Protein Synthesis: DNA → RNA → Protein

Overview of Protein Synthesis

Protein synthesis is the process by which cells generate proteins from genetic information encoded in DNA. This process involves two main stages: transcription and translation. Transcription produces messenger RNA (mRNA) from DNA, while translation uses mRNA to assemble amino acids into a polypeptide chain.

• Transcription: DNA → mRNA

• Translation: mRNA → Protein

Segments of DNA and Amino Acid Sequence

• Codon: A sequence of three nucleotides in mRNA that specifies a particular amino acid.

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

• Each amino acid is represented by one or more codons.

• Average gene size: ~8,000 nucleotides; average protein: ~400 amino acids.

Key Steps in Protein Synthesis

• Start Codon: AUG (codes for methionine) signals the beginning of translation.

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

• Transcription: Initiated by RNA polymerase binding to the promoter region; DNA is unwound by helicase.

• RNA Processing: Introns are removed, exons are spliced together; mature mRNA exits the nucleus.

• Translation: Ribosomes read mRNA codons, tRNA brings amino acids, and a polypeptide chain is formed.

Example: Codon-Anticodon Pairing

• mRNA codon UCC specifies the amino acid serine.

• tRNA anticodon AGG pairs with UCC.

Post-Translational Modification

• Polypeptides may be folded, cleaved, or modified after synthesis.

• Proteins destined for secretion or membrane insertion are processed in the endoplasmic reticulum and Golgi apparatus.

The Cell: Structure and Function

Major Components of the Cell

The cell consists of three main compartments: plasma membrane, cytoplasm, and nucleus. Each compartment has specialized functions essential for cellular life.

• Plasma Membrane: Composed of a phospholipid bilayer with hydrophilic heads and hydrophobic tails. Functions include separation of environments, communication, and selective permeability.

• Cytoplasm: Contains cytosol (mostly water with dissolved solutes), organelles, and cytoskeletal elements.

• Nucleus: Surrounded by a double membrane (nuclear envelope), contains DNA and controls cell activities.

Plasma Membrane Structure

• Fluid Mosaic Model: Membrane proteins and lipids are scattered in the membrane, allowing flexibility and movement.

• Functions: Receptors, enzymes, transport, structural support, and cell identification.

• Selective Permeability: Only certain substances can cross the membrane freely.

Transport Across Plasma Membrane

Cells use various mechanisms to move substances across the plasma membrane, including diffusion, osmosis, and active transport.

• Diffusion: Movement of molecules from high to low concentration. No energy required.

• Osmosis: Diffusion of water across a semipermeable membrane.

• Facilitated Diffusion: Movement of polar solutes via membrane proteins.

• Active Transport: Movement against concentration gradient; requires ATP.

Key Equations

• Osmotic Pressure:

• Membrane Potential:

Types of Solutions

• Isotonic: Equal solute concentration inside and outside the cell.

• Hypotonic: Lower solute concentration outside; cell swells.

• Hypertonic: Higher solute concentration outside; cell shrinks.

Cytoplasmic Organelles

Membrane-Bound Organelles

• Mitochondria: "Power plants" of the cell; site of ATP production. Double membrane structure with inner folds (cristae).

• Endoplasmic Reticulum (ER): Large folded membrane; rough ER has ribosomes for protein synthesis, smooth ER for lipid synthesis and detoxification.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids for secretion or delivery to other organelles.

• Lysosomes: Contain hydrolytic enzymes for digestion of cellular debris.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

Non-Membrane Bound Organelles

• Ribosomes: Sites of protein synthesis; composed of rRNA and proteins.

• Centrioles: Organize microtubules during cell division.

Cytoskeleton

• Actin Filaments: Support cell shape and movement.

• Intermediate Filaments: Provide structural stability.

• Microtubules: Facilitate intracellular transport and cell division.

Summary Table: Major Cell Organelles and Functions

Additional info:

• Some explanations and examples have been expanded for clarity and completeness.

• Equations for osmotic pressure and membrane potential are included for academic context.