Understanding Prions and Protein Misfolding Diseases

The central dogma of molecular biology describes how genetic information flows from DNA to RNA to protein. In the nucleus, transcription uses DNA as a template to synthesize mRNA, with RNA polymerase adding complementary base pairs (A-T, G-C). The resulting mRNA carries genetic information to the cytoplasm, where ribosomes read it in triplet codons (three-nucleotide sequences) to synthesize proteins. Each codon specifies one amino acid, enabling the translation of genetic instructions into functional proteins.

The central dogma describes how genetic information flows from DNA to RNA to protein. The cell nucleus transcribes DNA into messenger RNA (mRNA), which carries the genetic message to ribosomes. Ribosomes then translate the mRNA sequence into proteins by reading codons (triplets of nucleotides) and assembling corresponding amino acids. This process, called protein synthesis, repeats cyclically in every cell to produce the proteins necessary for cellular functions.

The central dogma describes how genetic information flows from DNA to RNA to proteins. DNA contains instructions for building proteins, which perform cellular functions. DNA replication copies the entire genome during cell division, while transcription copies only specific genes into RNA. The information flow is unidirectional: DNA to RNA to proteins, never reversing. Three RNA types facilitate protein synthesis: mRNA carries genetic code, rRNA forms ribosome structure, and tRNA delivers amino acids. The genetic code is read in triplets (codons), with 64 possible codons specifying 20 amino acids. The code is redundant (multiple codons per amino acid) and universal across all organisms. Translation occurs in three phases: initiation (ribosome assembles on mRNA), elongation (amino acids are added sequentially), and termination (stop codons signal completion).

The central dogma of molecular biology states that DNA serves as the template for RNA synthesis (transcription), and RNA serves as the template for protein synthesis (translation). DNA stores genetic information in genes, which direct the production of all cellular RNAs (messenger, transport, and ribosomal). These RNAs then direct protein synthesis in the cytoplasm. Without DNA, cells cannot produce the proteins necessary for life. This flow of genetic information from DNA to RNA to protein is fundamental to all living organisms.

The central dogma describes the flow of genetic information from DNA to RNA to protein. DNA contains genes that encode the instructions for building proteins. During transcription, RNA polymerase reads the DNA template and synthesizes messenger RNA (mRNA). The mRNA then travels to ribosomes where translation occurs, with the ribosome reading the mRNA sequence and assembling amino acids into polypeptide chains. Stop codons signal the end of translation. Newly synthesized proteins are transported to the endoplasmic reticulum via signal recognition particles for further processing.

Infectious agents are classified into four major categories. Bacteria include gram-positive cocci (Streptococcus pneumoniae, Staphylococcus aureus, Enterococcus), gram-negative bacilli (Enterobacteriaceae, Pseudomonas), and anaerobes (Clostridium, Bacteroides). Fungi are classified as yeast-like (Candida, Cryptococcus), filamentous (Aspergillus, Mucorales), and dimorphic (Histoplasma, Blastomyces). Viruses are classified by genetic material (DNA/RNA) and include hepatitis B, HPV, herpes viruses, HIV, influenza, and coronaviruses. Parasites include protozoa (intestinal parasites) and helminths (cestodes, trematodes). Understanding these classifications is essential for selecting appropriate antimicrobial therapy.

Infectious agents (pathogens) are disease-causing organisms that enter a host's body and multiply, causing infection. The main categories include: (1) Bacteria - well-known pathogens, (2) Viruses - consisting of a protein coat (capsid) and nucleic acid (DNA or RNA, but never both), (3) Viroids - infectious RNA particles without protein coating that cause diseases like potato spindle tuber disease, (4) Prions - misfolded proteins that cause diseases like Creutzfeldt-Jakob disease, (5) Protozoans - including flagellated and sporozoan types (e.g., Plasmodium causing malaria), and (6) Fungi - which can also act as infectious agents. Archaea are generally non-pathogenic except in rare opportunistic infections in immunocompromised hosts.

Microbiology studies microorganisms—small living organisms invisible to the naked eye. They are classified into four categories: bacteria (unicellular prokaryotes), viruses (non-cellular infectious agents), fungi (yeasts and molds), and parasites (protozoa and helminths). Cells are divided into prokaryotic (no true nucleus, DNA free-floating) and eukaryotic (true nucleus with membrane-bound organelles). Bacteria are classified by shape: cocci (spherical), bacilli (rod-shaped), and spirilla (spiral). Normal flora are beneficial bacteria performing vitamin synthesis, immune activation, and pathogen defense. Viruses require host cells to replicate, consisting of genetic material surrounded by a protein coat. Fungi are eukaryotic organisms classified as yeasts (unicellular, budding) or molds (multicellular, filamentous). Parasites include protozoa (single-celled) and helminths (multicellular worms).

Virus classification based on nucleic acid and replication: (1) dsDNA viruses - double-stranded DNA, (2) ssDNA viruses - single-stranded DNA, (3) dsRNA viruses - double-stranded RNA, (4) (+)ssRNA viruses - positive-sense single-stranded RNA, (5) (-)ssRNA viruses - negative-sense single-stranded RNA, (6) (+)ssRNA viruses with reverse transcriptase - like retroviruses, and (7) dsDNA viruses with reverse transcriptase - like hepadnaviruses. Each group has specific examples that are important to memorize for clinical relevance.

Viruses are classified into DNA and RNA viruses based on their nucleic acid type. DNA viruses are classified into six families: Poxviridae (largest DNA virus), Herpesviridae, Adenoviridae, Papillomaviridae, Polyomaviridae, and Parvoviridae (smallest DNA virus). Most DNA viruses have double-stranded DNA (dsDNA), with Parvovirus being the exception having single-stranded DNA (ssDNA). RNA viruses are classified into multiple families including Coronaviridae (contains SARS-CoV-2), Flaviviridae, Orthomyxoviridae (influenza), Paramyxoviridae, Rhabdoviridae, and Reoviridae. Most RNA viruses have single-stranded RNA (ssRNA), with Reovirus being the exception having double-stranded RNA (dsRNA). Some RNA viruses have segmented nucleic acid: Influenza virus has 8 segments, Reovirus has 10-12 segments, and Arenavirus has 2 segments.

Proteostasis is the cellular mechanism that maintains the proper folding and function of proteins, involving chaperone proteins that assist in folding nascent polypeptides into their native three-dimensional structures, and degradation pathways (ubiquitin-proteasome and autophagy) that eliminate misfolded proteins; when proteins misfold, they become unstable with exposed hydrophobic residues that cause them to aggregate either as amorphous aggregates or amyloid fibrils, leading to various diseases including lysosomal storage disorders (loss-of-function), amyloidosis, Alzheimer's disease, type 2 diabetes, prion diseases, and serpinopathies (gain-of-function).

Chaperones are proteins that assist in the proper folding of other proteins. They help proteins achieve their correct three-dimensional structure and can also unfold misfolded proteins. Chaperones are present in all cells and are essential for maintaining protein homeostasis. They are classified into five families based on molecular weight: Hsp60, Hsp70, Hsp90, Hsp104, and small Hsps. Chaperones do not participate in the final product but facilitate the folding process. This function is essential for maintaining cellular protein homeostasis and preventing the accumulation of misfolded proteins.

Proteins fold into their native functional state, which is energetically favorable (low energy). Misfolded proteins tend to spontaneously fall into aggregation-prone states (orange regions on energy landscape), which are extremely stable and difficult to reverse. Molecular chaperones prevent proteins from entering these misfolded states and target misfolded proteins to the proteasome for degradation, maintaining protein homeostasis (proteostasis).

This video explains the role of chaperones in protein folding and proteostasis. It identifies chaperones as molecular agents that assist in the proper folding of proteins, preventing misfolding and aggregation. The video specifically mentions Hsp70 and GroEL as key examples of molecular chaperones involved in this process. It states that chaperones contribute to proteostasis—the maintenance of a functional proteome—by ensuring newly synthesized or stress-denatured proteins achieve their correct three-dimensional structures. The function of chaperones is presented as essential for cellular health, as improper folding can lead to nonfunctional or toxic protein aggregates. The video frames chaperones as critical components in the cellular machinery that supports protein homeostasis, particularly under conditions of cellular stress or during protein synthesis. It does not elaborate on mechanisms such as ATP dependence, co-chaperone interactions, or structural details of Hsp70 or GroEL complexes. The focus remains on the general biological purpose of chaperones in facilitating correct protein folding and maintaining proteome integrity.

Proteostasis is the cellular system maintaining protein health by managing inherent challenges: imperfect biosynthesis, unavoidable stress, mutations, functional unfolding requirements, and age-related damage. This requires constant rescue operations through chaperone networks that prevent aggregation into pathological fibrils or amorphous aggregates. The folding landscape theory explains how proteins navigate energy landscapes with metastable intermediates susceptible to competing intermolecular interactions. HSP70 chaperones serve as central hubs in this network, performing multiple functions including maintaining nascent chains, facilitating membrane translocation, rescuing misfolded proteins, and assisting folding—all through the same molecular machine.