The Central Dogma of Biology: DNA to RNA to Protein

A mutation is a change in the nucleotide sequence of DNA. Substitution mutations replace one nucleotide with another, such as thymine being replaced by adenine. Since DNA and mRNA are complementary (with uracil replacing thymine in mRNA), these changes alter codons and can change which amino acid is incorporated during translation. Most mutations that alter amino acid sequences render proteins non-functional because the protein's three-dimensional structure is disrupted. This demonstrates how single genetic changes can have significant consequences for cellular function.

Silent mutations change the DNA sequence but do not alter the resulting protein, occurring because the genetic code is degenerate (multiple codons code for the same amino acid). Nonsense mutations change a codon to a stop codon, resulting in a prematurely terminated protein that is typically non-functional. These mutations demonstrate how specific changes in the genetic code can have different consequences for protein function.

Transcription copies DNA into mRNA using RNA polymerase, with uracil replacing thymine. Translation decodes mRNA at ribosomes using tRNA anticodons matching codons. Each three-letter codon specifies an amino acid or stop signal. The genetic code is nearly universal. Mutations are DNA sequence changes causing genetic disorders. Single base substitutions alter amino acid sequences, potentially producing nonfunctional proteins. Analyzing mutations requires tracing effects through the central dogma: identify mutated DNA base, determine resulting mRNA codon, find corresponding amino acid using genetic code, and predict protein function impact. Gene expression produces functional proteins; gene mutations produce nonfunctional proteins causing diseases. Cystic fibrosis results from nonfunctional CFTR chloride channels. Protein function directly determines phenotype: functional proteins yield normal traits, nonfunctional proteins cause abnormalities.

Mutations are changes in the DNA sequence. A point mutation (substitution of one base) may or may not change the amino acid due to code degeneracy. A deletion mutation (loss of one base) causes a frameshift, changing all subsequent codons and typically resulting in a non-functional protein. The effects of mutations depend on where they occur and what codons are affected. Scientists use base pairing rules (A-T/U, G-C) to determine DNA sequences from RNA sequences and vice versa.

Gene mutations affect protein synthesis through the central dogma: DNA mutation → altered mRNA during transcription → incorrect amino acid sequence during translation. Consequences include loss of amino acids, addition of incorrect amino acids, altered sequence, or premature termination. Most mutations are harmful, resulting in non-functional proteins. Very rarely, mutations can be beneficial by increasing protein activity.

The central dogma states DNA makes RNA makes protein, but this is not absolute. Two major exceptions exist: (1) Reverse transcription, where RNA serves as template for DNA synthesis, exemplified by retroviruses like Rous Sarcoma Virus (discovered the concept) and HIV (causes AIDS). The HIV replication cycle involves RNA-to-DNA conversion, integration into host chromosomes, transcription to RNA, and viral particle assembly. (2) Prions are proteins that can alter the 3D structure of other proteins, causing diseases like Mad Cow Disease, Creutzfeldt-Jakob Disease, and Scrapie. These exceptions demonstrate that genetic information flow is not always unidirectional.

The Central Dogma has several exceptions observed in nature and laboratory settings. Reverse transcription allows RNA to be converted back to DNA, primarily in retroviruses like HIV, using the enzyme reverse transcriptase. RNA can also serve as genetic material in some viruses. RNA-dependent RNA polymerase allows RNA to be copied directly from RNA templates. Prions represent another exception as infectious proteins that can convert normal proteins into misfolded forms without any genetic material. Unlike other biological entities, prions do not contain DNA or RNA. Instead, they are misfolded proteins that can induce normal proteins to adopt the same misfolded conformation. When a prion encounters a normal protein, it causes the normal protein to change its shape and become a prion as well. This process can continue, with each prion converting multiple normal proteins, leading to a cascade of misfolded proteins. In the brain, this accumulation of misfolded proteins causes tissue damage and neurodegeneration, resulting in fatal diseases like mad cow disease and Creutzfeldt-Jakob disease. Despite the diversity of human characteristics, approximately 99.9% of human DNA is identical among all individuals, with the remaining 0.1% accounting for individual differences.

Translation converts mRNA codons into proteins using ribosomes and tRNA molecules. The genetic code uses three-nucleotide codons to specify 20 amino acids. Exceptions include reverse transcription (RNA to DNA, as in HIV) and prion proteins (protein to protein information transfer). Despite these exceptions, the Central Dogma remains fundamental to molecular biology, enabling genetic manipulation and providing insights into life's fundamental mechanisms.

The Central Dogma of Biology, proposed by Francis Crick, describes the flow of genetic information: DNA replicates to make copies of itself, DNA is transcribed to make RNA, and RNA is translated to make proteins. This describes the one-directional flow of genetic information in cells. However, in 1970, Howard Temin and David Baltimore discovered reverse transcription - the process by which RNA can be used to synthesize DNA. This was discovered in retroviruses (like Rous sarcoma virus) and challenged the one-directional Central Dogma. Reverse transcription is now known to be important in HIV and other retroviruses, where the viral RNA genome is converted to DNA inside host cells.

The central dogma describes DNA → RNA → protein flow, but retroviruses (like HIV) can reverse this by converting RNA back to DNA. Prions are misfolded proteins that induce normal proteins to misfold, causing diseases like mad cow disease. These exceptions challenged the established paradigm and expanded our understanding of genetic information flow.

Key tools in biotechnology include: Recombinant DNA technology for inserting designed genes into vectors and host cells to produce desired products. PCR (Polymerase Chain Reaction) amplifies DNA. Gel electrophoresis separates DNA fragments by size. CRISPR-Cas9 is a gene therapy tool for treating various diseases. Monoclonal antibodies are produced in labs using B cells and cancer cells for disease-specific antibodies. Bioreactors and fermenters are used for tissue culture, cell culture, and producing products like antibodies, enzymes, toxins, and flavonoids. Real-world applications include: Cancer therapy using monoclonal antibodies like Rituximab, infertility treatment using recombinant FSH hormone, anemia treatment using recombinant erythropoietin, and thrombosis treatment using recombinant tissue plasminogen activator. The human insulin case study demonstrates how recombinant DNA technology replaced pig pancreas-derived insulin (which caused allergic reactions) with human insulin produced using E. coli bacteria, resulting in a safer, more effective, and scalable treatment.

Recombinant DNA technology produces pharmaceuticals like erythropoietin (anemia treatment) and human insulin (replacing animal-derived insulin). Subunit vaccines use genes coding for pathogen surface proteins inserted into yeast cells, producing antigens without the entire pathogen (e.g., hepatitis B vaccine). DNA vaccines use non-pathogenic viruses carrying pathogen antigen genes, tricking the immune system without infection risk. Gene therapy uses vector viruses to insert correct genes into patient cells (e.g., hemophilia treatment). CRISPR enables precise gene editing at specific locations, potentially treating genetic diseases like sickle cell anemia. Gene silencing uses siRNAs to bind mRNA and prevent translation, serving as natural defense against viruses.

Recombinant DNA technology has transformed medicine and agriculture. Recombinant insulin, the first human gene product created in 1978, is produced by cloning insulin gene subunits into bacterial plasmids. Edible vaccines insert pathogen genes into plant plasmids, creating bananas and potatoes that trigger immune responses—valuable in developing countries. The Human Genome Project revealed human genomes are over 99% identical between individuals. Transgenic animals are created by injecting DNA into fertilized embryos, demonstrating that gene function can be restored even when sequences differ between species. Knockout transgenic animals deplete gene expression using embryonic stem cells, modeling human diseases including cancer, obesity, and heart disease. CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats), awarded the 2020 Nobel Prize, enables precise DNA targeting, cutting, and gene replacement. Applications include creating knockout mice for disease modeling, correcting specific mutations using repair templates, and potentially curing genetic diseases like type 1 tyrosinemia in mice.

Recombinant DNA technology enables safe, large-scale production of therapeutic drugs. Recombinant insulin (since 1970) eliminates allergic reactions from animal-derived insulin. Interferons (IFN-alpha, beta, gamma) treat cancer, particularly hairy cell leukemia. Vaccines for Hepatitis B, Influenza, and Smallpox are produced through genetic engineering. Monoclonal antibodies (hybridoma technology) detect diseases like cancer and allergies. Antibiotics (penicillin, streptomycin) are produced biologically. Gene therapy corrects genetic defects by transferring normal genes. Molecular diagnosis uses PCR for cancer/AIDS detection and ELISA for STDs. Transgenic animals (mice, rabbits, pigs, sheep) have foreign genes inserted into their genomes, serving as models for studying gene function and disease mechanisms.

CRISPR-Cas9 is a gene editing technology that allows scientists to cut out portions of a genome and insert new material or proteins into that genome, which will then replicate. This technology can fundamentally change the DNA of a creature. The video notes that mRNA vaccines introduced to the world cause the body to begin to produce spike protein sequences that do not naturally occur in the human being, representing a real-world application of these technologies.