Gene Expression II: Protein Synthesis and Sorting (Chapter 19) – Study Notes

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Gene Expression II: Protein Synthesis and Sorting

Overview of Gene Expression: RNA to Protein

Gene expression is the process by which genetic information encoded in DNA is used to direct the synthesis of proteins, the functional molecules of the cell. This chapter focuses on the translation of messenger RNA (mRNA) into polypeptides (proteins), a central event in cell biology.

Central Dogma: DNA → RNA → Protein
Translation: The process of synthesizing proteins from mRNA templates.
Polypeptide Theory: Each gene encodes a specific polypeptide.

Genetic Code and Sickle Cell Anemia

The genetic code determines how nucleotide sequences in mRNA are translated into amino acid sequences in proteins. Studies of sickle cell anemia provided key insights into the relationship between genes and proteins.

Sickle Cell Anemia: An inherited disease caused by a single amino acid change in hemoglobin.
Electrophoresis: Used to separate normal and sickle cell hemoglobin, revealing differences in migration due to altered charge.
Protease Digestion: Trypsin cleaves hemoglobin into fragments for analysis.
Key Finding: A single amino acid difference distinguishes normal from sickle cell hemoglobin.
Example: Normal sequence: Pro-Val-Glu; Sickle cell: Pro-Val-Val.

The Genetic Code Is a Triplet Code

The genetic code is composed of triplets of nucleotides, called codons, each specifying a particular amino acid.

Triplet Code: Three nucleotides (codon) encode one amino acid.
Number of Codons: 4 bases (A, U, G, C) → possible codons.
Degeneracy: Multiple codons can specify the same amino acid.
Example: Coding strand: 5'-ATGGGCT-3'; Template strand: 3'-TACCCGA-5'; mRNA: 5'-AUGGGCU-3'; Amino acids: Met-Gly-...

RNA Guides the Synthesis of Polypeptides

During translation, mRNA serves as the template for protein synthesis. The coding strand of DNA matches the mRNA sequence (except T is replaced by U), while the template strand is used for transcription.

Template Strand: Used by RNA polymerase to synthesize mRNA.
Coding Strand: Matches mRNA sequence (with U instead of T).
Uracil (U): Used in RNA in place of thymine (T).

Possible Codons in Messenger RNA and Amino Acids

Codons in mRNA specify the addition of amino acids to the growing polypeptide chain. Some codons have special functions.

Start Codon: AUG (Methionine) initiates translation.
Stop Codons: UAA, UAG, UGA terminate translation.
Unambiguous and Universal: Each codon specifies only one amino acid, and the code is nearly universal across organisms.

Codon	Amino Acid	Function
AUG	Methionine	Start
UAA	None	Stop
UAG	None	Stop
UGA	None	Stop
Other codons	Various	Amino acid addition

Translation: The Cast of Characters

Several molecules and complexes are involved in translation, each playing a specific role in protein synthesis.

Ribosomes: Molecular machines that synthesize polypeptides.
tRNA: Transfer RNA molecules align amino acids in the correct order.
Aminoacyl-tRNA Synthetases: Enzymes that attach amino acids to their corresponding tRNAs.
mRNA: Encodes the sequence information for the protein.
Initiation, Elongation, and Release Factors: Proteins that facilitate different stages of translation.

Ribosomes: Machines in Polypeptide Synthesis

Ribosomes have distinct sites for mRNA and tRNA binding, which coordinate the addition of amino acids to the growing polypeptide chain.

mRNA-binding site: Binds the mRNA template.
A (aminoacyl) site: Binds tRNA carrying the next amino acid.
P (peptidyl) site: Holds tRNA with the growing polypeptide chain.
E (exit) site: Where tRNA exits the ribosome after amino acid delivery.

Site	Function
A site	Entry of aminoacyl-tRNA
P site	Peptidyl-tRNA (growing chain)
E site	Exit of tRNA

Wobble Hypothesis and Codon-Anticodon Pairing

The wobble hypothesis explains how some tRNAs can recognize more than one codon due to flexible base pairing at the third position.

Wobble Base: The third base of the codon allows non-standard pairing.
Inosine: A modified base in tRNA that can pair with multiple codons.
Example: tRNA with anticodon 3'-GCI-5' can pair with codons 5'-GCU-3', 5'-GCC-3', or 5'-GCA-3' (all coding for alanine).

Aminoacyl-tRNA Synthetases

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

Specificity: Each synthetase is specific for one amino acid and its tRNA(s).
Reaction:

Mechanism of Translation

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

Initiation: Assembly of the ribosome on the mRNA, recognition of the start codon.
Elongation: Sequential addition of amino acids to the growing polypeptide chain.
Termination: Release of the completed polypeptide upon encountering a stop codon.

Initiation in Prokaryotes and Eukaryotes

Prokaryotes: Shine-Dalgarno sequence aligns ribosome; initiator tRNA carries N-formylmethionine (fMet).
Eukaryotes: Ribosome scans for Kozak sequence; initiator tRNA carries methionine.

Elongation

Aminoacyl-tRNA Entry: tRNA enters A site.
Peptide Bond Formation: Peptidyl transferase links amino acids.
Translocation: Ribosome moves to next codon.

Termination

Stop Codon Recognition: Release factors bind to stop codon in A site.
Polypeptide Release: Completed protein is released from ribosome.

Polyribosomes and Translation Efficiency

Multiple ribosomes can translate a single mRNA simultaneously, forming polyribosomes (polysomes) and increasing protein synthesis efficiency.

Polyribosome: Complex of several ribosomes translating the same mRNA.
Advantage: Rapid and efficient protein production.

Protein Folding and Chaperones

Newly synthesized polypeptides must fold into their correct three-dimensional shapes to become functional. Molecular chaperones assist in this process.

Chaperones: Proteins that help other proteins fold correctly.
Examples: Hsp70 and Hsp60 families.
Mechanism: Chaperone binding is often coupled to ATP hydrolysis.

Mutations and Translation

Mutations in DNA can lead to changes in mRNA and protein sequence, potentially causing disease or altered cellular function.

Missense Mutation: A change in one nucleotide that results in a different amino acid.
Effect: May cause errors in protein structure and function.
Example: Sickle cell anemia is caused by a missense mutation in the hemoglobin gene.

Summary Table: Key Components of Translation

Component	Function
mRNA	Template for protein synthesis
Ribosome	Site of polypeptide synthesis
tRNA	Brings amino acids to ribosome
Aminoacyl-tRNA synthetase	Attaches amino acid to tRNA
Initiation/Elongation/Release Factors	Facilitate translation steps
Chaperones	Assist in protein folding

Additional info: Some details, such as the full mechanism of translation initiation in eukaryotes and the role of the Kozak sequence, were inferred for completeness.