Biochemical Similarities and the Unity of Life

Elemental Composition of Living Organisms

All living organisms share remarkable molecular uniformity, especially in their elemental composition and the types of biomolecules they utilize.

• Major Elements: Oxygen, hydrogen, and carbon constitute approximately 98% of the mass of living organisms.

• Carbon is the third most common element in organisms. Although silicon is more abundant in the Earth's crust, carbon-carbon (C–C) bonds are more stable than silicon-silicon (Si–Si) bonds, making carbon better suited for the complex chemistry of life.

• Biological molecules undergo combustion reactions to release energy.

• Carbon is uniquely suited for biochemical reactions due to its ability to form stable, diverse, and complex molecules.

Major Classes of Biomolecules

• Proteins: Polymers constructed from 20 different amino acids linked by peptide bonds. Proteins are the products of translation of genetic information and perform a vast array of cellular functions.

• Nucleic Acids: DNA (the genome) is transcribed into RNA. Nucleic acids store and transmit genetic information.

• Lipids: Smaller than proteins or nucleic acids, lipids have hydrophobic tails and hydrophilic heads, making them amphipathic and water-insoluble. They are essential for membrane structure and energy storage.

• Carbohydrates: Glucose is the most common simple sugar. In plants, carbohydrates are stored as starch.

The Central Dogma of Molecular Biology

Overview of Genetic Information Flow

The Central Dogma, proposed by Francis Crick, describes the flow of genetic information within a biological system.

• Replication: The process by which DNA is copied to produce identical genomes. DNA polymerase catalyzes the addition of nucleotides complementary to the parent DNA strand.

• Transcription: The process by which a segment of DNA is used as a template to synthesize RNA. RNA polymerase unwinds the DNA double helix and joins ribonucleotides to form a complementary RNA strand.

• Translation: The process by which the information in messenger RNA (mRNA) is used to synthesize proteins. Ribosomes read the mRNA sequence and assemble the corresponding amino acids into a polypeptide chain.

Central Dogma Schematic:

• DNA replication → DNA

• DNA transcription → RNA

• RNA translation → Protein

Equation (Central Dogma):

Gene Expression and RNA Types

• Gene Expression: The process by which information from a gene is used to synthesize a functional gene product (RNA or protein).

• Genome: The complete set of genetic information encoded by DNA, including all genes and non-coding sequences.

• Transcription: Always results in RNA. Some RNAs are translated into proteins, while others function directly as RNA molecules.

• Types of RNA:

  • mRNA (messenger RNA): Carries genetic information required for protein synthesis. The information in mRNA is realized during translation.

  • Functional RNAs: Act in the cell as RNA molecules and are not translated into proteins (e.g., rRNA, tRNA).

Cellular Organization and Organelles

Prokaryotic vs. Eukaryotic Cells

Cells are classified based on the presence or absence of membrane-bound organelles, particularly the nucleus.

• Prokaryotic Cells: Lack a nucleus; DNA is more exposed and susceptible to damage. Can absorb external DNA, contributing to genetic diversity and resistance.

• Eukaryotic Cells: Contain a nucleus, which serves as the information center and houses the genome. Possess various membrane-bound organelles.

Major Organelles and Their Functions

• Nucleus: Contains the genome and is the site of DNA replication and transcription.

• Chloroplast: Prominent in plant cells; site of photosynthesis.

• Mitochondria: Site of cellular respiration and energy (ATP) production.

• Endoplasmic Reticulum (ER): Involved in protein synthesis (rough ER, with ribosomes) and lipid synthesis (smooth ER).

• Golgi Apparatus: Processes and modifies proteins received from the ER.

• Secretory Granules: Membrane-bound vesicles that transport proteins to be secreted outside the cell.

• Endosomes: Involved in the uptake and sorting of materials brought into the cell.

• Lysosomes: Contain enzymes to break down proteins and other macromolecules.

• Vacuoles: Store water, ions, and nutrients, especially prominent in plant cells.

• Plasma Membrane: Regulates the entry and exit of substances; involved in nutrient uptake.

• Cell Wall (plants): Made of cellulose; provides structural support.

• Cytoplasm: Gives structure to the cell and contains the organelles.

Protein Structure and Dynamics

• Folded Proteins: Exhibit stable, high-order structures essential for function.

• Intrinsically Disordered Proteins: Lack a fixed structure and are highly dynamic, allowing for diverse interactions.

Key Chemical Bonds in Nucleic Acids

• Phosphodiester Bond: Forms the backbone of RNA and DNA, linking nucleotides together.

• RNA: Typically single-stranded.

• DNA: Double-stranded, forming a double helix.

Summary Table: Major Biomolecules and Their Functions

Example: Flow of Genetic Information

• Replication: DNA polymerase copies the genome during cell division.

• Transcription: RNA polymerase synthesizes mRNA from a DNA template.

• Translation: Ribosomes read mRNA and assemble amino acids into proteins.

Additional info: Some details about organelle functions and the distinction between prokaryotic and eukaryotic cells were expanded for clarity and completeness.