IB Biology B2.3 Notes

You previously learned about the basic components of eukaryotic and prokaryotic cells. In prokaryotes, this is all there is to the organism. However, in eukaryotes are larger multicellular organisms, requiring many different types of specialized cells. These all originate from a zygote, which undergoes mitosis into unspecialized cells. These are called stem cells, which are capable of dividing endlessly and specializing along different pathways via a process called differentiation.

The process of differentiation in the early-stage embryo is directed by external chemical called morphogens. These function as follows:

1. Source cells secrete morphogens at different parts of the embryo as a positional guide.
2. This will form a side with a low morphogen concentration and another side with a high morphogen concentration, forming a gradient.
3. The morphogen then act on nearby cells to generate a change in gene expression, causing differentiation.
4. Thus, cells on the high concentration side differentiate into the target tissue, whereas cells on the low concentration side do not.

This is thus a gradient of gene expression, will results in the formation of tissues in specific locations. As these differentiate and grow as according to the gradients, they end up forming organs, and then the whole body.

Thus, this process is necessary in embryonic development, growth into adulthood, and continual cell renewal throughout life.