Protein Synthesis: DNA → RNA → Protein

Overview of Protein Synthesis

Protein synthesis is the process by which cells generate proteins from genetic instructions 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

Genetic Code and Codons

The genetic code is composed of nucleotide triplets called codons, each specifying a particular amino acid. Codons are read during translation to assemble proteins.

• Codon: Sequence of three nucleotides in mRNA that codes for an amino acid.

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

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

Transcription Process

Transcription occurs in the nucleus, where the enzyme RNA polymerase binds to DNA and synthesizes a complementary mRNA strand. The DNA double helix is unwound by helicase.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Exons: Coding regions of mRNA that are spliced together.

• Introns: Non-coding regions removed during RNA processing.

Translation Process

Translation converts mRNA into a sequence of amino acids, forming a protein. This occurs at the ribosome, with the help of transfer RNA (tRNA).

• Ribosome: Organelle where translation occurs; consists of large and small subunits.

• tRNA: Molecule that brings specific amino acids to the ribosome, matching its anticodon to the mRNA codon.

• Peptidyl Site (P site): Where the growing polypeptide chain is held.

• Exit Site (E site): Where tRNA exits the ribosome after delivering its amino acid.

Post-Translational Modification

After translation, polypeptides may undergo folding and chemical modifications to become functional proteins. These processes often occur in the cytosol or within organelles such as the endoplasmic reticulum.

The Cell: Structure and Function

Major Components of the Cell

Cells are composed of three main compartments: plasma membrane, cytoplasm, and nucleus. Each compartment has specialized functions essential for cell survival and activity.

• Plasma Membrane: Phospholipid bilayer that separates the cell from its environment. It is selectively permeable and contains proteins for transport, communication, and structural support.

• Cytoplasm: Gel-like substance containing water, solutes, organelles, and cytoskeletal elements.

• Nucleus: Membrane-bound organelle containing DNA and the nucleolus. Surrounded by a double membrane (nuclear envelope).

Plasma Membrane Structure and Functions

• Hydrophilic heads: Face outward toward water environments.

• Hydrophobic tails: Face inward, away from water.

• Functions:

  • Separates extracellular and intracellular spaces

  • Communication and cell identification

  • Structural support

Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, where proteins and lipids move freely within the bilayer, allowing flexibility and dynamic interactions.

Transport Across Plasma Membrane

Transport mechanisms regulate the movement of substances into and out of the cell.

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

• Osmosis: Diffusion of water across a semipermeable membrane.

• Facilitated Diffusion: Movement of molecules via membrane proteins.

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

Equation for Diffusion Rate:

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.

Membrane Potential

The difference in ion concentrations across the plasma membrane creates an electrical potential, with the inside of the cell typically negative relative to the outside.

Cytoplasmic Organelles

Membrane-Bound Organelles

• Endoplasmic Reticulum (ER): Network of membranes for protein and lipid synthesis. Rough ER has ribosomes; smooth ER does not.

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

• Lysosomes: Contain digestive enzymes for breaking down waste and cellular debris.

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

Non-Membrane Bound Organelles

• Ribosomes: Sites of protein synthesis; found free in cytoplasm or bound to ER.

• Centrosomes and Centrioles: Organize microtubules during cell division.

Mitochondria

Known as the "power plants" of the cell, mitochondria generate ATP through oxidative phosphorylation. They have a double membrane and their own DNA.

• Outer and Inner Membranes: Inner membrane has folds called cristae to increase surface area for ATP production.

• ATP Production: Occurs via the electron transport chain and citric acid cycle.

Equation for ATP Synthesis:

Cytoskeleton

The cytoskeleton provides structural support, enables cell movement, and organizes organelles.

• Actin Filaments: Thin filaments involved in cell movement and shape.

• Intermediate Filaments: Provide mechanical strength.

• Microtubules: Hollow tubes that help with cell division and intracellular transport.

Summary Table: Major Cell Organelles and Functions

Additional info:

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

• Equations and tables have been added to reinforce key concepts.