Lecture Review on Molecular Biology and Evolution

LECTURE 1–2: CENTRAL DOGMA, TRANSCRIPTION & mRNA PROCESSING

  • The central dogma explains the flow of genetic information from DNA to RNA to protein.
  • Transcription is the process where a gene's DNA sequence is copied into RNA.
  • Unlike DNA replication, transcription only uses the template strand of the DNA, which is read in the 3′→5′ direction.
  • RNA polymerase synthesizes RNA in the 5′→3′ direction.
  • RNA polymerase binds to a promoter, a specific DNA sequence that signals the start of transcription.
  • No primer is needed for transcription, contrasting with replication.
  • Transcription has three stages:
      - Initiation: RNA polymerase binds to the promoter and unwinds the DNA.
      - Elongation: The RNA strand is built.
      - Termination: RNA and RNA polymerase are released.
  • Termination can involve intrinsic mechanisms like hairpin loops or proteins like rho factor, which provides additional regulation in bacteria.
  • In eukaryotes, the primary RNA product is pre-mRNA, which undergoes processing before translation.
  • Processing includes:
      - Addition of a 5′ cap for ribosome recognition and stability.
      - Addition of a poly-A tail for protection and export.
      - Splicing, which entails the removal of introns and joining of exons.
  • Alternative splicing increases functional diversity by allowing a single gene to encode multiple protein variants without expanding genome size.

LECTURE 3: TRANSLATION

  • Translation is the conversion of the mRNA sequence into a protein and occurs at the ribosome, which consists of ribosomal RNA (rRNA) and proteins.
  • The ribosome has three functional sites:
      - A site: where new transfer RNA (tRNA) enters.
      - P site: where the growing polypeptide chain is held.
      - E site: where tRNA exits.
  • Translation begins when the ribosome recognizes a start codon (AUG).
  • In prokaryotes, recognition is facilitated by the Shine-Dalgarno sequence; in eukaryotes, the ribosome binds the 5′ cap and scans for the start codon.
  • tRNAs deliver specific amino acids to the ribosome, carrying anticodons that pair with the mRNA codons.
  • As the ribosome advances along the mRNA, peptide bonds form, extending the polypeptide chain from the N-terminus to the C-terminus.
  • Translation concludes when a stop codon is reached, prompting release factors to complete the polypeptide release.
  • A critical difference between prokaryotes and eukaryotes is that transcription and translation occur simultaneously in prokaryotes but are separated in space and time in eukaryotes.

LECTURE 4: MUTATIONS

  • Mutations are alterations in the DNA sequence that can affect gene function and phenotype. They can occur at two main levels:
      - Nucleotide level: changes in individual nucleotides.
      - Chromosomal level: larger structural changes in chromosomes.
  • Point mutations can be classified into:
      - Substitutions:
        - Transitions: Purine ↔ purine or pyrimidine ↔ pyrimidine.
        - Transversions: Purine ↔ pyrimidine.
      - Insertions and deletions: often lead to frameshift mutations that alter the reading frame, typically resulting in nonfunctional proteins.
  • Classifications based on effect:
      - Silent mutations: do not change the protein structure.
      - Missense mutations: alter one amino acid in the protein.
      - Nonsense mutations: introduce a premature stop codon.
  • Mutations may arise spontaneously during DNA replication or be induced by environmental mutagens like radiation or chemicals.
  • Germ-line mutations are heritable and passed to offspring, while somatic mutations affect only the individual.

LECTURE 5: OPERONS (PROKARYOTIC REGULATION)

  • Gene expression in prokaryotes is primarily regulated at transcription initiation through structures known as operons.
  • An operon consists of:
      - A promoter for initiation.
      - An operator that regulates access to the promoter.
      - Several structural genes that are transcribed together.
  • The lac operon is an inducible system activated by lactose presence; lactose binding to the repressor allows transcription of genes necessary for lactose metabolism.
  • The trp operon is repressible and is deactivated when tryptophan is abundant; tryptophan acts as a co-repressor activating the repressor.
  • Regulation can be:
      - Negative: Repressors block transcription.
      - Positive: Activators enhance transcription.
  • Sigma factors assist RNA polymerase in recognizing various promoter sequences, facilitating coordinated gene expression based on environmental conditions.

LECTURE 6: TRANSCRIPTION FACTORS (EUKARYOTIC REGULATION)

  • Eukaryotic gene expression is regulated more complexly compared to prokaryotes, mainly through transcription factors.
  • These proteins bind to specific DNA sequences (promoters, enhancers, and silencers) to modulate transcription initiation.
  • Activators increase transcription, while repressors decrease it.
  • Enhancers can influence transcription from a distance by looping the DNA.
  • Unlike operons, eukaryotic genes are dispersed throughout the genome, but coordinated expression is achieved via shared transcription factors regulating various genes.
  • This mechanism allows for precise control of gene expression spatially and temporally during development and differentiation.

LECTURE 7: POST-TRANSCRIPTIONAL REGULATION & EPIGENETICS

  • Eukaryotic gene expression is regulated after transcription through several mechanisms:
      - RNA processing
      - RNA interference
      - Epigenetic modifications
  • Epigenetics refers to modifications in gene expression without changing the DNA sequence, commonly through:
      - DNA Methylation: typically represses gene expression by hindering transcription factor binding or recruiting repressor proteins.
      - Histone Modifications: such as acetylation, which opens chromatin structure and enhances transcription.
  • Chromatin exists in two forms:
      - Euchromatin: loosely packed, active form.
      - Heterochromatin: tightly packed, inactive form.
  • RNA interference utilizes small RNAs (miRNA and siRNA) that can bind to mRNA to either degrade it or block its translation.
  • These mechanisms allow cells to finely regulate protein production in response to varied internal and external signals.

LECTURE 8: VIRAL REGULATION

  • Viruses rely on host cells for gene expression and modulate their gene expression based on their lifecycle.
  • In the lysogenic cycle, viral DNA is integrated into the host genome, replicating without producing new viruses.
  • In the lytic cycle, viral genes are actively expressed, resulting in the creation of new viral particles and eventual host cell lysis.
  • Viral genes are divided into early genes (involved in host machinery takeover) and late genes (encode structural components for viral assembly and release).
  • Viral promoters often mimic host promoters to ensure recognition by host RNA polymerase.

LECTURE 9–10: NATURAL SELECTION & EVOLUTION

  • Natural selection is the mechanism where certain traits enhance reproductive success, leading to increased frequency in a population.
  • Charles Darwin's observations include:
      - Overproduction of offspring: leads to competition for resources.
      - Limited resources: sets a cap on population growth.
      - Variation among individuals: some variations confer advantages.
      - Heredity: advantageous traits can be passed down.
  • Individuals with favorable traits exhibit higher fitness, resulting in more offspring.
  • Natural selection acts on phenotypes but results in changes to allele frequencies, constituting evolution.
  • The Hardy-Weinberg equilibrium is a null model serving to detect evolution; deviations suggest active processes like selection, mutation, migration, or genetic drift at work.

LECTURE 10: PATTERNS OF SELECTION

  • Natural selection can manifest in different patterns based on fitness relative to trait values:
      - Stabilizing selection: favors intermediate phenotypes, reducing variation.
      - Directional selection: favors one extreme, shifting the population mean.
      - Disruptive selection: favors both extremes, potentially increasing variation which can lead to speciation.
  • Such patterns can be visualized through graphs mapping trait distribution and fitness, necessitating accurate interpretation skills for exams.

LECTURE 11: PHYLOGENIES

  • Phylogenetic trees represent hypotheses on evolutionary relationships among species, indicating ancestry and descent patterns.
  • Nodes signify common ancestors, while branches symbolize lineages.
  • A clade comprises a common ancestor and all its descendants.
  • Traits may be classified as homologous (inherited from a common ancestor) or convergent (similar due to independent evolution).
  • The principle of parsimony suggests selecting the explanation that requires the fewest changes.
  • The orientation of a phylogenetic tree does not alter the relationships it illustrates.

LECTURE 12–13: SPECIATION & ISOLATION

  • Speciation occurs when populations become reproductively isolated, leading to genetic divergence.
  • The biological species concept defines species as groups capable of interbreeding and producing fertile offspring.
  • Alternatives to this concept may rely on morphological or evolutionary lineages.
  • Reproductive isolation mechanisms can be classified as:
      - Prezygotic: prevent mating or fertilization (e.g., temporal, behavioral, mechanical, habitat, gametic).
      - Postzygotic: reduce viability or fertility of offspring (e.g., hybrid inviability or sterility).
  • Reinforcement strengthens reproductive barriers, promoting further divergence when hybrids exhibit low fitness.

LECTURE 14–15: LEARNING & BEHAVIOR

  • Behavior results from a mix of genetic and environmental influences, classified as either innate or learned.
  • Innate behaviors are genetically programmed (e.g., reflexes and instincts), while learned behaviors develop through experience.
  • Types of learning include:
      - Imprinting: occurs during critical periods.
      - Habituation: a decreased response to repeated stimuli.
      - Sensitization: an increased response to stimuli.
  • Behavior can be studied through various frameworks, including ethology (study in natural contexts) and behavioral ecology (evolutionary significance focus).
  • Concepts like proximate causes (how behaviors occur) and ultimate causes (why behaviors occur) provide insight into behavioral phenomena.

LECTURE 16–17: BIOGEOGRAPHY & ECOLOGY

  • Biogeography studies species distribution shaped by evolutionary history and ecological factors.
  • Dispersal refers to organisms moving to new locations, while vicariance involves barriers that fragment populations.
  • Ecology focuses on organism-environment interactions. Key climatic factors include solar radiation, latitude, and precipitation, determining biomes defined by climatic conditions and vegetation types.
  • Community ecology analyzes species interactions and energy flow through trophic levels.

LECTURE 20–21: COMMUNITIES & COMMUNITY FUNCTION

  • Ecological communities are shaped by species interactions, characterized quantitatively by species richness, evenness, and diversity.
  • Trophic levels depict energy flow from primary producers to consumers.
  • Community function results from interactions, including competition, predation, and mutualism.
  • Keystone species exert a notably large ecological impact, while foundation species and ecosystem engineers fundamentally shape the environment.
  • Trophic cascades illustrate how alterations at one trophic level influence several levels below, emphasizing ecological interconnectedness.

Final Advice

  • Understanding rather than memorization is crucial: focus on cause and effect.
  • Anticipate evaluation methods such as:
      - Graph interpretation.
      - Mechanism questions.
      - Hypothetical scenarios (e.g., "What happens if…?").