Fundamentals of Genetics and Evolution: Genes and Genomes

Coding vs. Non-coding DNA: - Coding DNA: Accounts for merely ~ $1.5\%$ of the total DNA in the genome. - Non-coding DNA: Accounts for ~ $98.5\%$ of the DNA. A significant fraction of this is functionally critical, including: - Regulatory Elements: Enhancers and promoters which control gene activity. - RNA-producing Genes: Genes that transcribe into functional RNAs such as ribosomal RNA (rRNA) and transfer RNA (tRNA).

Gene Architecture: Eukaryotic genes consist of coding sequences called exons separated by non-coding segments called introns.
RNA Splicing: The process by which introns are removed and exons are joined together. - Machinery: Splicing is performed by spliceosomes and small nuclear RNA (snRNA) molecules.
Exon Variation: - Approximately $10\%$ of human protein-coding genes consist of a single exon and do not undergo splicing. - The remainder of genes can give rise to splice variants, allowing a single gene to code for multiple protein isoforms.

Genome Uniformity: Every cell in a multicellular body originates from a single-celled zygote through repeated divisions to become an adult with 30-40 trillion cells. Consequently, every cell contains the same genome.
Cellular Differentiation: Although the genome is identical across cells, cells differ in function and appearance because different genes are switched on or off (expressed).
Reproductive Cycle: - Diploid Parents: Possess two sets of chromosomes. - Haploid Gametes: Produced through meiosis, containing half the parental genome. - Diploid Offspring: Formed when two haploid gametes fuse.

Definition of Meiosis: A specialized cell division that reduces the chromosome number by half to create haploid gametes, ensuring that when gametes fuse, the diploid number is restored.
Key Stages and Processes: - Duplication: Chromosomes duplicate into sister chromatids. - Pairing: Homologous chromosomes (pairs with the same genes but potentially different alleles) align. - Crossing Over (Recombination): Homologous chromosomes exchange genetic material at points called chiasmata. - Large chromosomes may have multiple chiasmata (e.g., 2), while smaller ones may have only one (e.g., 1). - Alignment and Separation: Pairs align on the metaphase plate at the center of the spindle apparatus. Spindle fibers contract, drawing chromosomes toward opposite poles. - Rupture of Chiasmata: Occurs as homologous chromosomes are pulled apart.
Meiosis I and II: - Meiosis I: Segregates the homologous chromosome sets into different germ cells via cytokinesis. - Meiosis II: In males, this occurs in each primary spermatozoa to yield four identical, haploid secondary spermatozoa. In females, the process results in the maturation of the egg, with other products becoming discarded polar bodies.
Independent Assortment: The random distribution of maternal and paternal homologs during meiosis generates unique genetic combinations.

Mendelian Perspective: Genes were originally viewed as indivisible "units" of heredity that determine phenotypes (e.g., color alleles $R$ and $r$ resulting in genotypes $RR$ , $Rr$ , and $rr$ ).
Molecular Perspective: The gene is now understood as a continuous nucleotide sequence. Mutations transform ovih sequences, and variation can occur in the coding region or regulatory regions.
Complexity of the Gene Unit: The "gene" remains an elusive unit because heritable phenotypic variation can be caused by differences in protein coding, levels of expression, or the specific nature of mutations.
The Central Dogma Path: $DNA \rightarrow mRNA \rightarrow Protein \rightarrow Phenotype$ .

Context: The bacterium E. coli digests lactose by producing the enzyme beta-galactosidase.
System Components: - lacZ gene: The sequence coding for beta-galactosidase. - Promoter: Located upstream ( $5'$ ), where RNA polymerase binds. - Termination sequence: Signals the end of transcription. - Repressor Protein: Coded for elsewhere in the genome; it binds to the promoter to block RNA polymerase when lactose is absent.
Induction Mechanism: - When lactose is present, it binds to the repressor protein. - The repressor undergoes a conformational change and detaches from the promoter. - Transcription: RNA polymerase transcribes the DNA into mRNA. - Translation: mRNA is translated by ribosomes into amino acids to form beta-galactosidase.
Conclusion: The process continues until all lactose is metabolized, at which point the repressor re-binds. Mutations in any part of this regulated process will alter the phenotype.

Definition: Genetically Modified Organisms (GMOs) contain genes inserted from other organisms.
Biological Vectors: The most common method for gene insertion.
The Gene Cassette: To ensure a transgene functions in a new host, it must be part of a cassette consisting of: - Promoter: Initiates transcription. - Transgene: The actual coding sequence of interest. - Terminator: Ends transcription.
Common Regulatory Elements: - CaMV35S Promoter: Derived from the Cauliflower Mosaic Virus. It is a strong promoter that keeps the transgene expression "always on" at high levels. - NOS Terminator: Derived from the Ti plasmid in Agrobacterium tumefaciens. It contains a polyadenylation signal which directs the cell to add a poly(A) tail of adenine nucleotides to the mRNA, protecting it from degradation.

Concept: PCR emulates DNA replication to amplify specific segments of DNA for visualization and study.
Required Components: - DNA template. - Water and pH buffers. - Free deoxynucleotide triphosphates (dNTPs): dATP, dGTP, dCTP, and dTTP. - Polymerase Enzyme: e.g., Taq polymerase. - Primers: Short sequences (Forward and Reverse) complementary to the regions flanking the target sequence.
The PCR Cycle: 1. Denaturation: Heating to ~ $95^\circ\text{C}$ to break hydrogen bonds and separate the double helix into two template strands. 2. Annealing: Cooling to ~ $56^\circ\text{C}$ to allow primers to bind to their complementary bases. 3. Extension: Warming to ~ $72^\circ\text{C}$ (optimal for Taq polymerase) to synthesize new strands by adding dNTPs to the $3'-OH$ end of the primers.
Exponential Growth: The quantity of DNA doubles with every cycle ( $2, 4, 8, 16...$ ). After ~ 30 cycles, millions of copies are produced.

Agarose Gel Electrophoresis: DNA is dyed and run on a gel to separate fragments by size, allowing for visualization.
GMO Testing Example: - Genomic DNA is extracted from food samples. - Specific primers are used: Plant primers (control) and GMO primers. - Results are compared against a molecular ladder (e.g., markers at $100\,bp$ , $200\,bp$ , $500\,bp$ , $700\,bp$ , $1,000\,bp$ ). - Non-GM food will show a plant-specific band (e.g., $455\,bp$ ) but no GMO band. GM food will show both the plant band and the GMO-specific band (e.g., $200\,bp$ ).