QUESTION 9 Most complex organisms develop from specialized reproductive cells (e

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QUESTION 9 Most complex organisms develop from specialized reproductive cells (eggs and sperm in animals) Two reproductive cells meet, then they grow and divide to form every type of cell in the adult organism. In order for this process to occur the epigenome must be erased through a process called reprogramming At certain times during development (the timing varies among species), specialized cellular machinery scours the genome and erases its epigenetic tags in order to return the cells to a genetic "blank slate Yet, for a small minority of genes, epigenetic tags make it through this process and pass unchanged from parent to offspring A List two types of epigenetic tags as discussed in the class. Be specific. B Please explain the mechanism of how epigenetic tags affect the fate of cells Your answer has to reflect the level of a junior level course and show good evidence of learning from our lectures, specfically what was covered in Chapter 9 or 10 T T T Arial Path p

Explanation / Answer

Embryonic stem cells exhibit dramatic and complex alterations to both global and site-specific chromatin structures. Lee et al. performed an experiment to determine the importance of deacetylation and acetylation for stem cell differentiation by looking at global acetylation and methylation levels at certain site-specific modification in histone sites H3K9 and H3K4. Gene expression at these histones regulated by epigenetic modifications is critical in restricting the embryonic stem cell to desired cell lineages and developing cellular memory.

For mammalian cells, the maintenance of cytosine methylation is catalyzed by DNA methyltransferases and any disruption to these methyltransferases will cause a lethal phenotype to the embryo. Cytosine methylation is examined at H3K9, which is associated with inactive heterochromatin and occurs mainly at CpG dinucleotides while global acetylation is examined at H3K4, which is associated with active euchromatin. The mammalian zygotic genome undergoes active and passive global cytosine demethylation following fertilization that reaches a minimal point of 20% CpG methylation at the blastocyst stage to which is then followed by a wave of methylation that reprograms the chromatin structure in order to restore global levels of CpG methylation to 60%. Embryonic stem cells containing reduced or elevation levels of methylation are viable but unable to differentiate and therefore require critical regulation of cytosine methylation for mammalian development.

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