Cell Structure and Protein Synthesis

Cell Structure: Plasma Membrane and Protein Synthesis

• Cells have a plasma membrane, a selective barrier.
• Phospholipid bilayer forms the core forming a hydrophobic barrier
• Phosphate heads interact with water.
• Fatty acid tails are hydrophobic.
• Proteins and phospholipids move laterally within the membrane.
• Fluid mosaic model describes the dynamic nature.
• Membrane proteins allow selective passage of substances.

A Typical Cell Components

• Nucleus: Contains DNA, controls cell activities.
• Nucleolus: Site of ribosome production.
• Rough endoplasmic reticulum (RER): Ribosomes attached, processes proteins.
• Smooth endoplasmic reticulum (SER): Lacks ribosomes, synthesizes lipids.
• Golgi apparatus: Modifies, sorts, and packages proteins.
• Mitochondria: Powerhouse of the cell, produces ATP.
• Lysosomes: Contain enzymes, break down waste.
• Ribosomes: Responsible for protein synthesis.
• Centrioles: Involved in cell division
• Secretory vesicles: Transport molecules outside the cell.
• Microtubules: Part of the cytoskeleton, maintain cell shape.

DNA in the Nucleus

• Chromatin: DNA is packaged with proteins (histones).
• Histones: Positively charged proteins, interact with negatively charged DNA.
• Euchromatin: Active in transcription, less condensed.
• Heterochromatin: Inactive regions, highly condensed.

Genetic Code & Protein Synthesis

• DNA directs protein synthesis
• Gene: DNA sequence coding for a protein or mRNA.
• Genome: Total genetic information in a cell. Human genome ~25,000 genes.
• DNA triplets (codons) code for specific amino acids.
• Genetic code is universal.

Basic Steps of Protein Synthesis

• DNA is transcribed into mRNA in the nucleus.
• mRNA moves to the cytoplasm.
• mRNA is translated into protein by ribosomes.

Protein Synthesis: Transcription

• Transcription is DNA-directed RNA synthesis.
• mRNA is synthesized from a DNA template.
• mRNA carries the genetic code from DNA to the ribosomes.
• Steps of transcription include initiation, elongation, and termination.
• RNA polymerase binds to a promoter region in DNA.
• Elongation involves addition of complementary RNA bases to the template strand.
• Termination occurs at a stop codon.
• Pre-mRNA needs modification before translation.

Pre-mRNA modification

• Removal of introns.
• Addition of a 5' cap and a 3' poly-A tail.
• Alternative splicing.
• Splicing of introns to generate mature mRNA.

RNA Interference

• Some RNA molecules can block translation or destroy specific mRNA molecules
• Silences genes, blocking protein synthesis.

Protein Synthesis: Translation

• Ribosomes are complex particles
• mRNA binds to the ribosome.
• tRNA brings amino acids to the ribosome.
• Enzymes catalyze peptide bond formation between amino acids.
• The polypeptide chain grows until the stop codon is reached.

Ribosome

• Complex structure made of two subunits (small and large).
• Three sites for tRNA binding: A site, P site, E site.
• Ribosomes are responsible for linking amino acids together to form polypeptide chains.

Protein Synthesis: Translation Steps

• Initiation: Initiation factors, small subunit, initiator tRNA, large subunit form complex at start codon.
• Elongation: tRNA brings amino acids, peptidyl transferase links them, ribosome moves.
• Termination: Stop codon, polypeptide released.

Post-translational Modifications

• Chaperones help proteins fold correctly.
• Cleavage, addition of lipids or carbohydrates, methylation, phosphorylation.
• Protein modification can affect protein functionality and destination.

Destination of Proteins

• Proteins are targeted to specific compartments within the cell.
• Leader sequences direct protein's destination.
• Proteins ending up in cytoplasm, organelles (mitochondria, etc), or secreted from the cell.
• Proteins are transported throughout or out of the cell.
• proteins travel using transport vesicles.

Role of Rough ER in Protein Synthesis

• Ribosomes on RER synthesize proteins.
• Proteins enter RER lumen for processing.
• RER directs protein processing, folding and transport.
• Proteins can be packaged or become membrane proteins for insertion into the cellular membrane or transported outside of the cell.

Proteome

• Proteome refers to all proteins expressed by a cell or organism at a given time.
• The proteome's composition is dynamic and regulated based on a variety of factors (environmental and internal).
• Proteins can be modified
• Post-translational modifications
• Protein modification can impact protein function and fate.

Major Regulation of Protein Synthesis

• Transcriptional regulation.
• Translational regulation
• Factors affecting protein synthesis efficiency.

Regulation of Protein Synthesis

• Regulation of protein synthesis occurs at various points in the process.
• Various mechanisms used to turn genes on and off, and enhance or limit protein synthesis as needed.