Epigenesis and Epigenetics

• Epigenesis looks beyond inherited genes, examining environmental effects on development.
• It studies positive and negative impacts of surroundings on genes, going beyond inherent traits.

Epigenetics

• Since the 19th century, epigenesis has been known as epigenetics.
• Epigenetics studies the impact of external (environmental) or internal (behavioral) factors on DNA.
• This impact modifies gene expression without altering the nucleotide sequence of DNA.
• Such modifications may be heritable.
• Epigenetics is essentially epigenesis + genetics.

Epigenome

• The genome is the complete set of DNA within a cell.
• Every cell contains the same genome, with the potential to specialize into different cell types like muscle or nerve.
• The epigenome examines internal and external factors causing chemical modifications in genes.
• These modifications can either activate or inactivate genes.
• This helps in understanding environmental influences on genes, psyche, toxins, genetic disorders, and behavior.
• Monozygotic twins showcase epigenetic effects, displaying increasing differences, phenotypically and psychologically, over time due to varied experiences

History of Epigenesis

• Aristotle developed the Epigenesis Theory, stating organism development depends on sequential growth stages.
• Caspar Friedrich Wolff reinforced this theory in the 18th century.
• He observed embryonic development in plants and animals, noting creation arises from systems of cells.

Epigenesis vs. Preformationism

• Epigenesis suggests an organism develops in stages without a pre-existing complete form.
• An embryo differentiates from an egg cell to a zygote, then into various cell systems, organs, and finally an infant.
• Preformationism posits that development occurs from a pre-existing complex form that simply grows in size.
• Followers once believed miniature organs and cells existed within the egg cell.

Examples of Epigenesis

DNA Methylation in Gene Transcription

• DNA methylation involves enzymes adding methyl groups ( ) to DNA bases, silencing genes by turning off transcription.

Dosage compensation/X-chromosome inactivation

• In dosage compensation, larger gene or chromosome areas may be methylated, inactivating one X chromosome in somatic cells.
• The inactive X chromosome forms a Barr body due to tight winding.
• This random process occurs during embryonic development.
• DNA methylation here inactivates gene transcription.
• Dosage compensation is seen in calico and tortoise-shell cats' fur color, where mosaic expression is due to X-chromosome inactivation.

Genomic Imprinting

• During gametogenesis, an inherited maternal or paternal allele that is methylated (imprinted) is not transcribed (silenced).
• Gene expression is determined by the other parent's non-methylated allele.
• Methylation of the silenced allele is maintained during DNA replication to keep it dormant in future generations.
• Prader-Willi Syndrome results from the loss of the paternal gene.
• Angelman Syndrome is caused by the loss of the maternally inherited gene.

Chromatin Structure/Remodeling

Histone Binding

• DNA (as chromatin) wraps around histone proteins in the nucleus.
• Tight binding of chromatin around the histone switches off the gene, while loose binding allows gene expression.
• This is important in regulating transcription initiation.

Acetylation

• Histone tails are either acetylated (activating chromatin) or not acetylated (inactivating chromatin).
• Non-acetylated histone tails form a tight bond with DNA, switching it off.
• Histone acetylation plays a crucial role in DNA replication and repair, and for the functioning of chromosomal processes in eukaryotes.

Epigenesis studies environmental impacts on gene development beyond inheritance. Epigenetics, since the 19th century, examines how external and internal factors modify gene expression without DNA sequence alteration, sometimes heritably. The epigenome explores chemical gene modifications, explaining environmental influences.