, UGA.
Untranslated regions (UTRs): 5' and 3' UTRs are essential for mRNA function and regulation.
5' cap and 3' poly(A) tail: Important for stability and translation initiation in eukaryotes.

Table: mRNA Features

Feature	Prokaryotic mRNA	Eukaryotic mRNA
5' End	Ribosome-binding site	5' Cap
Start Codon	AUG	AUG
Stop Codon	UAG, UAA, UGA	UAG, UAA, UGA
3' End	Untranslated region	Poly(A) tail

Monocistronic vs. Polycistronic mRNAs

Monocistronic: Most eukaryotic mRNAs encode a single polypeptide.
Polycistronic: Some bacterial and archaeal mRNAs encode multiple polypeptides (operons).

The Mechanism of Translation

Stages of Translation

Translation proceeds in three main stages: initiation, elongation, and termination.

Initiation: Assembly of ribosome, mRNA, and initiator tRNA at the start codon.
Elongation: Sequential addition of amino acids to the growing polypeptide chain.
Termination: Release of the completed polypeptide upon reaching a stop codon.

Initiation in Eukaryotes

Start codon: Specifies methionine.
Initiation factors: eIFs (eukaryotic initiation factors) bind to the 5' cap and facilitate assembly.
43S preinitiation complex: Includes small ribosomal subunit, initiator tRNA, and initiation factors.
Kozak sequence: Consensus sequence (e.g., ACCAUGG) around start codon enhances recognition.
GTP hydrolysis: Facilitates joining of large ribosomal subunit.
Poly(A)-binding protein (PABP): Binds poly(A) tail and interacts with eIF4G for efficient initiation.

Elongation

Step 1: Binding of aminoacyl-tRNA to the A site.
Step 2: Peptide bond formation between amino acids at the A and P sites.
Step 3: Translocation of mRNA and tRNA, moving the ribosome along the mRNA.

Termination

Stop codon: Recognized by release factors, not tRNA.
Release factors: Bind to stop codon, triggering release of the completed polypeptide.

Genetic Code Table

Table: Standard Genetic Code

First Base	Second Base	Third Base	Amino Acid
U	U	U	Phe
U	U	C	Phe
U	U	A	Leu
U	U	G	Leu
...	...	...	...
U	A	A	Stop
U	A	G	Stop
U	G	A	Stop
A	U	G	Met (Start)

Additional info: Full genetic code tables are available in textbooks and online resources.

Summary

Translation converts mRNA information into polypeptides using ribosomes, tRNA, and aminoacyl-tRNA synthetases.
mRNA structure and regulatory elements are essential for proper translation.
Translation is a highly regulated, energy-intensive process.