Comprehensive Study Guide: DNA, Genetics, and Biotechnology

DNA as the Basis of Heritable Information

  • Heritable information provides for the continuity of life.

  • Genetic transformation is transmitted from one generation to the next through DNA or RNA.

  • DNA and RNA Role in Storage:     * Genetic information is stored in and passed to subsequent generations through DNA molecules and, in some cases, RNA molecules.     * DNA is used in almost all organisms as the primary storage molecule.     * Some viruses, such as the HIV virus, use RNA as their primary genetic material.

Comparison of Prokaryotic vs Eukaryotic Genetic Organization

  • Genetic Information Units:     * Prokaryotes generally have circular chromosomes.     * Eukaryotes have linear chromosomes.     * Plasmids are small extra-chromosomal DNA molecules found primarily in prokaryotes, but also in some viruses and eukaryotes.

  • Detailed Characteristics of Prokaryotes:     * Possess a single chromosome plus plasmids.     * The chromosome is circular and made only of DNA.     * Found in the cytoplasm.     * Copies its chromosome and then divides immediately after.

  • Detailed Characteristics of Eukaryotes:     * Possess many chromosomes.     * Chromosomes are linear and made of chromatin, a nucleoprotein consisting of DNA coiled around histone proteins.     * Found within a nucleus.     * Copies chromosomes, then the cell grows, then goes through mitosis to organize chromosomes into two equal groups.

  • Nucleosomes (Eukaryotes Only):     * A nucleosome consists of DNA wrapped around an octamer of histones.     * The H1 Histone is part of the stabilization of this structure.

  • Plasmids:     * In Prokaryotes: Carry characteristics other than basic survival information, such as genes for antibiotic resistance. They are self-replicating small supplemental circles of DNA containing 50005000 to 20,00020,000 base pairs and carry 22 to 3030 extra genes. They can be exchanged between bacteria (referred to as "bacterial sex") facilitating rapid evolution.     * In Eukaryotes: Plasmids exist in specific instances (though less common than in prokaryotes).

Chemical Structure of DNA and RNA

  • DNA Molecular Structure:     * DNA is a double helix made of nucleotides.     * Watson and Crick developed the double helix model.     * Dimensions:         * 2 nm2\,nm diameter.         * 0.34 nm0.34\,nm between each base pair.         * 3.4 nm3.4\,nm length of each full twist of the double helix.

  • Nucleotide Composition:     * Subunits consist of a phosphate group, a deoxyribose (sugar), and a nitrogenous base.     * Phosphodiester Linkages: These links form each strand's sugar-phosphate backbone. The 3′3' carbon of each sugar is joined by the phosphate group to the 5′5' carbon of the next sugar.     * Antiparallel Strands: The two sugar-phosphate backbones run in parallel but opposite directions (5′5' to 3′3' and 3′3' to 5′5'). One strand is essentially upside down compared to the other.

  • Nitrogenous Bases Types:     * Purines: Two-ringed nitrogenous bases. Includes Adenine (AA) and Guanine (GG).     * Pyrimidines: One-ringed nitrogenous bases. Includes Thymine (TT), Cytosine (CC), and Uracil (UU) in RNA.

  • Base-Pairing and Hydrogen Bonding:     * Adenine (AA) forms two hydrogen bonds with Thymine (TT).     * Guanine (GG) forms three hydrogen bonds with Cytosine (CC).     * Chargaff's Rules: State that the amount of Adenine equals Thymine (A=TA = T) and Guanine equals Cytosine (G=CG = C).     * Pairing Rule: A purine always binds with a pyrimidine to maintain the consistent diameter of the helix.

  • RNA Structure Differences:     * Formed from nucleotide subunits containing ribose sugar (instead of deoxyribose), a base, and three phosphates.     * Subunits are joined by a 5′5' to 3′3' linkage forming a sugar-phosphate backbone.     * Nitrogenous bases for RNA: Adenine (AA), Guanine (GG), Cytosine (CC), and Uracil (UU). Uracil replaces Thymine (TT).     * Function: While DNA stores information permanently, RNAs serve various functions including temporary genetic messaging and catalysis.

DNA Replication: Process and Enzymes

  • The flow of genetic information typically follows the pattern: DNA→RNA→protein\text{DNA} \rightarrow \text{RNA} \rightarrow \text{protein}.

  • Semiconservative Replication: DNA replication ensures the continuity of hereditary information. Each of the two strands of the double helix unwinds and serves as a template for a new strand. The final product consists of one template strand and one new complementary strand.

  • Key Enzymes in DNA Replication:     * DNA Helicases: Open the double helix by disrupting hydrogen bonds holding the strands together.     * Topoisomerases: Break one or both DNA strands to prevent excessive coiling (supercoiling) during replication and then rejoin them.     * DNA Polymerases: Link nucleotide subunits together. They can only add new nucleotides to the 3′3' end of an existing strand; thus, New DNA is synthesized in the 5′5' to 3′3' direction.     * DNA Primase: Synthesizes short RNA primers on the lagging strand and begins replication of the leading strand.     * DNA Ligase: Links Okazaki fragments by joining the 3′3' end of a new DNA fragment to the 5′5' end of the adjoining DNA.     * Telomerase: Lengthens telomeric DNA at the ends of chromosomes.     * SSBP (Single-Strand Binding Proteins): Keep the DNA strands open and stable during replication.

  • Leading and Lagging Strands:     * Leading Strand: Synthesized continuously in the direction toward the replication fork.     * Lagging Strand: Synthesized discontinuously as short Okazaki fragments in the direction away from the replication fork.

  • Replication Characteristics:     * Bidirectional: Replication starts at an origin of replication and proceeds in both directions (forming replication bubbles).     * Replication Fork: The site where strands replicate.     * Speed and Accuracy: Human cells have 4646 DNA molecules and about 66 billion base pairs. Replication takes just a few hours with only 11 error per 11 billion nucleotides.     * Repair Mechanisms: DNA polymerase proofreads each nucleotide. If an error is found, the enzyme deletes and replaces it. Over 100100 DNA repair enzymes exist (Excision repair involves nucleases cutting the error, polymerase adding the correct base, and ligase connecting the strand).

  • Telomeres:     * Short, non-coding repetitive DNA sequences at chromosome ends.     * They shorten slightly with each cell cycle, which may be a cause of cell aging.     * Cancer cells often have telomerase activity to maintain telomere length and resist apoptosis.

Protein Synthesis: Transcription and Translation

  • The Central Dogma: DNA→Transcription→RNA→Translation→Protein\text{DNA} \rightarrow \text{Transcription} \rightarrow \text{RNA} \rightarrow \text{Translation} \rightarrow \text{Protein}.

  • Types of RNA:     * Messenger RNA (mRNA): Transcribes information from DNA.     * Ribosomal RNA (rRNA): Main component of ribosomes, the site of polypeptide building.     * Transfer RNA (tRNA): Delivers specific amino acids to ribosomes. It has an anticodon on one end and the corresponding amino acid on the other.

  • Transcription Process:     * Initiation: RNA polymerase attaches to the DNA at the promoter region.     * Elongation: DNA opens and RNA polymerase adds RNA nucleotides, reading the DNA from the 3′3' to 5′5' end. The mRNA is synthesized in the 5′5' to 3′3' direction.     * Termination: RNA polymerase reaches a termination sequence, stops, and detaches the mRNA.     * Strands: The 5′→3′5' \rightarrow 3' DNA strand is the coding strand; the 3′→5′3' \rightarrow 5' strand is the template (non-coding) strand used to build mRNA.

  • Translation Process:     * mRNA interacts with rRNA to initiate translation at the Start Codon (AUG - Methionine).     * Codons: Three-nucleotide sequences on mRNA that specify an amino acid. There are 6464 possible codons, including 33 stop codons (UAA, UAG, UGA).     * Ribosome Sites: The large subunit has three sites (E, P, and A).     * Steps: Initiation, Cycles of Elongation (tRNA bringing anticodons/amino acids), and Termination (release factor binds to stop codon, polypeptide chain is released).

  • Protein Structure Levels:     * Primary Level: Sequence of amino acids bound by peptide bonds.     * Secondary Level: Alpha helix or beta-pleated sheets bound by hydrogen bonds.     * Tertiary Level: Complex folding due to side-chain interactions (ionic, hydrogen, covalent).     * Quaternary Level: Interaction between multiple peptide chains.

Gene Regulation and Modification

  • Eukaryotic mRNA Modifications:     * Addition of a Poly-A tail at the 3′3' end (protects from enzymes).     * Addition of a GDP cap at the 5′5' end (stabilizes).     * Excision of introns (non-coding regions) using spliceosomes.

  • Bacterial Gene Expression:     * Coupled Transcription and Translation: Unlike eukaryotes, translation of bacterial mRNA usually begins before the 3′3' end of the transcript is even completed.

Viruses and Retroviruses

  • Virus Structure:     * Capsid: Head made of proteins and glycoproteins.     * May have a membrane/envelope that fuses with cell membranes.     * May have a tail region for attachment and injection of genetic material.

  • Life Cycles:     * Lytic Cycle: Immediate entry, degradation of host DNA, synthesis of viral components, assembly, and release.     * Lysogenic Cycle: DNA integrates into the host and has a dormant stage (e.g., Shingles).

  • Retroviruses:     * Flow of information is reversed: RNA→DNA→RNA→protein\text{RNA} \rightarrow \text{DNA} \rightarrow \text{RNA} \rightarrow \text{protein}.     * Use Reverse Transcriptase to synthesize DNA from an RNA template.     * Example: HIV-1 (AIDS virus).

Genetic Engineering and Biotechnology

  • Bacterial Foundations:     * Bacteria reproduce by binary fission (mitosis-like) with a generation time of ~2020 minutes.     * Archaea vs. Eubacteria: Both are prokaryotes with similar shapes/sizes and flagella. Archaea are often extremophiles, lack peptidoglycan in cell walls, and have complex RNA polymerases like eukaryotes. Eubacteria contain peptidoglycan and simpler RNA polymerases.

  • Biotechnology Tools:     * Gel Electrophoresis: Identifies length of DNA fragments. DNA is cut with restriction enzymes, fragments are placed on a gel with current applied. DNA travels from negative to positive; smaller pieces move further than large ones.     * Bacterial Transformation: Bacteria import external DNA and express its genes. This creates recombinant DNA and Transgenic Organisms (e.g., human insulin produced in bacteria).     * Selection: Antibiotics (like ampicillin) are used as selecting agents; only transformed bacteria containing the plasmid with resistance genes will grow.     * PCR (Polymerase Chain Reaction): Method used to make millions of copies of DNA using denaturation, annealing (with primers), and extension.     * Restriction Enzymes: Cut DNA at specific sequences to allow joined DNA pieces (pasting human genes onto plasmids).

  • GMO Examples:     * BT Corn: Produces bacterial toxin to kill corn borer caterpillars.     * Fishberries: Strawberries with an anti-freezing gene from flounder.     * Golden Rice: Rice enriched with Vitamin A.     * GFP (Green Fluorescent Protein): Derived from jellyfish, used to transform vertebrates (like fish) to glow.

Questions & Discussion

  • How do you feel about getting a genetic test that can tell you about genetic diseases that you may have?

  • If you are expecting and a genetic test tells you that the baby has a serious disease, what would you do?

  • Would you marry someone that has a genetic disease that could be passed on to the children?

  • DNA as Heritable Information: Genetic information is stored in DNA molecules, passed to subsequent generations; DNA is the primary storage molecule in most organisms. Some viruses use RNA as genetic material.

  • Prokaryotic vs Eukaryotic Genetic Organization: Prokaryotes have circular chromosomes and plasmids, while eukaryotes have linear chromosomes found in the nucleus with complex packaging (chromatin and nucleosomes).

  • DNA Structure: DNA is a double helix made of nucleotides, with specific base-pairing rules (A pairs with T, G pairs with C). RNA differs by having ribose and uracil instead of thymine.

  • DNA Replication: Involves enzymes like DNA polymerases and occurs semiconservatively, producing one old and one new strand. Leading and lagging strands are replicated differently.

  • Protein Synthesis: Transcription (DNA to mRNA) and translation (mRNA to protein) are key processes. RNA types include mRNA (messaging), rRNA (structural), and tRNA (transfer).

  • Gene Regulation: Eukaryotic mRNA undergoes modifications (5' cap, poly-A tail) and intron removal.

  • Viruses: Structure includes a capsid and possibly an envelope. Their life cycles can be lytic or lysogenic. Retroviruses reverse the flow of information (RNA to DNA).

  • Genetic Engineering: Techniques like PCR and transformation create recombinant DNA and genetically modified organisms (GMO).