Gene Expression and Protein Sorting: Chapters 19 & 12 Review

Gene Expression II: Protein Synthesis and Sorting

Functional Parts of Chromosomes

Chromosomes contain specific regions that are essential for their function and for the proper transmission of genetic information.

• Centromeres: Regions where sister chromatids are joined and spindle fibers attach during mitosis.

• Telomeres: Protective ends of chromosomes that prevent degradation and fusion.

• Origins of Replication: Specific sequences where DNA replication begins.

Typical Flow of Information in the Cell

The central dogma of molecular biology describes the flow of genetic information:

• DNA → RNA → Protein

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

• Translation: mRNA is translated into a polypeptide (protein).

RNA Processing Steps in Eukaryotic Cells

After transcription, eukaryotic pre-mRNA undergoes several processing steps:

• 5' Capping: Addition of a methylated guanine cap to the 5' end.

• Splicing: Removal of introns and joining of exons.

• 3' Polyadenylation: Addition of a poly(A) tail to the 3' end.

tRNA Function and Charging

tRNA (transfer RNA) brings amino acids to the ribosome during translation.

• Charging: The process of attaching an amino acid to its corresponding tRNA, catalyzed by aminoacyl-tRNA synthetases.

• Aminoacyl-tRNA synthetases: Enzymes that ensure the correct amino acid is linked to the correct tRNA.

Equation:

$\text{Amino acid} + \text{tRNA} + \text{ATP} \xrightarrow{\text{Aminoacyl-tRNA synthetase}} \text{Aminoacyl-tRNA} + \text{AMP} + \text{PP}_i$

Ribosome Structure and Function

Ribosomes are the molecular machines that synthesize proteins.

• Composed of rRNA and proteins.

• Consist of two subunits: large (60S in eukaryotes) and small (40S in eukaryotes).

• Sites: A (aminoacyl), P (peptidyl), and E (exit) sites for tRNA binding.

Translation and Protein Targeting

Proteins are directed to their correct cellular locations by signal sequences.

• Signal sequence: Short peptide at the N-terminus that directs the protein to the endoplasmic reticulum (ER).

• Signal recognition particle (SRP): Binds to the signal sequence and directs the ribosome to the ER membrane.

ER Targeting and Translocation

Proteins destined for secretion or membrane insertion are translocated into the ER.

• SRP receptor: Recognizes SRP and facilitates docking of the ribosome to the ER.

• Translocon: Channel through which the growing polypeptide enters the ER lumen.

Protein Import into Mitochondria and Chloroplasts

Proteins are imported into organelles via specialized translocase complexes.

• TOM (Translocase of the Outer Membrane): Imports proteins into mitochondria.

• TIC (Translocase of the Inner Chloroplast Membrane): Imports proteins into chloroplasts.

Signal Transfer Sequence

A signal transfer sequence is a peptide sequence that directs the transport of a protein to a specific cellular compartment.

Rough vs. Smooth Endoplasmic Reticulum (ER)

The ER is divided into two types based on structure and function:

• Rough ER: Studded with ribosomes; involved in protein synthesis and processing.

• Smooth ER: Lacks ribosomes; involved in lipid synthesis and detoxification.

Functions of the ER

• Protein synthesis (Rough ER)

• Lipid synthesis (Smooth ER)

• Calcium storage

• Detoxification of drugs and poisons

Golgi Apparatus Structure and Function

The Golgi apparatus modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Consists of flattened membrane sacs called cisternae.

• Has cis (entry), medial, and trans (exit) faces.

COP Proteins and SNAREs in Vesicle Transport

Vesicle transport between organelles is mediated by coat proteins and SNAREs.

• COPI and COPII: Coat proteins that mediate vesicle budding from the Golgi and ER.

• SNAREs: Proteins that facilitate vesicle docking and fusion with target membranes.

Protein	Function
COPI	Retrograde transport (Golgi to ER)
COPII	Anterograde transport (ER to Golgi)
SNAREs	Vesicle docking and fusion

Summary Table: Key Components in Protein Synthesis and Sorting

Component	Main Function
tRNA	Transfers amino acids during translation
Ribosome	Synthesizes proteins
SRP	Targets ribosome to ER
Golgi Apparatus	Modifies and sorts proteins
COP Proteins	Mediate vesicle transport
SNAREs	Facilitate vesicle fusion

Example: Insulin is synthesized in the rough ER, processed in the Golgi apparatus, and secreted via vesicles using COPII and SNARE proteins.

Additional info: Some details, such as the specific steps of protein targeting and the roles of translocase complexes, were expanded for clarity and completeness.