Epigenesis
All cells of an organism share an identical genome – each cell contains the entire set of genetic instructions for that organism
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The activation of different instructions (genes) by chemical signals will cause the cell to differentiate
The transcriptome represents the genetic instructions that have been actively transcribed into RNA molecules
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It includes all rRNA, tRNA and mRNA sequences (including all variants produced via alternative splicing)
The proteome is the totality of proteins expressed within a cell, tissue or organism at a particular time
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The proteome is significantly larger than the transcriptome as proteins may be modified (e.g. glycosylated, phosphorylated, etc.) to promote further variations
While the genome will be identical between the individual cells of an organism, the transcriptome and proteome will vary due to the influence of chemical signals triggered by internal or external stimuli
Genome → Proteome
Epigenesis
The activation of different genes in different cells of a multicellular organism will lead to different patterns of development in these cells
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This causes these cells to have different characteristics even though their base sequences are identical (the phenotype is altered but not the genotype)
Epigenetics is the study of how chemical and environmental triggers influence gene expression patterns (‘epi’ = above ; ‘genetic’ = genes)
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Unlike genetic changes (mutations), epigenetic modifications do not alter the DNA sequence and are potentially reversible
The specific development of an organism from an undifferentiated egg cell into a complex multicellular organism via differential gene expression is called epigenesis
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In humans, the process of epigenesis involves the following steps:
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A fertilised egg (zygote) undergoes several divisions to form a solid ball of cells (called a morula)
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These cells will differentiate to form a blastocyst, composed of a fluid-filled cavity containing an inner cell mass
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The inner cell mass will continue to divide and differentiate to form an embryo (which will develop into a foetus)
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Embryonic Development
