Higher Biology Unit 1: DNA, Gene Expression, Genomics, and Evolution

Chemical Structure and Properties of DNA

• Nucleotide Subunit Composition: Deoxyribonucleic acid (DNA) is a macromolecule composed of repeating monomeric subunits called nucleotides. Each individual nucleotide consists of three chemically bonded components:

  • A deoxyribose sugar molecule.
  • A phosphate group.
  • An organic nitrogenous base belonging to one of four types: adenine, thymine, cytosine, or guanine.

• Formation of the Sugar-Phosphate Backbone: Nucleotides join together via covalent bonding between the phosphate group of one nucleotide and the deoxyribose sugar of the adjacent nucleotide, creating a robust and repeating sugar-phosphate backbone.

• Double-Stranded Helix: DNA is organized into a double-stranded helical structure formed by two polynucleotide chains coiling around a central axis.

• Complementary Base Pairing and Hydrogen Bonding: The two individual polynucleotide strands are held together by hydrogen bonds established between specific complementary base pairs:

  • Adenine pairs exclusively with thymine via complementary hydrogen bonding.
  • Guanine pairs exclusively with cytosine via complementary hydrogen bonding.

• Antiparallel Orientation of Strands: The two complementary strands run in opposite directions to one another (antiparallel):

  • Each strand possesses a directional orientation defined by its chemical ends.
  • The $3'$ end terminates with an exposed deoxyribose sugar molecule.
  • The $5'$ end terminates with an exposed phosphate group.
  • One strand runs in a $5'$ to $3'$ direction, while the complementary opposing strand runs in a $3'$ to $5'$ direction.

Cellular Organisation of DNA Across Domains

• Prokaryotes vs. Eukaryotes: Organisms are differentiated fundamentally by the structural compartmentalization of their genetic material:

  • Prokaryotic organisms do not possess a membrane-bound nucleus.
  • Eukaryotic organisms possess a true membrane-bound nucleus containing their nuclear DNA.

• Genomic Organisation in Prokaryotes:

  • Prokaryotes contain their main genetic material packaged as a single circular chromosome.
  • Additionally, prokaryotic cells frequently carry auxiliary extrachromosomal rings of double-stranded DNA known as plasmids.

• Genomic Organisation in Eukaryotes:

  • Within the eukaryotic nucleus, DNA is organized into multiple linear chromosomes.
  • Linear eukaryotic chromosomes are packaged tightly by being wound around specialized structural proteins termed histone proteins.
  • In addition to nuclear linear chromosomes, eukaryotic cells possess circular chromosomes within semiautonomous metabolic organelles: mitochondria and chloroplasts.
  • Yeast serves as a notable exception among eukaryotes, as yeast cells contain plasmids in addition to their linear nuclear chromosomes.

Molecular Mechanism of DNA Replication

• Biological Purpose and Timing: DNA replication occurs immediately prior to cell division, ensuring that an exact, faithful copy of the genetic material is transmitted to each subsequent daughter cell.

• Template Strand Unwinding: The double-stranded DNA unwinds and the hydrogen bonds between base pairs unzip, exposing two single-stranded DNA templates.

• Replication Sites: DNA replication initiates simultaneously at several locations across a single eukaryotic DNA molecule, forming replication forks that expedite duplication of large genomes.

• Primer Requirement: The principal synthesizing enzyme, DNA polymerase, cannot initiate nucleotide polymerisation without an existing starter sequence:

  • DNA polymerase requires a primer to initiate strand replication.
  • A primer is defined as a short strand of nucleotides that anneals via complementary base pairing to the $3'$ end of the template DNA strand.
  • The presence of the primer provides a free deoxyribose group, allowing DNA polymerase to bind and commence adding DNA nucleotides.

• Polymerase Action and Directionality:

  • DNA polymerase synthesizes new DNA by adding complementary free DNA nucleotides to the growing strand.
  • Nucleotides are added strictly according to complementary base-pairing rules (adenine to thymine, guanine to cytosine).
  • DNA polymerase can add incoming DNA nucleotides solely to the deoxyribose ($3'$) end of the new forming strand.
  • Consequently, strand elongation proceeds exclusively in the $5'$ to $3'$ direction.

• Leading vs. Lagging Strand Synthesis:

  • Leading Strand: Synthesized continuously because DNA polymerase moves in the same direction as the unwinding replication fork, adding nucleotides continuously to the growing $3'$ end.
  • Lagging Strand: Replicated discontinuously in fragments because the strand runs in the opposite direction; as the template strand is progressively exposed, synthesis must initiate repeatedly with new primers and extend away from the fork.

• Fragment Ligation: The separate Okazaki fragments generated on the lagging strand are covalently joined together by the enzyme ligase to form a continuous polynucleotide chain.

Polymerase Chain Reaction (PCR)

• Definition and Function: The polymerase chain reaction (PCR) is an in vitro laboratory technique designed to amplify specific target sequences of DNA, generating millions of identical copies from minute initial quantities.

• Role and Design of Primers: In PCR, primers are short synthetic single-stranded nucleotide sequences engineered to be complementary to specific target sequences flanking the two opposing ends of the DNA region selected for amplification.

• Three-Step Thermal Cycling Profile: PCR operates through repeated automated cycles consisting of three discrete temperature phases:

  • Denaturation: The reaction mixture is heated to a temperature between $92\text{--}98^\circ\text{C}$. This high thermal energy breaks the hydrogen bonds linking complementary base pairs, denaturing and separating double-stranded DNA into single strands.
  • Annealing: The mixture is cooled to a temperature between $50\text{--}65^\circ\text{C}$, allowing specific primers to bind and anneal to their complementary target sequences at each end of the target DNA region.
  • Extension: The mixture is heated to a temperature between $70\text{--}80^\circ\text{C}$. At this optimal temperature, a heat-tolerant DNA polymerase synthesizes new complementary strands by adding free DNA nucleotides to the $3'$ ends of the bound primers.

• Exponential Amplification: Repeated thermal cycles of heating and cooling sequentially double the quantity of target DNA per cycle, rapidly yielding high concentrations of the targeted sequence.

• Practical Applications:

  • Forensic science to solve crimes by analyzing trace DNA recovered from crime scenes.
  • Legal paternity testing to resolve biological parental relationships.
  • Medical diagnostics to detect and diagnose genetic disorders and pathogenic infections.

Gene Expression: Transcription and RNA Processing

• Concept of Gene Expression: Gene expression is the fundamental cellular mechanism whereby genetic information stored within DNA sequences is transcribed and translated to produce functional proteins. In any specialized cell, only a fraction of the total genes within the genome are actively expressed.

• Chemical Structure and Properties of RNA:

  • Ribonucleic acid (RNA) is a single-stranded nucleic acid.
  • Composed of RNA nucleotides containing a ribose sugar (possessing an additional hydroxyl group compared to deoxyribose), a phosphate group, and one of four nitrogenous bases.
  • The four bases of RNA are adenine, uracil, cytosine, and guanine.
  • Uracil replaces thymine and acts as the complementary base to adenine.

• The Three Types of Functional RNA:

  • Messenger RNA (mRNA): Carries a complementary copy of the genetic code from DNA in the nucleus out through the nuclear pores to ribosomes in the cytoplasm. It is synthesized during transcription and translated by ribosomes during protein synthesis. A linear sequence of three consecutive bases on mRNA is termed a codon, each coding for one specific amino acid.
  • Transfer RNA (tRNA): Folds into a distinctive cloverleaf three-dimensional conformation stabilized by internal complementary base pairing. Each tRNA transports a specific amino acid from the cytoplasm to the ribosome. It features an anticodon (an exposed triplet of bases complementary to an mRNA codon) at one end and a specific amino acid attachment site at the opposite end.
  • Ribosomal RNA (rRNA): Combines structurally and catalytically with ribosomal proteins to assemble the operational ribosome units.

• Transcription by RNA Polymerase:

  • Transcription begins when RNA polymerase binds to DNA, unwinding the double helix and breaking the hydrogen bonds holding the complementary base pairs together.
  • RNA polymerase moves along the template strand, adding free RNA nucleotides according to base-pairing rules (uracil pairs with adenine; guanine pairs with cytosine).
  • RNA polymerase synthesizes a continuous primary transcript of mRNA.

• RNA Splicing and Transcript Maturation:

  • Primary mRNA transcripts contain alternating protein-coding and non-coding segments:
  • Introns: Non-coding nucleotide sequences present in the primary transcript that do not code for protein products; they are enzymatically excised and removed.
  • Exons: Coding sequences that specify the amino acid sequence of the final polypeptide; they are spliced and joined together.
  • Order of Exons: The natural sequence and linear order of exons remains unchanged during splicing.
  • RNA splicing joins the remaining exons together to produce a mature mRNA transcript, which subsequently exits the nucleus.

• Alternative RNA Splicing: A single gene can code for multiple distinct proteins through alternative RNA splicing. Different mature mRNA transcripts are produced from a single primary transcript depending on which exons are retained and spliced together into the final sequence, enabling substantial proteomic diversity from a single gene.

Gene Expression: Translation and Protein Conformation

• The Translation Process:

  • Translation is the synthesis of a polypeptide chain from an mRNA sequence, carried out by ribosomes in the cytoplasm with the assistance of tRNA molecules.
  • Initiation: Translation commences when the ribosome binds to the mature mRNA transcript at a specific start codon.
  • Elongation: Transfer RNA molecules transport their attached amino acids to the ribosome. The anticodons on the tRNA molecules align with and bond to their complementary codons on the mRNA transcript via complementary base pairing.
  • Peptide Bond Formation: Adjacent amino acids brought into close proximity within the ribosomal complex are joined together by peptide bonds, generating an extending polypeptide chain.
  • tRNA Release: Once an amino acid has formed a peptide bond with the growing chain, its corresponding tRNA molecule detaches from the codon and leaves the ribosome to collect another amino acid.
  • Termination: Translation terminates when the ribosome encounters a stop codon on the mRNA transcript, signaling the release of the completed polypeptide.

• Polypeptides and Three-Dimensional Protein Conformations:

  • Amino acids link sequentially via peptide bonds to form polymers termed polypeptides.
  • Polypeptide chains fold into intricate three-dimensional shapes that are stabilized by hydrogen bonds and various secondary chemical interactions between individual amino acid side chains.
  • The broad variety of three-dimensional shapes assumed by proteins dictates their specific functional characteristics (such as structural support, enzymatic catalysis, receptor signaling, or immunological defense).

• Determination of Phenotype:

  • An organism's physical and biochemical phenotype is determined predominantly by the distinct suite of proteins produced as a direct result of gene expression.
  • Phenotype is also influenced and modulated by external environmental factors.

Cellular Differentiation and Stem Cells

• Definition of Differentiation: Cellular differentiation is the developmental process through which an unspecialised cell selectively activates and expresses specific sets of genes to synthesize proteins characteristic of a specific cell lineage. This biochemical specialization enables the differentiated cell to perform dedicated physiological functions.

• Plant Meristems: In plants, differentiation occurs in localized regions called meristems. Meristems consist of unspecialised plant cells that maintain the capacity to divide (self-renew) and differentiate into the diverse specialized tissues of the plant body.

• Animal Stem Cells: Animal stem cells are unspecialised cells capable of ongoing mitotic division (self-renewal) as well as differentiation into specialized cell types.

• Pluripotency in Embryonic Stem Cells:

  • Cells harvested from the very early embryo are designated as pluripotent.
  • Pluripotent stem cells possess the developmental capacity to differentiate into ANY specialized cell type found in the adult organism, because all genes in an early embryonic cell can be switched on.

• Multipotency in Tissue (Adult) Stem Cells:

  • Tissue stem cells reside within mature tissues and organs, playing essential roles in the ongoing growth, repair, and replenishment of damaged or worn-out cells.
  • Tissue stem cells are multipotent; their differentiation potential is restricted to giving rise to the cell types characteristic of the particular tissue in which they reside.
  • For example, hematopoietic (blood) stem cells located within the bone marrow can differentiate into all the varieties of blood cells (red blood cells, platelets, and various white blood cell lineages), but cannot give rise to non-blood cell types.

• Therapeutic and Research Applications of Stem Cells:

  • Therapeutic Uses: Stem cells are utilized directly in regenerative medicine to repair or replace damaged tissues and organs (such as skin grafting for severe burns and corneal repairs). Further cellular therapies are actively in development to treat degenerative and metabolic conditions such as diabetes.
  • Research Models: Stem cells serve as biological model systems to investigate basic cellular processes, including regulation of cell growth, gene expression, and mechanisms of developmental differentiation.
  • Drug Discovery and Disease Modeling: Stem cells are utilized to model disease progression in vitro and provide human cell lines for drug efficacy and safety testing.

• Ethical Considerations: The extraction and experimental use of embryonic stem cells necessitates the destruction of early human embryos. This practice generates major ethical controversies centered on debates regarding the status of an early embryo and whether its destruction constitutes the termination of a human life.

Structure and Non-Coding Regions of the Genome

• Definition of the Genome: An organism's genome represents the entirety of its hereditary genetic information, fully encoded within its DNA sequence.

• Coding Regions: Coding regions represent distinct sections of DNA that contain functional genes coding for amino acid sequences that yield polypeptides and proteins.

• Non-Coding Genomic Regions: A large proportion of eukaryotic genomes is composed of non-coding DNA sequences that do not code for protein products.

• Roles of Non-Coding DNA:

  • Transcriptional Regulation: Non-coding DNA sequences function as regulatory elements that control, promote, or inhibit the transcription of adjacent and distant protein-coding genes.
  • Non-Translated RNA Sequences: Non-coding genomic sequences code for functional, non-translated RNA molecules essential for cellular metabolism, including transfer RNA (tRNA), ribosomal RNA (rRNA), and diverse RNA fragments.

Genomic Mutations and Chromosomal Abnormalities

• Definition of Mutation: A mutation is defined as any random, physical change occurring in the nucleotide sequence of an organism's genome.

• Origins and Mutagenic Agents:

  • Mutations can arise spontaneously at random due to cellular replication errors.
  • Mutation frequency can be induced or accelerated by mutagenic agents, which include:
  • Forms of radiation (such as ultraviolet light and X-rays).
  • Chemical mutagens (such as mustard gas).

• Levels of Mutational Occurrence: Mutations can manifest at different genomic levels, ranging from single nucleotide alterations up to changes affecting whole alleles, genes, or large chromosomal structures.

• Point Mutations (Single Nucleotide Substitutions): Point mutations involve the chemical substitution of one nucleotide base for another in a DNA sequence, resulting in the direct alteration of at most ONE amino acid codon:

  • Missense Mutations: A nucleotide substitution causes a single codon to be changed such that it specifies a different amino acid. This single amino acid alteration may produce a non-functional protein, or it may have little to no measurable effect on overall protein structure and enzymatic activity.
  • Nonsense Mutations: A nucleotide substitution changes an amino acid codon into a premature stop codon. Translation terminates prematurely, generating an abnormally shortened and generally non-functional polypeptide.
  • Splice-Site Mutations: A nucleotide substitution occurs at an intron-exon junction, disrupting normal RNA splicing. This can cause one or more non-coding introns to be mistakenly retained in the mature transcript, or coding exons to be improperly excised, substantially altering the resulting translated protein.

• Frameshift Mutations: Additions (insertions) or deletions of one or two nucleotides alter the reading frame of the entire genetic message downstream of the mutation. Every single codon and its corresponding amino acid following the mutation site are completely altered, leading to profound disruption of the synthesized protein's tertiary structure and complete loss of function.

• Chromosomal Structural Mutations: Chromosome-level mutations involve substantial structural rearrangements occurring within a single chromosome or between non-homologous chromosomes. Due to the massive quantities of genetic material affected, chromosomal mutations are frequently lethal:

  • Duplication: A segment of genetic material from a homologous partner chromosome is added to a chromosome, resulting in repeated sequences.
  • Deletion: A structural section of a chromosome is detached and lost completely.
  • Inversion: A chromosomal segment is broken, rotated through $180^\circ\text{C}$ ($180^\circ$), and reinserted in the reverse orientation.
  • Translocation: A chromosomal segment breaks off and joins onto a completely different, non-homologous chromosome.

• Evolutionary Importance of Gene Duplication: Duplication events provide an important mechanism for evolutionary innovation. Because the original copy of the duplicated gene continues to produce the functional protein necessary for survival, the extra duplicate copy is free to accumulate novel mutations without compromising the organism's immediate viability, potentially evolving novel beneficial functions over evolutionary timescales.

Evolutionary Mechanisms and Speciation

• Definition of Evolution: Evolution is the progressive phenotypic and genetic change occurring in populations of organisms across successive generations as a direct consequence of genomic variations.

• Modes of Genetic Inheritance:

  • Vertical Gene Transfer: The direct transmission of genetic sequences down lineages from parent organisms to their biological offspring. This occurs via sexual reproduction or asexual reproduction (including binary fission in prokaryotes).
  • Horizontal Gene Transfer: The lateral exchange of genetic material between independent organisms belonging to the same generation. Highly prevalent in prokaryotes and viruses, horizontal gene transfer frequently involves the physical transmission of whole or partial plasmids between bacterial cells, driving rapid evolutionary adaptation (such as the rapid dissemination of antibiotic resistance).

• Natural Selection: Natural selection is the non-random increase in the frequency of adaptive DNA sequences that enhance an organism's survival and reproductive success, alongside the corresponding non-random decrease in the frequency of deleterious genetic sequences.

• Phenotypic Selection Dynamics:

  • Stabilising Selection: Natural selection favors intermediate phenotypes within a population distribution while selecting against phenotypic extremes, thereby reducing overall phenotypic variance without altering the mean value.
  • Directional Selection: Natural selection favors phenotypes located at one extreme of the phenotypic distribution curve, causing a progressive directional shift in the population mean toward that favored extreme.
  • Disruptive Selection: Natural selection favors two or more distinct phenotypic extremes simultaneously while selecting against intermediate phenotypes, potentially splitting the population into two distinct phenotypic groups.

• Concept of a Biological Species: A biological species is defined as a group of interbreeding organisms capable of mating to produce viable, fertile offspring.

• The Process of Speciation: Speciation is the evolutionary generation of new biological species driven by the interaction of isolating mechanisms, novel mutations, and natural selection:

  • An isolation barrier physically or biologically divides an ancestral population into separate, non-interbreeding subpopulations, halting gene flow.
  • Distinct random mutations arise independently within each isolated subpopulation.
  • Differing selection pressures operate upon each subpopulation, driving natural selection to adapt each group to its specific ecological niche.
  • Over generations, the genetic and phenotypic divergence becomes so profound that if the two subpopulations reunite, they are unable to interbreed to produce fertile offspring, establishing distinct species.

• Allopatric vs. Sympatric Speciation:

  • Allopatric Speciation: Occurs when populations are physically separated by geographical barriers, such as expanses of water, mountain ranges, or vast deserts, preventing physical contact.
  • Sympatric Speciation: Occurs when reproductive isolation emerges without geographic separation, taking place within the same geographical territory. Sympatric isolation is mediated by ecological barriers (such as divergence in habitat or breeding preferences) or behavioral barriers (such as divergence in mating rituals or courtship displays).

Genomic Sequencing, Phylogenetics, and Pharmacogenetics

• Genomic Sequencing Principles: Genomic sequencing entails determining the precise linear order of nucleotide bases across individual genes, targeted genomic loci, or entire organismal genomes.

• Methodology and Bioinformatics:

  • Modern genome analysis requires two interdependent steps:
  • High-throughput laboratory sequencing methodologies to read DNA base sequences.
  • Comprehensive analysis of sequence datasets using bioinformatics, an interdisciplinary field integrating computer science, statistical modeling, and mathematics to organize and interpret complex biological data.

• Phylogenetics and Molecular Clocks:

  • Comparing genomic sequences across diverse taxa reveals homologous regions and mutations, allowing the construction of phylogenetic trees that illustrate evolutionary lineages and shared common ancestry.
  • Assuming that nucleotide mutation rates remain relatively constant over deep geological time (the molecular clock hypothesis), scientists calculate the precise timing of evolutionary divergences between lineages.
  • Molecular clock timelines are calibrated and validated by cross-referencing genomic sequence divergence against fossil record dates.

• The Three-Domain System: Comparative genomic analysis has led to the taxonomic classification of all living organisms into three major biological domains:

  • Domain Bacteria.
  • Domain Archaea.
  • Domain Eukaryotes (Eukarya).

• Evolutionary Chronology: Genomic data and fossil records trace the lineage of life through key chronological milestones: the emergence of primitive cellular life forms $\rightarrow$ unicellular prokaryotes $\rightarrow$ complex eukaryotes $\rightarrow$ multicellular animals and plants. Within animal evolution, invertebrates evolved prior to vertebrates; across terrestrial evolutionary history, land plants evolved last.

• Comparative Genomic Discoveries:

  • Genomes of diverse organisms have been sequenced, including the human genome, diverse pathogens responsible for diseases in humans, domestic animals, and agricultural crops, as well as classic scientific model organisms.
  • Comparisons across diverse lineages demonstrate that key regulatory and functional genomic sequences are highly conserved (exhibiting virtually identical base sequences across vast evolutionary distances).

• Personalized Medicine and Pharmacogenetics:

  • Sequencing individual human genomes allows clinical identification of specific genetic errors, mutations, and risk factors associated with complex medical conditions, including hereditary predispositions to cancer, psychiatric disorders, and drug dependencies.
  • Pharmacogenetics (or pharmocogenetics): The clinical application of genomic information to inform the rational selection, design, and administration of pharmaceutical drugs.
  • Personalised Medicine: Using an individual's specific genomic sequence profile to select the most therapeutic pharmaceutical agents and determine exact customized dosages, maximizing therapeutic efficacy while minimizing adverse drug reactions.