ANTHP 105 9.8.25 (Lecture 5)

Epigenetic Regulation and Gene Expression

• Impact of environment on gene expression occurs at multiple levels, including DNA structural level, transcription, RNA processing, and translation; each step is differentially sensitive to environmental conditions.
• DNA structural level:
  • Chromosome vs. chromatin: chromatin must unwind to read DNA and transcribe it into messenger RNA (mRNA).
  • Epigenetic marks on chromatin (histone modifications and DNA methylation) regulate how easily DNA can unwind from histones; marks can either amplify or inhibit accessibility.
• Histone modifications and DNA methylation:
  • Modifications added to histones can make unwinding easier or harder, influencing transcriptional activity.
  • DNA methylation can have regulatory effects on gene expression (context-dependent), potentially affecting which genes are transcribed.
• Regulatory genes and HOX genes:
  • HOX genes (homeobox genes) are key regulators of development in organisms with backbones; they control when and where body parts develop during development.
  • HOX genes can regulate transcription to turn on or off production of RNA segments, affecting whether portions of the messenger RNA are coding or noncoding.
• RNA processing and splicing:
  • Splicing enzymes remove noncoding chunks (introns) from pre-mRNA to produce mature mRNA.
  • Splicing is sensitive to cellular environmental conditions; adverse conditions can hinder splicing, potentially preventing formation of mature mRNA.
• Translation and gene expression:
  • After transcription and splicing, mRNA is transported to the ribosome for translation into protein; translation is another regulatory point.
• Plasticity and development:
  • Plasticity can occur at the individual level (short-term responses) and developmental level (long-term changes in form or function).
  • Developmental plasticity involves changes during development that can permanently alter the adult phenotype.
• Epigenetic marks shaping phenotype: example setup
  • Hormone signaling (e.g., norepinephrine) can influence a specific tissue, such as brown adipose tissue (BAT).
  • BAT deposition can increase in response to hormonal cues, promoting heat production via energy-efficient brown fat.
  • Hormone-induced histone modifications can make it easier to read a particular DNA region, influencing BAT development and function.
  • These histone modifications may be transmitted to subsequent generations in some contexts, though typically traits acquired over a lifetime are not passed to offspring.
• Gene-by-environment interactions:
  • The transcript notes emphasize the complexity of gene–environment interactions and the debate over how much of a complex trait is determined by genetics versus environment.
• Key question raised:
  • Is it possible to determine the degree to which complex traits are shaped by genetics, environment, or their interaction?

Foundational Concepts: Taxonomy and Species

• The foundational concept introduced: taxonomy and the idea of a species.
• Species definition used in this course: a group of organisms that can interbreed and produce fertile offspring.
• Acknowledgement that there are many definitions of species; this is the one used in the class.
• Origin of terms: the idea of genus and species was coined by John Ray in the 1600s.
• Binomial nomenclature:
  • Binomial nomenclature is the two-name naming scheme used to classify organisms (genus + species).
  • Example: Homo sapiens; genus = Homo, species = sapiens; the two-part name identifies the organism.
• Taxa and taxonomy terminology:
  • Taxa (plural) are the named groups in taxonomy; a single taxon is a taxon at any level (e.g., a species is a taxon).
• Descent from a common ancestor:
  • The concept that members of the same species descend from a common ancestor.
  • Proponent highlighted in the transcript: Erasmus Darwin (grandfather of Charles Darwin), who was a proponent of descent concepts and engaged with fossil interpretation.
  • Erasmus Darwin was known for his interest in flowers and breeding, and he contributed early ideas about descent from ancestors.
• Historical context: Darwin and Lyell (as referenced in the transcript):
  • Erasmus Darwin contributed to ideas about descent from a common ancestor.
  • Charles Lyell, a geologist, studied rocks in Scotland and argued that geological processes operating today have also acted in the past, informing ideas about deep time and gradual change (often linked to uniformitarian thinking).
  • The transcript notes Lyell’s work on rocks and the implication that the same processes are at work today, informing broader evolutionary and geological contexts.

Taxonomic Hierarchy and Nomenclature Basics

• Taxonomic levels and their relationships (as a framework relevant to the discussion):
  • Species → Genus → Family → Order → … (hierarchical organization used in classification)
• Binomial name and the convention of Latin naming:
  • Each species has a two-part name consisting of genus and species (e.g., Homo sapiens).
• Special terms:
  • Taxon (plural: taxa): any named group in the taxonomy system; the term is used for a single group or a set of related organisms.

Descent from a Common Ancestor and Historical Figures in Biology

• Descent from a common ancestor as a foundational idea in biology, attributed to Erasmus Darwin in the transcript as an early thinker on this concept.
• The transcript notes the historical role of Darwin (the famous Charles Darwin) in association with these ideas, but emphasizes Erasmus Darwin as the originator of the concept within this lecture context.
• Role of Charles Lyell in shaping thinking about time scales and processes:
  • Lyell’s geological perspective highlighted ongoing processes that shape the Earth, informing how scientists think about change over long time scales relevant to evolution.
• The transcript presents these as foundational ideas that connect biology with geology to understand development and evolution over deep time.