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Which part or parts of the the genome are the sequences located. Are they spread across the chromosomes? If so how are they accessed sequentially with precision during embrygenesis?

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2 Answers 2

This is a very general question. The "developmental sequences" are just genes like any other. Like all genes they are semi-randomly distributed through the genome. While there are gene-rich and gene-poor areas in the genome, with some exceptions --notably the homeobox genes--, genes are not grouped by function.

As to how they are accessed sequentially, that is a huge question but it is not specific to embryogenesis. Genes will always need to be turned on/off at specific times and in response to specific stimuli. This is particularly obvious during development but is not exclusive to it.

The basic way this happens is through feedback and control loops. Gene A activates the transcription of gene B which activates gene C which then deactivates A. If you manage to turn on A, the rest will follow. One of the ways this can happen in developing embryos is through maternal mRNA. These are mRNA molecules that are present in the fertilized egg and which begin to be translated. This gives us a starting point to activate gene A, which then turns on B etc.

This answer is a huge simplification but a full answer could take up a couple (or 40) of PhD projects.

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Terdon is right on the money when it comes to how is the coordination done, and I know of at least one student working on this for her thesis. However, to answer your other question directly, there was a great review in 2010, which was taught in my genetics class that year.

It is important to note that we still don't know all the genes that are involved with meaningful certainty. Further, it would be exceedingly difficult to fund and conduct Hong et al's work in the US.

If you read the article you will notice that several genes that are involved are genes that are active through out life at various levels. Figure 4 gives a good example of how spread out the information.

Notice that the three categories of genes they used were even quite different: Stem Cell Specific Genes, Muscle, fat and connective tissue genes, Significantly Enriched GO term

So when you think about how diverse the genes are, it would be even odder for them to be clustered together. Again, they are probably not truly random, but they don't seem to have a pattern.

To quote their take home:

Furthermore, by comparing with published data, we identified three groups of maternal genes with distinct regulation profiles during wk 4–9 of development, which provided a molecular basis for the anatomically distinct developmental events that occur between early embryogenesis and subsequent organogenesis and histogenesis.

More important, our genome-wide and detailed developmental profiling of the patterns of gene expression has revealed that genes involved in the same biological processes are often regulated coordinately. This finding has led us to hypothesize that patterns of gene expression, if known in sufficient detail as in the present study, might be used to predict a role for the gene in a developmental process.

Hong Yi, Lu Xue, and Ming-Xiong Guo, et al. FASEB J. 2010 September; 24(9): 3341–3350. doi: 10.1096/fj.10-158782 PMCID: PMC2923361

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