Molecular

Genomes of Living and Non-Living Organisms and Their Significance

• Living Organisms: Cellular organisms (eukaryotes, bacteria, and archaea) possess genomes made of double-stranded DNA (dsDNA) that encode all proteins and cellular machinery required for metabolism, growth, and replication.

• Non-Living Entities: Non-living microbes (viruses, viroids, and prions) have unique genetic structures. Viral genomes consist of single-stranded (ss) or double-stranded (ds) DNA or RNA. Viroids consist solely of small, circular ssRNA without a protein coat. Prions lack nucleic acid genomes entirely and consist of infectious protein conformations.

• Significance: The genome stores hereditary information necessary for maintaining life processes, facilitating reproduction, driving cellular activity, and enabling evolutionary adaptation through mutation.

Structure of Chromosomes in Living and Non-Living Organisms

• Eukaryotes: Chromosomes are linear double-stranded DNA molecules tightly packaged around histone proteins to form nucleosomes, condensing into chromatin structure within a membrane-bound nucleus.

• Prokaryotes: Contain a single, circular chromosome located in the nucleoid region, associated with non-histone proteins and compacted via supercoiling by topoisomerases like DNA gyrase.

• Viruses: Viral nucleic acids exist in linear, circular, or segmented forms encapsulated within a protein capsid.

• Significance: Chromosome packaging protects genetic material from structural damage, regulates gene expression, and ensures equal segregation during cell division.

Organization of Structural Genes in DNA and Cellular Survival

• Structural Genes: Sequences of DNA that code for specific functional proteins or structural RNA (tRNA, rRNA).

• Prokaryotic Organization: Genes are organized into polycistronic operons under a single promoter, allowing coordinated regulation of metabolic pathways without introns.

• Eukaryotic Organization: Genes are monocistronic and contain exons (coding sequences) interrupted by non-coding introns that are removed via pre-mRNA splicing.

• Function for Survival: Direct the synthesis of enzymes for metabolic pathways, structural proteins, repair enzymes, and regulatory factors essential for homeostasis.

Structural Organization of Genome Types Across Organisms

• Eukaryotic Genomes: Composed of a nuclear genome (multiple linear chromosomes) and organellar genomes (mitochondrial DNA and chloroplast DNA, which resemble circular bacterial DNA).

• Prokaryotic Genomes: Consist of a primary chromosome along with extrachromosomal plasmids—small, circular dsDNA molecules carrying non-essential but advantageous genes like antibiotic resistance.

• Viral Genomes: Classified by the Baltimore system into seven groups: dsDNA, ssDNA, dsRNA, (+)ssRNA, (-)ssRNA, ssRNA-RT, and dsDNA-RT. They display high gene density with overlapping reading frames.

Mendel's Hereditary Factors and Transmission

• Hereditary Factors (Genes): Discrete particulate units that retain their structural integrity across generations without blending.

• Law of Segregation: Allele pairs separate during meiosis I gamete formation, ensuring each gamete carries only one allele per locus.

• Law of Independent Assortment: Alleles of unlinked genes segregate independently during gamete formation.

• Transmission: Gametes unite during fertilization to restore diploidy in offspring, generating phenotypic ratios such as $3:1$ for monohybrid crosses and $9:3:3:1$ for dihybrid crosses.

Mechanism of Recombination and Cell Cycle Timing

• Mechanism: Homologous recombination occurs via double-strand break repair (DSBR) or synthesis-dependent strand annealing (SDSA). Recombination proteins (RecA in bacteria, Rad51/Dmc1 in eukaryotes) catalyze single-strand invasion into a homologous duplex, forming a Holliday junction followed by branch migration and resolution.

• Timing: Takes place during the pachytene stage of Prophase I during Meiosis in eukaryotic cell cycles.

Studies Identifying DNA as Genetic Material

• Griffith's Experiment (1928): Discovered the "transforming principle" when heat-killed virulent Streptococcus pneumoniae transformed benign live strain cells into virulent cells.

• Avery-MacLeod-McCarty Experiment (1944): Selectively degraded proteins, RNA, and DNA in cell extracts, demonstrating that transformation occurred only when DNA remained intact.

• Hershey-Chase Experiment (1952): Labeled T2 bacteriophages with radioactive isotopes $^{35}\text{S}$ (protein) and $^{32}\text{P}$ (DNA). Showed that only $^{32}\text{P}$ entered host E. coli cells, confirming DNA as the genetic material.

Mechanisms of Microbial Entry into Living Cells

• Receptor-Mediated Endocytosis: Microbes bind to cell-surface receptors and induce endosome formation (e.g., Influenza virus binding sialic acid).

• Membrane Fusion: Enveloped viruses fuse their lipid bilayer directly with the host plasma membrane (e.g., HIV via gp120/gp41 and CD4/CCR5 receptors).

• Genome Injection: Bacteriophages attach to cell surface proteins and inject their nucleic acid directly into the cytoplasm while leaving the protein coat outside.

• Phagocytosis / Induced Uptake: Intracellular bacteria secrete virulence factors to induce engulfment by host cells.

Radioactive Isotopes for Labeling Macromolecules

• Phosphorus-32 ($^{32}\text{P}$): Used to selectively label nucleic acids (DNA and RNA) at their phosphate backbone.

• Sulfur-35 ($^{35}\text{S}$): Used to label proteins by incorporating into sulfur-containing amino acids (cysteine and methionine).

• Carbon-14 ($^{14}\text{C}$) and Tritium ($^3\text{H}$): Used for general labeling of biological molecules, including tritiated thymidine for DNA synthesis tracking.

• Nitrogen-15 ($^{15}\text{N}$): A heavy non-radioactive isotope used by Meselson and Stahl to demonstrate semiconservative replication.

Mechanisms of Entry by Foreign Invaders

• Direct Penetration: Non-enveloped viruses create pores in host cell membranes to translocation their genome.

• Type III/IV Secretion Systems: Syringe-like bacterial structures inject effector proteins directly into host cells to manipulate host actin cytoskeletons and facilitate invasion.

• Enzymatic Degradation: Pathogens secrete collagenases, hyaluronidases, and tissue-degrading enzymes to breach physical cell barriers.

Nucleic Acids Functioning as Genetic Material

• Deoxyribonucleic Acid (DNA): Serves as the primary genetic material in all cellular life forms and DNA viruses due to its double-stranded stability and lower rate of spontaneous mutation.

• Ribonucleic Acid (RNA): Serves as genetic material in RNA viruses (e.g., Retroviruses, Coronaviruses, Paramyxoviruses). RNA can store genetic information and exhibit catalytic activity (ribozymes).

Chemical Structures and Names of Nucleic Acids

• Nucleotide Structure: Composed of a nitrogenous base, a pentose sugar, and one or more phosphate groups.

• DNA Components: Contains 2'-deoxyribose sugar and purine bases Adenine (A), Guanine (G), alongside pyrimidine bases Cytosine (C) and Thymine (T).

• RNA Components: Contains ribose sugar (with a 2'-hydroxyl group) and bases Adenine (A), Guanine (G), Cytosine (C), and Uracil (U).

• Backbone Linkage: Nucleotides are connected by $5' \rightarrow 3'$ phosphodiester bonds.

Structures, Organization, and DNA Isoforms

• Primary Structure: Linear sequence of nucleotides in a single strand ($5' \rightarrow 3'$ direction).

• Secondary Structure: Double-helical anti-parallel strands held together by base pairing (A=T with 2 hydrogen bonds; G=C with 3 hydrogen bonds) and base stacking interactions.

• DNA Isoforms:

  • B-DNA: Standard right-handed double helix under physiological conditions ($10.5\,\text{base pairs/turn}$).

  • A-DNA: Dehydrated, compact right-handed helix ($11\,\text{base pairs/turn}$).

  • Z-DNA: Left-handed zig-zag helical conformation ($12\,\text{base pairs/turn}$) associated with transcriptionally active regions and specific GC-repeats.

Replication Processes and Protein Functions

• Initiation: Initiator proteins bind to origin sequences ($\text{oriC}$) to open the duplex.

• DNA Helicase: Unwinds the double-stranded DNA helix.

• Single-Strand Binding Proteins (SSBs): Stabilize single-stranded template DNA to prevent re-annealing.

• Topoisomerase / DNA Gyrase: Relieves torsional strain and supercoiling ahead of the replication fork.

• Primase: Synthesizes short RNA primers required to provide a free 3'-OH group.

• DNA Polymerase: Synthesizes new strands in the $5' \rightarrow 3'$ direction (continuous leading strand, discontinuous lagging strand forming Okazaki fragments).

• DNA Ligase: Joins Okazaki fragments by forming phosphodiester bonds.

Enzymes Digesting Nucleic Acids and Their Mechanisms

• Endonucleases: Cleave internal phosphodiester bonds within a nucleic acid chain (e.g., Restriction Endonucleases cut specific palindromic sequences).

• Exonucleases: Remove single nucleotides sequentially from either the 3' or 5' ends of a nucleic acid strand.

• RNase H: Specifically degrades the RNA strand in RNA-DNA hybrid duplexes.

• DNases and RNases: Catalyze hydrolytic cleavage of DNA and RNA phosphodiester backbones, respectively.

Genetic Material in Transcription, Translation, and Nucleic Acid Conversions

• Transcription: DNA is transcribed into messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA) by RNA Polymerase.

• Translation: mRNA directs amino acid assembly into polypeptides via ribosomes and tRNA complexes.

• Conversions:

  • Transcription: DNA to RNA via RNA Polymerase.

  • Reverse Transcription: RNA to complementary DNA (cDNA) via Reverse Transcriptase.

  • Replication: DNA to DNA via DNA Polymerase or RNA to RNA via RNA-dependent RNA Polymerase (RdRp).

Nucleic Acid Hybridization and Molecular Interactions

• Nucleic Acid Hybridization: Annealing of single-stranded nucleic acids with complementary strands from different sources based on sequence homology (used in Southern blotting, Northern blotting, and FISH).

• Intermolecular Interactions: Hydrogen bonding between complementary base pairs on opposite strands and ionic interactions with divalent cations (e.g., $\text{Mg}^{2+}$) that neutralize phosphate charge repulsion.

• Intramolecular Interactions: Base pairing within a single nucleic acid strand, creating secondary structures such as hairpin loops, stem-loops, pseudoknots, and cloverleaf structures (e.g., tRNA).